Very Basic Question Re Defrost

SaltiDawgDecember 3, 2011

I have a one-year-old Carrier HP and Carrier Oil Furnace.

All seems to work well.

I do not understand about the Defrost Cycle.

When does the unit defrost? What does it sense to tell it to defrost? When it defrosts, does the oil furnace operate while the HP is in defrost mode? Does the HP actually operate in A/C mode while defrosting?

If I am not mistaken, my outside unit was once running at the same time the oil furnace was operating - I may be mistaken on this point. would it have been defrosting at that time?


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Heat pump defrost is triggered via two methods, depending on the features of the unit in question. Lower-end systems have timed defrost. Defrost triggers after a specific amount of runtime has accumulated, usually adjustable for example to 1, 1-1/2, or 2 hours. Defrosting will occur whether or not it's needed. Note that this is actual accumulated system runtime, not for example that defrost occurs every two hours 'round the clock.

Better systems have demand defrost which works via temp sensors and other variable parameters. Defrost triggers only when needed, which of course is much more efficient.

Far as I'm aware the defrost cycle ends (even on timed systems) when a temp sensor on the outdoor coil determines it has heated sufficiently and the ice/frost is melted.

Yes, a heat pump switches to cooling (air conditioning) mode during defrost. Heat is pumped from inside the house to the outdoor coil (same process as during cooling in the summer) to warm the coil and melt the accumulated frost. The supplemental heat source, whether electric strips, gas, or in your case an oil furnace, does not directly contribute heat to defrost the coil. Supplemental/auxiliary heat runs during defrost for the purpose of warming the cold air that would otherwise be blowing inside the house (air conditioning). Defrosting will/would occur even if there is no supplemental heat.

I've had two heat pumps with electric supplemental/auxiliary. The compressor and auxiliary do run together during "normal" supplemental conditions when the heat pump alone can't meet the heating need ... the electric strips supplement the compressor. If the thermostat is set to Emergency Heat, as would be done if the compressor fails, then ONLY the electric strips will run. I am not familiar with oil or gas furnaces as supplemental/auxiliary so can't say if they function in the same manner as electric. I recall reading some long while ago that gas supplemental/auxiliary and the compressor do not run simultaneously but don't know if that's correct, or if perhaps it was correct info at the time I read it but is no longer.

    Bookmark   December 3, 2011 at 11:00AM
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Carrier does use time/temp defrosting. The Infinity is a bit better in that it bases the time interval on the previous length of defrost, not a preset time. Carrier does NOT use demand defrost.

Heat from electric strips will not aid a defrost but heat from a fuel burner will as that heat is being picked up by the indoor coil and that increases the heat transfer which causes the ice outside to melt. Or in the case of a timer, the coil to warm up even if there is no frost.

The only time you should see the oil on is in defrost. It should never be on with the heat pump heating.

    Bookmark   December 3, 2011 at 5:12PM
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To elaborate a bit: On most HP systems, the secondary source of heat will come on while the HP is defrosting so that you don't feel cold air coming out of the vents during the defrost cycle. On the HP that serves my basement, I've disconnected the secondary electric resistance heat, so it does indeed blow cold air during the defrost. Most HP systems have electric resistance heating, or "strip heat", as the secondary heat source. There are systems that use gas as the secondary, but to be honest, until I read this thread, I didn't know that anyone made a HP with oil as the secondary.

If you happen to be watching it, you can easily tell when your outdoor unit is running a defrost cycle because the outdoor fan will stop. The compressor is still running; the unit stops the fan so that the heat will be concentrated in the coil. When the defrost cycle finishes, the fan will start back up. Under the right weather circumstances, when the fan starts back up it will blow a big cloud of fog out of the unit.

    Bookmark   December 11, 2011 at 12:07AM
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fa f3 20,

Thanks! Your description of the defrost cycle, including the fact that the outside fan does not run during a defrost cycle, describes EXACTLY what I saw happening.

While oil would not be my first choice for new construction in my area, it was clearly the most economical approach in my home given that the infrastructure already existed and natural gas is not available.

Oil costs are only a small percentage of my heating costs now that I have a heat pump option for heating - previously I had only A/C and oil furnace.

Thanks to all for the replies.

    Bookmark   December 11, 2011 at 9:18AM
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"our description of the defrost cycle, including the fact that the outside fan does not run during a defrost cycle, describes EXACTLY what I saw happening."

The defrost cycle tries to heat up the outside coil.
running the fan would disperse the heat (needed during actual cooling, not desirable during defrost).

An H in defrost ode is running as an AC again to try and heat the coil, not remove heat from the house (though it does this also).

    Bookmark   December 11, 2011 at 2:10PM
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