Tip & How-To about Heating & Cooling

Why is Ice Forming on my Heat Pump

Many people have heat pumps and never realize that ice can form on the outdoor unit. Some have seen ice on their heat pumps but do not know how much is too much until they have a problem with not enough heat. Still others have a backup heat source and never know that they have a problem.

First off lets establish that heat pumps will form ice on the outdoor unit, usually whenever the outdoor temperatures go below about 40 degrees F. This threshold for ice depends a lot on the humidity of the air outside. If it is raining or snowing, the ice will form much more readily and rapidly. It is normal for ice to form on the outdoor unit because it is the cold side of the system when it is running in the heating mode. Because of this cooling, if the outdoor temperature is near or below freezing and there is moisture in the air, it will condense and freeze on the unit.

To keep the ice from building up to the point where it stops all the air flow, the outdoor unit will have a defrost timer board. This operates in conjunction with a sensor. Every 30 – 90 minutes the board will check the sensor. If the sensor tells the board that ice has formed on the coil it will then change the unit over from heating to cooling. This will make the outdoor unit the hot side for a short time to melt away the ice. While it does this, the unit will also energize the back up heat which is often an electric coil or a furnace. This will keep the inside air from blowing cold while the unit is defrosting.
When the sensor feels the coil temperature at about 50 degrees F it will then tell the board to return to the regular heating and switch the coils around again.

If this process fails to work for some reason the outdoor coil will become a big block of ice and look something like this.


When your outdoor coil becomes totally frozen over it stops all the air flow through the coil and stops the heating process. In extreme case it can be very difficult and becomes a very long process to defrost the coil. You need to practice a lot of patience and care when defrosting a badly frozen coil. A torch or preferably a heat gun (hair dryer) can be used to speed up the defrost process. If it continues to occur the timer needs to be set to defrost more often or either the timer or the sensor has gone bad.

http://www.fixya.com/support/r3563896-air_conditioning__cleaning_the_condenser

http://www.fixya.com/support/r3636724-r_410a_new_refrigerant_air_conditioning

http://www.fixya.com/support/r3636751-air_conditioner_seer_seasonal_energy

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2 Answers

I have a Mitsubishi heat pump that runs well but the outdoor unit does not defrost what could be the problem.


Why does my heat pump ice up in Winter?
Heat pumps naturally ice-up in the winter. It is normal for the entire coil to be covered in a white frost and even light ice, during cold weather conditions. However, it is bad for the entire unit to be encased in ice. This indicates ductless heat pump problems which should be addressed quickly to save energy and avoid serious damage to your ductless unit. These systems should periodically go into a defrost cycle. This keeps the unit running efficiently. If the coils are blocked by ice, proper heat transfer between the coil and the outside air will not occur.
How does the defrost mode work?
When the mini split heat pump goes into defrost, the reversing valve inside of the outdoor unit is energized, switching the system from heat to the air conditioning mode. The outdoor coil becomes the hot, the indoor coil becomes cold, and both - the outdoor and indoor fans shut off. This allows the outdoor coil to melt accumulated ice. When the built-in micro-computer analyzes that all ice have been melted, the heat pump heating system goes back to heating mode.
sanyo-mini-split-defrost.pngA cloud of water vapor may be seen rising over the outdoor unit and a "whoosh" sound can be heard as the refrigerant reverses direction. The entire process usually takes up to 10 minutes (depending on conditions).
How often does the system goes into defrost mode?
Ductless mini-split heat pumps have different ways of determining when to go into defrost. The built-in microcomputer determines outdoor temperature, refrigerant pressures, and several other factors. In colder temperatures the system will go into defrost more often than in warmer.
If a ductless mini split heat pump is severely iced-up in the winter it is possible that it isn't defrosting (though there could be many other causes). Let the manufacturer certified technician check your system

Oct 29, 2011 | Mitsubishi Mr. Slim MXZ30TN Air...

1 Answer

This unit keeps forming ice in the outdoor coil section and breaking the outdoor coil fan blade. Heats fine once replaced but is it compressor suppose to cut off when it gets a certain temperature outside and switch to electric heat to prevent this from happening?


If its forming ice it's either low on refrigerant of its not going into defrost mode. If it runs for a few minutes and the lines begin to ice up, it's low on refrigerant and needs to be serviced.

Jan 25, 2011 | GE AZ38H12DAB Thru-Wall/Window Air...

1 Answer

My Goodman GSC130361DE 60 Htz has problem with coils iceing up. Cleaned several times.


Hi, I am assuming you are talking about the indoor coil icing up when you are in cooling. The reason I am asking this as this appears to be a heat-pump and they will ice up in the winter at the outdoor unit. I will go with the indoor coil. If the indoor fan is blowing up to speed and you have cleaned the indoor coil but still have this ice forming, it could very well be that you are low on freon. This will also cause the suction line, the large copper line outdoors to ice and on into the indoor coil. This will happen if you have developed a leak some where. Also, make sure you have good strong return air coming back through the air filter, because if you have problems with the duct work like holes from varments, or you have had any repairman around the duct work, sometimes they will flatten it causing no air flow and you will have ice. My bet is you are low on gas. Shut the unit off to make sure it has no ice on the coil, and go to the outdoor unit. Have someone turn it to cooling and put your hand above the fan on the condenser coil. The air should start to get warm and then hot if the freon level is ok. This also depends on the outdoor temperatures. The hotter it is outdoors, the hotter the air will be. If it is rather cool, around lets say 75*F, and this copper suction line is starting to form ice, the problem is it is low on freon. In this case, this is something you will need to call a tech out to charge the unit. You have to be certified and be able to buy the right freon which is only available to a/c and refrigeration tech's. If this is a heat -pump, and it is winter where you are and the outdoor coil is icing up, let me know and I can go through the steps to check this out. I do believe this is a summer problem where you may be, but you never know unless you ask. Please don't forget to rate me on this post as I am sure you will be kind. I wish I could save you a tech call, but if its low and you have cleaned the indoor coils, and filter is clean, then this is the problem.
Sincerely,
Shastalaker7
A/C, Heating, & Refrigeration Contractor

Sep 26, 2010 | Goodman CLQ36AR49 Air Conditioner

1 Answer

outer unit produce ice after tempreture 21


Hi, what you have is called a Heat-Pump, and it also has a reversing valve that reverses the flow of freon through the unit. In the winter time, the outdoor unit becomes the evaporator coil and the indoor becomes the condenser coil. You have what is called a defrost board and also defrost thermostats on the outdoor unit. When the unit starts to build up ice on it, it has a defrost timer and along with the stats will terminate a defrost and melt the ice away. You should see the fan shut down and the unit will start to throw off steam, almost looks like smoke. Ice is a normal thing to happen on heat-pumps as long as it goes through a defrost cycle every so many hours to keep the ice from getting real thick and the unit stops heating your home. So, if your not having heating problems, and the ice does not keep building and getting thicker and thicker, this is normal, and there is nothing for you to worry about. If you have any more questions, feel free to contact me and I will help you out. Please don't forget to rate this post for me, as I know you will be kind.
Sincerely,
Shastalaker7
A/C, Heating, & Refrigeration Contractor

Sep 05, 2010 | Haier ESA3125 Wall/Window Air Conditioner

2 Answers

I just moved into this home and am wondering what EM heat is compared to just Heat and my outside unit is full of frost what do I need to do?


Hi, EM stands for emergency heat. You should only need to use it if the outdoor unit does not operate. You have what we call a heat-pump. If the outdoor coil is iced up, it should go into a defrost cycle to melt away the ice. You will know when it does as you will see steam coming up from the unit as though it is on fire. The outdoor fan will also shut down during this process. These units will defrost using time and temperature to defrost the coil. If the ice continues to build-up, and does not defrost every couple of hours, I would say you have a faulty defrost board or defrost thermostat. There is not alot you can do as these are very complicated systems to work on. You will more then likely have to call out a service tech to check this outdoor unit for you.It also should have heat strips in the outdoor unit for EM heat if the outdoor unit fails. If it is iced up it is on and running. Thats what I would do is keep an eye on it to see if it does defrost, and if not, you will need to get a tech out to check it. It takes special equipment to check these units. Please keep me posted.
Sincerely,
Shastalaker7

Feb 01, 2010 | Honeywell Programmable Thermostat Heater

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