Merry CHRISTmas! Thank you in advance for any suggestions/recommendations you may have! 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)

When you say "Hybrid Electric Heat Pump/Tank Water Heater" I am visualizing a domestic water heater with a small heat pump on top? If that is what you are thinking, I do nt think it is going to work for you. The largest HP model I have seen only has about a 9000 BTU HP on top. The electric backup element on those is the typical 4500W hot water heater element. 1KW = about 3400 BTU so that element is only good for about 15000 BTU/hr, and that is running at 100% which they were not designed to do... A HP water heater takes heat from a room and concentrates/pumps it into the water tank. Where would this type get it's warm room air from, the room you are trying to heat?

I didn't validate any of your numbers, but that "boiler load" number indicates you need to provide at least 46KBTU/HR, I am assuming when it is 0 degrees F outside. That means a boiler larger than that so as not to be running at 100%. At those numbers, that is a very large electric boiler or outside air source HP(must move a lot of air). For this volume of heat a fueled boiler probably makes more sense...

Thank you for your quick response!

yes i was thinking of the type of water heater with the heat pump on top. Good point on the asking where is the heat going to come from. Just a thought, what if i would duct the cold air out of the building and only have it sucking warm air from in the building, maybe at the ceiling? I realize that until the building got warmed up this would be a wasted option on the water heater.

I have read, that you can order tank water heaters with the 2nd element "more active" either that they both kick on a the same time or that the second one kicks on at a specified time after the first?

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!!

Use a different web browser so we can get some white space. That's downright unreadable!

First things first: Run your own heat load calculation on the building at your 99% outside design temp (=+2F at the Ft. Wayne airport, not 0F) , and your 60F indoor temp. The 46K number seems a bit high, but damned if I'm going to try to unscramble the text to make it readable. I'm taking a WAG that it'll be more like 35-40K.

You'll NEVER get there with an electric water heater. 46000 BTU/hr is (/3412=) 13,480 watts. You can get there with a 15kw electric boiler (at a COP of 1), or you can use a reversible air source chiller- probably a 3-tonner, but maybe a 4, and in your climate probably use only about 1/3 the amount of power as an electric boiler.

Like Dana says you will NEVER get what you need out of a domestic water heater! As for pumping air outside, that air WILL be replaced by more air from outside. Air infiltration can be a major player in any building heat loss calculations. I didn't notice any accounting of that in your numbers. With garage doors, I am guessing it WILL be a major player for your building also...

I agree 15KW for an electric boiler, IF your numbers are valid. Can the electric service to the building handle that? 15KW @ 240V is nearly 63 amps all by itself... A heat pump would deliver the heat 3 times more efficiently(20A or less).

You can do this yourself, but there is a learning curve that you appear to be behind on... It might be worth your time and money to bring in a 3rd party who does this type heating design and at the very least confirm your required heat loss/load...

If you have natural gas available, that is the only fuel I would consider for heating this building. Your electric costs will likely be very high to heat that large of a building with resistance type heaters and as mentioned above, heat pump water heaters are simply not a viable option.

Also, is this an occasional use building? Do you plan on maintaining the temperature at setpoint all of the time or do you plan to turn down the heat when you are not in the building? If the answer is that you will turn the heat down when not using the building, then a radiant concrete slab alone is a poor heating choice unless you have a set schedule you will using the building and you put it on a programmable thermostat to ramp up the heat before you need it. It will take several hours for that large concrete mass to bring the room temperature up to setpoint from a lower temperature.

With natural gas, you might use a gas boiler for heating the slab combined with one or more forced air fan coil units for more rapid heating of the air in the building. Maybe strategically place the fan coil unit(s) in areas most used.

You could look at a Chiltrix CX30 plus some supplemental heat. But seriously consider nat gas, perhaps via overhead radiant.