Not All Mini-Split Heat Pumps Are Ductless!
Mini-split heat pumps and ductless go together like peanut butter and jelly. Or do they? If all mini-splits were ductless, then why were those rigid elbows sitting on a jobsite I visited recently? That home is going to be heated and cooled entirely with mini-split heat pumps.
Mini-split heat pumps and ductless go together like peanut butter and jelly. Or do they? If all mini-splits were ductless, then why were those rigid elbows sitting on a jobsite I visited recently? That home is going to be heated and cooled entirely with mini-split heat pumps.
The answer, of course, is that there is such a beast as a ducted mini-split heat pump. We’ve actually done quite a few HVAC designs where we specified them for the job, as is the case with the jobsite mentioned above. When designed, installed, and commissioned properly, they work beautifully and efficiently. The homeowner may not even know their HVAC system is any different from their neighbors.
Well, that’s not 100% true. I’m sure
they’d notice that their backyard isn’t nearly as noisy as their neighbors’ because mini-split condensing units aren’t nearly as noisy as conventional systems.
I like ductless mini-splits, and they’re great…for some applications and some clients. An objection some people have, though, is that they don’t want the mini-split head hanging on their wall. I personally don’t have a problem with them, but I realize they’re not for everyone.
Some people just don’t want to see mechanical systems hanging on their wall. Crazy, I know! But that’s where ducted mini-splits come in. The head is just a mini air handler that hides behind the drywall or under the floor somewhere and does its thing (i.e., heating and cooling efficiently and quietly).
What you see above is not the rover we recently landed on Mars or a Turbo Thermo-Encabulator Max with its hydrocoptic marzel vanes exposed. It’s the air handler and ductwork attached to a ducted mini-split heat pump. This one’s in a different home (for which we also did
the HVAC design), but it’s the same animal. The photo at left shows what the air handler looks like before it’s installed. This one was waiting to be installed at the jobsite I visited last week.
Yes, the efficiency is a bit lower because pushing air through a duct system takes more energy than releasing the conditioned air into the room right there at the unit, as a ductless mini-split does. When the ducts are designed, installed, and commissioned properly, however, it works very well and can still be very efficient.
Speaking of good duct design, let me show you the beautiful photo I took of the the trunk-and-branch duct design in this house. The photo below shows the supply trunkline, carrying the conditioned air to be distributed to the various rooms, with branches coming off and sending conditioned air through the branches. The trunk is made of rigid sheet metal, for better air flow, and the branches are made of flex duct, for better noise control. Beautiful, isn’t it? (That’s a nice tongue-and-groove ceiling above it, too!)
The ducted system overcomes another difficulty of ductless mini-splits, too. It can sometimes be difficult to get enough air flow throughout the house with ductless because it can get very expensive to put heads in every bedroom. With the ducted system, you can give each room just what it needs.
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Nice article! One word of
Nice article! One word of caution to first time designers: Ducted heads in the smaller sized units (less than 3 tons, such as Mr.Slim M Series) have almost no blower power (typically a max of 0.20″WC external static), so don’t even think of using flex.
The only way I’ve found to make these work is to 1) toss foam filter that comes with the unit, 2) build return plenum with over-sized ceiling filter grille, 3) limit the supply side to a full cross-section reduction trunk (same initial dimension as supply opening) with curved, flat Y take-offs, and 4) bootless supply outlets (branch and grille are same dimension).
BTW, I hope the duct system depicted in your final image is connected to one of the larger, more powerful heads 🙂
Great article Allison!Most
Great article Allison!Most have no idea that there are options.
It may be an optical illusion, but it looks like the condensate drain for that air handler is coming out on top of the air handler rather than on the bottom.
IMHO the ducted version of
IMHO the ducted version of the minisplit makes the most sense for smaller rooms where a head per room would add too much to costs. For large rooms just use a regular minisplit. For our house we would require 3 units. 12,000BTU for the living/dining/kithchen, 6,000 for the master bedroom, and a 6,000 ducted unit to serve the kids bedrooms. The kids rooms are right next to each other so ductwork static pressure for the 6,000 BTU unit would be minimal. Then the question becomes cost vs replacing the existing conventional system.
These systems are amazing, I
These systems are amazing, I installed a ducted mini-split in my own home. A modest home of 1,100 sq.ft. So far we have been very happy with it. I would like to try using the non-ducted version in my next home.
In Nashville these systems are great all year, but what would a person do in colder climates where the temperatures drop into the single digits and lower for extended periods of times? What is the emergency heat option for these systems…space heaters?
The manufactures of my Mitsubishi said it will heat down to 4 degrees. I doubt I’ll ever see that temperature in Nashville and if I do it may only be for an hour in the middle of the night.
Here’s the challenge. Assign
Here’s the challenge. Assign the Apple community to re-designing the heads and you’ll have everyone wanting one in every room.
@Ryan, as with any heat pump,
@Ryan, as with any heat pump, mini-split heat capacity drops with outside temperature. Whether or not supplemental heat is required depends more on unit size and how tight and well insulated the house is (capacity vs. load). In most cases, supplemental heat will be needed well before the outside air reaches the heat pump’s minimum operating temperature (many units operate down to -10F).
Supplemental heat can be provided by various types of radiant electric or hydronic methods depending on the situation. Each house is different.
In any case, I typically only specify mini-splits for super-efficient homes (loads < 1 ton) with a minimal supplemental heat fraction. For homes with larger loads, I’ve found that conventional equipment is usually a better value.
“Assign the Apple
“Assign the Apple community to re-designing the heads and you’ll have everyone wanting one in every room.”
I think you’re onto something Pat!
Thanks Allison…for the
Thanks Allison…for the record, our project is the second picture, the pretty one with the tongue and groove ceiling and trunkline design, the first picture is NOT OURS and we cannot be associated with that install. 🙂
That does look like an awful
That does look like an awful lot of ductwork to put on a mini-split.
A few Passive House projects
A few Passive House projects (my bungalow remodel included) are experimenting with using the combination of an ERV to exchange air and a ducted minisplit to condition air. They share the same duct work. I would like to add a small Fantech inline boost fan to supplement the wimpy minisplit fan and allow for more manageable duct sizes. Any experiences or thoughts on syncing a secondary fan with the built in fan?
I am using the Samsung 1-ton ducted model EH035 and the UltimateAir RecoupAerator.
In Kansas, we have significant latent loads during the summer, so I will have stand alone dehumidifier. Several HVAC contractors have also recommended the option of increasing air circulation–which would not be part of the ERV cycle, so as not to increase indoor humidity. The boost fan could be used for this task as well, without running the minisplit.
Our house will be very similar to the ones Carter Scott is building–compact design with around 2000 ft2, heat load of 8000-10000 Btu/hr, Passive House levels all around.
Perhaps I should have just used a single or double-head ductless minisplit, or waited for a magic box, but I have the ducted model already, so I want to try to use it.
That’s literally not a nice
That’s literally not a nice ductwork at all.
Jason, you shouldn’t need a
Jason, you shouldn’t need a separate dehumidification system, especially if your house is anywhere close to meeting PH air tightness spec. Most mini-splits are smart about managing the sensible-latent split based on actual conditions.
If you’re experiencing high summer RH levels (above 60% for an extended period), then something else is wrong. You need to identify and fix the problem rather than installing a band-aide. An energy-guzzling dehumidifier sorta defeats the purpose of building a high performance home, no?
As for your ducted head, I’d say wimpy is a fair description — the EH035 can only handle 0.125″ WC. I can’t imagine anyone trying to supply an entire house with that unit! That’s barely enough static to supply a couple of bedrooms, and even then, the duct system would have to be almost non-existent (I hope your filter is at least 2 ft2). I’ve been successful using the SEZ-KDxx to supply as many as four bedrooms in a straight line (it has 0.20″ WC available static), but I had to scrutinize each fitting and diffuser to shave off every possible hundredth of an inch of static.
A booster fan would help, but it would need to change speeds in tandem with your air handler, and calibrated to restore the approximate design airflow for each of your unit’s 3 speeds. Otherwise you can throw your system’s efficiency out the window, as well as it’s ability to manage latent-sensible split (coil temperature). I’ve never attempted such a thing, but I understand how it could be done.
Having your ERV and mini-split connected to the same duct system is problematic. At a minimum, you should make sure the ERV is set up to shut down when the EH035 is calling. I can’t be more specific without knowing how they’re interconnected, but the ERV may be contributing to your moisture problems. My brother is facing $20k mold in remediation and his ERV turned out to be the culprit.
Finally, using the ERV’s blower to recirculate (I didn’t know the RecoupAerator has this feature) is another large waste of energy. ERV’s are VERY inefficient as air handlers, since the blower must overcome the high static pressure drop of the heat exchange core. The RecoupAerator uses a high efficiency ECM motor, but yet it consumes 270 watts on high (200 CFM). That’s only 3/4 CFM per watt, or about 10 times less efficient that an conventional ECM blower. In general, recovery ventilators should not be operated at a higher CFM than required to meet the ventilation target.
David, thanks for your
David, thanks for your response. I will send you a message through your website.
David B.:
David B.: Yes, this is one of the more powerful heads. We’ve had success using a little bit of flex and still getting good air flow with them.
David R.: Thanks. Actually, the drain does come out a little higher because there’s an internal pump.
Bob: They also make sense where the homeowner doesn’t want to see any ductless heads, but I agree in general with your statement.
Ryan: Looks like David Butler did a good job answering your question already.
Pat: I love that idea! Maybe that’s what the Nest thermostat designers can work on next.
Ryan S.: Hear, hear!
Lance: Yes, I did mention in the article that was a different project, but as you point out, it was also a different HVAC contractor who did the install.
Thomas A.: Looks like a lot, but it works. The house you see above where the ducted head is installed in an attic has about the same amount of ductwork (or possibly more) as the one that’s going in now and it has great air flow. Once we commission the new one, I’m sure it will be fine, too.
Jason M.: I agree with David Butler’s response. You shouldn’t need a separate dehumidifier with a tight envelope and an AC that’s doing its job, not in Kansas anyway. And yes, 0.2″ is static is low, yours allows for almost no ductwork at all. By the way, will you be in Denver at the Passive House conference next month? I’ll be there.
Federor: Really? And you say that because…? Could it be that you prefer the smooth look of ductboard from the outside, as shown on your website, despite the problems that ductboard has? We never spec ductboard in our designs.
I’ve sold hundreds of these
I’ve sold hundreds of these in both residential and Light Commercial, new construction and existing structure applications. 100% success, 100% satisfaction!
A friend and builder recently
A friend and builder recently a completed an approximately 2000 sq ft house and used one interior DHP in the main area. He used a Panasonic bathroom vent fan unit in conjunction with ducting to circulate air for the bedrooms. Works like a champ. Extremely quiet and highly energy efficient.
I am a little late to this
I am a little late to this conversation but I thought I would give my experience having a passive house in the humid deep south. Our latent loads are 7-8 times our sensible loads. If you size your mini-split for the sensible (where it is most efficient) you are not likely to manage your latent loads efficiently or completely. If you size your minisplit for the latent load (where it is significantly less efficient) you will be well oversized for your sensible loads (which has its cycling problems). Depending on your location/latent load, supplemental dehumidification will potentially be unavoidable. And as pointed out, they are horribly inefficient for the scale of typical residential projects. While you can easily get mini-splits in the low to mid 20’s SEER the most efficient stand alone dehumidifier tops out at about 5.5 SEER. Good luck.
@csaft, thanks for posting
@csaft, thanks for posting that. As I alluded to in my previous comment, ERV may be contributing to your problem. I have an issue with the common practice of using ERV’s as the primary exhaust for high moisture areas such as baths, especially in homes with large moisture loads (e.g., teenagers) and/or small sensible loads.
Do you have an ERV exhausting from your baths? The problem is that the ERV will end up recycling a large portion of the bath moisture since a shower will generate exhaust air with a higher dew point than the outside air (except during rainstorms). ERV’s either reject or recycle moisture depending on which side has the highest partial vapor pressure, as represented by dew point.
As you point out, it’s definitely more efficient to use the HVAC system to remove moisture than any stand-alone system. In my experience, unitary air conditioners (without reheat) max out at roughly 50% latent capacity before getting into frost conditions. This can be accomplished with conventional HVAC equipment using a programmable ECM blower. However, with mini’s, airflow and compressor capacity are inter-related and the controls are proprietary. Some models have a dehumidification mode, but it won’t get anywhere close to 50% latent capacity.
We had a 3 ton Carrier heat
We had a 3 ton Carrier heat pump installed in our home in 1983. Believe it or not, the compressor is still doing fine but we recently had to replace the air handling unit. I’m wondering if a mini split system is feasible for us when it’s time to replace the outside compressor unit. Our air handler is in the attic and we would use the duct system that was installed for the original heat pump. What do you think?
@Tim: Without knowing more
@Tim: Without knowing more about your system and load, I can’t be certain, but it’s doubtful you could swap out your system with a residential ducted mini-split since the blower only has a fraction of the power of a conventional air handler (measured in available static pressure, 0.2″). Your duct system was most certainly not designed to operate at such a low static.
Mitsubishi, and I believe LG, produce more powerful ducted mini-split heads in their commercial lines, but the matching outdoor units are variable refrigerant flow (expensive) which you won’t benefit from with a single-zone duct system. Minimum size is 3 tons.
The more important question is why? The only reason to consider ducted mini-splits is when you want to break up your home into zones with separate air handlers, or the total load is smaller than the smallest conventional system.
One more thing… there’s a misconception about mini-split efficiency. While small single-zone ductless mini-splits (one compressor matched with one indoor unit, less than 1.5 tons) are available with very high efficiency ratings, the larger systems, especially those with ducted heads, only top out at about 15 SEER.
The Fujitsu ARU series of
The Fujitsu ARU series of ducted air handlers are good to .36″ ESP with about a 15% loss in rated capacity. .25″ will net you the full rated capacity.
21 SEER for single head models with the ductwork in conditioned space.
I am looking to have a ducted
I am looking to have a ducted system installed in a 1926 Spanish style home with very limited attic space. If you or anyone you know would like to place a bit, please email me directly. Home is in San Diego, CA