Best efficiency upgrade options for radiant floor heating
Hi,
I just bought a 2700 sq ft house just north of Seattle (climate zone 4c). 1900 sq ft of that space is heated by a radiant floor heating system that is serviced by two 50 gallon electric water heaters. (They are also tied into the domestic hot water system, but that’s a separate issue). We don’t has a gas hookup. I like the radiant floor heating but would like to make the system more efficient if possible.
Is it possible to upgrade to a heat pump water heater to serve the radiant floor heating? A contractor mentioned that he didn’t think that’d work because the heat demands of radiant floor would be too much. Is that true?
Are there any better options for this configuration, such as an external air to water heat pump? I was originally considering getting a 65 gallon HPWH for the radiant floor and a 50 for the water heating. Any recommendations to maximize efficiency in this case?
Thanks,
Ted
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Replies
The physics of that approach don’t make sense to me. A heat pump removes heat from one area and transfers it to another. In this case, you’d be drawing heat from one part of your building (garage? Utility room? Closet?) and depositing it into the radiant tubing. You are going to make one part of your home much colder (potentially running out Of usable heat in the process!).
That's what I was thinking, it didn't really make a ton of sense. Would an air to water heat pump drawing external air be the best option?
I would think so. But be ready to be on the bleeding edge of that as far as US residential applications. Seems like it would make a lot of sense but just hasn’t been adopted here. I’ve never seen one. I’d love to know if you go that route as I am facing a similar choice (though I have gas so that is probably the most economical choice).
Will the seller share a year's worth of power bills? If yes, it's at least possible to figure out what the actual heat load is, which is a good starting point for any heating solution. A primer on how to do that lives here:
https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new
How many watts of water heater are currently serving the heating system, and what is the storage temperature of those water heaters?
If it's all low/very low temp there are reversible hydronic chillers that could be used here, but sizing them (and the buffer tanks, if any) would be important go get a handle on up front:
https://www.arcticheatpumps.com/?gclid=EAIaIQobChMI7rfHooeL5gIVh4rICh34xg10EAAYASAAEgILi_D_BwE
https://www.chiltrix.com/
If the loads are low enough one (or two) of Sanden's split heat pump water heaters might be used even at somewhat higher water temps, but for a non superinsulated 2700' house that's not too likely.
+1 on an external air to water heat pump. But calculate the numbers to see if it's worth the cost and complexity.
In the absence of electricity use history, run a Manual-J type heat load calculation using aggressive rather than conservative assumptions on R-values and air tightness, etc. Even a whole-house calc using Loadcalc.net can ballpark it reasonably, if you use the tightest construction type options and zero ventilation air. (Even then it usually overshoots by double-diget percentages, but it won't be 2x.)
The wattage of the existing water heaters puts a fairly firm upper bound on the heat load, but I would expect there is quite a bit of margin build in. Assuming 4500 watts each that would be 9000 x 3.412= 30,708 BTU/hr which for 1900' of space is a ratio of 16 BTU/hr per square foot, which at an outside design temp of 20-25F would be quite a bit on the high side for a 2x6 framed house with low-E windows, or a reasonably tight 2x4 framed house with older wood sash single panes + clear glass (not low-E) aluminum triple track storms.
Wood stove with hxchg or supplemental solar hot water (antifreeze loop)?