Radiant vs. Mini-Split
Experts on this website advise against installing radiant floors in a high performance house. However, that feature is a homeowner request for a project that will have exposed concrete floors anyway, with or without the tubing. On the other hand, I’m on a roll changing all kinds of long decided features. To recap some arguments against hydronic floors:
• Pumping equipment draws significant electrical load. The system may not be particularly efficient to operate.
• Equipment can be complex and expensive to install.
• Floors may never be noticeably warm if the building enclosure holds in the heat, so no real difference in perceived comfort. There’s no concern for chilly floors with adequate sub-slab insulation (R-20 specified at ground level).
The advice often repeated here is to forget about radiant floors and redirect budget toward windows. Fair enough. For heat, choose a super efficient mini-split system such as the Mitsubishi Mr. Slim series. A quick search turns up many online vendors offering competitive prices on these.
The homeowners for this project actually lived for many years in the Houston area without air conditioning (!!) and do not consider it necessary for the mild PNW climate. However, based on some Manual J calcs with EnergyGauge software (free 15 day demo) the heat load is 20,800 Btu/h, and cooling load would be 15,000. That’s a good match for Mr. Slim. Predicted annual cost is $87 heating and $65 cooling. Yes, annual. If accurate this leaves radiant looking foolish—and the designer looking brilliant.
Searching for Mr. Slim turns up many glowing reviews including HVAC contractors saying these things are bulletproof. Lingering questions:
1. Aesthetics. The wall units look funny. They have a ducted unit for concealed use, although it seems to reduce efficiency somewhat. Go with ducted for subtlety?
2. Multi-zone vs. single. The best numbers are from single zone units, but for a 1,600 SF house surely it’s better to have zones. For example one zone downstairs (living area) and two upstairs (bedrooms). Bathrooms get electric baseboards or towel bars. Makes sense?
3. Any other reasons not to propose this as a change from radiant?
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Replies
As a homeowner I love radiant and hate it when the[much smarter than me] guys say to ditch it. They are of course probably correct. It does seem like it would be the best way to integate solar hot water heat tho........
IMHO barrier pex is so cheap, put it in the slab.
I have several of the 26 SEER mitsubishis and the smallest ones are way to big [ac wise anyway] for an average bedroom. If these are the kind of people that will leave the doors open, with this low an ac load, a little one down and a little one up, unless they cook a lot. I looked at the mult's and the hidden ones, but they were much more expensive. I was thinking of building a trim 'box' around at least one, but eventually you get used to looking at them
Another factor with the concrete floors: theoretically the concrete should better absorb solar heat if it's not already heated from within. Of course this may be in conflict with the third bullet point, if the floor isn't significantly warmer with the radiant system. Still, the role of the concrete topping becomes partly finish surface and partly thermal mass and solar heat sink.
Thomas,
I just had a series of conversations with Marc Rosenbaum -- some of which will be excerpted for future blogs -- and all field reports from ductless minisplit installations remain very positive. No problems, no complaints, above-spec performance, good cold weather performance, extraordinarily low energy bills.
Michael Chandler says some homeowners don't like the remote controls, because they can be confusing. Some homeowners don't like the looks of the wall-mount units. Hey, if you the looks bother you, and you want to spend $10,000 more on a different heating system so you can hang a pretty picture on your wall instead of looking at a component of your energy-efficient heating system, go ahead and blow the money on what you want. It's a free country.
On the issue of bedroom temperatures -- door open versus door closed -- and variations of 2 to 5 degrees between common areas and bedrooms without heating or cooling units -- this is VERY occupant-specific. Educate your clients and they should be happy. If you get a fussy client who wants to keep the door closed all the time and freaks out if the temperature is 5 degrees different in the bedroom, then they'll have to pay for a heating unit in the bedroom (or choose a ducted minisplit with slightly worse efficiency).
If the homeowner is willing to leave bedroom doors open during the day, and doesn't mind slight temperature variations in the morning -- and if you build an impeccable thermal envelope -- they'll be delighted with a single unit in the living room.
I am an ex-HVAC tech that is now a green remodeling contractor. I get similar quandaries here in northern Utah where we don't really need A/C. I have installed many Mr. Slim units and highly recommend them, but if you don't require A/C, the best choice for most of my customers is the Rinnai wall furnace. Surplus City has the 20,500 BTU/HR units for $800. They are small and have an attractive direct vent flue. I am also a big fan of transoms to ensure a single unducted unit can heat the whole house with doors closed, but the transom can be closed for privacy/noise.
I'll pile on against radiant as an owner resident with radiant floors:
1. The floors are cold 98% of the time. But remember, even a 75F concrete or tile floor feels chilly to bare feet.
2. You can't cool or ventilate with radiant floors (practically speaking).
Re: solar hot water: It's rarely cost effective to heat the house with it... a case of you get the least when you need it the most. I highly recommend it for DHW, though.
Radiant heat can work better in a superinsulated home. Just reduce the area heated to equal the proper floor sqft'age where floor temp is warm enough for comfort.
No more heating perimeters of rooms. Heat only foot paths. Push heat into bath floors an hour before wake times etc.
Innovate, experiment, live and learn.....
As to the wall mounted heat pumps, why not recess the mounting area and add an attractive grill and or display shelf......
Get out of the same old thinking....
THIS IS WAY RELEVANT, so thanks for the post and remarks......@AJ, those things run on remote control, so hiding them is not a good approach. Also, if you heat perimeters, the unheated slab areas would detract from effectiveness and efficiency, seems like you're shooting from the hip on that last post. I am strongly considering the comments of such knowledgeable and respected building scientists on this site, but I think that some of the opinions are not taking everything into consideration.
Hey Thomas, let me guess, you talked them out of straw bale construction?
Mathew.... Read slower....... I said NO perimeter heating. Heat only the actual walked on areas. It is a very progressive innovative better way to go when using radiant in a superinsulated home that does not require the heat that the entire floor would put out as we used to build radiant... for the last century.
Less heated floor is smart. Sorry that you don't understand.
And controlling anything these days is very easy. There are all kinds of remote control extenders that myself and friends have used for many many years. Not a problem either.
Catch up with your knowledge Matt