Heat Pump Turns Into A Block Of Ice
a Tempstar 5000 heat pump unit in my home. I live near Tyler, Texas about
75 miles east of Dallas (we are in Maryland). The temperature here rarely
gets too far below freezing. At this time of the year I am experiencing
"freeze-up" of the unit. What I mean is that frequently during the heating
cycle the unit quits putting out warm air and begins putting out cold air.
This occurs at various outside temperatures even 40 degree or higher. The
unit is clean, free of debris and has only been in service for three years.
I find it hard to believe that this is a normal occurrence for this part
of the country. Is there something that can be done? Peggy Schreiber
A: I am quite familiar with your problem and
from what I have seen you have
one of two problems, possibly two. In the normal course
of making heat your heat-pump gets cold and WILL make ice depending on
outdoor temperature and humidity. The textbooks all say that most ice production
is produced at 40 degrees F. But this will vary with actual humidity and
the temperature of the coils especially if it is snowing.
To get rid of this ice, most heat-pumps have a time-temperature-defrost
system. This system uses a timer; either a clock-motor or an electronic
timer. The timer will have mechanical or electronic jumpers with stops
at 30,60 and 90 minutes.
When the compressor runs the clock runs and time is accumulated.
At the selected intervals the clock will energize a defrost event through
a thermostat or a sensing bulb, If the temperature of the sensor is below
28 degrees F. When the temperature of the sensor rises to 50 or 70 degrees
F Or 10 minutes elapses and the defrost cycle is terminated.
The way the defrost cycle works is the unit is forced
into A/C mode, the out-door fan is stopped, the indoor fan is stopped or
electric heat is activated. The ice is melted off the coils and when the
temperature sensor is satisfied, or 10 minutes elapses if it is not satisfied
or failed, the outdoor fan comes back on and blows the water vapor away
and at the same time the unit is reversed back into heat-mode.
If this function is not working your unit can and WILL
become a solid block of ice to the point of stopping the outdoor fan and
making the unit useless. This is also very hard on your compressor. Most
units come from the factory with the time set at 90 minutes. I would recommend
setting the time to 30 minutes.
What causes the ice: Your heat-pump is basically an air
conditioner run in reverse. It works by boiling refrigerant in the outdoor
coil and condensing it in the indoor coil. The way the refrigeration process
works is liquid refrigerant is metered to the outdoor coil through a TEV.
(Thermostatic Expansion Valve) or an orifice (Carrier calls them accurators)
. Units like Trane, Rheem or York use TEVs most all the others use orifices.
At one extreme is if the orifice is too large. All the
liquid refrigerant will be blasted through the system without a chance
for it to pick up latent heat or build head pressure at the indoor-coil.
At the other extreme, if the orifice is too small
or the TEV. is stuck, the outdoor coil will form ice just as the refrigerant
enters the coil and build ice very quickly but not pick up much heat. If
a pressure gauge is hooked to the unit on the suction side you will find
a very low pressure like 10-30 psi and the compressor will draw significantly
low current. If the unit is short
of refrigerant this symptom will occur in both heat and A/C modes.
Other things that will cause ice to form faster: filthy
dirty unit; grass, dirt, mud, leaves, pet hair if the animal is near the
unit, a stopped fan or fan blade on backwards or fan motor turning the
wrong direction, a replacement fan motor of a lower rpm (850 instead of
1550), a flat pitch fan blade (22 degrees instead of 33 degrees), bad motor
bearings. Any and all of the previous causes will cause poor performance
in cooling mode and high current and head pressures or low current and
pressures in heat mode.
Chances are you have a problem with your defrost cycle
but you could have other problems too.
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