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

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.

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

You are wrong.

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.

Pre 2005 or so, the y terminal was just a pin. Nothing more than a marrett. When the thermostat called for cooling , it always has to send power to y (only the condensing unit) and g, (the furnace fan). Things are different now adays because constant fan is lower speed than a/c fan so the furnace need to decipher between a call for g and a call for g and y. But thermostats are all built the same, and always will be, a call for cooling will energize g and y. This inherent "flaw" is why you cannot jumper any furnace (r to g) with a/c to run the fan manually with no relay. It will always turn on the a/c as well.

I think we are both right here. Just pulled a new t stat out of the truck. The y and g are common when the t stat is in cool position only. When the t stat is in off or heat those two points are isolated. So the hrv will kick the Ac and furnace fan on if in cool. However op said the problem exists when the t stat is off or heat as well.

Have a look here The diagram will show why g and y are always jumped in auto. Today's digital thermostats are different than yesterdays. They are separating them on some but this would still cause an issue with no relay as you could never put the stat in cool with the HRV on.

So if this is the case than why would the thermostat have a Y and a G if both points are common to each other, what is the point in having 2. I have never questioned this issue because I have never ran into it. However acording to Venmar( Most Units are similar)

For a furnace connected to a cooling system:

thereby turning on the cooling system. If you identify this type of thermostat, you must use the ALTERNATE FURNACE INTERLOCK WIRING.

So the OP could change his thermostat out for a new one, and keep his wiring exactly the way it is now or, if there is a spare conductor between the furnace board and the HRV, than he can use the diagram at http://www.venmar.ca/DATA/DOCUMENT/2...Exchangers.pdf

PG 16
and connect the unit using Alternate Furnace Interlock wiring.
but in no case will he need a relay added in.

It is a long history lesson involving the the dawn of AC and single speed belted fans. Coles notes: the thermostat operator can choose to run the fan or AC. This is important today because with ECM motors, it is advantageous to run the fan.




No because, as you stated a few posts back, he would not be able to set the stat to cool and auto.

Look closer at that diagram, "NC C NO" stands for relay and this brand of HRV has one built in. Therefore, he does need the relay. The HRV may have one though.

Yes that is a relay. All hrvs have this relay. All I was saying was op did not need to add an additional relay as the hrv would take care of it. This is the relay that turns the furnace fan motor on. And I have checked my setup. It is wired diagram one and my furnace fan runs independent of my cooling. And yes we need a y and a g terminal so the t stat operator can run the fan and the Ac independent of one another. However if y and g are common, as you say than anytime one runs the other would run as well. Even switching the summer fan to on would cause the Ac to run at all times

Well I bit the bullet and bought another tstat since mine was fairly old. With the new tstat installed I cannot seem to trigger the a/c anymore when I pop my air exchanger. Yeah!!! I will still turn off the breaker to the a/c in the winter just in case. Thanks for all the help.