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jnjadcock · Oct 15, 2007, 12:35 PM

For various reasons, I want to have the house HVAC fan running all the time. Continuous movement of air, keeps the house from getting "stuffy", white noise in the background, etc. This is fine during the warmer months, as the system allows that. But even if I command the fan to always be on during the Heat mode, it only comes on with the furnace.

We have a gas furnace. Ruud I think. I've searched quite a bit across the Internet, and actually found one source that said that all gas furnaces control the HVAC fan, regardess of what the thermostat commands. Is this true? It would appear so, since we've had two different thermostats in the house, and neither one allowed me to have the fan on all the time, even though both appeared to let me select that via the controls.

Because I have a gas furnace, am I just simply "out of luck" in the colder months, in terms of having the fan always on?

Thanks!

KISS · Oct 15, 2007, 12:46 PM

The "G" terminal needs to be connected on the thermostat. For heat/cool/fan, you normally need 4 wires.

The fan can be overridden to be on by the furnace and the stat. The fan positions are usually AUTO and ON. The furnace is commanded to turn off, but the fan will continue to run until the plenum temperature drops. The fan may not come on right away until the plenum gets warm, so to avoid drafts.

jnjadcock · Oct 15, 2007, 01:46 PM

Originally Posted by KeepItSimpleStupid

The "G" terminal needs to be connected on the thermostat. For heat/cool/fan, you normally need 4 wires.

The fan can be overridden to be on by the furnace and the stat. The fan positions are usually AUTO and ON. The furnace is commanded to turn off, but the fan will continue to run until the plenum temperature drops. The fan may not come on right away until the plenum gets warm, so to avoid drafts.

The way I understood it is that the fan will not generally run when in Heat mode if Heat is not called for, to avoid drafts as you say. But if I connect the "G" terminal to the thermostat, then I CAN override this and have the fan always on, even in Heat mode.

Yes, on our thermostat (a simple digital Hunter), there's only AUTO or ON for the fan. When in the Heat mode, the fan acts like it's in AUTO, even if it is ON.

Turning on, the fan acts as I would expect... That is, the gas furnace will light and the fan will come on 30-60 seconds later. So all operations are normal as far as I can tell. I'd simply like to override this fan programming to command it ON all the time, even in Heat mode.

Can you confirm that this could be a simple wire connection at the thermostat?

Thanks much!

KISS · Oct 15, 2007, 02:34 PM

Yep, I'd look for a connection to the "G" terminal. Hopefully, you have enough wires in the cable.

You need to turn of the heater and remove the thermostat from the wall. Find the number of wires available and the ones that are connected and to what terminal.

labman · Oct 15, 2007, 02:41 PM

I just tried setting my thermostat to heat and the temperature well below room temperature so the furnace wouldn't come on. When I turned the fan switch to ON, the fan come on. Now it would be possible for the furnace controls in the heat mode to disconnect the relay powered from the G terminal. My Bryant doesn't. If your Ruud does, the advice above won't work. Not the first time for him.

My furnace has some little switches to program its behavior. If you have, or can find the manual on the net for yours, it might be as simple as flicking the right switch. Otherwise, the easiest thing might be to identify the hot wire to the blower motor and connect a switch between it and the hot from the house hot. Since most furnaces use different winding for heating and cooling, you may want to use a relay to keep the heat winding from having power when the cooling winding does.

eschuen · Nov 3, 2007, 01:19 PM

You never mentioned the year of this furnace. Not knowing this makes a lot of difference. You also never positively said the fan runs continuesly in the cooling mode. Need to know both of the above

T-Top · Nov 3, 2007, 03:35 PM

If you are using a two transformer system one for the heat and one for the cooling. The blower will not come on if you turn the thermostat fan switch to on. The G circuit is energized in the cooling mode. When the thermostat is switched to heat only the W is energized. Its one of the down falls of a two transformer system.

jnjadcock · Nov 4, 2007, 07:12 PM

Originally Posted by T-Top

If you are using a two transformer system one for the heat and one for the cooling. The blower will not come on if you turn the thermostat fan switch to on. The G circuit is energized in the cooling mode. when the thermostat is switched to heat only the W is energized. Its one of the down falls of a two transformer system.

This must be how our system is set up. In cooling mode, we can lock the fan on with the fan switch on the 'stat. In off mode, again, we can lock the fan on with the switch (no conditioning, just fan). When put in heat mode, the fan will immediately switch off if the fan switch was on, and it will act as if the fan switch was on auto, even if the switch is on.

Everything is still original to the house, which was built in 1993.