furnace [RE-wrenches]

[Windy Dankoff, Dankoff Solar]

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>Greetings all,
>
>I just got an off-grid job that is a new, manufacturered home.  It was
>delivered to the site completely wired and with most of the
>(inefficient) appliances and systems installed.  The home is all set up
>for a forced air furnace (ducts installed) although the furnace has not
>been purchased yet.  ......

Kurt,

Here is part of an article "Surviving Winter with a PV Power System".
The whole article is at:
http://www.dankoffsolar.com/reference/surviving.html

Windy



FURNACES AND CONTROLS

THERMOSTAT CIRCUITS and POWER USAGE: Most central heating systems use
a low voltage circuit through a wall thermostat, to tell the furnace
when to turn on and off. The low voltage is derived from a small
transformer which is powered constantly. It consumes only a few
watts, but in an alternative energy system that may be a significant
load -- if it is the only AC device that's running, it is adding
constant additional draw just to keep the inverter "up". That amounts
to the wintertime energy output of $300-$500 worth of solar array,
plus battery capacity to match!

If yours is a system where the inverter spends most of its time
turned off (relatively little AC power usage) it is worth the small
modification of adding a line voltage thermostat * to your furnace
circuit. Have it installed IN THE AC LINE to the furnace controls.
(Also bypass the original thermostat.) This way when heat is not
needed, all power is cut to the furnace transformer. A small "limit
switch" thermostat may also be added to sense heat in the furnace and
keep the blower on until residual heat is exhausted. Material cost of
these modifications is under $50 and the wiring is simple.

LOW TEMPERATURE SETTING: When nobody's home, you may need your
furnace only to prevent your home from reaching freezing temperature
(so that water pipes, fixtures and bottles won't freeze). Most
heating thermostats stop at 55°F, but a lot of fuel may be saved if
the temp can be lowered to 40°. or less. Electric power is saved too,
since the furnace blower will run much less. The thermostat listed
below will set down to 40°.



PS. There's also info in that article about making low voltage heat
tapes.   -- WD

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