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