Radiator Average Water Temperatures

arborus14

I have noticed, when sizing fin-tube radiators, that manufacturers of high efficiency products provide ratings for average water temperatures (AWT) down to 90F. Other manufacturers of standard radiators provide correction factors down to the same number. I am a bit skeptical of a radiator operating effectively at these low temperatures especially at our design temperatures of -13F here in the North. The convection off these radiators has to counteract the cold air spilling off a window and I would think you would not want to go below 130F as an AWT. Does anybody know where there is some independent technical information on performance of baseboard vs AWT? Does anyone have experience with going low with AWTs - how low can you go? This would be for a "code" building, not a super insulated - triple glazed project. Thanks Cam

SWEI

Cast iron radiators function well when they are fed by anything a hot water or steam system can supply (from ~85°F to ~250°F.) Their output falls as the supply temp drops.

The output of fin-tube baseboard or convectors falls off faster, and it's non-linear -- the output drops faster and faster as the temp goes down. Roughly speaking, it looks something like this:

SWT Output
180°F 443 BTU/ft
170°F 391 BTU/ft
160°F 330 BTU/ft
150°F 278 BTU/ft
140°F 226 BTU/ft
130°F 183 BTU/ft
120°F 139 BTU/ft

Numbers above derived from Slant/Fin performance tables for Fine/Line 30 -- after removing the "15% heating effect" and converting AWT to SWT, assuming a 20°F ΔT.

SWEI

The fine print footnoting their table makes the data even more questionable:

*Ratings at 140°F and lower temperatures determined by multiplying 150°F rating by the applicable factor specified in Table E in the I=B=R Testing and Rating Standard for Baseboard radiation.

My experience is that the perceived output falls off much faster than a linear relationship would predict. I wonder where they measured (for those values they actually did measure.) If the sensor was conveniently located above the convector outlet, it might actually look pretty good.

hot_rod

arborus14 said:

I have noticed, when sizing fin-tube radiators, that manufacturers of high efficiency products provide ratings for average water temperatures (AWT) down to 90F. Other manufacturers of standard radiators provide correction factors down to the same number. I am a bit skeptical of a radiator operating effectively at these low temperatures especially at our design temperatures of -13F here in the North. The convection off these radiators has to counteract the cold air spilling off a window and I would think you would not want to go below 130F as an AWT. Does anybody know where there is some independent technical information on performance of baseboard vs AWT? Does anyone have experience with going low with AWTs - how low can you go? This would be for a "code" building, not a super insulated - triple glazed project. Thanks Cam

If you are bent on low low temperature fin tube operation , one option is to force the convection.

Jaga has a product that matches to some of their fin tube offering that adds small microprocessor driven fans. They show performance #s down to 30C.

An interesting company with a unique niche product.