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re: tankless hotwater system
26 nov 2000
gary slusser wrote:
>...the boiler water ranges in temp between burns and that heat
>is transfered into the water in the coil
sounds more like a "summer-winter hookup" for a house heating furnace
than an tankless instantaneous water heater...
>and when the burner comes on it has to heat the boiler water first
>and then that heat transfers to the water in the coil raising its temp.
>with no scale in the coil we'll say that time period from burner
>on to off is 2 minutes and it's just over that by the time the
>domestic water is usually at its highest temp.
hmmm.
>now we add an insulator, the scale, to the heat transfer formula and
>find that the scale increases the time it usually takes the water
>[in the coil] to reach max temp.
seems like this would also increase the boiler cycle time.
>now at the same time we've decreased the id of the coil tubing while
>the main line water pressure of the system has stayed the same.... the
>velocity increases as the id decreases...
and the thermal conductance increases with more velocity and roughness.
>which usually means we don't reach our normal recovery rate (gpm @ x
>degrees temp rise).
i was thinking of the "recovery rate" of a water heater as the time required
to heat a batch of water, say 50 gallons, from 40 to 130 f. but maybe that's
"recovery time." the grainger catalog specs "recovery gph at 90 degree rise
d.o.e. method" for tank-type water heaters, eg 18 gph for a 4500 watt heater
vs 76 for a 75,500 btu/h gas tank heater and 409 for a $4300 65 gallon 360k
btu/h input version. hmmm... 18gphx8btu/g-fx90f = 12,960 btu/h or 3.8 kw...
jade mountain says their $360 10"x9"x3" 80ax240v (19kw) eemax ex190t "whole
house heater" can do 2.5 gpm with a 50 f temp rise, eg heat 60 f water to
110 f for a continuous 2.5 gpm shower. their $26 12"x6"x3" 110 v powerstream
heater provides a 20 f rise at 1 gpm, so that 2.5 gpm shower would require
2.5x(110-60)/(1x20) = 7 of them...
nick
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