Radiator Sizing

[John Molyneux]

I need some additional radiation in my basement family room to replace a meager amount of baseboard that just isn't cutting it at the low water temps I'm running with the new modcon.

The room is about 15 X 22 with 7 foot ceilings. Walls are below-grade concrete with a layer of beadboard covered by an additional layer of sheetrock. Ceiling and rim joists are insulated with Roxul.

I can get a good deal on a 36 X 32 Pensotti panel by someone who bought it but can't use it, and can't return it. It's a pretty big panel, but does it really matter given the low water temps I'm running? And it will have a TRV on it anyway.

 

[Dana]

If you have the Manual-J, what was the calculated heat load of the basement?

A 32 x 36" Pensotti at an AWT of 110F (say, 115F out, 105F back or similar) it would deliver something between 2250-2500 BTU/hr with 65F air near the slab, more if it's colder.

With fin-tube baseboard, maybe 125-150 BTU/hr per running foot at an AWT of 110F. If it's say, a 10' stick o' baseboard you're talking at most 1500 BTU/hr. I suspect your basement load is a bit higher than the combined ~4000 BTU/hr of output given that the foundation isn't insulated, which would mean higher water temps to get it fully up to 68F under design conditions. But adding the radiator certainly can't hurt, even if it doesn't keep up under all conditions.

With maybe R1 or so of beadboard & sheet rock between the concrete and the room air it wouldn't take a lot to peel another 2500 BTU/hr off the heat load with rigid insulation, but it would be a bigger project. Even doing it one wall at a time you'd be able to gauge the result by the basement temperature.

If there's even 1' of above grade exterior exposure on the concrete, you're looking at a U-factor of about U0.3- U0.5 for the concrete + beadboard + gypsum stack up, and 75 square feet of exposed wall. At 0F outdoors and 68F indoors that stripe alone contributes at least U0.3 x 75' x 68F= ~1500 BTU/hr to the heat load, more than half the low temp output of the radiator. Bringing that stripe up to current code min would cut that to less than 150 BTU/hr, less than half the output of a conscious standing adult human.

The fully below grade numbers are squishier and harder to model but they're still a real load, it just changes less with hourly outdoor temp.

 

[Sponsor]

 

[John Molyneux]

Good point about the Man. J. Duh. Once I saw that it confirmed the DIY method I didn't look at the room-by-room detail.