thermostat operation

R to G turns on blower R to Y starts a/c compressor R to W starts heat

"Nukie Poo @verizon.net>"

So when the A/C is running, would that mean R-G-Y are all connected together? (same for the heat, but with W)?

Thanks, (I've always wanted to know this) Romy

Correct

Actually, for heat, you merely connect R and W. There is another thermo-switch at the furnace (on the heat-exchanger) that will turn on the blower *after* the burner has warmed up the heat-exchanger. This also keeps the blower running for a few seconds after the burner shuts off.

IIRC, similar is true for cooling. When you connect R to Y, the A/C starts and it starts the blower directly.

R to G is only connected when you turn the 'Auto-On' fan switch to 'On'. The G is open-circuit any time the fan switch is in auto.

daestrom

Other research I did suggests what you say, almost. . In my case I have a water pump activated with the R to W . It then activates the blower but it appears that if I had manually set R to G I might change the speed of the blower (can be wired for dfferent speeds, mine isn't but friend' is). I read that some thermostats might activate both Heat and Blower for some applications. It said that for cooling the thermostat activated both the compressor and blower. In my case based on wiring I see that as the only way the blower could be started for cooling. ie R to Y or compressor and R to G for blower indicating again that cooling would be different speed than heating if I had multiple speeds wired. Thanks

Not sure there if there is only ONE standard or not. I have a propane forced-air furnace here with an add-on A/C compressor. In this case the fan is controlled by the furnace controller (a PIC, by chance) in heating mode, so the thermostat only makes the R-W connection when heat is required. In cooling, the furnace controller isn't really involved and so the thermostat effectively connects Y and G together so that the fan is forced on when coooling is required (R-Y-G).

Of course, if you change the fan control from Auto to On, it connects R-G regardless of mode.

Interesting. It would be easy to test this with a typical mechanical stat. However, is it ok to use a DMM to test a digital setback stat (ie. any current or polarity issues)?

Thanks, Romy

On my newer furnace, programmable time delays are used for fan on and off

Dave M.