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[ph10west]

I have a Central Boiler. I'm using a second T'stat to control my fan in my HVAC (heatpump), but the primary t'stat has the compressor signal wire "Y" connected to the fan control wire "G".  Is there a t'stat that I can use that does not connect the Y and G wires? 

If the fan switch is "ON" on (the heatpump t'stat) the wires are not connected. So I have considered turning the fan switch to the on position on the HVAC t'stat and using the second t'stat to break the fan wire connection so the compressor would not come on unless the inside temp went too low.  When cooling I would have to switch the fan to auto and bypass the second t'stat.  Since I live in the South I'd like to be able is switch between cooling and heating easily.  Less steps is better for the wife and kids.  Anybody figured this out?

[Scott7m]

New stat should be jumped over from the red r to your new stat, from the w setting on your new stat run a jumper to the g green post.  New stat will work the fan only and old one will work like it always has.

If that don't work u may have to use a relay as well to stop it from back feeding and running the outside unit

[RSI]

New stat should be jumped over from the red r to your new stat, from the w setting on your new stat run a jumper to the g green post.  New stat will work the fan only and old one will work like it always has.

If that don't work u may have to use a relay as well to stop it from back feeding and running the outside unit

You have to remove the wire from the G on the original thermostat and splice it to the W wire on the new thermostat. Otherwise it runs the compressor.

[Scott7m]

That's what the relay would be for, I never unhook any wires really.

[ph10west]

Thanks for the replys. 

I think I wired it like you guys said.  I ran the r old to r new and the w new back to g old.  However since the old has y and g common the compressor runs.  Soooo... Tell me how to wire in the relay. Pleeeease

[RSI]

If you just take both wires off the G and splice them together it will work.
If you want to do the relay instead you will have to wait for Scott to reply. I don't do it that way and not sure how he wires it.

[Scott7m]

Yoder eating could tell us all about this, he's who told me about the relays

Rsi, what do u do when you run into a variable speed fan?

[RSI]

Yoder eating could tell us all about this, he's who told me about the relays

Rsi, what do u do when you run into a variable speed fan?

Don't they just have a fan switch on the thermostat the same as a regular? If they don't then I haven't run into one yet.

[ph10west]

I see how taking both the wires off g on the old t'stat would work fine for heating. Fan gets its signal and you are NOT sending the signal to the old t'stat so the is no feedback.  But what happens when you call for cooling?  I think you would have to put the g wire back on the old t'stat.

[RSI]

The G has nothing to do with cooling. It is the fan only signal.

[yoderheating]

I will try and explain how to wire it with a relay. Start with the new and old Tstats r to r and c to c, this will power the thermostat. Now, you need a extra wire in the existing thermostat wire. Use the extra wire to run from the W on the new tstat down to the relay in the blower unit. I use a white-rogers 90-293 5 pole relay. On the relay #3 pole is wired to c from unit. #1 and #2 on relay gets wired to the tstat wire from the W on new tstat. You split the tstat wire going from the g on old tstat to g on unit (normally green wire). The wire going to unit goes on #4 and the wire going to g on old tstat goes on #5. #5 and #4 is normally closed so you can use old tstat whenever you are not using the outdoor furnace. When the new Tstat activates the w it turns on the relay by powering #1 and which closes between #2 and #4 therfor opening #4 and #5. This turns on the fan but does not allow power to flow back to the g on the old tstat.
 I'm sure this makes no sense but at least I tried.    

[ph10west]

RSI, you say g has nothing to do with cooling.  Then when cooling is called for the fan gets its signal from the the cooling wire?  I've never messed with a t'stat much.  So I don't know how all the signals work inside the HVAC unit.  I realize the signals are just 24v circuit getting completed.

[RSI]

The thermostat doesn't control the fan in either heat or cooling mode. It is controlled from the furnace itself. All the G wire is for is the manual fan switch on the thermostat.

[ph10west]

OK thanks! Now if I can get my bypass valve wired to bypass the heat exchanger.  I have all the bugs worked out.  I wanted it to open when I switch the t'stat to cooling.  Any ideas on how to do that.

[RSI]

What type valve do you have? Does it have a motor that just hits the end and stalls or does it drive both ways?
If it is using power all the while it is bypassing you probably don't want it running all summer.

You should be able to use a relay to do what you want. Just connect the coil between the C and Y wires. I never tried it but if the coil is enough to make the compressor run a solid state relay would probably work.