In-Floor Heat System – Electric Energy Source – Hydronic or Electric Resistance?
I am planning a remodel of a 100+ year old house in Seattle. We are planning on removing the existing forced air furnace and replacing with in-floor heating. The energy source will be electricity. It seems to me the most efficient method would be electric resistance instead of hydronic. Wouldn’t there be a loss in efficiency (and a more complex system) if you use electricity to heat the water in a hydronic system versus the near 100% efficiency of electric resistance? The only benefit I could see to having a hydronic system would be to possibly tie it in with a solar water heater.
Everything I’ve found online comparing hydronic versus electric resistance heating makes the assumption that you have an energy source other than electricity to drive your hydronic system (gas, propone). In Seattle we are lucky to have fairly inexpensive electricity. Plus I plan to install a PV system in the near future.
I’m leaning towards the electric resistance heating for the whole house (900 sf main floor and 800 sf basement) but would love to hear other’s thoughts.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Not far from you, and I see homes with electric heating mats under floating floors or tile on a fairly regular basis. In some cases they are providing the primary heat.
Questions you need to answer: what is the BTU output per SF that you need in order to heat the house on the coldest days of the years, and can the electric mat(s) provide that much output?
With a hydronic system, you can almost always get the output you need. If the boiler is inside the house, there's not really waste heat from it per se, although it might be overly concentrated in an area where you don't want all of it. You have to consider distribution loss from the piping and tubing if it runs outside the conditioned space. I have seen lots of lots of houses with radiant piping running in the crawl space, which carries a large penalty.
An electric mat is going to have some loss through the subfloor or slab, and that would have to be compared to losses from tubing heating the same floor.
Nothing's simple, eh?
Sean,
There's no difference in efficiency between electric resistance floor mats and an electric resistance boiler connected to hydronic tubing. But the hydronic alternative is unnecessarily complicated and expensive to install, and you introduce moving parts (the circulator) which requires maintenance and replacement.
The simplest electric resistance method is plain old fashioned electric resistance baseboard heat.
The best way to heat your house with electricity, though, is with ductless minisplits -- which would probably be less expensive than your proposed system with a boiler, circulator, and hydronic tubing.
For high efficiency electric radiant floors in a Seattle climate you would do very well on efficiency with a Daikin Altherma air-to-hydronic heat pump (with a mini-split technology compressor & outdoor coil) but the installed price tag might be a bit daunting!
If you run mini-splits and bump it up to a steady 73-74F indoors 24/7 all winter long for added comfort you'd still only use ~1/3 of the power that you would with resistance-radiant floors.
If going with resistance heating, radiant cove heaters have a comfort margin over electric baseboards, and are more "instant on" than radiant mats since there's only the thermal lag to the elements (which run without the orange glow you might be thinking of), and the fact that it's heating up the humans & objects in the room first, not the room air. That makes them well suited for using only when the room is occupied, letting the room cool off for lower thermal losses on average. But if you're just heating it with a "set and forget" strategy there not much of an operating cost benefit, only comfort.
If you think mini-splits are just a side-step on comfort from a force-hot air you'll be surprised. Yes, they do move air as the heating medium fluid, but it's at much lower speeds/volumes and much quieter than with on/off hot air systems, and the room temps are much steadier. Being fully modulating systems with maximum efficiency at part load, they do better with a "set and forget" strategy on temperature, letting the compressor & blower adjust speed to match the heating load. On the comfort scale ductless mini-splits come only after low-temp panel radiators and radiant floors. For a 68% reduction in operating cost I'd be willing to "suffer" without the cushy radiant floor, though you may want to put in radiant mat or coves (or both) in doored-off bathroom spaces.
If you don't have an insulated basement slab heating the basement with electric mats would be out-of-sight expensive. If you're actively heating it all, insulating the foundation walls is also an important first step. Insulated and air-sealed, with at least some storm windows it's likely that you could heat the whole space with a 1.5-2 ton multi-split or a couple of 3/4 ton mini-splits.
But every successful heating system design starts with a room-by-room heat load calculation based on the U-factors of the exterior surfaces of that room (windows & walls, roof for the upper floor) at intended interior temp and the 99% outdoor design temp (which in your case is 25-28F for lower altitude King County. see: http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf). If you have a heating fuel use history you can use the former heating plant as the measuring instrument to come up with the whole-house heat load, but if you're micro-zoning or load balancing the colder rooms with resistance heat in an ductless heat pump application you'll need to know the heat load of each room to get it right.
I would go with Electric floor systems as it gives a feel of an electric blanket. These systems are great for making especially cold floors foot-friendly. They also enhance the temperature of chilly room by a few degrees. The warm floor in a bathroom makes getting out of the shower a cozier prospect on a cold day.