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What size of Hybrid Electric Heat Pump/Tank Water Heater for 28’x96’ concrete slab?

What size of Hybrid Electric Heat Pump/Tank Water Heater for 28’x96’ concrete slab?

After waiting over 25 years, I was able to get a building built that will support my many hobbies. In designing the building, we tried to make the building pleasing to the neighbors and low cost to operate. One of those things is floor heat. Right now I have ½” pex in the concrete floor, but I am getting mixed answers on what I can use for a heat source.

I would like to use a Hybrid Electric Heat Pump/Tank Water Heater and instead of running one pump and 3 zone solenoid valves, I would like to run 3 separate pumps for the zones. I also plan to mount metering valves on all 9 pipes going into the floor – hot side and on the cold end of the pipes put thermometer/pressure gauges to “tune” the system and make sure everything is balanced

The building specs:
- Located just south of Fort Wayne, Indiana (elevation 791 feet)
- Outdoor temp 0 degrees – indoor temp 60 degrees
- 28 x 96 with 10 foot walls (24” on center studs and rafters)
- 7/16” OSB, Tar Paper, then Metal Roof
- 7/16” OSB, House Wrap, Then Vinyl Siding
- 2x6 wall studs with R19 insulation and 5/8” drywall
- Metal siding for ceiling with approximately 12” of cellulose blow in insulation (R36?)
- 3 – 3’x4’ Jeld –Wen double hung windows (2-west side, 1 south side)
- 2 – 10’w x 8’t Garage doors R-18.4 (North side)
- 1 – 8’w x 7’t Garage door R18.4 (west side)
- 1 – 36 x 80” entry door (west side)
- 5” Concrete with rebar 2’oc (“slab on grade” or “monolithic slab”)
- 2” foam under slab
- 1/2" Pex 12” oc (3 zones – 9 loops)

Here are the specs that the contractor gave me on the floor
- Heating Load Summary:
- Total System Head = 5.6 ft
- Boiler Load = 46830 BTU/h
- Total System Volume = 27 Gallon
- Total Heated Area = 2688ft2
o Zone 1
 Tubing Spacing = 12
 Intensity (BTU/H*ft2) = 17.64
 Back/Edge Losses (BTU/h) = 2.43
 Total Required (BTU/h) = 15802
 Total Provided (BTU/h) = 15838
o Zone 2
 Tubing Spacing = 12
 Intensity (BTU/H*ft2) = 11.56
 Back/Edge Losses (BTU/h) = 1.88
 Total Required (BTU/h) = 15532
 Total Provided (BTU/h) = 15881
o Zone 3
 Tubing Spacing = 12
 Intensity (BTU/H*ft2) = 22.33
 Back/Edge Losses (BTU/h) = 1.78
 Total Required (BTU/h) = 10003
 Total Provided (BTU/h) = 10041


- Zone 1 28x32 (Garage Area – North side)
o # of Circuits = 3
o Circuit Length = 300’
o Supply fluid temperature = 93.3 degrees Fahrenheit
o Delta T = 20 degrees Fahrenheit
o Gallons per minute = 1.98
o Head = 4.9 feet
o Radiant capacity = 15838 BTU/H
o Required Heat = 15802 BTU/H
o Panel Load = 17978 BTU/H
o Glycol = 45%
o Slab Conductivity = .81 BTU/(h*ft* degrees Fahrenheit)

- Zone 2 28x48 (Machining Area – Middle)
o # of Circuits = 4
o Circuit Length = 300’
o Supply fluid temperature = 85.2 degrees Fahrenheit
o Delta T = 20 degrees Fahrenheit
o Gallons per minute = 1.98
o Head = 5.6 feet
o Radiant capacity = 15881 BTU/H
o Required Heat = 15532 BTU/H
o Panel Load = 18052 BTU/H
o Glycol = 45%
o Slab Conductivity = .81 BTU/(h*ft* degrees Fahrenheit)

- Zone 3 28x16 (Storage Area – South side)
o # of Circuits = 2
o Circuit Length = 300’
o Supply fluid temperature = 97.1 degrees Fahrenheit
o Delta T = 20 degrees Fahrenheit
o Gallons per minute = 1.19
o Head = 5.4 feet
o Radiant capacity = 10041 BTU/H
o Required Heat = 10003 BTU/H
o Panel Load = 10799 BTU/H
o Glycol = 45%
o Slab Conductivity = .81 BTU/(h*ft* degrees Fahrenheit)

Comments

  • Zman
    Zman Member Posts: 7,433
    edited December 2016
    First off, congratulations, you started with a plan. The insulation tubing lengths and tubing spacing looks good as does the heat loss calc. I don't think you need that much glycol but no biggy.

    Water heaters in general are not designed to be boilers and will not function as well or last as long as a true boiler.

    Heat pump water heaters can save electricity by stealing some heat out of the air around the heater and putting it back into the water. They do save some money over electric resistance water heaters provided you don't mind cooling the air a bit (think summer time). In your case, you will be cooling the air only to heat it back up with the water you are trying to heat to begin with.
    Translation: More expensive heater and no chance to be more efficient than a regular water heater.

    You will also have trouble finding an electric water heater big enough. To convert KWh to Btu multiply by 3,412.

    What other fuel sources are available? What are your local prices?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    JDmachinist
  • JDmachinist
    JDmachinist Member Posts: 7
    Thank you for your quick response!

    We do have Natural Gas at the house, but would have to bury lines to the building. I would like to go with electric for a couple of reasons: 1) The water heater will be close to the breaker box in the building and 2) Most importantly, i could eventually add solar panels or a wind power to help with the electric bill, but that would not with a natural gas unit.

    How big of heater do i actually need? The contractor told me a 18000BTU unit would be enough. When I look at the numbers the overall need for the entire building is way over that??? Am I understanding the numbers wrong? I have read so many horror stories online about people putting in units that were too small and they ran constantly (expensive) or too big and they short cycled (short life). i hope to get this right the first time with help from great guys like you!!

    Thanks again!!
    bulldoglax
  • Hilly
    Hilly Member Posts: 418
    Based on your heatloss numbers you should look for an Electric Boiler rated at 14kWH
  • Zman
    Zman Member Posts: 7,433
    I attached a spreadsheet which allows you to easily compare energy costs.
    In my market, electricity cost 3x as much as gas in the residential market.
    I would recommend a properly sized natural gas condensing boiler.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • JDmachinist
    JDmachinist Member Posts: 7
    thank you, that was very helpful!
    have a great CHRISTmas!
  • A.J.
    A.J. Member Posts: 257
    With a heat pump water heater your taking BTUs out of the building to heat it, if could pull that off we would never have to burn any more hydrocarbons. Your starting off good just start thinking about doing a water to water heat pump that way you will be taking BTUs from the ground and putting them right into your building . It will work all you need to do is to match the heat loss of the building to the output of the heat pump at design temperatures . Your going to spend more up front but resistance heating is the most expensive way to heat .
    If your just south of Fort Wayne you should be able to find a Water Furnace dealer to help you, they make a great product.
    A.J.
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,542
    I'd also recommend a gas condensing boiler. Unless your gas and electric rates are vastly different than my region, then as mentioned gas will cost roughly 1/3 to operate.

    I'd also recommend a boiler such as weil mclain eco70, or lochinvar whn055. Both of those, and there are many others, are wall mount so you loose no floor space as a bonus. If it's a huge deal to get the NG to the building, you could consider propane, be pending on local costs, that should still be significantly cheaper than Electric. Also if you do go with electric a 14kw boiler will draw 60amps. How big is the electric service to this building? Is it a sub panel off your house, or does it have its own entrance and meter? How far is it from your house?

    Merry CHRISTmas to you as well!

    Taylor
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    Gordy