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Old 07-25-2009, 02:41 PM   #1
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Strange fan wiring on Heat Pump


I installed a new t-stat for my heat pump last winter. It worked fine in winter. But, when I tried it for the first time this summer, the blower fan would not come on in the cooling cycle EVEN THOUGH it comes on in the heating cycle.

I have done a lot of troubleshooting and this is what I found. The t-stat is working fine. When it is in heating mode, the t-stat is putting current out from the G-fan and Y-heating connections. When it is in cooling mode, it is putting current out of the G, Y and O-changeover connections.

However, I tested the G and found that the wire was not connected to a closed circuit. I verified this by crawling under the house and shimmying to my air handler with all of 2.5' of clearance and, sure enough, the green wire from the thermostat was not connected to anything.

But, my question is, SHOULD the green wire be connected to anything? Why does the fan work in Heating mode, but not Cooling mode, and, if that is the case, should I just be reconnecting the G-fan wire? Or, if the fan comes on in heating mode WITHOUT the G connecting between the t-stat and the blower, should I not want to connect it to get the fan running in the cooling cycle?

When I went down to the air handler last week, one of the nuts on the panel was rusted and will need a work to get off. So, while I realize their may well be a schematic on that panel, I want to have as much info before venturing down there again and hacksawing off the nut and if there is a schematic. Also, for all I know, my FORMER HVAC guy, who was not entirely reputable, might not have followed the schematic. Any ideas? Thanks in advance, Joel

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Old 07-26-2009, 10:39 PM   #2
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Strange fan wiring on Heat Pump


"G" is the green wire controlling the fan from the thermostat. Some thermostats have a setting "elec"/"gas" to allow the furnace to control the fan in heat mode. Sounds like your thermostat may be set to "gas" so the fan works in heat (activated by the furnace which is getting its call from the heating side of the furnace) but not in cool (as the G wire isn't connected). If your thermostat has a Elec/Gas setting and it's set to Gas, try setting it to electric and connecting the green wire.

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Old 07-27-2009, 01:37 AM   #3
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Strange fan wiring on Heat Pump


Thanks! On my second trip to the air handler, I not only saw the loose Green wire, but also saw that there was an electrocuted mouse in there. (Before, I had the one last screw to take off, so I could not completely remove the panel.) I am thinking that the mouse knocked out the Green wire, where I think it was under a wire cap with the Black wire from the thermostat and the Green wire hooked directly to the air handler. The Black wire should be my emergency (electric) heat, which uses the strips/coils in the air handler and therefore does not rely on a connection from the heat pump to get the blower going. I did see that there was also a Blue wire from the heat pump that was tied into a Blue wire from a circuit on the air handler, which is how the Heat mode of the heat pump must be running the fan, as the blue wire to my thermostat is not in use. So, it looks like my AC is meant to run on the same fan setting as my emergency heat, while the Heat pump generated heat is supposed to run on another setting. I had seen that a slower fan speed is usually called for when one is cooling in a humid climate, which I presume Massachusetts is deemed. So, I hope this is the right set up. Thanks again.
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Old 07-27-2009, 07:18 AM   #4
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Strange fan wiring on Heat Pump


Do you have access to the wiring diagrams for your thermostat and heatpump? (The blue wire is usually the alternative heat/cool switching mode wire for your reversal valve). An incorrect connection in a heat pump can do serious damage!

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Old 07-27-2009, 01:25 PM   #5
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Strange fan wiring on Heat Pump


I appreciate your concern. I am currently gathering estimates for a low-profile gas furnace to go down into the crawl space and serve as either the primary or backup for winter. The heat pump really does not do the job nicely during a cold winter day in Massachusetts. I am going to make sure the electrician checks out and explains to me all the wiring that is being done, and you have added a question to that list. I must confess, that I know that the orange wire from the thermostat, which hooks up with the orange wire to the heat pump, is the changeover from heat to cool. But, indeed, I was surprised to see a blue wire from the heat pump tapping into the blower fan circuitry when the blue wire to the thermostat is not being utilized, although the blower fan seems to use two color wires, black and blue. So, I should probably do some more exploration on that. The blower panel upon removal does not have a schematic, but there is probably a schematic on a panel of the heat pump. Thanks!
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