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Old 03-24-2010, 06:43 PM   #1
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Odd Thermostat "Fan On" issue


Here's my setup... Hunter 44550 thermostat wired to new Dayton (Luxaire) furnace. All works fine. If I set the fan switch from "auto" to on, the fan comes on... no problem there. However, if I short R & G (to turn on fan from separate control), the A/C condenser kicks on, even though it is in Heat mode. It will only do this if the thermostat is connected. With the thermostat disconnected, only the fan comes on. It is sort of like the signal is backfeeding through the thermostat and turning on the A/C.
Reason for doing this... the thermostat does not have an intermittent fan run schedule, and I want to run the fan once every hour for 15 minutes, to normalize temps through the house. I wanted to connect an X-10 module on a schedule to do this. So far, X-10 is not the issue, not even in the picture yet.
Is there a way to isolate the signal with the current setup? I was thinking some sort of diode, but that is more for DC voltage, and we are talking about AC here. Also, I am afraid that if I spend the money on a new thermostat, I am only going to end up with the same problem, and lose the one feature I like about the Hunter (run-time timer- shows how long the system was heating/cooling for the day).
I know I can do all of this plus more with a new thermostat from BayWeb, but $220 is a bit more than I can spend right now.
Thoughts and ideas appreciated.

TJ

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Old 03-24-2010, 07:10 PM   #2
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Odd Thermostat "Fan On" issue


Additional info...
The thermostat is wired 4- wire, with a jumper for RH and RC. I removed the jumper from the RC (to isolate the 24vac from the cool side), but still the same. With the thermostat connected, jumping RH and G at the control board kicks the contactor.

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Old 03-24-2010, 07:13 PM   #3
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Odd Thermostat "Fan On" issue


If you send a signal backwards from the furnace to G on the tstat it will internally hit Y in the tstat and start the A/C. In the old days the tstat energized G to start the fan with A/C and Y at the same time so it can still happen as the tstat companies have to build them that way for the existing older units. New furnaces energize the cooling fan with the Y signal at the circuit board and G can be used for low speed operation. Long story short, you cannot have more than one wire on G at the board at ANY time or use it for more than one purpose.
Read this:Unexpected wiring found on existing thermostat
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Old 03-24-2010, 07:17 PM   #4
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Odd Thermostat "Fan On" issue


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Originally Posted by yuri View Post
If you send a signal backwards from the furnace to G on the tstat it will internally hit Y in the tstat and start the A/C. In the old days the tstat energized G to start the fan with A/C and Y at the same time so it can still happen as the tstat companies have to build them that way for the existing older units. New furnaces energize the cooling fan with the Y signal at the circuit board and G can be used for low speed operation. Long story short, you cannot have more than one wire on G at the board at ANY time or use it for more than one purpose.
Energizing G at the thermostat kicks the fan on "high" or the cool speed. When the unit calls for heat when the fan is in "on" mode, it will kick the fan to the lower speed.
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Old 03-24-2010, 07:20 PM   #5
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Odd Thermostat "Fan On" issue


The more expensive higher end models of some brands use the G for low speed (Lennox does) same principle about the backfeed for all brands. Not sure how yours is configured.
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Old 03-24-2010, 07:24 PM   #6
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Odd Thermostat "Fan On" issue


Just tried this out, and seems to go along what you said.
I disconnected G from the thermostat, and tried jumping R & G at the control. Bingo! We have a winner. Fan comes on without also pulling in the contactor. So, as you said, "Long story short, you cannot have more than one wire on G at the board at ANY time or use it for more than one purpose."
Thanks for helping think this through Yuri. I guess I will have to disable the fan switch on the thermostat (disconnect the G), and only control the fan over-ride from the X-10.

TJ
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Old 03-24-2010, 07:38 PM   #7
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Odd Thermostat "Fan On" issue


Yep. This problem has been known for many years in Canada, read the other post. Lifebreath has built an extra circuit into their HRVs for this but not the tstat companies.
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Old 03-30-2010, 09:27 PM   #8
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Odd Thermostat "Fan On" issue


Well, I was able to solve the problem. All I did was wire in an isolation relay. Now I can connect my X10 relay to the R & G, and it does not back-feed to the Y, and turn on the A/C. And, it cost less than $10 for the solution!
I used a 4E658 from Grainger.

This may also be helpful to those up North, that need to wire in controls for a Bath or Range fan.

Here is a Schematic:

Last edited by TJ_in_IL; 03-30-2010 at 10:06 PM. Reason: Removed incorrect image.
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Old 03-30-2010, 09:59 PM   #9
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Odd Thermostat "Fan On" issue


The diagram does not work from what I see as the coil needs to go to C etc. I can imagine what you did and I can use a R8222D relay to create special projects. We don't do a lot of that for liability reasons. Once you deviate from the UL and AGA/CGA rating/approval, problems can happen. Glad you got it working.
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Last edited by yuri; 03-30-2010 at 10:01 PM.
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Old 03-30-2010, 10:05 PM   #10
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Odd Thermostat "Fan On" issue


WOW! I can't believe I did that! \Thanks for catching that.
Here is the correct version:

Thanks Yuri.
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Odd Thermostat "Fan On" issue-fan-switch-isolation.jpg  

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