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Steve-P 11-09-2010 10:05 AM

Thermostat with separate fan and heat relays
Looking for a thermostat that has separate relays for its Fan G and Heat W teminals. The Honeywell TH4110D1007 apparently does not. Those two terminals are always connected together (even when the thermostat is not calling for heat) when the fan operating mode is electric. An older Chronotherm III I have does not operate like that and seems to have distinct relays for each control output. I am interested in upgrading though to a model with display of both current and setpoint T as well as backlight.

Another issue is that in Heat mode when the fan operating mode is Gas, the TH4110D1007 connects its Fan G terminal to the Cool Y terminal. I need a thermostat that does not make that connection unless it is oeprating in Cooling mode.

These are issues for me because I have a hydronic hot water forced air system with a simple blower that just uses a relay controlled by the thermostat's Fan terminal to turn the blower on. It also has a temperature sensor switch on the water line that applies 24 V to the same Fan terminal wire to turn ON the blower's relay when the water warms enough. The problem then is that the 24 V from the water temperature sensor gets applied back to the thermostat's Fan terminal and since that terminal is shorted to the Cool Y terminal inside the thermostat, that ends up turning the compressor ON when the system is heating - not good when it is cold outside.

beenthere 11-09-2010 06:14 PM

This may not be what your asking for.

But. I wire the aquastat up as a line voltage control. And let it power the fan that way.

Then if the thermostat ever has to be changed out at night time, or a weekend. It doesn't take a special thermostat to make it work right.

TJ_in_IL 11-09-2010 07:22 PM

I had a similar issue with my system. I was able to correct it with a relay.
Here is the link that explains what was found, what I did to correct, and any warnings from those with far more knowledge than me:
From the sounds of it, this will work.

biggles 11-09-2010 08:15 PM

why are you going on to the Y terminal to get the fan ..since the fan ON is the heating point why don't you use W on the stat as the heat call (FAN ON) to relay then put the temp disc in series with the 24 volts for the fan relay coil..if the stat calls for heat the W terminal calls in the fan relay only after the pipe heats up and closes and the fan runs with hot water in the coil.this way you are totally on the heating side of the stat R/W only and no Gs involved.on a normal furnace the stats G isn't part of the furnace cycle in the heating mode it is called in from a heat disc or time delay within that furnace..:whistling2:

beenthere 11-09-2010 08:26 PM

He's using an air handler.
And what he is describing is something that is common. Many stats, if the G terminal is back fed, will energize the Y terminal also and run the condenser.

I'm not sure anymore. But I think the Th8000 series has the G and Y isolated from each other.

yuri 11-09-2010 08:32 PM

I doubt if any of them do. May be able to invent a system using a R8222D or K relay and feed thru a n/c contacts etc.

Steve-P 11-10-2010 11:16 PM

Honeywell customer service emailed me to say that the RTH6350, RTH6400, RTH6450, TH6110D and TH6220D do not connect the G and Y terminals together. They did not answer the other part of my question on connecting G and W in Electric Heat mode.

beenthere 11-11-2010 03:31 AM

Thats selectable on the TH6000 series. In that you tell it if you want it to bring the fan on for a heat call. Or if the equipment will bring the fan on during a heat call.

Steve-P 11-11-2010 04:21 PM

According to Honeywell customer service the RTH6350, RTH6400, RTH6450, TH6110D and TH6220D do not have an interconnect between W and G terminals.

Perhaps the TH4110D1007 is less expensive because it does not use individual relays on G and instead connects them directly to the W and Y outputs and just uses those outputs also for G.

beenthere : When the fan operate mode is set to electric, the TH4110D1007 thermostat turns the fan G on. But it seems to only control one line and that line is connected to both G and to W. It does not have separate relays that connect R to G and another relay connecting R to W.

beenthere 11-12-2010 04:46 AM

Use a TH6000 series, not a RTH6000 series.

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