I have 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 and freeze 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 when it is raining, snowing or
sleeting the problem will aggravate itself tremendously. When it is very
dry you will not get any ice.
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 stops or electronic jumpers with times at 30, 60, 50, 70 and 90 ect. minutes depending on manufacturer of the defrost equipment.
Some systems use a pressure switch to detect that the coils have become stopped up with ice. I have seen this system on a 10 Ton York unit but it is not very common on residential equipment. I have also added 300 PSI pressure switches to units to problem units to assure a proper termination.
Defrost Guidelines: The unit must defrost regularly when icing conditions occur and long enough to get rid of the ice and short enough to not waste energy. There is usually a best place for the sensor or thermostat on the outdoor coil just after the metering. Some manufactures have a tubing stub for the thermostat and others a well. If you remove or change the sensor try to mark where it was. Failure to place it in the correct place will cause the unit to either not defrost or the cycle will be too short and the unit will build ice anyway. Place it at the wrong end of the coil and you will get no defrost activity.
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 (refrigerant filled sensing bulb on the Ranco E-15) is below 28 degrees F. the defrost will take place. When the temperature of the sensor rises to 50 or 70 degrees F Or 10 minutes elapses the defrost cycle is terminated. A typical defrost cycle should be from 1.5-3 minutes.
NOTE: When I use the word "sensor" I am referring to an electronic input such as a thermistor or thermocouple. When I use the word "switch" or "thermostat" I am referring to a bistate device that is either on or off depending on temperature.
My opinion: Some moron somewhere
came up with the idea to terminate a defrost after 10 minutes and every
heatpump control has that time built in. I have found from working with
functional units that after 3 minutes of defrost in a fully functional
unit all the ice will be gone (from the coil not the case) even if the
unit hasn't been defrosting in weeks, at 5 minutes the high pressure switch
(you do have one don't you?) will cut out with a liquid line temp of 150F+
at 10 minutes the compressor will have gone off on internal relief and
and if you are lucky tripped the internal overload after taking a few months
off the life of the compressor.
If your unit has a GENERIC electronic timer board (used on Newer Heil/Tempstar, Rheem/Ruud and others) The way the system should work is every 30, 60 or 90 minutes the control board will put out a 24 volt ac signal for 10 minutes (read my explanation). Then if the defrost thermostat is made (below 28 degrees) it will send a "24 volt ac high" signal to the defrost relay (a 3 pole relay) to reverse the system (go into air-conditioning mode), shut off the outdoor fan, turn on the electric heat. When the defrost thermostat gets above 50 or 70 degrees it breaks the circuit to the defrost relay and the unit goes about its business. Some systems have a "reset" pin that restarts the time after the defrost thermostat opens. If there is a problem with this board it can cause rapid cycling of the defrost function.
You can test this board with the power (high voltage outside) off to the outdoor unit by making the thermostat call for heating (contactor must be pulled in) and jumping out the test pins. this will speed up the cycle by 256 times. If at the instant the unit goes into defrost you can remove the jumper from the test pins and the board should revert back to heating in 10 minutes (you will hear the relay click). You can test the output signal by connecting an 1819 bulb from the output terminal to 24 volts ac not to "Common" (see the wiring page). The output terminal connects itself to "Common" on a defrost function and not to 24 volts ac like you might expect! When the system is running any time you should get a signal for 10 minutes every time the cycle comes around. This happens in summer too.
Other defrost schemes have the relays
(a total of 3 contacts) mounted on the control board (Goodman/Janitorol,
Nordyne/Frigidare/Kelvinator ect.). Some will have one relay mounted on
the board and another one external. If the fan relay on your defrost board
fails add a 90-340 relay and share the reversing
valve or the electric heat instead of replacing the board. a 90-340
is much more rugged than the fan relay on the board. These can also be
tested with the line voltage power to the outdoor unit removed.
Note: there are a few systems that read the line voltage
as part of the control function, but not many.
1819 light bulbs and sockets available at Radio Shack
can be used to test the 24 volt ac. control voltage when connected with
clip leads to the different parts of the control circuit.
A brief description: The way the defrost cycle works is the unit is forced into A/C (cooling) 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 because no freon is feeding back to cool the compressor making for high Discharge Superheat. Most units come from the factory with the time set at 90 minutes. I would recommend setting the time to 30 minutes in humid climates, 90 in dry climates. The shame of it is that when it is very dry your unit can go for weeks without needing a defrost wasting a lot of energy and minutes when it is humid.
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/American Standard is the only brand that I know of that always includes a TXV for their heat cycle. Of the few Coleman units I have seen, even very old ones they have TXVs. With other brands it is a "crap shoot".
At one extreme is if the orifice is too large. All the liquid refrigerant mixed with hot gas 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. 10 to 20 is considered low and 30 to 40 is normal depending on the outside temperature. If the unit is short of refrigerant this symptom will occur in both heat and A/C modes. See Superheat on the charging page.
A unit that uses a TEV/TXV in heating or cooling mode or both, will always perform better than a system that uses fixed metering only, another reason to only purchase equipment with these features or have them added. In this world where everyone is concerned about energy savings and the environment, if you have a heat pump that has fixed metering instead of a TXV set you are pissing energy away, that is a fact!!! Add them before trying to do anything else. Any competent refrigeration mechanic can do the job.
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. To learn more see the charging page.
Chances are you have a problem with your defrost cycle
but you could have other problems too like a bad reversing valve or coil.
 |
 Contact us to order this device. |
If you need to replace your defrost
unit. There are several types General Electric, older Rheem and others
use a Ranco E-15 (combination timer and sensing probe), There is an electronic
retrofit available from ICM. that costs less than the mechanical unit.
Older Trane, Comfortmaker, Miller, Nordyne, and others use either an electronic
or mechanical timer unit and a defrost thermostat.
Many newer units like Carrier or Trane will use
a proprietary control board. These guys can be expensive and a pain to
find. In some cases an after market control board (shown
above) can be used. One of the problems with replacing defrost boards
is they are designed to fit in a specific "footprint" and give a place
to connect all the wires.
Good Luck,....Scott Meenen
The main problem with the Time/Temperature system that is on 95% of all heat pumps is that they will continue to defrost regardless of whether ice is present or not as long as it is cold out (Winter is known for it's cold). Without getting into the details several manufacturers are or have been including electronic defrost systems on their heat pumps.
York has had the "York Guard module" for decades which has always included a demand defrost system and other features, it is the best system ever in my opinion. If you asked me what is the best electronic control for a heat pump I would have to say the "York Guard".
Rheem has recently been using a system made by Ranco (Invensys) that works well and can be retrofitted to other systems (reversing valve issues need to be addressed).
Trane has used a demand defrost on most units made since the mid 90s.
There are systems out there that look like a demand defrost but are simply an electronic Time-Temperature system. I would recommend that any system that is having defrost problems be retrofitted with one of these systems, they will pay for themselves right away because it will eliminate regular defrosts that occur with conventional systems. By not having nuisance defrosts you will stop wasting energy in electric assist and disruption of the heat cycle.
This text written by: Scott Meenen * G & S MechanicalWritten By: Scott Meenen N3SJH of:
G&S MECHANICAL SERVICES.
Specializing in Mechanical, Controls and Electrical Modifications Of
Heating, Air-conditioning, Refrigeration, Cold storage,
Ice Production and Food preservation. Anything having to do with Heat and Energy.
Serving MD, DC, and Northern VA.
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