Why Your Hot Water Takes So Long
A lot of people complain that it takes a long time to get hot water at one or more of their hot water fixtures. In my case it’s the kitchen sink. I’ve measured it many times, and it takes two and a half minutes for hot water to arrive there. While I’m waiting for the hot water, 3 gallons of water go down the drain. Then after I turn off the water, another 3 gallons of hot water is left stranded in the pipes between the kitchen and the water heater. So I waste water on the front end, energy on the back end, and spend time waiting that I’d rather use doing something else. Does it really need to be so bad?
The basic problem
When you think it through, it’s easy to see that hot water distribution suffers because of one basic problem. When three gallons of water spill out of my kitchen faucet while I wait for hot water, that tells me something about the volume in the hot water line between the water heater and the kitchen faucet. To waste less water, I need less water to be sitting in that pipe. That’s it. That’s the problem.
The actual amount of wasted water, however, is often greater than the water sitting in the pipes. That’s the structural waste. But when hot water flows through pipes that are too large, it doesn’t just push everything in front of it until all of the structural waste is out of the pipe and the hot water arrives at the tap. The hot water mixes with the water already in the pipe and sometimes rides on top of the water in the pipe. That lengthens the time to get hot water as well as the amount of water wasted. (See Figure 2 in this Journal of Light Construction article by Gary Klein (pdf) for a visual.)
If you look at some of the plumbing company’s websites, they’ll give you the right answer…partly. But they add several other things, including sediment buildup, undersized water heater, and outdoor temperature, that have nothing to do with how fast the hot water arrives. The real reason it takes so long is the volume of water that has to be flushed from the pipe before hot water flows from the tap. To get less water in the pipe, you need shorter, smaller pipes.
Stay with me as I explain this. Yes, you can get hot water faster without changing out your hot water distribution system. I know that can be difficult and expensive in existing homes. But it’s possible to do some partial measures that can help.
Pipe length
The first part of the problem is excessive pipe lengths. That results from the design of a house and its plumbing system. The big reason is that the water heater and the wet rooms (i.e., the rooms where water is used) are too spread out. When you put a water heater in an attached garage, for example, and a bathroom on the opposite end of the house, there’s a whole lot of pipe for the hot water to traverse on its way to the showerhead. The solution is shorter pipes through designing a floor plan that clusters all the wet rooms in one part of the house…or at least a design that doesn’t spread them out as far as possible.
Another reason for excessive pipe lengths is the design of the water distribution system that connects the water heater to the fixtures. Hot water pipes rarely take the shortest distance from the water heater to a fixture. In my house, a pipe going straight from the water heater to the kitchen faucet would be about 40 feet long. The actual hot water pipe is about 80 feet long.
Pipe diameter
The second part of the problem is that the pipes are too big in diameter. Why are pipes too big? Because our plumbing fixtures have gotten more efficient while the way we design distribution systems hasn’t kept pace with fixture changes. Flow rates of plumbing fixtures are far lower than they used to be, especially since the 1992 Energy Policy Act went into effect. Showerheads, for example, went from flow rates of 3.5 or more gallons per minute (gpm) to 2.5 gpm, a drop of 37 percent or more. Dishwashers and washing machines cut their flow rates in half. When you cut the flow rate in half, you double the time it takes for hot water to reach the fixture.
The low flow fixtures we use now certainly save water. They even save hot water, but only after the hot water arrives and you’re actually using it. Without an efficient hot water distribution system, you still waste at least as much hot water as the high-flow fixtures while you’re waiting for it to arrive at the tap. We wasted water faster in the old days because that water sitting in the pipes got pushed out faster with higher flow rates.
Putting a low flow fixture at the tap hasn’t changed the volume of water in the line, the structural waste. It simply reduced the flow rate while leaving the same amount of water to push through before hot water can arrive. As mentioned earlier, though, the reduced flow rate may result in even more wasted water.
With low flow, the hot water doesn’t come rushing through the pipe, pushing everything in front of it all at once. That kind of displacement flow, or plug flow, doesn’t happen with low flow rates in larger pipes. The velocity is low enough that mixing occurs, which increases the time it takes to get hot water and the amount of wasted water.
The keys, then, are to decrease the amount of water sitting in the pipes and also size the pipe diameters for good flow patterns.
What can you do?
Most of the websites offering solutions to this problem online are from plumbing companies. Be wary of their advice because many of them want to sell you a tankless water heater or recirculation pump. Some even tell you to install fixtures with higher flow rates. That last one is really annoying because we’ve had low-flow fixtures for nearly 30 years now. Yes, the early low-flow fixtures weren’t as effective, but engineers have solved those problems. My kitchen faucet (shown above) works really well at 1.2 gallons per minute.
Here are a few solutions to the hot water delivery problem, starting with the most comprehensive.
- Reduce the length and diameter of your hot water pipes. This is easy and inexpensive if you’re designing a house. For existing homes, do what you can. If you have an opportunity to change the floor plan during a big renovation, cluster the wet rooms.*
- Move the water heater to a more central location. Again, easier to do in the design phase.
- Use two or more water heaters. If you’ve got one hot water fixture far from the water heater, consider a small water heater just for that use.
- Install a demand hot water recirculation system. Continuous recirculation can add a lot to your water heating costs. Putting a continuous recirculation system on a timer can help. Best is to use the demand-type reciruclation system. Push a button, wait a few minutes, and your hot water is ready to flow from the tap without wasting water in the meantime. The Building America Solutions Center has a good description of how demand hot water recirculation works.
When I remodel my basement next year, I’m going to replace the hot water distribution system. I’m not changing the floor plan, so I’ll be limited in how much I can reduce the pipe lengths, but I can certainly cut the pipe diameter significantly. Right now, I have a 3/4″ copper pipe running all the way to the kitchen. That’s the last fixture and a good 20 feet past the second-to-last fixture. I should be able to cut the pipe diameter to 3/8″ for the last section. Along with the little bit of length reduction I can get, I should be able to cut my wait time from 2.5 minutes to 30 seconds or less. That’s still too long, though, so I’ll also install a demand recirculation system.
Always try to address the real source of any problem. In the case of hot water taking too long to get to the tap, the primary problem is the volume of water in the pipes. Reducing that volume with shorter, smaller pipes is the best way to go. If that still doesn’t get your wait time down enough, use a demand hot water recirculation system.
Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and the founder of Energy Vanguard. He has a PhD in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.
* Caution: I’m giving general advice here. If you want to reduce the diameter of your plumbing pipes, it’s a good idea to pay attention to plumbing codes and, more important, physics. If you’re not a licensed plumber or building professional, find someone who can help you make the right decisions.
Related Articles
Water Heating Is a System, Not Just a Water Heater
The 3 Types of Energy Efficiency Losses in Water Heating
Down the Drain! — Heat Recovery for Your Hot Water
NOTE: Comments are moderated. Your comment will not appear below until approved.
This Post Has 54 Comments
Comments are closed.
What about Convective heat Loop(s)
When I designed my house, I put the three of the bathrooms within a few feet of the HPWH in the crawlspace. This gives me great HW within seconds.
The other 1.5 baths and laundry are on the other end of the house. as is the kitchen. For these I use a thermo-loop system that recirculates the HW without a pump. The level of circulation is regulated by a valve at the WH and the pipes to and from the kitchen are well insulated to minimize loss. This gives me warm water instantly and hot water eventually. I can live with that.
I also installed a DWHR system that runs the pre-heated water to my water heater and to the cold side of my sinks in the three baths, since really cold water is seldom needed there.
Norman, tell me more about this system. I don’t quite understand what you’re doing.
I used this as a starting point: https://www.buellinspections.com/re-circulating-hot-water/
But I finally went with a very simple solution, avoiding all the check valves and such.
The hot water line to the other end of the house is 3/4″ per code. This runs to right below the kitchen sink, before splitting off the feed washing machine and a lavatory.
At the kitchen sink connection I had a 1/2″ return line run, This line goes into the bottom of the HPWH and has a shut valve there,
This loop allows hot water to slowly circulate through the being reheated as needed at the WH.
Yes, there is some loss, but the HPWH is very efficient and I can control the amount of circulation (and thus loss) through the valve at the WH. As it is now, the valve is barely cracked open.
The result:
Cold water: 75 F (remember conditioned crawlspace)
Recirculated water: 90-95 F instantly
Hot water: 120 F
Also the HPWH is scheduled to run 120 when we most need it, 110 the rest of the time, except a one hour 140 each date to kill any germs in the tank.
Preach it, brother!
Having had this conversation numerous times in the past, Id say that the California plumbing code requires a 1/2″ minimum pipe for domestic water. It seems to me that a bypass valve at the remote point using the existing umbing is going to offer a more environmentally friendly solution than adding a second water heater in many cases.
Matt, when you say “bypass valve,” are you referring to a demand-type hot water recirculation system?
There is another hot water distribution retrofit that should be mentioned: bypassing the existing 3/4″ and 1/2″ pipes with 1/4″ pex tubing directly from the water heater. I did this in my house and get hot water at my kitchen faucet in 8 seconds compared to over a minute I had to wait before. For this retrofit to work properly, fixture flow rates should be 1.8 gpm or less and pex pipe length should not be over 25 feet. Is this code you might ask? Code requires you to put in over-sized piping but it does not require you to use them! The original piping is still installed and can be used with the turn of a valve.
David, great idea. I’m going to redo my whole hot water distribution next year when I remodel my basement, but that fix would work in my house.
Interesting reading of the code!
David, I’m interested in doing the same for my kitchen faucet because it takes too long to get hot water due to low flow (1.8 gpm) and about 20 ft of 1/2″ tubing. Would you mind explaining a bit more about how you did this? In particular, what brand of tubing did you use? I see that Uponor sells 1/4″ tubing but does not sell 1/4″ expansion heads or rings. How did you connect the 1/4″ tubing directly to your water heater?
Ken, good question. Here is what I did:
– Attach a 3/4″ to 3/4″” copper Tee as close as you can to the water heater outlet
– Then create a manifold consisting of 3/4″ to 1/2″ copper Tees connected by 3/4″ stubs. Put as many Tees on it as you need. End the manifold with a cap, not a 90 degree bend
– Then attach 1/2″ to male adapters to each branch of the manifold (see https://www.supplyhouse.com/Elkhart-30310-ELKHART-1-2-Copper-x-Male-Adapter)
– Then thread on a Sharkbite reducing adapters (https://www.supplyhouse.com/SharkBite-U066LF-1-4-Sharkbite-x-1-2-Female-Reducing-Adapter-Lead-Free)
– The PEX tubing then slips into the adapter. This is the tubing I used: https://www.lowes.com/pd/SharkBite-1-4-in-x-100-ft-PEX-Pipe/1002859060
– At he other end find a 1/2″ pipe as close to the fixture as possible, install a ball valve then install a 1/2 to 1/2″ Tee down stream and repeat steps 3 and 4 above.
– This insulation works well with this tubing: https://www.grainger.com/product/TUNDRA-1-2-in-Thick-2CKG5?searchQuery=2CKG5&searchBar=true
I was at first worried about the slip-on connectors leaking but they work very well and have had no leaks. However, these connectors should not be buried in a wall or floor. They will need an access door if not in an open basement. By the way, if you need to go further than 25 feet to your faucet, I suppose 3/8″ PEX tubing would work also with different adapters. I have not tried that. Besides saving lots of water and waiting time, this setup allows multiple hot water outlets to be used without loss of flow in any. I don’t know how big a difference it makes, but we use pressure compensating shower heads that deliver 1.5 gpm from 30 to 80 psi. Happy plumbing!
Thanks very much for your detailed explanation! This helps a lot.
I am also considering using 3/8″ tubing. According to a presentation by Gary Klein, 1/2″ PEX holds twice as much water as 3/8″ tubing. Since it may be easier for me to accomplish this task with 3/8″ PEX (instead of 1/4″ PEX), and 3/8″ would still halve my water delivery time, I may choose to go that route.
I’m also going to contact Uponor tech support and ask about how their 1/4″ tubing is typically used since they don’t make much in terms of fittings.
Thanks again!
And 3/8″ pipe holds 2.25 times as much water as 1/4″….
Look at “home run” piping for domestic water – both hot and cold. Many codes reference a requirement for a minimum residual pressure at the fixture under full load. With current low-flow fixtures, David has it right – 3/8” or ¼” tubing can do the job and give you almost instant hot water delivery.
Expect an argument from some code officials however, who think it just ain’t raht!
Branch and tee distribution systems do not make any kind of sense in the average residential structure. Looming water shortages require us to design smarter.
Just sayin’
Allison,
Great article.
This is a pet peeve of mine and so many homeowners.
A lot of our bath remodels include multiple shower heads, body sprays, rain heads and handheld showers.
We tee off the water heater and homerun to the master shower, then the vanities with a 3/4 inch manifold to feed all the heads and reduce to 1/2 inch to the vanities.
3/8 would work to the vanities for sure.
Elbows are critical, especially with multiple heads in a shower. Pex allows some sweeping turns instead of an elbow. Elbows do create water pressure issues.
I discovered this the hard way.
I had to tear out the whole tile shower as we only had a dribble on the handheld shower. The master shower was on the second floor over the kitchen.
The water heater was in a pantry closet off the kitchen. we teed a 3/4 line off the water heater and got in the master vanity base above.
We attached the 3/4 feed to the 1/2 feeding the showers and BINGO! If only I had tried that before I tore the shower out.
As you stated, the shower was on the very end of the hot water feed. After two other bathrooms, laundry and kitchen.
The wife complained about low pressure before we started. I missed that comment. When we added the handheld shower it was worse at the shower and real bad at the handheld.
Too bad it is all the way to trim out that you discover things like this issue. They say mistakes can build your character.
After the change to the 3/4 line the wife was ecstatic. I got paid.
Lesson that I learned? A handful, but I did figure out that somewhere in the plumber’s code, you must put the master bathroom on the end of the hot water line, so the contractor will receive the maximum amount of complaints about how long it takes the hot water to get to the shower and the pitiful flow.
It has been awhile Allison, but I always enjoy your articles.
Gene, I guess they violated that part of the plumbing code in my house. The kitchen is the one at the end of the line. Good to hear from you!
When we travel in Europe, it is very common to see small (electric) tankless water heaters in the kitchen and one in each bathroom. Why not go that route?? Is the technology for electric tankless not there yet in the US? This is what we’re planning for our remodel. Would love to hear if that’s not a good idea
Luba, tankless electric water heaters use a lot of current and aren’t very efficient. Heat pump water heaters are too large for point-of-use application like that. Small electric tank water heaters can work, but again, they aren’t very efficient. And clustering the wet rooms eliminates the need for multiple small water heaters.
But it can make sense in some situations. In my house, for example, the bathrooms are all close to the water heater and the kitchen is far away. I could use a small water heater just for the kitchen. I want to get maximum use out of my heat pump water heater, though, so I’ll improve the distribution system instead.
@Luba, Electric tankless heaters require a LOT of power . In my experience, European showers have MUCH lower flow rates than we’re accustomed to, and their homes, especially older ones, typically have fewer bathrooms. Hotels that rely on POU water heaters have to be wired to support that. This is a major expense, which is why they go with smaller heaters with what we would consider uncomfortably low flow rates.
The flow rate for a given water heater depends on inlet and outlet water temperature. A heater must be sized for the coldest expected inlet water temperature, which varies widely by region and water source. For example, a 115F shower at 2 gpm and 50F minimum inlet water requires a 19kW heater , which translates into an 80 amp dedicated circuit per bathroom, plus you’d need additional (smaller) heater(s) for the kitchen and laundry. Even with 1.6 gpm shower heads, the cumulative electrical load would quickly exceed the capacity of your electrical service entrance.
Keep in mind that a bathtub requires a MUCH larger heater to support the typical 4 to 5 gpm fill rate. Note that higher flow rates are required to fill a tub so the water doesn’t cool too much before the tub is filled.
Another option for a remote bath: install a small 120V 1800W POU heater plumbed in series with the main water heater. 120V POU heaters are typically used for commercial lavatories (i.e., hand washing only) and have a small well-insulated storage tank (up to 7 gal). In a remote bathroom, the POU heater provides enough instant hot water to last until the water arrives from the central water heater.
David, we’ve had years of experience using a POU 4-gal 120V 1800W water heater for our master bath, which is at the opposite end of the house from our water heater. The hot water line comes zig-zagging from the WH to the laundry, then the kitchen, then the wet bar, then the guest bath, and finally to the master shower, followed by the master vanities. A really stupid layout, but it is what it is. The original owner or the plumber or builder must have realized that it was going to be problematic, and seemingly last-minute, added a loop and 120V outlet in a closet near the master shower to allow the installation of a small POU water heater.
When we bought the house, no POU WH was installed, so I bought a good-quality new one (Bosch). The 4 gallons routinely required up to 30 min to come up to the set-point heat, which I set on the hot side (135F) to reduce the amount of hot water coming from this small reservoir that was required for a comfortable shower. It would barely last through my wife’s hot showers, at which point the incoming water to the mini-tank from our main WH would be “warm” not “hot.” Being 85 ft from the main WH, with uninsulated 3/4″ copper pipes buried in the soil under the slab (cold and hot close to each other), it is no wonder. But here’s the kicker: I was also using a recirculator with a hot/cold return crossover in the guest bath, which meant that the water in the line up to that point was generally “warm” already. We also originally were using an outlet timer to automatically turn on the minitank so that hot water would be ready when needed for a morning or evening use, but the hard water killed the heating element and anode in only 3 years. So, after installing a new element and anode, we changed to just turning it in for shower use, and had to wait for the tank to heat. When both of us showered, we had to wait again for the re-heat. If there was an event for which we needed to shower, we had to plan ahead and start the process about 1.5 hours in advance.
The only realistic solution was to run a new power cable to the closet from the nearby service entrance (just outside of the master bath), and install a 12 kW tankless water heater in place of the mini-tank. Set for the usual 120F output level, it works like a charm, with hot water in seconds. And the electrical power (kWh) used for a shower is now LESS than the power required for the mini-tank WH for the same length of showering, although the kW spike is significant (but drops rapidly as the shower proceeds). This also resolved the problem of the drop in temperature of the incoming hot water to the shower valve during the shower, since it is now quick and constant. A win-win, especially since we do not find it necessary to take showers every day, and only use a master bath vanity’s hot water perhaps 10 times a week.
You are indeed correct… length times area (pi times radius squared), will be the total volume of water… so shortening the length, and reducing pipe diameter reduces the volume of water…
but… other than reducing the length, most folks should be INCREASING the diameter of their pipes… unless you like water hammer… or have less than 60 psi of pressure. if you are over 60 psi, you should either do the calculations for flow velocity in the pipe, and keep it under 3-4 ft/sec, or just run 3/4″ to all your fixtures.
the worst is long runs of 1/2″ pipe in my home… and no water hammer arrestor will help you…
get a recirculation setup, and save your plumbing and appliances. get rid of small diameter line… refrigerators, and dishwashers especially!
I have heard this argument before for not using smaller diameter pipes for your hot water. But the funny thing is I use 1/4″ pex tubing to all of my hot water faucets directly from the water heater and get no water hammer what so ever. I do have an expansion tank installed on the cold water pipe just before the water heater inlet.
David, I think the key here may be the use of PEX. Did you use Uponor ProPEX (which is PEX-A) or some variety of PEX-B (e.g., Watts, Viega, and others)? As you know, PEX-A is relatively soft compared to PEX-B, and PEX-A is very soft compared to rigid pipes like copper and galvanized.
My understanding is that the ProPEX lines are able to absorb the shock caused by water hammer in many cases without the need for arrestors. This is because PEX is “stretchy” and the line itself acts like an arrestor. However, I believe most codes still require the use of arrestors with PEX.
I had water hammer with 3/4″ and 1/2″ galvanized lines but the hammer disappeared when I replaced all of the old galvanized with Uponor ProPEX.
Ken, see my post above. I used Sharkbite PEX-B tubing.
You can do a home run in 3/8″ pex to any fixture.
The best source on this subject is Gary Klein:
https://www1.eere.energy.gov/solar/pdfs/sda_saving_water.pdf
Correction: “to any fixture allowed by code” and that you are concerned about wait times.
Good correction, Kevin. The plumbing code used in California and some other places (IAPMO) requires 1/2″ minimum, I believe. Also, if you’re filling a large tub, you may need a larger pipe.
Tub fills are really not part of the problem, it is the repeated short draws for lavs and kitchen sinks that take forever and waste energy and water. You can go ahead and run larger sized piping to a tub IF you go home-run from the water heater and cold main without being concerned about these issues. When you are running the faucet for 10 minutes to fill a 20 gallon plus tub, the initial wait time is insignificant. Smaller tubing is not appropriate for tubs and many showers, but with home-run piping you can mix and match sizes appropriately. BONUS – no fittings or connections inside wall and ceiling assemblies.
Allison: Great article and reminder; that we can substantially improve delivery time and reduce waste, if we take a closer look at that actual ’cause’ of the delay.
Your write up is right on!
Having taken Gary Klein’s class several times on ‘Structured Plumbing’, I’m convinced of the many benefits of hot water ‘planning’ at the design stage.
It’s not uncommon to have home owners ask about their water bill which here in Wisconsin includes the water purchase AND the sewer charge for the same. Look how much clean water gets dumped into the sewer system that then needs to be processed at the local water treatment plant.
Keep up the good work !
Thanks, Joe! Yes, I’ve been in several of Gary’s presentations. His Journal of Light Construction article (link to pdf below) is the best introduction to this topic for people who are new to it.
Efficient Hot-Water Piping, by Gary Klein (pdf)
Seems to me that it would be cheaper and more efficient to go with electric tankless at the kitchen faucet vs redoing pipe and installing a return.
This article is timely. I’ve been chewing on this problem with my kitchen faucet for a while.
I’ve already ran a return line of pex, but held off buying a return pump and mechanical timer. I don’t like the idea of pumping the hot water through a 40 loop of unconditioned garage, and I don’t want to run any more wires for a controller back to the water heater. Could be wrong but that system seems more complex than a POU heater..
At this point; I’m just trying to decide between a small electric tank or tankless to serve a kitchen faucet and dishwasher. Any recommendations for the DIY guy?
Personally, I would never consider a POU heater or recirc system for the kitchen since I can always make good use of the cool or tepid water until the hot water arrives. Moreover, most dishwashers have an internal heater that raises the water temperature as necessary. Some models can be connected to the cold water line, but that only makes sense if the primary heater is conventional electric.
Dave, thanks for the input. But I am working with less than 60psi (well water), 80 feet from the water heater, and 20′ is in an unconditioned garage. I insulated it, but it made no difference. And by the way, our well is not a stellar performer so we would like to save water when able.
In the winter, it takes 4-5 minutes to get up to temp. Not only that, pressure drops to almost nil in the kitchen when we are filling the tub. I’m planning on upgrading the 1/2 to 3/4 behind all the showers (clustered above the heater), possibly doing homeruns.
So a POU will be another tool solve the pressure issue since I have a 3/4″ cold line under the kitchen in the garage.
The house is all 3/4″ on the cold and 1/2″ on the hot. In this case, I’m not sure the small pipe is helping us in the branch configuration.
I’m actually the opposite. I don’t trust my dishwasher to efficiently heat very cold water enough for the sanitizing cycle , plus dishwashers are energy star rated and I think some have a power limit imposed to retain that rating.
My dishwasher uses the not yet hot enough water in the first cycle , doesn’t wait for it to get hot either. So the dishwasher manual actually tells me to run the adjacent faucet and wait for hot water before starting the dishwasher.
Also I don’t have good use for those gallons of cold water. Especially knowing that for dishwashing , running water is several times more wasteful than the dishwasher. Cant drink mine either.
Get a tank. You can plumb the inlet to your existing hot water line.
Good points all.
Sone turn of the 20th century homes, perhaps others earlier, placed the bathroom next to the kitchen. Our 1903 Vic did and it seemed odd until we realized it addressed this very issue, time to get hot water to the place of use quickly with little loss of heat. In this case from the wood stove to the tub and sink. Efficient design.
Should I consider putting a heat pump water heater in my basement? In CA cooling my main problem, and it should aid my HVAC by pumping heat into the water. I recirculate the warm water, and plan to soon make it on demand rather than timer.
I have a 24-year-old home in Durham, NC, and need to replace both the HVAC and hot water systems, as they died at the same time. In working these projects, and realizing all the remedial work necessitated by the new energy conservation codes, I became curious to know if there was any way that HVAC and domestic hot water systems could somehow be tied together such that both systems benefit? It’s just a question that has been in my mind, probably since I am thinking of both systems. I don’t see any connection. For the hot water, my current thinking is tankless with demand recirculation.
How about insulating hot water pipes? My pipes run under a slab and is uninsulated, when is the EPA going to make insulating hot water pipes mandatory in New home construction? They establish efficiency standards for hot water heaters, HVAC, and home wall and attic insulation but let efficiency down the drain with hot water. If I turn off the hot water for a minute while I soap, lather and scrub the hot water is already chilled as it goes through moist or ground water under the slab. My home is so bad I have installed a recirculating pump that pushes cold water sitting in the hot water pipe into the cold water line I remotely turn on and off when I use the shower.
@Stan, hot water pipe insulation is regulated not by the EPA but by local building codes, which are typically based on the “I codes” (i.e., International Code Council). The pump you had installed is similar to the “on demand” re circulation system referred to in the article, except there’s a sensor that automatically turns off the pump when the hot water arrives.
If this were HVAC ductwork we would be talking about the heat lost to the piping and from there to the attic as the fluid slowly runs through..
What about heat mass? The hot water also has to fight against the cold pipes and warm those up for 80 ft, right? And like Stan wrote, keep losing heat based on how well pipes are insulated or not. I’m wondering if this is a negligible part of the problem or not.
As a homeowner if I want hot water, I have no use for the first 3 minutes of cold water at all. Suppose I’m ok waiting. I’d love for the not-yet-hot water to NOT go down the drain and the fixture just give me the hot water after it’s arrived at said fixture. Is there a system to recirculate the not-yet-hot water back to the tank and only give me the hot water?
Deniz wrote: “Is there a system to recirculate the not-yet-hot water back to the tank and only give me the hot water?”
A traditional hot water recirculation pump requires a return loop. A high speed ‘on demand’ pump can use the cold water line for the return. Without a pump, the only way to draw hot water water to the fixture is to open the faucet, which of course sends the water down the drain.
And yes, recirculation heat losses can be significant, depending on how many minutes the pump operates, whether hot water pipes are insulated and the R-value, whether hot water pipes are routed through unconditioned space, and how cold that space gets. I know of cases where recirculation systems doubled water heating costs in winter.
Great idea Deniz, but as always it comes down to “how much money you got kid?” Perhaps some day when the cost of wasted water and energy becomes a bigger expense (or more people actually give a sh-t), it will result in innovative ideas like you are suggesting. Unfortunately the world currently seems to be running on the financial payback principle – and the planet is pushing back with the reality that money don’t buy you love or a sustainable balance.
To elaborate further on my last comment, the concept you raised has been realized a couple of ways. Potable water ECM circulators are available for domestic recirculation lines with one or more “hot buttons” where the user pushes the button prior to using the fixture. The cool water in the branch(es) is flushed back into the water heater as hot water is advanced to the fixture. Also, some on-demand heaters (Navien comes to mind) have integral circulators and optional “hot buttons” to do exactly the same thing. Retrofit can be fairly easy in many cases if plumbing is accessible in basements or crawl spaces.
The Water Demand Calculator is a useful tool for new builds (along with a Manual-J, etc.).
https://www.iapmo.org/water-demand-calculator.
That helps to size the distribution pipes.
Ooh! I’m doing this right now. Full basement reno, so it’s time to replace our polybutelene pipes from 1995. I’m pretty excited about it, but nobody else cares, so I’m glad to share here.
From the street, I start with 2 separate shutoffs- spigots/everything else. That way I can leave sprinklers on when I go out of town.
I have manifolds/home runs for basement & ground floor, with all pipes run in the basement ceiling.
I’ll keep my water heater around 140 F. The hot manifold is fed from a thermostatic mixing valve (TMV).
I have 1″ pipes running to the 2nd floor, cold and hot (140 F). That’s about 1 gallon of wasted hot water, in the pipes so I plan to have a 6 gallon water heater as a booster tank, feeding a TMV and then manifolds upstairs.
I’ve considered using a gravity loop instead of a booster tank. I may run an extra 1/2″ recirc line just in case. I would only run recirc to the upstairs manifold. I’d install that with a swing check valve and a gate valve to control flow. However, I think the booster tank is simpler. I may get a 240v tank and run it at 120v.
I was considering a few 3/8″ lines to some of the farther fixtures. I think you convinced me to run it in parallel, just so I have the option if I get frustrated with delays in the future.
I ALSO considered a drain water heat recovery unit, but they’re just too expensive for the marginal benefit (longer hot showers for our family of 6). The TMVs already do that to some extent.
I noticed that SupplyHouse recently started selling a drain water heat recovery unit made in Canada for a pretty good price (brand name is Thermodrain). The cheapest option is about $380 and is 42% efficient. The payback will depend on how you heat your water, but these units make so much sense – no maintenance, electronics, or mechanical parts to go wrong.
I’ve been rehabbing a house in PA for sometime. Built in 1968, it has several interesting and stupid features. Since starting, I have redone everything in the house. I have a tankless boiler-DHW. The boiler feeds a underfloor hydronic heating system. I like hot showers. Now. My town demanded that I connect to the local sewer and meters my water use to bill for sewer. I have well water that is coming into the house at below 50 degrees. All of this made for an expensive wait to get my hot shower. But not anymore. On the way to getting the house up and running, I used a 20 Gallon electric water for DHW. I now pipe well water into the 20 gal electric tank and use a timer to warm up the water to lowest setting (90 degrees) 1 hour before I get up. I also installed a water recycle system that is located under the last sink in my house. This turns on, at a button press in my bathroom, and draws hot water from the line, which cycles up the tankless heater, and directs the flow back to the heater through the cold water line. When the sensor in the pump detects, 109 degrees, it stops recirculating the water. So I press a button in my upstairs bath. Get my towel and shower stuff together. Step into the shower and cycle the water mixer all the way to hot to get roaring hot water instantly. I actually need to cycle it back to near 50-50 mix to cool it down. All of this without giving my town a dime for wasted, cold water. Here is the link for the pump: https://www.amazon.com/gp/product/B08TQJHPXK/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1. I did a lot of research on these and purchased a less efficient system on the way to doing this. I am also reconsidering the piping size of my house. I wanted to do a manifold with homeruns for all fixtures but didn’t have enough space in the house. Currently, I run 1 inch from well, through pressure tank and filters which splits to 3/4 inch T splitting water cold out and water to tankless heater. Mains are 3/4 pex B to 1/2 pex B to fixtures.
Insulating the hot water pipe with rubber pipe insulation will make a huge difference in cutting down the time for hot water arrival at the tap. Best if done during time of construction but if not and pipe is accessible in crawl space or basement this will still improve the time over uninsulated pipe.
At 231 cubic inches per gallon and 2.5 gallons going by before hot water arrives it would mean if you’re using 1/2″ pipes there is 279′ of travel (98′ per gallon) or if you have 3/4″ pipes 107.5′ of travel. (43′ per gallon) Or some combination of the 2.
Hopefully you can figure out where the issue is and improve that substantially.
Josef: My estimate is that the pipes from the water heater to my kitchen sink are about 80 feet long and 3/4″ in diameter until the last few feet. I wrote in the article above, though, that the amount of water that comes out of the tap while you’re waiting for the hot water to arrive is greater than the water in the pipe. Near the end of the pipe diameter section, I wrote:
“With low flow, the hot water doesn’t come rushing through the pipe, pushing everything in front of it all at once. That kind of displacement flow, or plug flow, doesn’t happen with low flow rates in larger pipes. The velocity is low enough that mixing occurs, which increases the time it takes to get hot water and the amount of wasted water.”
My pipes were both too long and too big. I just finished a retrofit and get much better performance now. Here’s my article on the improvement in my bathroom shower and lavatory:
How I Got Faster Hot Water
I did the kitchen last weekend and now get warm water in less than 20 seconds and hot by 30 seconds. The amount of water that comes out in the meanwhile is about 9 cups, a little more than half a gallon.
I’m happy to hear you got it sorted out! I know the mixing definitely causes more water than the pipe volume to come out, figured some quick mathematical musing on it would be fun. I’m glad to hear your estimates are around the range of varience I’d have expected. Thank you for the thought provoking articles.
My problem is that the pipes are under a slab and with water under the slab so the pipes cool rapidly in about a minute so I have to keep the shower running while I soap, shampoo, and wash. I have installed a recirculating pump that pushes cold water sitting in the hot water pipes back to the water heater through the cold water pipes. I use Amazon’s Alexa to turn on the recirculating pump plugged into the Alexa smart plug when I shower. Irritating thing is the cold water at the sink gets warm when the recirculating pump is running. However, it is a lot better not wasting my time and water waiting for it to get hot.
I have a continuous hot water recirculation system, on a timer, installed in my house. I plugged the timer into an electrical strip with a lighted on/off switch. This electrical strip is located in my centrally located water heater closet. When I need hot water at any faucet, I turn on the circulation pump at this switch and just wait a few minutes . I turn the pump off after use so that I don’t have hot water continuously circulating in the pipes throughout the day .
Good to turn off recirculation, continuous recirculation eats up energy keeping the pipes hot, especially uninsulated pipes, mine runs under a slab and through ground water since it cools off in a minute. My water heater is in the garage so I plugged the recirculation into a smart plug and tell Alexa to turn it on and off. Building codes should set the maximum time and volume it takes to get hot water to the faucets; and insulating hot water pipes.