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First Step: Sizing the boiler
Good morning. We are in the process of installing staple up 1/2 " Pex-Al-Pex to heat our home. We live in Maine. We'll be heating <1100 sf of living space in 3 zones. (Zone 1=3 loops, Zone 2=2 loops, Zone 3=2 loops initially, with option to add a third if 2 aren't adequate - this zone will be in the walls of a 200 sf basement room). I don't have exact figures yet for each loop size, but all will be between 200 and 300' max.
We are looking at Bosch Greenstar LP boilers. (Heat only, we have a hybrid electric/heat pump water heater).
We've been given a wide range of BTU recommendations from several installers. Anything from 40k to 100k.
I'd done some research online and thought we should end up in the 50-60k range, but after poking around this forum some, I'm afraid even that will be too big and we'll end up with short cycling.
A little more info: On the main floor, we're stapling to sub floor, and it has a combination of porcelain tile over plywood and cement board (small bathroom), vinyl tile directly over subfloor (kitchen and dining), and 3/4" hardwood directly over subfloor (living, hall, 3 small bedrooms).
In the basement we are stapling to 1/2" plywood, which is used essentially as paneling, so no other barrier between the PEX and the room other than primer and paint.
We are using 4' omega channel aluminum plates over the PEX and using rFoil reflective bubble insulation fit between the joists/studs, leaving an air pocket between the rFoil and PEX.
Joist ends have been insulated with 2" EPS sealed with spray foam. The house was built in 1989 and has Rockwool type insulation in the attic crawlspace. Its a 1 story home of around 900 sf after deducting interior walls (988 total) and as mentioned we'll also be heating the 200 sf basement room.
For the main floor, the radiant floor will be used to supplement 2 heat pumps in Spring and Fall, but used as primary heat in the colder months. It will be the only heat source for the basement room, and that room will be heated continuously throughout the heating season.
So, I'm hoping you fine people might be able to give me some insight into how to size a boiler using only 1/2" PEX for heating ~ 1100 sf in Maine. I appreciate any help you can give.