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scrappy_dude · Nov 29, 2006, 10:54 AM

I leave in the Kanas City, MO area and have a heat pump for primary heat with natural gas forced air for backup. When the temperature gets down to the high 20's, the outside unit (condenser?) frosts up and the fins are clogged with frost. It seems to me when the fins are clogged with frost it cannot be working correctly. Is this frost normal? The gas furnace seems to kick in when the temperature drops to around 25 degrees.

Is there a thermostat or switch somewhere on the unit that tells the heating system when the heat pump should run and when the gas heat should kick in. If there is, I would like to set it to have the gas kick in sooner at a higher temperature (maybe around 32 or 35 degrees).

NorthernHeat · Nov 29, 2006, 02:16 PM

Yes, frost is normal, ice is not. They frost up the worst in the 30's-40's when there is enough humidity in the air to condense on the coil. It should only get an inch or 2 thick though. Does your thermostat have an emergancy heat selection, on those very cold nights just do that. I'm not sure, but I think Kansas gets cold enough you would have an aux. heat source, either gas or electric.

scrappy_dude · Nov 30, 2006, 07:27 AM

Originally Posted by NorthernHeat

Yes, frost is normal, ice is not. They frost up the worst in the 30's-40's when there is enough humidity in the air to condense on the coil. It should only get an inch or 2 thick tho. Does your thermostat have an emergancy heat selection, on those very cold nights just do that. I'm not sure, but I think Kansas gets cold enough you would have an aux. heat source, either gas or electric.

When frost clogs up the fins, how in the world does the heat pump work? It seems to me that once the unit frosts up it no longer can pull air across the fins and pull heat out of the outside air.

My thermostat does have an emergency heat selection, put I travel and I can't depend on my wife or teenager kids to think an move the switch to gas heat. I would like this to be automatic. There must be some sort of switch already because when the temp's get in the low 20's, the unit automatically switches to gas heat. I would like to know how the system knows to do this and if there is anyway to set it so that it kicks into natural-gas heating when the temp hits around 32 degrees.

esquire1 · Nov 30, 2006, 08:30 AM

Look at your condensing unit. You may have to open it up. There are different type on controls for this.

NorthernHeat · Nov 30, 2006, 03:06 PM

Yes there is a low ambient switch you can use to tell the condenser not to come on at a set temperature, then it will switch over when to cold and send power to the furnace, so you need 1 more thermostat wire very possible they ran 8 wires and one is still free for use. And believe it or not a heat pump can pull heat through frost. The older heat pumps where time defrost whether the unit needed it or not. For quite a few years the have been using demand defrost, which cause the frost problem to be more pronounced. From an efficiency stand poin you don't want it to defrost very much, when iyt goes into defrost mode it is actually air conditioning your house and the aux heat turns on so you can't feel the air conditioned air.

elephantears · Oct 19, 2010, 04:09 PM

There are two pieces of wisdom that will help you most. (1) There is probably a pin selection on the defrost board in the outside unit that gives a choice of 30 , 60 , 90. These are for different sections of the country. In TN we use the 90 . You need the 60. In Canada (yes, there are many heat pumps in canada) they use 30. This selection detrmines when the system will look to see if there is ice on the coil ( actually a temperature switch that makes or breaks at a given temperature)
(2) you have a hybrid or dual fuel system. All of these have a setpoint on the thermostat where the thermostat decides when to changeover from fossil fuel to heat pump. Old school guys will remember the outdoor thermostats which are adjustable. This would be found on the outside unit,http://stores.ebay.com/Bill-Porter-T...Pump-Made-Easy A man named Bill Porter sold a lot of outdoor thermostat kits for heat pumps. You can still find them today and they are excellent outdoor thermostats not only for dual fuel but also for increased efficiency on electric backed heat pumps.
If you have a dual fuel thermostat that does not give an outdoor temperature reading, you probably have a mechanical outdoor thermostat.
White rodgers built an IF95 thermostat that was adjustable in the configuration. It too has now followed suit with honeywell and requires an outdoor temp sensor.