Could electric in-floor be the best heating source for a high-efficiency home?
Working on a nearly passive rated house that will only have electricity ( no NG or Propane). Current options are an electric boiler for baseboard heat with a sidearm for domestic ( any recommendations for elec. boilers for this option?) Second option was possibly doing electric resistance in-floor heat. Some of the companies I have called have said they do not recommend using this as a primary heat source. I think this is mostly due to the average house not being highly efficient. Anyone have any experience of feed back about this? What about longevity. I remember something about thermostats and senors going out but you could double-up on those. Customer will be trying to offset elec. use with as many PV panels as possible. It seems like the elec. resistance option would have less up front cost. Architect has originally detailed one of the sub-zero heat pumps with back-up elec. boiler. House will be at 9,000 feet in the Colorado mountains. We are climate zone 5. The cost and the how complicated the original design by the architect is, has made the customer and I look for other options… Any help would be welcome! House will be around 1600 sf, two stores ( second is more of a loft) will be a second residence, Walls will be R-37, Roof will be R-72, foundation will be R-20. There will be an ERV installed.
Thank you in advance!
Jesse in Colorado
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Replies
Heat pumps will generally get you more heat per kilowatt than resistance heat since a heat pump is only “moving” the thermal energy, not making it outright. It’s the old physics problem: what happens in a room with a 1500w heater and a 1000w air conditioner both running? The temperature drops.
Radiant heat does have some comfort advantages though. If you aren’t worried about costs, you could install both radiant and heat pump heating systems.
Bill
Without the room by room load numbers there is no way to make the call on this, and there is no way to infer those numbers from the R-values given. (What, windows and framing fractions and 99% outside design temperatures matter? :-) )
Depending on the floor plan it may be worth doing a ductless heat pump + radiant floor solution as the best compromise between upfront and ongoing cost/efficiency.
You may need to calculate the U-factors of non-standard assemblies first to be able to calculate the loads, but there is no substitute for running the load numbers. There is no other way to get it right.
Thank you for your response. We are currently having the heat load calcs being done. I can report back with the information.
Jesse, I am in Colorado at 8000 feet and which I am pretty sure is Zone 6. Not sure where one can be at 9000 feet and in zone 5 here, but Colorado is full of surprises. Beyond that, my own house is all electric with slightly lower insulation levels for basement and roof. Windows are U.15 fixed and U.19 operable, and about 15% of exterior wall area. I am using roughly 1.3 BTUs per sq foot per degree day for the heating season. The heated footprint is nearly twice your projected 1600sf, so it might be reasonable to estimate that you will likely use about 4250 Kwh of electricity for heat only.
An electric water heater will be a very variable consumer of KW if the home is only occupied intermittently. We enjoy hot water (probably too much) and the annual baseline electric usage for hot water, freezer, fridge, induction cooktop, ovens and everything else is actually higher than our heat usage.
We could do much better on heating the house by using mini-split units as Dana mentions. For a number of reasons involving installed costs and lack of repairmen/installers in our area we did not go that route. During the design process I did look at the in floor heat option from many angles and simply could not justify the sharply higher costs of doing any form of in floor heat.
Water based systems were not only costly to install but also presented the risk factors of leaks and not being very compatible with highly insulated home. The thermal behavior of such systems gets tricky with very low loads. Electric mats, wires, and films all posed very high front end costs per sq ft and limited flooring options. I even considered under rug electric mats as an option, but the cord issues killed that.
Failure of any of the choices under a tile or wood floor would be rather unfortunate. Test floor installations I did in a prior home suffered multiple failures of temperature sensors and thermostats. Additionally, the warm feet idea only becomes real once the floor surface is about 82F which in most cases will be way over the needs of a high insulation design if maintained at that temperature.
Ultimately, I went with radiant cove heaters in all rooms with exterior walls. The up front cost of units and additional electric work was less than half the projected cost of a single mini-split which I might not get repairmen for. There are some downsides to this plan. Some think they are ugly. We don't even see them anymore. Heat is strictly KWH and minis will beat them every time. Minis have the ability to cool as well. We did not plan for cooling being at 8000 ft and so far have not needed any. The climate shifting may cause regret in years to come.
So for what it is worth, I would not recommend any under floor heat except perhaps the luxury of a warm tile floor in the master bath. Your feet will not notice if the house is really as well insulated as you plan for. All surfaces inside will (should) be pretty much the same except for the window glass. We do not miss the in floor heat ( or the problems) and I would suggest that paying more attention to getting the best triple glazed windows will pay much better comfort dividends.
Edit: For once it wasn't Dana about the minis, but Bill...my bad
Jesse,
I agree with previous posters who note that hydronic heat distribution connected to an electric boiler makes no sense -- it's the worst of both worlds. You end up with high energy bills, expensive installation costs, and worries over leaks.
If you end up going with electric-resistance heaters in most rooms, why not install at least one ductless minisplit in your largest common room -- perhaps the living room? No downside -- and you'll get cooling for free.
Jesse,
I say go with a minisplit heat pump (or two, depending on your room / floor layout) and put electric resistance heaters in your corner rooms. Put in-floor electric heat under your bathroom floor if you feel the need. Don't put in a boiler for backup heat! That's overkill. I live at 9,400' in Colorado (zone 7 - but it's a warm zone 7 with a design temp of -6°F). My design temp heat load is about 21,400 btu/h for 3,150 sq. ft. on two levels. I put in two separate one-head Fujitsu cold weather mini-splits, one on the main floor and the other in the walk-out basement. The basement unit is oversized for the space, but a lot of its heat ends up in the floor above. I'm very happy with the result (note - I moved in Dec. 2018 so I only have 4.5 months of experience with the coldest night at -3°F so far).
Why two separate units? I did this for redundancy because the Fujitsu installer is 3 hours away. I sized the mini-splits to 105% of my design heat load and sized the total resistive cove heaters plus master bath in-floor heat at 50% of the design heat load (remember to de-rate the heat pumps by 30% because of your altitude). Maybe two separate units was paranoid, but if one heat pump goes down, I can still heat the house while I wait for the HVAC guy to schedule a trip out to me. Also, you can easily argue that a couple electric floor heaters would have been the least expensive and easy backup heat option and a multi-split would have been fine. The other reason I chose to pair one indoor minisplit head with one outdoor unit was that they were more efficient: the difference seemed to be ~ 10%, depending on the multisplit you select.
I also have a small woodstove with the intake vented to the outside. This is to be used when we want a pretty fire and provides power-out backup heat that works with my open floor plan. I chose the Morso 7110 as it has a relatively small heat output and it has been working well. Other background information: no final blower door test, but I expect 1.5 ACH. FYI, I'm all electric, I installed a HPWH that is ducted to a 50°F crawlspace thermally connected to the ground and I included a 36" tall drain water heat recovery unit in the basement level for all the main floor water use (includes the master shower). Insulation levels are: 11” cellulose filled walls east, west and south, 13.5” cellulose walls on the north side, R20 under the slab and R20 – R30 on all buried basement walls, attic is >R75, the windows run between R5 and R6 and the doors run R2.5 for the main entry door and R5 for all the others. Overall, my insulation values match yours fairly well so I think my house’s performance might match yours.
The result of this winter's performance? When the doors are open, the corner bedrooms, 20' down a hall from the open area, run 3 to 6°F cooler than the main room at night This includes the cooling effect of the ERV pumping cool air at 20 cfm into the rooms. The 6°F cooler occurs when I didn’t run the resistive heaters and the night temperatures are around zero. Close the doors when it's 15°F outside and the cove heaters are needed. During the day, if it's sunny, those rooms run reliably above 68°F with the cove heaters off. The electric heat has only been used in the corner rooms when those rooms go below 65°F, and that's been rare. The main room thermostat is set to 72°F, but the wall thermostat is mounted far from the windows. Near the living room / dining room windows, where we sit and eat, ends up at 68 - 69°F.
I live an hour from Roger who wrote post # 4 above and my electric bill is ~55% of his. He has the same windows, similar insulation values, similar # of sq. ft., but is 1,400’ lower with a different floor plan and construction technique – so a straight up comparison isn’t possible. What causes my lower bill: minisplits or the HPWH? I don’t know, both most likely, but my point to you is that the minisplit heat pump option is viable for your 9,000’ all electric home. As Martin said in post #5, even if your floor plan isn’t the best for whole-house heating with minisplits, using one or two for the bulk of your heating needs will save you some money on your bills and consume less of whatever your grid uses for fuel. If you add solar PV, then you don’t have to spend as much to get to net zero. Payback on the minisplits with respect to resistive electric? Depends on the cost of the install and that can vary a lot. Compared to Roger from post #4, I think my payback is about ten years.
Will you miss the radiant warm floor feeling by switching to heat pumps? I say the answer is no because your house won’t have very warm floors. A super insulated house doesn’t require floors warm enough to “feel warm” to your bare feet. In my house, the resistance heat in our master bedroom floor is set to floor temperature of 71° and was never on during the day and only barely on at night over the last winter. The bathroom does have a big south facing window and the sun on the floor does more for the warm floor feeling than the heater. At night in winter, the bathroom air temperature is comfortable but the tile there never feels warm.
I hope this helps you make your decisions!
Bill