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Heat loss calcs, and how to size a water heater for hydronic heating

bobt14 | Posted in Mechanicals on

We are building a home using slab hydronic heating in Central Oregon and our HVAC guy recommends 2 electric water heaters to carry the load (no gas avail). He says that our demand will be about 52kBTU/hr when the temp is 0F outside and the slab is heated to 70F. So 2 x 50gal water heaters rated at ~30kBTU each provides enough with buffer. He says he used Wrightsoft Manual J to calculate. we don’t really want 2 water heaters if not needed.

The 2000 sqft house house is supposed to be pretty tight, does this demand seem reasonable (all discussions on the sensibility of using hydronic heating in tight homes notwithstanding)? When i ran my own spreadsheet based on the design, i only come up with about half.

my assumptions: 70F deg inside, 0F deg outside, ground temp under slab 50C

windows: ~450sft, U=0.28 avg. Q=8000 BTU/hr
walls (minus windows): ~4000 sqft, R26. Q= 11kBTU/hr
Stem walls: 195 ft perimeter, U=0.83, Top 2″ in contact with slab. Q = 2000 BtU/hr (drops to 600 BTU/hr for 1/2 inch added thermal break)
Slab: 1360 sqft, R22 below, ground temp 50F. Q= 1300 BTU/hr
Raised Foundation Floor: 700 sqft: TBD
Roof: TBD
Doors: TBD (garage, front, french,etc)

the walls will be closed cell foam + blown in FG, 2×6″ construction. The TBD’s i’ll get to later in my model, but even if they add up to ~ 4000 BTU/hr, that won’t take my overall estimates much over 26kBTU/hr.

I’ve never ran real life estimates like this before, can i be that far off in my calcs, and do we need 2 water heaters?


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  1. GBA Editor
    Martin Holladay | | #1

    I agree that 52,000 Btuh sounds high for your design heat loss. Here are some links to articles with more information:

    How to Perform a Heat-Loss Calculation — Part 1

    How to Perform a Heat-Loss Calculation — Part 2

    It doesn't make much sense to use electric resistance elements to heat water, and then to circulate the water through pipes in your slab. It would make more sense to simply install electric elements in your floor (many manufacturers make such elements), to avoid the expense of PEX tubing and pumps.

    Or, better still --- why not just install a few electric baseboard units? You would save thousands of dollars.

  2. Expert Member
    Dana Dorsett | | #2

    What kind of ventilation/infiltration rate went into the Wrightsoft calc?

    BTW: With a flash'n'fill R26 center-cavity R value, with 16" o.c. framing (25% framing fraction typical), you'd be at about R16 for a "whole wall" R after factoring in the thermal bridging of the framing, and that's giving it another R1 or so for the combined gypsum + sheathing + siding. That's a U-factor of about U0.063, so for 4000 square feet and 70F design delta-T you're looking at design temp wall losses of about 70F x 4000' x 0.063= ~17.5 BTU/hr which is quite a bit more than 11K.

    If instead of putting the spray foam on the inside of the cavities you put 2" of EPS on the exterior of the sheathing, with blown cellulose or 1.8lb Spider/Optima/L77 in the cavities you'd be hitting around R23, and probably for less money:

    Closed cell foam is about 17 cents per R per square foot, or about a buck a square foot for a 1" flash layer, $2./sq.ft. for a 2" layer. To hit R26 center-cavity I'm assuming you're talking 2" of foam + 3.5" of fiber(?).

    EPS is about 10cents/R-foot, and R4.2/inch so you'd be looking at about 85 cents per square foot. But with a 25% framing fraction you're looking at only 75% of the square footage of the EPS. Still, that 75% coverage is costing you about 0.75 x $2= $1.50 per square foot of gross wall area, to the EPS cost of 85 cents/foot. And the spray foam solution underperforms the exterior-foam approach by a substantial margin: The 2" EPS sheathing approach loses 30% less heat. In a US climate zone 5 climate R8.4 (2" of type-II EPS) gives plenty of dew-point margin for being able to skip the interior side vapor retarders for 2x6 framing, using only the standard latex paint. The IRC prescriptive minimum is only R7.5:

    A 0F design temp is probably a hair too conservative- the 99% outside design temp for Bend is still +4F, so unless you're more 1000' higher than Bend, 0F is probably more like a 99.5% temperature bin.

    BTW: If the dirt under the house is really 50C you shouldn't build there, since it means you're on a volcanic vent or hot spring. :-) Mid winter subsoil temps of 50F would seem more likely in central OR, not 50C.

  3. Expert Member
    Dana Dorsett | | #3

    If your heat load comes in at anything like 50K (or even 30K) you'll be better off with an electric boiler than any electric hot water heater solution. Electric tank HW heaters are not designed for that sort of duty cycle, and will require a lot more maintenance, whereas electric boilers are fully ruggedized and purpose-built for space heating. Hot water heater solutions only make sense if your design heat load is something like half the full-power rating or lower. A 50KBTU/hr electric boiler is ~$1.5K at internet pricing, and while that's on the order of a grand more than a pair of hot water heaters, it's less plumbing & wiring to install, and outlast hot water heaters by at least a decade.

    But note, a cold-climate ductless mini-split or two could cut your heating costs by about 2/3, and pay for themselves in the short years on reduced power use. If you wanted to retain the cushy feel of the radiant floor you could set the mini-split to 70F, and use a floor-thermostat and set the floor to 72F, which would be emitting ~4 BTU per square foot into a 70F room. A pair of 1.25 ton mini-splits delivers over 35,000 BTU/hr @ 0F (see: ), covering 100% of your average load. Most of the time the ductless would be handling the lion's share of the load, but as the loads fall off during the shoulder seasons the less-efficient electric boiler would dominate the figures.

    But get the heat load nailed down no matter what, to get the sizing of the electric boiler &/or mini-splits right.

  4. bobt14 | | #4

    Outstanding, this forum and you guys are an incredible resource, wish i had found it and engaged sooner... before breaking ground!. I'm going to need to digest this for a bit, talk with the architect, builder and HVAC guy. I have it written down somewhere that the HVAC guy said the savings for 2 electric water heaters over an electric boiler (installed cost i assume?) was about $2100. .

  5. Expert Member
    Dana Dorsett | | #5

    A 50KBTU/hr electric boiler costs about $1500. A bargain basement electric HW heater is $200, times two is $400. The only way it comes to a $2100 delta is if he's marking up the hardware 100% on top of the installation labor. The labor for installing the electric boiler is LESS than installing two hot water heaters.

    Smaller electric boilers are a few hundred cheaper, and there's no point to oversizing it, so run the heat load calculations and get it right.

    A mini-split capable of delivering 18-22,000BTU/hr @ +5F using less than 1/3 the amount of power per unit heat of an electric boiler runs about $2-2.5K. As a DIY, add about $200-500 for related mounting hardware and tech-time for charging & testing the unit, but for full turnkey all-pro installation roughly double the hardware cost:

    The M-series MSZ/MUZ-FH15NA is more efficient than any of the above, but not yet available through internet stores yet. They probably will be by next summer.

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