Is this system Gas or Heat Pump with Gas Backup?


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Old 05-09-21, 04:40 PM
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Is this system Gas or Heat Pump with Gas Backup?

I'm replacing a thermostat on what I thought was a natural gas system with cooling (vs. heat pump):

* There are only 4 wires (red, white, yellow, and green).
* The old thermostat has the green wire going to the "O" terminal
* The old thermostat has NO wire going to the G (fan) terminal.
* The old thermostat switches are set to "Heat Pump" (not Non-Heat Pump) and "Electric" (not "Gas").
* The old thermostat calls for heat and the compressor/condenser does not turn on.
* The compressor/condenser is not on risers and the model number sticker is completely faded.
* The model number for the gas furnace is: York FG9B06010UP11A

QUESTIONS:
(1) Does that fact that there's only 4 wires prove that it's NOT a heat pump? I thought this was gas only for heat, but now I'm second guessing myself because of the green wire going to the "O" on the old thermostat. So someone REALLY screwed up if this is not a heat pump, and I do not think it is.

(2) I attached the green wire to the new thermostat's G terminal, but the fan does not turn on when setting the fan switch to "On." Why?

Any replies are welcome ...thank you!
 
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Old 05-09-21, 05:09 PM
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What was the old thermostat ?
A picture would be VERY helpful...... how-to-insert-pictures.

Most thermostats are labeled "heat pump" and "conventional".
It's up to you to choose the right label.

If there are only 4 wires it's probably not a heat pump.
 
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Old 05-09-21, 05:43 PM
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The old thermostat is a Robert Shaw 9500, but I don't think it would matter which one it is, would it?

...because my questions are generic:
(1) Should a 4-wire system ever have the green wire going to the "O" terminal and no wire going to the "G" terminal?
(2) If I attach the green wire to the "G" terminal on the new Tstat, why doesn't that fan come on when set to?
 
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Old 05-09-21, 11:13 PM
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The colors of the wires are not always used in the typical or accepted norms.

That thermostat has separate O and B connections which means if there was a wire on either one you have a heat pump.

What terminals were the other wires connected to ????

If you have a gas furnace as backup heat then your stat was probably wired as follows:
one wire to R
one wire to Y
one wire to O
one wire to W
At the equipment end..... the G wire will be connected to Y. This is done so that on a call for cooling.... the Y terminal activates the condenser and the blower. Same for heat pump heating..... the Y terminals activates the condenser and the blower. The backup heat call would be for the gas furnace and that has its own blower control.
 
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Old 05-10-21, 08:39 AM
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PJmax, thanks for helping with this! I appreciate it.

With the old Tstat, the 4 wire colors to the terminals were:
Red wire-->R
White wire-->W
Yellow wire-->Y
Green wire-->O

...and the internal switches for that old Tstat were set to "Heat Pump" and a 2nd switch to "Electric," not "Gas."

I'm tempted to believe it's a heat pump with gas backup based on the old Tstat wiring, but:
* The compressor/condenser is not on risers
* I cannot see a horizontal copper piece in that outdoor unit (reversing valve)
* The compressor/condenser is as small as one can get and this is a 1600 SF townhouse in PA
* I could have sworn when I called for heat 1 degree above room temp, the outdoor unit did not go on, now I'm second guessing myself - that I remembered wrongly.

The gas furnace/air handler is a York FG9B06010UP11A. Would the manual for that (which I have) reveal whether it's a heat pump setup or cooling only?

My ultimate question is how to wire up my new Tstat. The old one didn't allow the circulating fan to run when switched to "On" instead of "Auto" so I decided to get a new one. Now I have the Green wire going to the G terminal on the new Tstat and I have the SAME problem; the circulating fan will not go on when switching the Tstat switch to "ON."
 
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Old 05-10-21, 09:26 AM
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Can you see the wiring to the condenser ?
A basic condenser would only have two wires.
A heat pump would have at least five wires.
There should be a splice point at the furnace.

Model of furnace wouldn't reflect if a heat pump used.
 
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Old 05-11-21, 06:24 AM
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Hi, what does compressor/ condenser not on risers mean? can you take some pics of the inside of the condenser, you should see a reversing valve if itís a heat pump.
Geo 🇺🇸
 
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Old 05-11-21, 08:22 AM
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PJ: I will be checking how many wires go to the outdoor unit today and will post the results.

Geo: I did mention above that I checked for a reversing valve and do not see one. But sometimes it's hidden, so it's not a sure thing. Also, I'm under the impression that a heat pump needs to be on risers for defrost mode and to keep it off the snow. Is this not true?

-Tony
 
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Old 05-11-21, 10:15 AM
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Most ductless heat pump condenser units are off the ground as they are light.
A full size heat pump condenser would benefit from being mounted off the ground but many aren't.
 
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Old 05-11-21, 02:19 PM
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OK, the outdoor unit clearly has 2 wires (18 gauge) going to it. I also found the model number of the outdoor unit when removing the cover: AC024M1021A. So it's looking like a cooling only unit.

(1) So why did the old Robert Shaw 9500 tstat work with the settings on "heat pump" and "Electric" not "gas?" And with the Green wire going to the "O" terminal and no wire going to the "G" terminal?

(2) More importantly, why does my new thermostat allow the circ fan to blow when switching it on when I have the green wire going to the G terminal? So my new t-stat wiring is:
Red-R terminal
Yellow-Y
White-W
Green-G

I can check the furnace too to make sure it looks right after consulting wiring diagrams. Thanks!
 
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Old 05-11-21, 03:06 PM
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Most thermostats put 24v on the reversing valve wiring (O) when entering the cooling mode..... not necessarily when calling for cool.

BUT some thermostats apply 24v to the O terminal just before a call for cooling. If this is how your old thermostat operates..... then the O terminal would technically act just like a G terminal except that "fan on" mode would not work.

Your current wiring on your new stat looks correct.,
 
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Old 05-11-21, 03:50 PM
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OK, I took a photo of the wires in the furnace. GET THIS: the orange wire is connected to the furnace G terminal but the orange wire at the thermostat is unused! And the green wire connected to the thermostat terminals is not attached to the furnace terminal!

This would explain why the circ fan never runs when turned on, right?

I'm about to attach the green wire to the furnace terminal G and leave the green wire attached to the new thermostat's terminal G. But I wanted to get confirmation from you all that this is the correct way to set it up.






 
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Old 05-11-21, 04:58 PM
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Yes.... connect the green wire to G.
 
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Old 05-11-21, 06:05 PM
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OK, everything's running as it should with the green wire finally connected. The circ fan now runs on demand and the heat and cooling both work properly.

I can't believe a builder had the old thermostat set up as a heat pump when it's clearly not.
I can't believe a builder had wires attached to terminals that were not attached at the other ends.
I can't believe the system worked all those years. But I guess the white, yellow, and red wires were the important ones and they were OK.
I can't believe a layman like me (with your help) had to correct the wiring of a professional installer.

Thanks a lot for all the help, I learned a lot on this mystery! And that's my goal.
 
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Old 05-11-21, 06:14 PM
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Good job. Glad you got it all worked out.
 
 

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