Air conditioning for a radiant heated home?
We’re planning a new two-story house build in Ottawa, Ontario, probably equivalent to zone 6/7 on the US chart. We are planning to use hydronic heated floors on all three levels.
What are the most common approaches for adding air conditioning to a house with radiant floors? We will have a fairly comprehensively ducted HRV/ERV system.
The house will be superinsulated (~R40 walls, R60 attic, R20-30 below grade) with southern exposure and proper window shading to limit solar radiation. Finished plans should be around 2500 sqft.
A large portion of the second floor will be open to the first floor (above the living and dining room areas and main staircase), leading me to think ductless minisplits would be a bad idea.
Is there a practical way to utilize the HRV ducting to supply air conditioned air? I’m planning to have the supply ductwork set up to supply fresh air throughout all the major areas of the house as well as a supply for each bedroom.
Any help is appreciated! It would be nice to avoid doubling up on the amount of ductwork running through the house.
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Replies
Lance,
For a superinsulated house, the easiest way to provide air conditioning is with an air-source heat pump (ductless minisplits, ducted minisplits, or a conventional air-source heat pump with forced-air ductwork).
Once you've installed your air conditioning system, you can use the heat pump for heat as well. That means you don't need any in-floor radiant heat. This approach saves many thousands of dollars.
For more information, see All About Radiant Floors.
Lance,
I don't understand your statement that the open floor plan leads you to dismiss a mini-split. Typically, an open floor plan is favorable for a centrally located ductless mini-split, because there is so much natural air circulation around the house.
Do you have a manual J with cooling loads? If the cooling loads are less than 10K btu/hr, then Minotair (http://www.minotair.com/home_en/) may be able to satisfy your desire to use your ventilation ductwork for cooling.
Mini-splits work fine for air conditioning spaces that are open from one level to another. I know of a house in Minneapolis that is cooled with a single 1.5 ton mini-split head located in the upper level near the top of the stairwell.
Running the cooling load numbers with Manual-J or similar is important for getting the equipment sized correctly for the loads. Super-insulation does a lot more for reducing the peak heating loads than it does for the cooling loads, that are often dominated by solar gains through the windows. The sensible cooling loads aren't very severe in Ottawa, with a 1% outside design temperature of 83F (28C) but there is also a real latent load (21 grains @ 50%RH). The higher the ventilation rate with the HRV, the higher the latent load, and it will be important for comfort and dust-mite control to dial back the ventilation rates a bit when the outdoor dew points are north of 56F (13C), which is most of the time in the July-August time frame, since the sensible load may not be high enough that the cooling system will dehumidify sufficiently. Right-sizing a modulating AC for the load will make a real difference on that front. See the dew point graph near the bottom of this page:
https://weatherspark.com/averages/28316/Ottawa-Ontario-Canada
Future hydronic heat homeowner, welcome to the loneliest fraternity here on GBA.
I didn't catch whether you told us your water based heat source. Small boiler, air source heat pump, ground source heat pump? That data point may drive your direction.
Our house is heated and cooled by a ground source (geothermal) heat pump. We make hot water in the winter and chilled water in the summer. We have two levels of in-slab heat and one second story area with panel radiators. Like your home our second story is partially open to the first floor.
For summer air-conditioning chilled water runs through a Unico high velocity fan coil unit that feeds a main trunk "spine". Most rooms have one, two, three or more flexible takeoffs from that spine.
Different than what you propose for an HRV, we have a distributed exhaust ductwork system from all bathrooms and only one HRV supply outlet. That HRV discharge is near the peak of our home, proximate (but not directly connected) to the free air return for the Unico fan coil. The nice thing about that is we aren't dumping cold air (the HRV is only partially efficient) close to any living space; that fresh air is well tempered before anyone is near.
My impression is that radiant heat combined with high velocity air-conditioning is a "somewhat common" approach in parts of the western US. I don't think it's a "most common approach" though. (See sentence one.)
My impression is that many GBA members like ground-sourced systems but are often wary of the install costs and one-off nature of the designs.
My development requires spec homes to be GSHPs, and most of the new homes are specs. Based on conversations with neighbors, there seem to be quite a few comfort and reliability issues. I considered installing a GSHP myself but opted for an air-source heat pump at one third the cost. This savings meant I could do a bit more to improve the performance of the structure, which also further reduced my heating and cooling requirements.
About a year ago, the spec builders were having trouble justifying the added cost of installing GSHPs (even with the Federal tax credit). The developer's solution was to set up a program where new owners lease the system for several years and then pay a one-time lump sum to own it. Crazy, no?
The house referenced in #3 in Minneapolis cooled with a single 1.5 ton mini-split is heated with hydronic radiant floors using an electric boiler (at time-of-use rates.) Even though the mini-split can deliver a large fraction of the heat, it isn't distributed well. When the owner is going to be away for more than a day he turns off the radiant, lets the mini-split run as freeze-up control.
Since a large fraction of the cooling load in Ottawa is latent, an HRV with a recirculation mode would distribute the drier air to all rooms quite well, even though the amount of sensible cooling delivered by an HRV duct system would be paltry.
If you are heating the water for hyrdonic heat with a heat pump, air source or water source, you can reverse it to generate chilled water. You can have one central fan coil and ducts to distribute the A/C air, but you can also use mini fan coil units that are sort of like mini-split heads but can be smaller and sleeker, and distribute water to them. Chiltrix, HTP, and Jaga make them. You have to insulate the chilled water pipes well to prevent condensation on them, and finding an installer who gets that is hard, but once you get over that hump, installing the hydronic distribution system is easier than installing ducts.
Given that most of the load is latent (dehumidification), it's actually quite attractive to put a chilled-water coil on the incoming HRV air. When that incoming air has higher humidity than your interior air, you can remove a kg of water vapor from it at lower energy cost than removing a liter of water from the interior air. And it's satisfying to stop the humidity from entering, rather than removing it after it enters. That's probably all you need for cooling. The chilled-water coil hardware is readily available, and if you already have a heat pump that can supply chilled water you are pretty well set. But if you don't have a heat pump and want to do this, you'd need to find a small chiller, and it seems like most of the small chillers that would be sized right for just that HRV air would be designed for indoor use only, which defeats the purpose.
But a single mini-split for the whole house is probably fine too. Although I don't have any real data on this, I suspect that distributing dehumidification evenly around a house is less important and much easier than distributing heat or cooling around a house.
Thanks all for the fantastic conversation! I'll try to reply individually to each point brought up.
Martin, I had previously read your (excellent) article on heated flooring and it actually put to rest my main concern; that heating the floors without a Warmboard-type setup would leave me without enough heat. I think I actually passed out when I got the WB pricing, which would litterally double or triple the cost of my building envelope (!?!). But after reading your article I realized my required Btu/sqft would likely be low enough that even a staple-up system under carpet would supply enough heat. It would certainly be cheaper to install than an above floor system.
Realizing that I likely wouldn't get the "warm floor" sensation in a well insulated envelope was a bit of a bummer, but I guess it makes sense. Even with that, heated floors seem to offer benefits that would be nice to have even if one of those benefits may not be energy savings.
I plan to do all of the mechanical work myself, so I'm not as concerned with the labor typically associated with a heated floor installation. (LOL on the lonely "Frat" comment!)
As far as the ductless mini-split goes, I'll list my concerns:
1. Zoning. With all second story rooms (4 bedrooms, 1 bath, 1 laundry) opening to the main floor below, heat rising to the top of the structure would surely keep them very hot, no? I guess keeping the doors closed all the time and relying on air supplied from the HRV would have some effect, it's just hard to see how it would be adequate? The open area is facing the South wall, and even with proper window shading there will be some solar gain. Nothing worse than a hot bedroom when you're trying to sleep...
2. Zoning. With a ductless mini-split and multiple indoor heads, enough to handle proper distribution, I would assume the cost would start to climb pretty quickly. It would seem a good single indoor unit mini-split sells for about $4000 in Canada, larger units with additional indoor units can't be cheap? I haven't yet gotten any quotes, but I have seen a few prices for single unit models.
3. Cold. While the historical weather data for Ottawa (thanks for the link D. Dorsett!) only indicates an average low of -15C for January, it is not that uncommon to have several days in a row with night time lows below -30C (-22F). Even the newest and highest efficiency Fujitsu and Mitsubishi mini splits are not very efficient at those temperatures. True, I'm more concerned about cooling with the mini splits, but an investment of that magnitude (point #2) would probably displace my budget for a high efficiency gas boiler.
You have to trust me when I say that NOTHING would please me more than targeting this build at going 100% electric and planning to be off the grid, but I'm afraid that won't be in the cards for us until battery storage and solar are quite a bit cheaper than they are today. Maybe in 10-15 years time, but not by the time we're hoping to move in.
4. Cosmetics. While I am not against new ideas and sacrificing a little style for some good old-fashioned geekery and efficiency, my wife will have none of it! :-) I'm a one man show when it comes to making this build as efficient as possible, and I'm already thinking of ways I can hide some of my attempts during the build. (Hint: She likes the idea of heated floors because everyone she talks to thinks they're great even though they probably know very little about them. As soon as I start talking technical she glazes over.)
The main cosmetic issue would be the mini split indoor units mounted on the walls. I could literally tell her those white boxes generated rainbows and puppies and I doubt she'd care... this is one of those things I don't even have to show her to know what her reaction would be.
Another cosmetic faux-pas with her is ceiling fans. She hates them. So that's a no go for circulating air which is a bummer because some of the newer ones certainly are good looking and highly efficient ways of moving air around.
I looked at the hidden ducted indoor units, but based on the specifications I've read you can't get anywhere near the highest efficiencies with those units, the regular mini splits all seem to offer much better specifications as well as working in much lower temperatures. Maybe I'm reading the specs wrong? The newest smaller units are at or above 30 SEER ratings... the thought of ducting one of these into an HRV has crossed my mind!
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So are my thoughts/concerns somewhat realistic with regard to mini-splits? I really do like the idea of using them, if for no other reason than install simplicity and super high efficiency.
Reid Baldwin,
No I haven't done Manual J calculations for the envelope yet. Is there a decent online worksheet I can use or some decent freeware? I've worked through some rough heating load calculations, which seem pretty straight forward, but cooling is a bit more involved with latent loads and solar gain.
That Minotair looks EXACTLY like the unit I was dreaming about... the lovechild of a high efficiency dehumidifier and an HRV! It makes perfect sense to me; have the option of ducting the heat from the dehumidifier outside in the exhaust stream of the HRV in summer time and get the air conditioning benefits of humidification. Why there are not a bunch more units like this on the market is a little beyond me?
The larger "12000" unit is advertised as pulling up to 88 pints of water a day. That seems like a lot. Would that do for an average four bedroom household? In my head I'm planning an above average strategy for both the range hood and bathroom shower exhausts that would likely operate separately from the main house HRV system.
I'm really excited to find this system, thanks for the link! The fact it's "Made in Canada" should make acquiring one pretty easy as well, as long as the price isn't too much of a shock!
Lance,
Performing Manual J load calculations is Step One. For cooling load calculations, you should really use software. If you don't want to buy Manual J software and learn to use it, you should probably hire an energy rater, energy consultant, or mechanical engineer to perform the Manual J calculations.
For more information, see Calculating Cooling Loads.
After you've calculated your cooling loads, you can design your cooling system. A variety of approaches are possible, including a fully ducted system, a ducted minisplit system, or one or more ductless minisplits. Since you are sharing many anxieties and uncertainties, you would benefit from hiring a mechanical engineer to design your system.
Just because mini-splits aren't very efficient at -22C (Ottawa's 99% outside design temperature (or even -22F) doesn't mean you shouldn't be using them for heating- they're nearly 2x as efficient as an electric boiler at that temperature, and 99% of the time it's warmer than that in Ottawa. An electric boiler controlled by floor thermostats to keep the floors warm, and one or two mini-splits to control the air temperature (until/unless can't keep up) can work reasonably efficiently, as long as you keep the floor temperatures only 1C or so above the room temperature (which you can bump up to 22C if you like, since it's more than 2x as efficient as the floor, on average). Again, a Manual-J would tell how much of the load could be covered by the mini-splits. Even sized for the air conditioning load, there is a strong chance that the mini-split(s)would have sufficient capacity to cover the whole heating load of a superinsulated house, or at least the lion's share thereof.
If you're using something else for heating the floors you don't need a ductless head in every room for air conditioning. At the modest cooling loads of most homes in Ottawa one ductless head per floor (at most), or just one on the upper level should cover it. Even though the HRV would not do much for distributing the cool air, simply dropping the temperature in the open area to 23-24C should keep temperatures in the other rooms well bounded. A recirculating HRV system would distribute the dry air, which is more important than cooler temps at your likely loads. The exception would be a doored off room with a high cooling load, say an upper floor bedroom with a west facing window with late-day solar gains, but keeping the door open until after the sun is off the windows on the hotter days would cover most of those cases too.
Lance,
The Minotair is something I investigated when I built my house but didn't end up choosing. My manual J predicted cooling load was 50% higher than the Minotair's capability. Therefore, I don't have any personal experience with it. My knowledge about it is limited to what I could find on their website and this GBA article: https://www.greenbuildingadvisor.com/articles/dept/musings/another-north-american-magic-box. Another unit that operates on the same principles is a CERV as described in this article: https://www.greenbuildingadvisor.com/articles/dept/musings/balanced-ventilation-system-built-heat-pump.
Thank you all for your very thoughtful input. I agree that pursuing a Manual J would be the right thing to do. One of the things I look most forward to about building this house myself is the opportunity to understand everything that's going into it and not having to rely on advice from others, like lazy HVAC technicians. :-)
Can someone recommend a good reasonably priced Man J software? I looked at a few and the prices are anywhere from reasonable to full-on commercial product $$$ (I am just a homeowner after all). Having experience with the software or knowing someone who does would be a great asset.
I should add that I'm a Mechanical Technologist and have no problem learning technical things. Most topics in HVAC are things we studied in school, though that was 20 years ago.
Thanks!
Lance,
There are lots of inexpensive pencil-and-paper ways to perform heating load calculations, but accurate cooling load calculations require decent software. But perhaps GBA readers will suggest an online tool or inexpensive software for cooling load calculations.
-- Martin Holladay
Lance,
You should check out today's Q&A Spotlight:
Adding Air Conditioning to Radiant-Floor Heat.
That's incredible! Thanks SO much for putting that together! The timing was a little freaky... this must have already been in the breach when I posted my reply!
I will definitely be looking into the CC6 software! As it seems to point out, dehumidification will be very important.
Still looking forward to anyone's input regarding a reasonably priced option for some capable Man-J software.
Thanks again! I'm sure I'll be back with more questions as my ideas evolve into a design.
Can you tell me if using the mini-splits in an open plan (2100 on first floor and 500 in a open loft) would circulates the air to the other rooms? Do I need an ERV for circulation to the rooms off of the area where the ductless heads would be? I am trying to use radiant heat floors for heat and the mini-splits for AC. Planning to insulate well and use good windows. Just don't want hot areas in the summers. I'm in Oklahoma.
Thanks.
Beth,
Q. "Can you tell me if using the mini-splits in an open plan (2100 on first floor and 500 in a open loft) would circulate the air to the other rooms?"
A. A ducted minisplit will circulate air to individual rooms. A ductless minsiplit will not.
Q. "Do I need an ERV for circulation to the rooms off of the area where the ductless heads would be?"
A. An ERV is not designed to circulate air or equalize indoor temperatures. An ERV is designed to introduce a little bit of fresh air into a house from outdoors and to exhaust stale air from certain rooms. For more information on ERVs (a type of ventilation device, not a heating or cooling device), see Designing a Good Ventilation System.
Q. "I am trying to use radiant heat floors for heat and the mini-splits for AC."
A. You can do that if you want. However, once you install your minisplits, you should be aware that these minisplits can provide space heating as well as cooling -- so you won't need your radiant floor heating system.
-- Martin Holladay
ERV & HRV systems can be designed to guarantee an exchange of DRY conditioned air into the spaces, providing a real amount of latent cooling, but the amount of sensible cooling would be almost too low to measure.
To know if a particular doored-off room would be susceptible to overheating requires a room by room Manual-J cooling loiad calculation.
In an open floor plan use mini split + ceiling fan. You can get 2000cfm with only 3 watts.
Who would I contact to help me with these load calculations? You all are way above my pay grade! Would my local mini-split supplier be able to help? Do you know of any builders in my area that are building these types of homes? Thanks!
Beth,
You should contact an energy consultant, HERS rater, or mechanical engineer to perform your Manual J load calculations. For more information on this issue, see these articles:
Saving Energy With Manual J and Manual D
How to Perform a Heat-Loss Calculation — Part 1
How to Perform a Heat-Loss Calculation — Part 2
Calculating Cooling Loads
-- Martin Holladay