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Heating, Air Conditioning, Fridge, HVAC/YP9C with YJHF48 heat pump on defrost


QUESTION: Hi - I was wondering if you could help answer the following query of mine. I had a York propane furnace YP9C in combination with a YJHF48 heat pump installed about 2 years ago. It has been working ok but in winter like now the defrost would kick in and the incoming temp would be around 40F - a little chilly and I'm the one who sits next to the vent! I often wondered why the furnace wouldn't kick in at the same time to offset the cold defrost cycle. So I called the contractor but they wanted $120 to come out. I'm pretty handy with wiring since I work on high end analytical instrumentation. So I called York tech and they told me to check the wiring schematics but that basically W1/66 from the on-demand defrost should be wired into the fossil fuel unit W1/66. So I checked and sure enough they'd wired it wrong. Now it works and the air coming in is about 56F on a cold evening, better. Here's what I'm wondering: the YP9C comes on at the same low rate as if just kicking on the furnace normally. There is mention in the manual of the burners coming on at 80% whilst HP defrosts in the manual if the furnace can detect the presence of O being energized so I fed a wire from the heat pump (fossil fuel kit) to the furnace O and changed the jumper on the furnace from (originally) heat pump OFF to heat pump ON but it doesn't seem to have a made a difference. I would expect that if burners were firing at 80% even with 40F air being blown in I should see say (110F + 40F)/2 = 75F.
I know this is quite specific to these products. Hope you can help.
Thanks in advance.

ANSWER: Rich there are a number of things that come to mind with this problem. I totally agree that in defrost you should be getting much warmer air than when the heat pump is providing the heat. In defrost mode the outdoor unit actually is switching to a/c mode to defrost the outdoor coil. You know during the summer the heat you feel coming off of the outdoor unit, this is what defrosts the coil is the coil is heated up in a/c mode. When in this mode several things happen. The outdoor fan should be cutting off to enhance defrost. The furnace should energize to keep from having the cool air entering the ducts. I am not sure about the indoor fan. In normal a/c mode the fan will operate at a higher speed to push the denser cool air into the ducts and in heat mode the fan runs at a slower speed to allow the heat exchanger to heat up enough to provide heat, but in defrost mode I am not sure about the fan speed. Check with the York tech again to see what speed the fan should be running at during defrost. I know the fan must run at high enough speed for the outdoor unit to operate efficiently for defrost but in reverse it must run slow enough for the heat exchanger to heat up enough to provide the compensating heat. This is all I can think of as far as the mechanics of the system. Another thing to check is the ductwork. Trace the ducts to be sure there are no open gaps especially in the entire return duct. I found during the original installation of my own ductwork the return line made a 90 degree turn into the box the filter was housed in underneath my home and it was in the lowest part of the house and very difficult to get to, but the 90 degree elbow was only attached with duct tape and had fallen away from the grill box allowing return air from under the home to enter the ductwork, so I was basically returning outside air and trying to heat this air during the winter. You may have some loose ductwork, but check this and the fan speed for defrost mode. Thanks Rich and let me know. Thanks J

---------- FOLLOW-UP ----------

Fossil Fuel kit wiring
Fossil Fuel kit wiring  
Thanks for the response. A few follow-up bits of info. Originally the W1/66 wire from the heat pump was not connected to the fossil fuel board. Consequently frigid air would blow into the house - I think the fan was at the same rate as warm air is blown in when the heat pump is not in defrost mode.
The duct work is ok. Register temp is 90F is the family room right now with the heat pump running, delivers 130+F when the furnace is running.
Something is not quite right since in the manual they describe the YP9C furnace as perfect for a hybrid setup, see
The setup which is most like my setup is on p18 except mine has the fossil fuel kit between heat pump. See attached image.
When I attach W1/66 from HP (was not attached originally) to W1/66 on FF kit board, furnace comes on during defrost. However fan speed is low, temp is at lowest 56F rather than 40F without furnace. If I were to switch to emergency heat (propane), furnace cycles on a low speed (maybe 35% of max) and ramps up slowly. It seems when in defrost it's doing something similar.
Now according to the manual on p21 if you set the heat pump jumper to YES (mine was set originally to NO), if a W and Y2 as well as O signal is hooked to the furnace burners should come on at 80%.
I tried to reach out to York tech but no response. I called the York installers but they want me to take out a service contract and will not discuss anything with me. I feel that's a tad cheeky - they don't set it up right then want $220 to put right what they should have put right in the first place.
So right now it's wired as it was but with W1/66(HP) to W1/66 (FF kit). Also of interest was when it was in defrost yesterday the furnace stays on a little bit longer than when O is energized (so effectively HP and furnace are on together for a short time but I figure with the bonnet sensor this shouldn't be an issue). Sounds like the YP9C is on a run2 schedule when it does that.
Sorry for the long response - any light you could cast on this would be appreciated.

Rich I wish I could help more but this one is a problem that really requires a site visit to really get into the unit to try to determine the problem. One concern is with the furnace. In any fuel burning furnace the fan is energized by either a timer or a temperture switch that brings on the fan after the heat exchanger has warmed enough to heat, but in tandem with a heat pump I would think the fan must immediately energize for a proper defrost, but maybe not depending on the system requirements, so you really need a hands on technician to diagnose the problem. Sorry I could not help any further. J

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J Cook


With 25+ years experience, I am familiar with residential, commercial, and industrial HVAC equipment including but not limited to boilers, chillers, reciprocating and screw compressors. I am trained in all manners of control wiring.


I currently have three HVAC licenses and Refrigeration license by the State of North Carolina. I have been in this field for over twenty years. I have been a service technician for a contractor and also worked at a state college in the repair and maintenance of steam lines and equipment. I am currently the Building Maintenance Superintendent for a municipality.

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