To Zone or Not to Zone…and How?
Unless you have a simple, small home, you probably want more than one thermostat to control the heating and cooling. That lets you control the conditions separately in different areas. And that’s where zoning comes in. A common example is a two-story house with one thermostat on each floor. But there’s a lot more to it than just adding thermostats.
Dampers or equipment?
When you adjust a thermostat, it sends a signal that changes the conditions in the area served by that thermostat. It sends more or less conditioned air to that part of the house, depending on which way you adjusted it. But how that happens depends on whether your multiple thermostats are connected to one piece of heating and cooling equipment or one piece of equipment for each thermostat.
In the first case, you have a zoned system. When one thermostat calls for conditioned air, it opens a damper to allow air to flow into the zone that’s calling. Of course, it also has to turn on the heating and cooling system if it’s not already on.
In the other case, the one piece of equipment controlled by a thermostat has only one zone to serve. That keeps things simpler, especially with air flow.
The downside of zoning with dampers
The problem of using dampers on a single piece of equipment is what to do about the air flow. Let’s say you have a heating and cooling system with fixed capacity. It doesn’t have a variable speed blower. So when, say, only one of two zones is calling for conditioned air, what happens to the excess air the blower is trying to move? Meet the bypass duct.
The most common way installers deal with the excess pressure created when not all zone dampers are open is the bypass duct. It sends air from the high pressure supply side back to the return plenum. That air re-enters the system having already been conditioned. So it gets colder or hotter with each successive pass. At best, that will reduce the efficiency of the system. At worst, it can freeze the coil in cooling mode or crack a heat exchanger in heating mode.
Good HVAC companies understand that a bypass duct shouldn’t be part of a zoned system. There are ways to do it properly and some good controls available for that purpose. And of course, the first thing to do is use variable capacity or multi-stage equipment that can ramp up or down depending on the number of zones calling.
Zoning best practices
At Energy Vanguard, we do third party residential HVAC design all over the US (and a bit outside, too). Nearly every house we do a design for has multiple zones. And most of the time, we have a separate piece of equipment for each zone. Our preference is to zone with equipment rather than dampers.
But we do some designs with zoned systems that have one piece of equipment serving multiple zones. On those jobs, here’s what we do:
- Use multi-stage or variable capacity equipment.
- Never use bypass ducts!
- Balance the air flow in the zones.
- Allow excess pressure to bleed into dormant zones.
I explained the first two above. Balancing the air flow in the zones means making sure you don’t have a zone that needs a lot less air than the system can ramp down to. For example, if you have a variable capacity system whose minimum air flow rate is 500 cubic feet per minute (cfm), you shouldn’t have a zone designed for only 150 cfm.
The last tip is advice we pass on about setting the dampers. When a damper closes, it has options for how much it closes. You can set it to close all the way or stay open a little bit. By leaving it open a bit, any excess pressure after the blower ramps down has an escape path…and it’s one that doesn’t have the downsides of the bypass duct.
Smaller zones and multiple thermostats are the way to go for comfort and efficiency. And of course, as I’ve said before, five tons is never the right answer. (Well, almost never.)
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can subscribe to Energy Vanguard’s weekly newsletter and follow him on LinkedIn.
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One classic zoning issue is either a 3-story townhouse or 3-story colonial. With the cooling unit in the basement the top floor is warm in the summer and cool in the winter. One solution is to install a separate unit for the 3rd floor.
Have you had success with zoning in lieu of 2 units in the above scenarios?
I am sure most of the houses you design the HVAC for are the more top end houses.
Zoning with equipment is great when the budget allows but zoning with dampers (so long as at least two rules are never broken 1- No Bypass duct. 2-Your smallest zone meets “or at least closely meets” your systems minimum operating capacity.
I’ve had great success installing multi zone systems, But I’ve had the displeasure of having to fix more zone systems done improperly than ones I’ve seen done properly.
Another important part of zoning with one piece of equipment is to understand the buildings heat/gain losses throughout the day.
If at any given time the needed capacity is 3 tons BUT the house as a whole needs a 5 tons YOU DO NOT need a 5 ton (plus the 5 never works anyways)
We’ve lived with a zoned system in our townhome for over 24 years.
It had a bypass duct
Dampers which do not bleed air into dormant zones.
Fixed capacity heating and cooling.
The original evap coil did not have a TXV, so our compressor died an early death around year 10.
We ended up replacing the AC which inclia TXV at the evap coil. Then a new 80 percent gas furnace 5 yrs after that. The bypass duct has been sealed off and the damper for the third floor is permanently opened which convertsthe floor as the dumping ground for the excess air. Unfortunately businesses will no longer touch our flex duct because of the material it was made from circa 2000. This prevents use from having the system properly balanced.
Gotta love production builder quality.
What is your ducts made of? I’m unfamiliar with any modern flex that has anything other than fiber
We’ve been zoning heat pump systems since our beginning in 2006. Allison nails it here – no bypass ducts – ever! We nearly always require at least a multistage system for zoning, though we make occasional exception with small (1.5 – 2 ton) single stage systems with only two zones – such as a typical 2 story townhouse. In those cases we try to both somewhat oversize each individual zone’s ductwork AND ‘crack’ (elevate the zero stop) both zone dampers.
We always use variable speed air handlers and commercial grade power open / power close motor dampers, not the cheap spring loaded units. The ramping and enhanced dehumidification functions of variable air handlers both help manage excess static / noise concerns.
Two stage systems we deploy up to 3 zones (I personally owned and lived in a 4 zone two stage system, but that’s a bridge too far, IMO) We’ll go as many as 5 zones on a true variable capacity system such as Trane XV.
While I agree that 5 tons is rarely the right answer, we did deploy two 5 ton systems once with 10 temperature control zones…but it was an 8200 SF house with some poor envelope choices…the site super bragged how tight it was, but I had to spend an extra $1700 on a 2nd fan for our blower door rig just to get to -50 Pascals…not so tight!
Zoning is also important on small houses as well. You have to take into consideration the use of the rooms not just treat the manuals as gospel. In our (1960)1750sqft house I have thought about zoning because the room used as an office gets insanely hot very quickly with two of us in there and two computers running. It will go from 74-85+ very fast. And no the door is not open and will not be open and cannot be open. I do need to add a return duct into the office which is easy since the unit is right next to it and easy to run a small return through the wall which a return would help some when the ac is running.
Then there is the master bedroom where we would like to keep that room at 65-68* at night. Oh and the kitchen which we rarely cook in the summer because the system is sized correctly( technically undersized for actual temps here in TX)
Zoned would improve things in our house but when I go over it all what makes sense is to just put a minisplit in the rooms that have the extra heat load and then in the future downsize the main AC unit. Yes is all sounds strange but in our use case it would be the best overall option and in TX you want backup sources of cooling. Heating is not an issue for us as heating is far easier than cooling though the office does still get too hot in the winter and I have opened the window many times to cool it off in there or closed the vent to stop heat from coming into the room.
Unfortunately few of us actually live the way Manual J etc think we live and every ones requirements are different from a fixed formula that doesn’t take those things into consideration. Really it might be better to start using commercial formulas for some residential uses as it is closer to what many are using their houses for now.
And lets also not forget that smaller rooms are going to require more action on behalf of the AC since they are small and the heat builds up faster than in a larger room that has the mass to steady the temps out.
Another option is “follow me” zoning. At least one thermostat manufacturer has the ability to control from remote temperature sensors which you can schedule to be used during different parts of the day. So if you are empty nesters like us, you can have the thermostat control off the living area temperature during the day and the master bedroom at night. No dampers or multi-capacity systems needed.
Run around (return) bypass on direct expansion coils is a no no… there is a few nice multi-zone single condenser systems out there but run of the mill A/C Tech’s typically do not have the wherewithal to set them up or the customer doesn’t have the pocketbook to get it done right by someone who can. Wife caring for her mom in her 6100 SF house has 4 A/C units/zones. We live in Phoenix, her mom had under floor returns installed – absolute nightmare for cooling control. Air balance had supply air from one zone blowing air on another zones T-stat for two zones.