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In Sept. I had a new HVAC system installed. Not duct work, just unit. I went with a Trane heat pump with a propane gas furnace as backup heat. Trane XL16i 4 ton unit and the furnace is a S9X2 model.

Here are the model numbers of what was installed.

HP 4TWX6048H1000BB

F S9X2C100U5PSBAA

COIL 4TXCC007DS3HCAA

Problem is this: When furnace comes on the temp in house will drop 3 or 4 degrees before it starts to come up to where the thermostat is set. The temp. drop happens within about 10-15 minutes, so pretty quickly, but it takes maybe an hour for the temp to get back up. I have had the service techs out here 4 times so far trying to figure out what is going on. I say that this is not normal. 1st, I don't think the temp should drop like that, and 2nd it doesn't seem like it should take that long for a propane gas furnace to heat the house up a few degrees.

The problem is not associated with the heat pump because when the outdoor temp is below 35 the heat pump is locked out. So it has to be either with the furnace or some other associated part of the entire system.

The service tech says that the unit is operating as it should and has checked a lot of setting and programming. He did make a few adjustments on during one of the trips such as:
1. made 2nd stage heat come on sooner.
2. turned Smart optimization off
3. change to aggressive recovery or made it a more aggressive recovery. not sure which. My notes just say changed to aggressive recovery.

These things didn't help. We then found that air was being sucked into the return from the attic. I have since fixed that problem and the temp drop issue is still there.

The furnace unit is in the attic. I have a cape cod style house and the supply duct work is in the attic as well.

I am very frustrated with at this point and I am looking for input from others to see what they think may be the cause. Any input is helpful. Thanks.
 
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