Have a 1926 San Francisco home with a Carrier WeatherMaker 8000 gas
furnace heating system installed. We wanted to update the old mercury
thermostat with a digital programable one, mostly to avoid having one
person wake up early, turn on the heat, and come back to bed every
We purchased a Honeywell RTH7400D unit (5+1+1). Now I'm feeling like
an idiot because I can't get the damn thing to turn our heat on!
The old set was very simple: two wires connected. One to "R" and the
other to "4". The manual says to remove the jumper between RC/R and
install the two wires with "R" = "RC" and "4" = "R". Done. Doesn't
work. Hook the old up and it works fine. Bump the wires together and
it works fine.
Tried different settings and adjustments to no avail. I hear the
internal 'click' when the Honeywell turns "on". But it doesn't start
the heat unit.
Does anyone have any ideas??? I'm feeling pretty incompetent as my
wife's girlfriend took her new thermostat and two wires and was hooked
up and running within 30 minutes.
There are two wires running to the mercury from the "R" spot on the old
thermostat if that makes any difference.....
I am not too familiar with your exact model. Some of these require a
compatible system by where the thermostat is working an electronic device,
rather than a hard contact relay, which may be required or your particular
heating system. You would be best to contact Honeywell, and also the
manufacture of your furnace to know about any possible details for
compatibility with their thermostat, and your model of furnace.
The only other thing I can see, is that the thermostat you have may be
Old R - transformer power - connect to R on RTH7400
Old 4 - call for heat - connect to W on RTH7400
Don't ask why Honeywell shows it differently in their owner's manual.
For your application, their instructions are incorrect.
Glad that everything is working for you. The RTH7400 is really a very
good thermostat, easy to use and quite accurate as to temperature
control. Just don't forget to change the batteries once a year or so.
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