Hi Folks, I am in the process of finishing two basement suites with radiant floor heat in a building in Southeastern BC, Canada. Based on the size of the spaces and the location, my supplier recommended two 30,000 BTU heaters (each unit will have it's own boiler system so utilities can be kept separate). Two 30K boilers seems like overkill to me so I have done a heat loss calculation and I wanted to run it by the experts here to make sure I am not in left field with the numbers. Each unit has the same number and size of windows and doors and is identical in size so I did one calculation for the whole area and the divided the heat loss in half to get each unit heat loss. The basement is 5' below grade with three feet of the walls above grade. The lowest temperatures we see here are -22F. The winter soil temperature 5' down is 40F. Walls are R20 and I put R12 rigid foam under the 1200 sq ft slab with 1/2 pex tubing. I calculated delta T based on 70F inside. Also, I used a soil temperature of 40F for the floor and for the walls two feet up from the floor and then used -22F for the remaining 6 feet of the wall surface. I used the equation: 1/R * delta T * area. I calculated infiltration by using the area of the upper three feet of the wall assembly multiplied by .52. After totalling the heat loss I added 25%. Here are my calculations: Floor: 1/20 * 30 * 1200 = 1800, Doors: 1/5 * 92 * 46 = 846, Windows: 1/2 * 92 * 40 = 1840, Lower wall: 1/20 * 30 *280 = 420, Upper Wall: 1/20 * 92 * 840 = 3864, Ceiling: 1/60 * 92 *1200 = 1840, Infiltration = 300, Total Heat Loss = 10, 910. Plus 25% (2727 btu) for a total of 13,637. Based on these calculations I was thinking of using either the 2.5 kW (8500 BTU) or 4.5 kW (15,000 BTU) Mini Electro Boiler. I don't want to oversize either. So, have I royally messed up somewhere or do these numbers make sense? Have I missed something important? What boiler would you recommend I go with? Thanks, Zad.

that's not a proper way to do a basement calc, the equations are much more complex.

Infiltration, no idea what you were doing there. it's not usually a big deal in a basement though you do need to account for it.

In short: your numbers have little bearing on reality. Not to be harsh, but you sound like you "know enough to be dangerous". Might want to get some real help on this or at least download a canned program with real inputs for basements and infiltration calcs.

you can't really oversize electric boilers. they are generally infinitely modulatable.

... and even in bang-bang mode there are no cycling losses the way there is with combustion equipment. They're inherently low mass, and aren't doing flue purges, etc.

What will you do for domestic hot water?

Get a pro to do the heat load if it is important.

Yes, I do know just enough to be dangerous, very true. I used the Slant Fin heat loss calculator and found the heat loss to be around 22500 btu's. However, local contractors who do this all the time in ICF basements in this area recommend 25 btu's per square foot which puts me at 30,000 btu's. There are two units in this basement so I am going to put in a 15,000 btu unit in each one. Now, the next question. For units this small, what would you recommend for cost and functional efficiency for radiant and DHW. We have hydro electric power up here and it is clean and cheap so the building is 100% electric. I have considered an electric hot water tank with a heat exchanger, I have also considered a boiler and storage tank with a heat exchanger, and I have considered a boiler and hot water tank. Any thoughts? Zad.

25 BTUs a square foot for an ICF basement is proably at least double and maybe triple the load you'll really have in most cases, assuming the floor is properly insulated and it's not walkout.

It's pretty hard to find an electric tank with the output you need. Could do a tank for hot water and an electric boiler for heating, that's lowest amp draw. Or an electric on demand for each, if the DHW loads are low that's not bad either.

You can do an on demand plus heat exchange etc but at that point you are pretty close to the cost of separate units.

Hi Rob, What btu sizing would you recommend for a very tight 1200 square foot ICF basement with R12 under slab that is not walkout (two stairwells), 42 square feet of insulated metal door, 40 square feet of double pane low E glass and a delta T of 92 (we get down to -22F)? Zad.

I would have to do the calc to answer that, but if you broke 10 BTUs/sq ft I'd be surprised.