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LPG infloor heat with HWH with a heat exchanger in the HWH is too expensive to run!

fasteddie50 | Posted in GBA Pro Help on

I have the above home on a monolithic slab. …LPG to run the HWH and a heat exchanger pipe running through the HWH to heat my floor…TOO DANG EXPENSIVE TO HEAT MY HOME THAT WAY! Suggestions on how to make it more efficient would be appreciated. A air to water heat pump? A heat pump water heater with a heat exchanger pipe running through the HWH? A solar system of some sort? Any help from professionals or folks who have the same heat system as me and added “whatever” to make it more efficient would be greatly appreciated on Whidbey 🙂 would be/ whidbey…sorry, bad play on words 🙂 THANKS FOR ANY HELP! ED

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  1. iLikeDirt | | #1

    Is your floor slab insulated? How much? Regardless, propane is an expensive fuel and your water heater is an inefficient heating appliance (maybe < 60% efficiency). A heat pump water heater likely won't produce water hot enough, and air-to-water heat pumps are very very expensive, so that may not be a good option. Do you already have ducted AC? If so, you could add a propane furnace or replace it with a conventional heat pump. If you don't have any AC or ductwork, it might be a better idea to install multiple ductless mini-split heat pumps, or else one ducted unit with new ducts in the conditioned space. First to a quick-and-dirty load calculation using this website: and tell us the heating and cooling loads it gives you at the end.

  2. fasteddie50 | | #2

    R 10 perimeter insulation around slab AND underneath slap...My sole source of heat is in floor heat. We don't need AC on whidbey island....I'm just trying to find the best way to heat the water for my infloor heat. Presently it's heated through the hot water heater/heat exchanger pipe. When I built the home....I installed 2x8 exterior walls.....double pane windows and sealed all penetrations such as electrical boxes (where wires entered and then around the box after the gyp was installed. I have raised heel trusses.

  3. user-2890856 | | #3


    You need an HE propane (lp) unit . Maybe just invest in a quality unit with an efficiency above 90% like this one or it's big brother , Phoenix . Don't know your loads so could not say which . Maybe you need someone to verify your radiant infloor design also for your house . it very well may have been done poorly . Seen that more than I care to discuss , it has kept me working though . Here are links to the units , we install them and they are what the claim they are . Newest for low load homes . Many piping options due to multiple outlets The original and oldest modulating condensing water heater

  4. GBA Editor
    Martin Holladay | | #4

    While R-10 insulation under your floor slab might be OK for a house with forced-air heating, it isn't much insulation for a house with embedded hydronic tubing in the slab. It would have been better if your builder provided 4 inches of sub-slab foam, because your slab gets hot.

    Double-glazed windows are code-minimum windows -- nothing to brag about. It sounds like your house meets code, but it doesn't sound like it is superinsulated.

    If your heating bills are too high, you may want to replace your heating system, as some commenters recommend. But it might just make more sense to upgrade your home's thermal envelope -- perhaps by performing blower-door assisted air sealing, or perhaps by adding some cellulose in your attic (especially if your attic is insulated with fiberglass batts, which don't perform well).

    A conventional propane-fired water heater isn't very efficient, so it's not too surprising that your heating bills are high.

  5. user-2890856 | | #5

    I kinda like Martin's approach . Reworking everything because the source at present is a pig does not make fiscal sense . Performing some envelope upgrades along with purchasing a pig that eats much less is a good 2 prong approach . Do have that design and install verified though as the radiant industry has evolved much in a couple decades . Few know how to properly do it now and there were less then .

  6. Expert Member
    Dana Dorsett | | #6

    A conventional propane water heater with a center-flue heat exchanger runs about 80-82% steady-state combustion efficiency, and if used for space heating, the heating AFUE would be about that. The fact that the thing tests 55-60% in an EF test is irrelevant, since the duty cycle in space heating mode is much much higher than when only heating domestic hot water. And if the water heater is indoors, the standby losses are supporting the heat load anyway, and not a loss the way it would be when measuring hot water heating efficiency.

    With a condensing hot water heater you might bring that up to 95%, which delivers about 15% more heat per gallon of propane. If it's unaffordable with a center-flue type propane tank, it's only 15% more affordable with a condensing hot water heater. Depending on the cost & quantity of fuel you're using, scrapping the fully functional ~80% efficiency tank for a condensing HW heater may or may not "pay back" withing the anticipated lifecycle of the new unit.

    On Whidbey Island WA it's pretty affordable to heat the house with mini-splits, as some relatives of mine have been doing on Whidbey for the past 4 years. They've been laughing every time they pass the propane truck on their way to the ferry, but maybe it's maybe worn off to just a smile and a shaking of the head by now. :-)

    If you're addicted to the comfort of a warm floor, set the floor temp with a floor thermostat to something a degree or so above your desired room temp, but set the HOUSE temp 3-5F above that temp using a mini-split or two. (One of my Whidbey relatives did his with a single 1.5 ton Mitsubishi, but now wishes he'd done it with a pair of 3/4 tonners, one per floor, due to stratification issues. Another nearby relative down in Port Orchard is doing just fine with a single-head in a 1-story home.)

    Air-to hydronic heat pumps are VERY expensive to install, but a Daikin Altherma could probably cover the whole load in your temperate climate. In some WA markets an electric boiler would be slightly cheaper to operate than condensing propane, but an Altherma would cost about 1/4 as much (a 75% discount), for an upfront equipment cost north of 15 grand. (A 1.5 ton mini-split would run about $4.5K in your neighborhood, if history is any guide.)

    What Martin said comes first though- search as hard as you can for the low hanging fruit, and pluck that first, then do a careful heat load calculation before adding more equipment.

    Active solar would be a poor investment in your foggy-dew climate. Spending the solar budget on rooftop PV and a ductless mini-split even delivers higher net solar efficiency than an active solar system would, since your mini-split coefficient of performance will average 3.5 or better, multiplying the ~15-20% collection efficiency of the PV to 50-70% efficiency, with a lot fewer system components (and fewer moving parts), a lot less maintenance, and lower system design risk.

  7. fasteddie50 | | #7

    Thanks gents! Found this site yesterday....Impressed by it and your helpful advice! I was the silly bldr. who did NOT put enough ins. under my slab....Didn't know any better....Did the code minimum at the time...I should a doubled up the ins. under my slab. I did seal all the typical "good cents" homes type of locations, elec. plugs/lights, gypsum at top of the wall was sealed when my drywall was installed, sealed the bottom plates, around windows (before gyp was installed) When I built the home about 14 or 15 years ago....there was nothing in the code with regards to window/air leakage. U value yes, but not air leakage as in on a slider type window, where the two edges meet up.... I figured it out when my drywaller put visqueen up around the windows and the plastic was blowing the plastic in by a huge amount. Anyway, YOU GUYS ARE GREAT AND THANKS FOR TAKING THE TIME TO ANSWER MY QUESTION AND GET GREAT IDEAS.... Was not aware of the HE hwh....will do....Will climb up in my attic and look for possible areas of air leakage (might have the wife do it as it won't be easy climbing around up there. ya, joke)....I will take a peek at what type of ins. i have up there too.....IF we stay in the home, I will do the mini splits (yes, cold foggy dew/over cast climate Dana)Was thinking if I ever build again, love the way the double stud walls look! KEEP UP THE GOOD WORK FOLKS ! :) Ed

  8. user-2890856 | | #8

    Ed probably should explore utilizing the equipment he presently has before adding other equipment . Odds are very high that the infloor can be properly utilized with a proper Modulating / condensing water heater very different from a stripped down center flue condensing water heater . The ECV ( energy conservation value) that a properly commissioned system offers can make adding even 5,000.00 worth of un needed stuff have a very unattractive payback . Taking care of the various things that are easy to address that may have been missed first time around are a given .
    The units I linked to have already lowered people's propane bills when done properly by 40 - 60% . That would be fuel , electric .
    Your slab should also not be hot Ed . It should be kept right where it needs to be on any given day , loading slabs full of BTUs is old thinking and the main reason why folks whom have never encountered a properly done slab say what they do about them . I do still question how solar PV would work or be recommended in the smae paragraph where it was stated that thermal won't ? Pick the low hanging fruit that makes sense and have someone perform a heat load calc using proper methods then repair what you have .
    While R 10 could have been a more robust number the fact that the floor above probably has an R value of 1.6 high end is plenty to drive the energy into the room and your downward losses will not be as great as you would believe . Considering the local climate climate , ground temps would never be low enough to achieve the Delta between earth and slab . R10 is enough for your area and to be used with radiant floor .

  9. Dana1 | | #9

    To be clear, I was not recommending PV as being cost-effective (without a lot of subsidy) in cheap-power Pacific Northwest, only that it is comparatively FAR more cost effective heating than solar-thermal in that climate when coupled with high efficiency air source heat pumps, and it would return higher solar efficiency in terms of BTU/annum per square foot of collector area. Mini-splits are extremely cost effective for offsetting propane use at Whidbey Island utility & propane pricing, and the financial math will stand on it's own merits, independent of PV. The amount of solar thermal it would take to make a dent in propane use that climate would be out of sight expensive- not enough sun, probably not enough rooftop to make it work (but maybe enough yard?).

    While the HTP systems really are exceptionally well designed and easy to design around, the notion that it will save 40%-60% compared to a cheap atmospheric drafted or power-drafted propane water heater in this application is not a likely outcome, an assertion that defies the raw combustion-efficiency physics. While there are many truly power-piggy poorly designed hydronic systems out there, one would have to go out of the way to design it & control it egregiously sub-optimally to make that happen. A mod-con boiler would be more expensive to install than a PAIR of 1-ton mini-splits, and would save in the neighborhood of ~15% on propane.

    The comfort/cost compromise would be to keep the slab at a constant comfortable temp by controlling the system with a floor thermostat rather than trying to heat the rooms with it, and controlling the room temp with comparatively cheap & effective ductless mini-split technology. As noted in the prior post, in many local markets in the Pacific Northwest even using an electric boiler (at a COP of 1) is comparable operating cost to (often cheaper than) heating with condensing propane. An electric boiler is also cheaper to install and lower maintenance than a modulating propane boiler. But I'm not recommending this as a solution, only pointing out that the local energy prices matter (a lot) when looking at different options. In the land of cheap electricity & moderate outdoor temps, air source heat pumps are impossible to beat with even the highest efficiency propane systems in a high propane-cost market like Whidbey Island. The problem is the high source BTU fuel cost of island propane, not marginal equipment efficiency. The solution is to use a different, cheaper(and to be sure GREENER, in legacy hydro based electric grids) energy source.

    Keeping the existing slab-heating system but not actually using it for the bulk of the space heating makes it's absolute efficiency less important. There may be several optimizations to be made beyond mere slab thermostat control, but the only way to make it affordable is to supplement it with cheaper heat &/or less heat load, not higher efficiency.

    The deep subsoil temps in the Puget Sound area are in the low 50s F(comparable to most US zone 5 locations), and at the local fuel prices R10 is in nowhere near the financially optimal balancing point at current propane pricing (though it might have been 14-15 years ago when propane was near it's historic low price point.) R10 meets current IRC 2012 code min slab-edge insulation for unheated slabs in Marine zone 4C, R15 would be required for heated slabs. But the heat loss through the R10 into the dirt isn't the major loss factor here, and correcting it with more sub-slab insulation would never be cost-effective. Once the concrete is poured, it is what it is.

    By the time the existing propane burner is toast there may be more (and cheaper) hydronic air source heat pump options to consider. But in the mean time turning that system off completely and running mini-splits will cut the heating cost by more than 2/3, maybe even more than 3/4, but you'd be sacrificing a bit of bare-foot comfort.

  10. fasteddie50 | | #10

    YOU GUYS ARE GREAT! "From the clash of differing opinions...comes a spark of truth" I realllllllly appreciate your time. I'm not worried about toasty feet. I usually keep the haus at 68 and wear slippers or shoes. I don't need a warm floor...I want CHEAP ENERGY/lower bills. I have a guest house on my 5 acres on whidbey. Dana or Richard, either one of you are welcome to come spend a few days anytime you like. No, I'm not kidding! Yep, I'm that appreciative of your advice/time/great ideas. Speaking of my guest house which has elec. wall heaters....All your info got me to thinking....I probably need to bite the bullet and put mini's into both homes....pull out the gas hwh...As propane is crazy expensive on the isle. I have read it can be over 1/3rd cheaper in other parts of the country. And elec IS cheap where we are..... Thanks again for great appreciated help! Sincerely, Ed Jones, Building suspector, McMurdo Station Aunt Arctica (for about another week :):)

  11. fasteddie50 | | #11

    p.s.....I forgot to put out the welcome mat for you Martin. I must warn the 3 of you though....I will pick your brain for more energy saving ideas. :) All the best to you gents :) Ed

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