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[RSI]

The fan switch on the old thermostat won't work. (I consider this a good thing because if it gets turned on it will overheat the house) There are a few furnaces and heat pumps that do require the G connection so I would turn on the heat and make sure the fan kicks on before leaving it that way. I have never seen one that did require it though but heatpumps are very rare around here.

[Scott7m]

The fan switch on the old thermostat won't work. (I consider this a good thing because if it gets turned on it will overheat the house) There are a few furnaces and heat pumps that do require the G connection so I would turn on the heat and make sure the fan kicks on before leaving it that way. I have never seen one that did require it though but heatpumps are very rare around here.

This place is full of heat pumps, I'm gonna have to do a relay to cover myself I'd think

[RSI]

Just disconnect the g terminal an turn the heat on and if the blower starts like normal then you are fine.

[Scott7m]

Just disconnect the g terminal an turn the heat on and if the blower starts like normal then you are fine.

So if the customer wants to use the heat pump, will his old stat still turn the fan on as normal

[RSI]

that is what you need to test. First thing is just remove the g wire and then turn on the heat pump and see if it works properly and the blower runs. Most will but some may require the G hooked up.

[jerkash]

Hey SumpterSmoker - Are you confused yet??
Don't worry, these guys will get you straightened out.  They have helped me a lot!

[Scott7m]

that is what you need to test. First thing is just remove the g wire and then turn on the heat pump and see if it works properly and the blower runs. Most will but some may require the G hooked up.

Ok makes sense, but if I'm 90 minutes from home and that don't work.  Then what?  I'll need to install a relay somewhere

I wasn't aware the diodes changed the current

[RSI]

Well,if I was going to use a relay, I would get a SPDT with 24v ac coil.
I would cut the wire going to the G on the original thermostat and run the furnace end to the common contact connection and the thermostat end to the NC connection. Then run a wire from the NO connection to the RH.

Then run one side of the coil to the C on the thermostat.

Now for the new thermostat run a wire from the W to the other side of the coil. You will also need a jumper wire between the two thermostats on the RH terminals (and the C terminals if it is a digital thermostat that doesn't use batteries)

[Scott7m]

Good grief lol

The ones I use do have batteries..

I thought it would be as simple as splicing into the yellow wire so the signal couldn't back feed and outdoor unit wouldn't come on

[RSI]

Lol it isnt really that complicated. You cant have the relay cut the connection to the y if it is a heat pump. If you do the heatpump wont work unless you shut off the owb thermostat.

If i get time i will try drawing up a diagram.

[yoderheating]

 I use relays on all my jobs, its the only way to be certain you make heat pumps do exactly what you want them to do. The other day I used 4 relays, a zone valve and a 24v ranco to make a unit work properly. As a wise man once told me, you can run the world off 24v if you have enough relays.
 SumpterSmoker, hopefully I can explain how to hook your system up with a relay. I use http://www.google.com/imgres?q=white+rodgers+90-293q+wiring+diagram&hl=en&sa=X&biw=1024&bih=613&tbm=isch&prmd=imvns&tbnid=r2DzaH9wuEWaPM:&imgrefurl=http://www.johnstonesupply.com/storefront/hvacr-parts/controls/electrical-controls/hvacr-parts/controls/electrical-controls/relays/switching-relays//prodL36-905.html&docid=PkH2qVJr1spNSM&imgurl=http://www.johnstonesupply.com/storefront/renderImage.image%253FimageName%253DWEB/10010/L36-905cl.jpg%2526width%253D407%2526height%253D268%2526padding%253D0&w=407&h=268&ei=Ni6PULGHE4zC0AHfmoHwCw&zoom=1&iact=hc&vpx=101&vpy=166&dur=550&hovh=140&hovw=184&tx=125&ty=101&sig=112355657484556333735&page=1&tbnh=139&tbnw=182&start=0&ndsp=18&ved=1t:429,i:71 a white rodgers 90-293q. Hopefully this link works.
 The new thermostat should be linked with the old c to c and r to r, this powers the new thermostat. I normally install the relay in the blower unit. Run c from the unit to the #3 on the relay. Run a wire from the w on the new thermostat to both the #1 and #2 on the relay. Splice the wire from g on the old thermostat and hook the thermostat side to #5 and the furnace side to #4.
 If you want to understand how it works I can explain further, it is very simple.

[Scott7m]

Yoder do you run a whole new line set down to the blower too?

[yoderheating]

If you are asking if I run a new thermostat wire then the answer is normally no. 95% of the time not all the wires are used in the existing wire and I just use one of those.

[RSI]

I use relays on all my jobs, its the only way to be certain you make heat pumps do exactly what you want them to do. The other day I used 4 relays, a zone valve and a 24v ranco to make a unit work properly. As a wise man once told me, you can run the world off 24v if you have enough relays.
 SumpterSmoker, hopefully I can explain how to hook your system up with a relay. I use http://www.google.com/imgres?q=white+rodgers+90-293q+wiring+diagram&hl=en&sa=X&biw=1024&bih=613&tbm=isch&prmd=imvns&tbnid=r2DzaH9wuEWaPM:&imgrefurl=http://www.johnstonesupply.com/storefront/hvacr-parts/controls/electrical-controls/hvacr-parts/controls/electrical-controls/relays/switching-relays//prodL36-905.html&docid=PkH2qVJr1spNSM&imgurl=http://www.johnstonesupply.com/storefront/renderImage.image%253FimageName%253DWEB/10010/L36-905cl.jpg%2526width%253D407%2526height%253D268%2526padding%253D0&w=407&h=268&ei=Ni6PULGHE4zC0AHfmoHwCw&zoom=1&iact=hc&vpx=101&vpy=166&dur=550&hovh=140&hovw=184&tx=125&ty=101&sig=112355657484556333735&page=1&tbnh=139&tbnw=182&start=0&ndsp=18&ved=1t:429,i:71 a white rodgers 90-293q. Hopefully this link works.
 The new thermostat should be linked with the old c to c and r to r, this powers the new thermostat. I normally install the relay in the blower unit. Run c from the unit to the #3 on the relay. Run a wire from the w on the new thermostat to both the #1 and #2 on the relay. Splice the wire from g on the old thermostat and hook the thermostat side to #5 and the furnace side to #4.
 If you want to understand how it works I can explain further, it is very simple.

Just curious, why do you run the W from the new thermostat to the #2 instead of the R? It will work the same, just wondering if there is a reason I am missing.

[yoderheating]

 You can do it either way, I just do it that way because its one less wire to hook up. The only time you need power on that terminal is when the relay is activated so I just run a jumper from #1 to #2 but you could get power for #2 from R. I have had a relay stick before and I guess the advantage there would be there would be no power on #2 if it did but you still wouldn't be able to activate the fan from the original thermostat if that happened.