[wagswvu]

Hello,
I recently bought a Home that was built in 1999 last September, and the Heat Pump is the same age of the home (15 yr). The original owners seemed to never use the Heat Pump much as the lockout temperature was setup pretty high around 50 degrees. However, with the cost of LP these days I want to run more of the heat pump rather than spend $ on fuel. So after spending last winter playing around with settings, I found that the sweet spot for cost and comfort for my unit is around 33-35 degrees. So I have the dip switches set to around that temperature.

My question and maybe somebody could explain. What I have noticed is that sometimes while the heat pump is running and icing up, instead of running a defrost cycle it will shut off and the switch to the furnace.

Here is an example of what happened this morning. It was around 33 degrees out, the heat pump kick on and ran for 45 minutes and looking pretty icy but then it shut off, and furnace kicked on. I turn the thermostat down and let everything sit for 15 minutes.

Then I turned the thermostat back up. The heat pump kick on and run for another 15 minute and then when through a defrost cycle. Why is doing this behavior? Does this have something to do with getting a false reading for the temperature sensor in the compressor, and making it think it’s colder than it really is outside? Is this just downfall of unites configured this way, in that on days when temperature is on the low end of it's configured lockout might switch over to furnaces when it gets icy.

My Setup: System is 15 years old
HEIL DC 90 Hybrid System (Heat Pump with a LP Furnace)
The system is configured with a dual fuel control module (with outside thermostat in the compressor) and the control board has the temperature lockout control on it.

I did have the unit serviced and inspected last spring by a HVAC tech, and it was given a clean bill of health.

 

[JES]

Sounds like it's running normally.

 

[wagswvu]

Sounds like it's running normally.

Thanks for the reply. Why would, in my example this morning, it just go though the defrost cycle and keep using the heat pump? or are you saying that it's running normally based on my assumptions of getting a false positive temp reading which then trips the lockout temperature setting and calls for furnaces to take over?

 

[JES]

I don't think your getting a false reading. When you getting that close to set point it could be switching back and forth.

 

[wagswvu]

I don't think your getting a false reading. When you getting that close to set point it could be switching back and forth.

Is that how these older systems are suppose to work? Because in that case it leaves a frozen/icy, since it didn't run a defrost cycle, coils and then your not running the heat pump anymore. I say false reading because I'm assuming when the heat pump gets icy the air that gets drawn in to the compress is colder. With the thermostat sensor right inside next to the coils, could cause the temp reading to be lower then it really is. This causes the system to think it's colder, so it switches to furnace.

 

[JES]

The sensor shouldn't be close to the coil. Are you talking about the defrost stat that is strapped the copper line?

 

[Bazooka Joe]

What happens if the outdoor temperature is above your balance point by several degree and the outdoor coil ices and closes the defrost sensor , does it initiates a defrost? if so what happens while your in defrost, does the LP come on to temper the discharge air , or the LP stays off and your basically in A/C mode with supply air temps in the 40's while in defrost? Those would be your two options as long as your above the balance point.

 

[wagswvu]

What happens if the outdoor temperature is above your balance point by several degree and the outdoor coil ices and closes the defrost sensor , does it initiates a defrost? if so what happens while your in defrost, does the LP come on to temper the discharge air , or the LP stays off and your basically in A/C mode with supply air temps in the 40's while in defrost? Those would be your two options as long as your above the balance point.

Yes, in that case it will go through a defrost cycle and the LP will kick on. Once the cycle is over, the LP turns off and the heat pump resumes operation. In my example this morning, when the coils got icy the LP kick on and the heat pump was shut off no defrost cycle ran. Was that caused because of the dual fuel control temperature sensor got a false positive and thought the outside air was below the balance point?

FYI: When I mean "dual fuel control temperature sensor" I'm talking about another sensor in the compressor housing that runs back into the furnace. In the furnace, their is a control board that has dip switches for temps. In my research, 20 years ago that's how they did hybrid systems with the old mercury thermostats.