Hey folks, After many attempts to model my house heat load with online calculators i give up. I am just computer dumb. I hope someone can help. I have a simple three story house with superior wall foundation r12, two more storys with 6.5 inch sips r22 +- and 10 inch roof panels r38 with lots of south and west casement windows. the house will have a high seer hvac system with j calc done by pros. What i am wondering is for my radiant what ballpark size water heater do i need for the 6-7 inch basement slab and the 1.5-2 inch middle floor slab? The loft will not have radiant. I hope to use one water heater for both domestic and radiant. My options are electric and propane. One day i hope to ad Solar hot water but i am running out of funds rapidly during construction. Thanks as always for any help. michael oh yea the house is 20 by 28 and 34 feet at the tallest point only parrtly buried basement.

I design hydronic heating systems with dedicated radiant heating software which produces complete room by room heat loads, tube size, length and design water temperature.

This is the first step towards success.

Combination water/space heating is best done with a ModCon and companion water heater and next best done with a Combi-Cor I.

"Open" systems are not installed by professionals which should be fair warning to a thinking man.

Personally I prefer external heat exchange based on based longevity issues with the combicor units, but the newer units might be better... IF water heaters are the right choice. but that's only true for very small heat loads and/or mild winter climates.

to do a room by room calc that means anything, you'll have to hire someone.

To clarify, the radiant will be redundant. Its a luxury for comfort. I think the biggest problem will be overheating the house and having the floor still feel warm. If radiant cant keep up i can just cut the heat pump on. Our winters are fairly mild and we like a cool house.This is a tiny house, less than 1350 sqft so would a mod con really be effiecient? I was thinking electric versus propane hot water heater with a heat exchanger for domestic HW not an open system.

depends on the heat load. 1200 square feet is small but in a real heating climate could still cost justify a mod/con in some cases.

I'm not a pro but if all you want is warm feet then it makes sense to me to start with what temp you want the floor to be for comfort and work backwards to calculate how many btus the floor will emit at that temp. Then you can also calc whether that will overheat your rooms or how much additional heat you'll need from the heat pump.

But again, I'm just learning.

Assuming your average COP is over 2 (efficiency of 200%+) on the heat pump, it should be your primary, not your backup heat for heating the place.  A mod-con's efficiency isn't likely to beat 95% efficiency, a propane tank HW heater might hit 80% on it's highest duty-cycle day, an electric tank used for space heating will give you a hair below 100%.   The economics will vary since the relative pricing of propane & electricity vary by location, but extremely rare is the local market where propane burned at even 95% efficiency will still be cheaper than electricity used in a heat-pump with a COP>2.

If the goal is to just avoid clammy-cold floors, using floor thermostats and setting the floors temp to 70F would keep your feet from freezing without losing too much in efficiency, but let the heat pump control the room temp for any significant heat load. In most local markets propane burned at 70-80% is still cheaper than electricity at 100%, but you have to do the math on current utility rates and look into a crystal ball on how they'll trend into the future to decide on which way to go there. The heat output required to keep floor at or barely above room temp in a room heated by other means won't strip the capacity of any full-sized HW heater (but it might if that's the primary source of heat.)

Ok,
You mentioned electric,
Understanding the heat pump is your primary and most fuel efficient the floor heat is a bump or luxury item.
My option would be an electric modulating boiler, small just for keeping floor warm. If you took sq footage and figured 25 BTU sq FT you would be fine. Ballpark at 28,000 BTU, 2 zones upper and lower. Key question is is there insulation under slab? is slab really 6-7 inches, what level is the pipe in the slab?
You may have problems with the slab.
Dan

Dana, that is exactly what i was lookin for. We have cheap electric here in southern N.C. and electric would be a little cheaper to install. If we used the floor as a tempering type thing instead of primary heat, What makes and models of electric water heaters would you recomend that coulsd do both domestic hot water and the two radiant zones? thanks Dana by the way im putting in a drain heat exchanger with two baths kitchen and washing machine all running through itdue to you and Brocks recomendation. Mucho gracias Dan, how does a modulating electric work and can you recomend a specific model. 1000 sqft of radiant is what i have . Downstairs is a slab that is at no point less than 5 inchs and at max probably 7. It has one inch dow foamboard everywhere and 2 inch near the walls that are not backfilled. 300 ft of 7/8 tubing is closer to the botom than the top, but it did try to float a good bit so i really dont know where most wound up, we punched it back down with rakes. The thin slab that will be on the middle level will be 1 and 1/2 inchs thick with six hundred feet of 1/2 inch pex in two loops of 300. The thinking if you can call it that for the thickend slab in basement was to be a bit of a heat sink for solar one day. Not the smartest thing ive done. Thanks for the help Dan, michael

Michael,
Regarding electric boilers we like the Thermolec brand, you can see it and a manual on our site.there are other electric boilers out there though. This unit modulates so if the actual demand is less the boiler will lower its load. If tempering is the only function than figuring at 20-25 BTU per sq ft will work fine.
7/8 pex is large, but sounds like a done deal. will work, what is the spacing? Looking at the picture and the 1,000 sq ft area you must have 500 sq ft per floor.600 feet of pex in the 1 1/2 inch area is a better spacing than what you have in the basement and will likely meet the heat load when your heat pump falls off in the bitter cold days.
so I would be thinking about a B-6UM 20,000 BTU or a B-8UM 27,300 BTU boiler.
Dan