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re: water heater in heated space?
16 jan 2001
don k. wrote:
>marty wrote:
>>...the lost heat from the water heater will cause the furnace to
>>run less and shut off sooner..
>...the house is at the desired temperature when the furnace cuts off.
>but the heater keeps running. so you are heating at least one room
>above the desired house temperature.
not if the room loses heat faster than the water heater can supply it,
including the heat that leaks from the room to the rest of the house.
>heat flow is proportional to temperature gradient. this hot room will
>lose more heat energy to the outside world because there's a higher
>temperature gradient to the outside world than without the water heater.
>therefore it's less efficient.
maybe.
>whether the difference amounts to a hill of beans is another story. :-)
say the 130 f water heater has 25ft^2 of us r10 insulation, so it supplies
about (130-70)25ft^2/r10 = 150 btu/h of heat (like a 44 watt light bulb)
to a room that's about 70 f. if the room has perfectly insulated walls and
an open 2'x8' door to the rest of the 70 f house, we have something like
150 = 16.6(4ft^2)sqrt(7')dt^1.5, using an empirical chimney formula, so
dt = 0.9 f, and the room will be about 70.9 f as 16.6(4)sqrt(7x0.9) = 167
cfm of air naturally circulates between the room and the rest of the house.
since the room walls are perfectly insulated, there's no heat loss from
the room to begin with, and no house heating efficiency loss if we put
a water heater inside the room.
now suppose the room has a 4'x4' r2 window and an electric baseboard heater
with its own thermostat, and it's 30 f outdoors. with no water heater, the
baseboard supplies (70-30)16ft^2/r2 = 320 btu/h of electric heat. if a water
heater supplies 150 btu/h, the baseboard only needs to supply 170 btu/h, and
the room remains 70 f, and there's no efficiency loss.
now suppose the room has a 2'x2' window, so it only needs 80 btu/h of heat
and the water heater makes 150, with the open door. then (150-80) = 16.6...
and dt = 0.54 f, so the 70.5 f room uses (70.54-30)4ft^2/r2 btu/h = 81.08
vs 80 btu/h. that's another 26 btu/day or 780 btu/month, ie 0.23 kwh worth
2.3 cents at 10 cents/kwh...
nick
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