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re: best setback temperature?
21 jan 2004
"marty" wrote in message
> i have forced hot air oil heat, small ranch house, loses about 60,000
> btus per hour. new jersey climate. what would be the optimum setback
> temperature for economy for night and unoccupied day time?
off, for greatest energy savings, imo. the drywall might crack at 50 f,
which could be a non-problem, with some flexible white caulk.
don't want to freeze the pipes.
> i used to select a nozzle and setback which would make the burner run
> for long periods, but now high electric costs make me rethink that.
electric costs? with no nozzle change, the blower runs fewer hours per day
with larger setbacks. it might use 400 watts, but a smaller nozzle and larger
oil-burning efficiency might outweigh more blower hours. a furnace supplying
60k btu/h at 80% efficiency with capacity rate ratio z = 0.01 (10 cfm flue
and 1000 cfm hot air) might have e = 0.8 = (1-e^-(n(1-z)))/(1-ze^(1-n(1-z)),
so n = 1.54 and au = 15.4. at 40k btu/h (cmin = 6.7 cfm, n = 15.4/6.7 = 2.3
and z = 0.0067), e = (1-e^(2.3(1-0.0067)))/(1-0.0067e^(2.3(1-0.0067))) = 0.9.
at 80k btu/h (cmin = 12.5 cfm, n = 1.23 and z = 0.0125), e = 0.71, so it might
supply 300k btu like this (viewed in a fixed font):
rate run time oil use elec. use cost
(btu/h) (h) (gal) (kwh) ($1.50/gal + 10c/kwh)
40k 7.5 2.6@90% 3.0 4.20
60k 5.0 2.9@80% 2.0 4.55
80k 3.75 3.3@71% 1.5 5.10
> i am thinking too large a setback makes it hard for the burner to reheat
> the house, yet too little may not save enough.
the most efficient nozzle might run 24 hours per day, but that
wouldn't allow any setback. what's the most efficient setback?
nick
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