Any HVAC experts?

[weyburnt]

Yes this would make sence except he said this still happens when the t stat is switched off. Also I don't think most t stats jumper the y and g when in auto. I'm pretty sure the furnace board takes care of this. Or else when the fan switch was in off, and the t stat was in cool, the fan would not run. Sometimes on older furnaces without a y point people will jumper these points but op said thier was no jumper. The r and the rh need to be jumpered out unless you have a system with separate heating and cooling transformers. Which you don't. If the unit does not do this with the t stat wire disconnected at the bored then double check the t stat wiring and replace the t stat. I do not believe a relay is the way to go. I Hooked 2 of these up last week alone. The only way you should need a relay is if it is an older furnace with no y point.

[Seriak]

Originally Posted by weyburnt

Yes this would make sence except he said this still happens when the t stat is switched off. Also I don't think most t stats jumper the y and g when in auto. I'm pretty sure the furnace board takes care of this. Or else when the fan switch was in off, and the t stat was in cool, the fan would not run. Sometimes on older furnaces without a y point people will jumper these points but op said thier was no jumper. The r and the rh need to be jumpered out unless you have a system with separate heating and cooling transformers. Which you don't. If the unit does not do this with the t stat wire disconnected at the bored then double check the t stat wiring and replace the t stat. I do not believe a relay is the way to go. I Hooked 2 of these up last week alone. The only way you should need a relay is if it is an older furnace with no y point.

Which wire do you want me to disconnect from the tstat or do you want me to disconnect them all?

[wingedfish]

Originally Posted by weyburnt

Yes this would make sence except he said this still happens when the t stat is switched off. Also I don't think most t stats jumper the y and g when in auto. I'm pretty sure the furnace board takes care of this.

If you have AC, try this on your furnace. Jump r to g at the furnace and no matter where the tstat mode is placed, if the fan switch is in auto, the AC will come on. It is a well known fact in the trade.

Originally Posted by weyburnt

Or else when the fan switch was in off, and the t stat was in cool, the fan would not run.

There is no off position for any fan switch on any thermostat.

Originally Posted by weyburnt

I do not believe a relay is the way to go.

You are wrong.

[weyburnt]

I was suggesting to remove all t stat wires. If the problem still continues u know the backfeed is in the furnace board ie a jumper. If not the backfeed is in the t stat end. And if you jumper between r and g the furnace fan will come on. If you jumper from r to y then the Ac and furnace fan will come on. The exception is if you have an old furnace with no y. Then you would have to jumper between y and g on the t stat to force the Furnace fan on when T stat was calling for cool. This causes a problem because when the t stat was set to summer fan on, or hrv went to on the Ac would come on. As I said op says no jumper is installed. In that case a relay needs to be installed in order to remedy the problem. If I'm incorrect what would the y terminal be for. The y terminal allows the furnace fan to kick in when cooling is called but isolates the condenser contactor when the fan switch or hrv connects r and g. However op said this furnace has a y position on the board.

Op sorry to hijack your thread hope we are able to help, I know how frustrating things can get.