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My electric furnace has no c terminal. Furnace is heat & cool both. (Lennox Electric Fan coil Model CBS18-21-3P).

I installed Honeywell RTH9080WF. I switched G to C on my thermostat and got the power.

Furnace has no C Terminal. The connections on furnace is "R" "W" "G" "Y"

Further on my thermostat I chose "fan controlled by heat pump" and it worked. Heating worked for the whole season.

In summers I switched to cooling and found that compressor is coming up but fan is not.

Fan comes up with heating but does not work with compressor. Kindly advice how to fix this problem.

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Your original wiring looked like this, where the top circles represent the thermostat terminals and the bottom the furnace.

enter image description here

To properly pull off the "G wire trick", you have to locate the transformer in the furnace. You then have to determine which leg feeds R, and which does not. The leg that does not feed R, will be where you connect your new C wire.

enter image description here

Once you have all that hooked up, then you'll install a jumper between the Y and G terminals in the furnace.

enter image description here

Once all that is completed, you'll have to make sure the motor is spinning at the correct speed when in cooling mode. If it's not, you'll end up with inefficient cooling. Which can cause poor performance, and/or damage to the system.

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  • thanks for the solution, I will try it and let you know. Kindly advise me transformer specifications and also if you can show me the other two wires of transformer and where to connect them as to my understanding transformer has 4 wires. Thanks again. – Fawad K Jun 4 '15 at 5:03
  • The thermostat I am using is Honeywell RTH9080WF, in its software settings it has an option "fan controlled by furnace", i chose that, I guess that is the reason the fan comes up while heat is turned on. Question if fan comes up with heat with the same settings i mentioned before then why not it is coming up with cooling? – Fawad K Jun 4 '15 at 5:06
  • Most older furnace control boards control the operation of the blower when in heating mode, but not when in cooling mode. It's just the way they are designed. The transformer should have 2 primary wires, which will be connected to the line voltage supply. And two secondary wires, one of which will be connected to the R terminal of the control board in some fashion. The other secondary wire will connect to a bunch of other things, likely including the control board. Without knowing anything about your furnace, or being able to see it, I can't advise any further than that. – Tester101 Jun 4 '15 at 9:22
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    If you don't feel comfortable poking around in the furnace, you could always contact a local HVAC company to hook it up for you. – Tester101 Jun 4 '15 at 9:22
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Most control boards need to have both Y and G energized, before the blower will come on in COOL mode. Did you install a jumper between the Y and G terminals in the furnace?

Did you move the wire that used to be G in the furnace, or is it still connected to the G terminal in the furnace? If you left the bootleg C wire on the Gterminal in the furnace, you'll have to move it to the correct location.

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  • No I did not install a jumper between 'G' & 'Y'. I called honeywell call centre and they did not advice me to do that. But I can try that, so should I?<br/> The wire in the 'G' at furnace is still at 'G' (not moved). <br/> I do not know about the bootleg 'C' wire, but I am sure that my furnace has no 'C' terminal. <br/> Kindly advice. – Fawad K Jun 1 '15 at 2:48
  • So all you did was move the wire that was on the G terminal, to the C terminal on your thermostat? And that Worked? – Tester101 Jun 1 '15 at 11:12
  • Yes, that is all i did. My heat pump and fan runs but when i switch on cooling compressor works but not the fan. Kindly advice a fix. – Fawad K Jun 1 '15 at 23:40
  • So should i try putting a jumper between G & Y at my furnace terminals? – Fawad K Jun 2 '15 at 21:47
  • @FawadK You shouldn't put a jumper between G and Y, if the wire you used for C is connected to the G terminal. Since I can't see how you've wired it, it's tough to advise you any further. – Tester101 Jun 2 '15 at 22:39
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Make sure your w terminal doesn't have the fan wire going to it. Only thing on w at furnace should be tstat w. Ran into this where technician wired the blower off w terminal. If there's a stranded wire at w terminal move it to g

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  • What sort of furnace did you run into this on? – ThreePhaseEel Jun 4 '18 at 2:07
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enter image description here

Here's a diagram that might help.

So you have two "old" wires with an "Add-a-wire" in order to control a furnace/ac set up with a Wifi Thermostat, but will have NO Fan Only option (unless you find a way to run a separate wire from G on Control to G on t-stat).

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