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Q: My Heat Pump Turns Into A Block Of Ice

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 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.

      Good Luck,....Scott Meenen

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Q: Furnace Duct Barely Warm

Everything looks fine, flames are nice and big, but even the duct closest to the furnace on the first floor seems lukewarm. Definitely not as hot as I think it should be! The furnace is about 17 years old. 

Toms Answer: It sounds as though you have forced air problems, as in the blower, and should be aware of the air filter. Check out Air Filters. I hope that it's something that simple. 

If it's not the air filter: Then you better have a look at the circulator blower. It should be running when the furnace is up to temperature. If not, it could be a dead motor or a defective fan control. 

Please check these things out and let me know if there's more to discuss. And I hate to mention this, but we find an on/off switch for the fan turned off occasionally. --Tom , The Furnace Man

Scott's Answer: 

If the vent is located on the side of a duct where air is flowing by then it is possible to have very little air flow. Because of the inertia of the air, it will not want to turn the corner. Larger systems use 'turning vanes' to get the air around corners. It was mentioned that a filter might be clogged. Every time I have seen a clogged filter in heat mode the air won't be moving very fast but if the burner is firing then the air coming out will be hot as ever. If the furnace is cycling on High limit then the burner will come on and off but the air will still be hot.

If the furnace has a non resetable safety-fuse or a snap-limit then, once it is tripped, there will be no heat until this is corrected.

But, in this case the burner was "on".

Also condensing furnaces (distinguished by exhaust vented with PVC pipe) do not get quite as hot as conventional furnaces.

One other remote possibility: Some furnaces, the way that they are manufactured, air gets trapped in the inner spaces between the heat exchanger and passes freely through the outer spaces.

We ran into this problem with a Carrier furnace where it would keep tripping the high limit but the air temperature leaving the furnace was reasonable, about 120 degrees. We were able to reach our hands into the unit and feel the air flow and found that there was a drastic difference between the amount of air flowing through the outer sections and the air flowing through the section where the high limit was. We had to put the highest set limit we could get and ended up taking out the pleated paper filter and replacing it with a conventional, to solve the problem.

Good luck, Scott Meenen

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Original message sent to Tom Berry, The Furnace Man

Good afternoon, Mr. Berry.

Thank you for your prompt response for my thermostat question.  It settled the problem.  If you have the time, may I ask a couple other questions on HVAC?

1.  What fuel(s) is used by a heat pump?  What all possibilities are there?  Are they only electric or can they be gas or oil or other?

2.  We're trying to categorize cooling systems, but we don't have enough information on certain products to do a good job.  So far, we have heat pump, self-contained system, evaporative cooler, and split system.  I'm not sure if these are good or accurate, because to my understanding a split system frequently contains a heat pump.  Would you be able to tell me where I could find a list of product descriptions, or better yet a list of product categories?

Thank you for your help with this.

Brenda Kirsch


    First lets define what a heatpump is.

A refrigeration system is usually used to move heat from a warm place to a warmer place. Or in other words to make a space colder. Without getting into the intricate workings of a refrigeration system, the amount of heat moved by the process is usually more than the amount of power consumed.

Every compression refrigeration system uses a compressor that is usually powered by an electric motor, but it could be powered by any other source such as a gas (natural or propane) or diesel engine such as in an automobile. 

A refrigeration system can also be used to move heat from a cool or cold place to a warm place for the purpose of making the warm place warmer not the cold place colder, such as the interior of ones house.

When a refrigeration system is used for this purpose it is generally called a heat-pump. A heat-pump can be used to move heat to or from air or water. Or to or from air to water then the water is used to heat or cool air. When used to heat a conditioned space such as your house the system is usually sized for cooling. When the system can't quite get the job done or if it is too cold to be cost effective or the heatpump system should fail to perform.

An alternate heat source must be used to fill in the difference. This heat source can be anything from electricity to a coal burning furnace. An electric furnace is an other name for an indoor heat pump unit that cools only or heats only  (no outdoor unit)

The heat system can be hot water (hydronic), oil forced air, gas forced air, hydronic fan coil, baseboard or cast iron radiation, or any combination in between. Most heat-pump systems are installed where there is no other option except electricity for the heat source and electricity is used for the supplemental heat. However, heat-pump systems are a great compliment to any other heat source because they work well in mild weather.

Heat-pumps can be one way (one directional). Such as you would find for heating domestic hot water. Heat-pumps such as you would find for space heating or cooling are two directional (reversing). That is they can move heat in either direction by operating an electric valve called of all things a reversing valve. So to answer your question a heat-pump is a refrigeration device that is used to make a warm space warmer as opposed to making it cooler.  I hope this answers your questions.

-Scott Meenen

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