Wednesday, November 26, 2008

Cottage heat loss coefficient

On my list of things to be thankful for, category wonk, subcategory building science, is a thing that happened during the first few hours of Nov 16 - the cottage settled into a thermodynamic steady state with the boiler running full out and all four radiant floor loops turned on.  (I was trying to warm the greenhouse floor for painting.) Because of steady state I know that the boiler electric power, the heat delivered by the distribution loops, and the heat leaking to the outside are all the same.  That plus the air and water temperatures from the monitoring system allow me to estimate the building "UA" or heat loss coefficient, and the flow rate of the hydronic circulation pump.

Data from the 2-3 am hour of Nov 16:

Boiler power 17061 Btu/h (5000 W)
Outside air temp 22.72-23.31 F
Inside air temp 70.3-73.78
Greenhouse air temp 55.53-57.69

Inside to outside temp difference 49.025 F, 27.236 C.

Building UA = 17061 Btu/h / 49.025 F = 348 Btu/hF
Building UA = 5000 W / 27.236 C = 183 W/C

Lower is better.  It would probably look better if I hadn't been heating the greenhouse, but in order to get steady state with just the cabin zones heated, it will have to get colder outside.

I'm actually more interested in the circulation pump right now as I'm trying to figure out the solar hydronic system.  The data I have is:

Boiler outlet 97.85 F
Cabin return A 90.27
Cabin return B 91.41
Greenhouse A 94.05
Greenhouse B 90.09

I really wish now I had a sensor on the boiler inlet to measure the mixed return water temp.  As it is I don't have much choice but to assume all the loops are carrying an equal share of the flow, and use the average of the four return temps.  

Distribution loop temp difference: DT = 97.85 - 91.44 = 6.395 F

Qdistrib = m cp DT = Pboiler, in steady state.

m = Pboiler/(cp DT) = 17061 Btu/h / (1 Btu/lbF * 6.395 F) = 2668 lb/hr * 1 gal/8 lb * 1 hr/60 min = 5.6 gal/min.

I looked up pressure-flow curve for the Taco 007 circulator, which is (irritatingly) labeled curve number five (dark green).

At 5-6 gpm the 007 is almost maxed out on pressure.  I wanted to put flow meters in the distribution loops but I can't find any (cheap) ones that drop less than about 1 psi @ 4 gpm, which is like two feet of water and it looks like that would choke this pump off entirely.

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