Service & Maintenance / Extra Edition

Btu Buddy 23: Handling A Frozen Outdoor Coil On A Heat Pump

February 18, 2005
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Bob is a service technician who is well trained and nationally certified. However, he sometimes suffers from the same confusion that all technicians occasionally do - the facts that he gathers may or may not point to the obvious cause of the problem or the best solution. But Bob has something that no one else has. He recalls his long-time HVACR mentor and imagines him accompanying him as "Btu Buddy," someone who reminds him to take time to stop and think before rushing to judgment, helping keep him on the right track, even with facts that are confusing.

The day has been very cold and damp when a customer calls the dispatcher and says that the coil on her heat pump is frozen solid. The fan is making a noise, which called her attention to the frozen coil. Bob tells the dispatcher, "Call her back and tell her to switch the thermostat setting over to emergency heat." Bob has other calls to make before he will be able to get to the frozen heat pump.

When Bob arrives, the customer meets him at the door and tells him that it is getting cold in the house and asks why the emergency heat did not heat the house up to temperature.

Bob explains that the emergency heat plus the heat pump is calculated to heat the house on a 20 degree day, but the emergency heat switch on the thermostat only shut down the heat pump to keep it from doing damage to itself and it also turned on the auxiliary heat which would help, but would not keep the house very warm. Bob tells the owner, "If you want to bring the heat up a little, you should go to the kitchen and put a pan of water on each burner of your stove. Boiling the water will add humidity to the house to improve the comfort inside. Make sure the pans have water in them to keep from overheating on the stove." She goes off to do this because she has an older person in the house.

Bob walks around the house to the heat pump and looks at it. It has a solid sheet of ice on the coils. He knows what to do first. He removes the top of the unit and clears the ice away from the fan blades that had been causing the noise the customer described. He then removes the control panel to get ready for a forced defrost. He goes to the thermostat and starts the unit. He then goes to the unit and starts the defrost cycle at the electronic circuit board. He hears the four-way valve change over to defrost, so he just watches as the defrost cycle progresses. After about 10 minutes, the defrost termination timer stops defrost. Bob takes a close look at the coil and cannot see that much of it defrosted. So he starts another defrost cycle and becomes more curious this time; it doesn't seem as though there is enough heat to defrost. He is scratching his head when Btu Buddy appears.

"You look confused," says Btu Buddy.

"Well, I guess I am confused. This heat pump will take all day to defrost the ice off of the coil at this rate. Something is not right," says Bob.

Btu Buddy gives him some advice. "Turn the heat pump off with the breaker while you go to your truck for gauges and your leak detector. By the time you get back, you will be ready to install them."

Bob has learned to follow Btu Buddy's lead, so he turns off the heat pump and returns with the gauges. When he starts to install them, Btu Buddy says, "Be sure to use the gauge ports that will give you the true pressure readings during both cooling and the heating cycle. Those are the ports that are on the outside of the cabinet up on the side."

Bob then asks a question. "Why did you suggest that we turn the pump off at the disconnect?"

Btu Buddy explains, "You could have gone in the house and turned the unit back to emergency heat, but then when you installed the gauges, you would have had to go in the house and turn it back on. You can control all of that from here with no trips to the house. The reason for shutting it off was to let the pressures equalize. I suspect there is not enough refrigerant in the system to produce enough heat for a proper defrost. If you install the gauges while in a vacuum, you may create another kind of problem. You may pull air and moisture into the system. It is always better to install gauges when there is a positive pressure. You should test the gauge ports before you remove the caps because the leak may be at the cap. Once you remove it, you have changed the conditions and will not know if it was leaking when you arrived."

Figure 1. This valve can be attached to a Schrader port and the valve core can be removed and valved off. A new core can then be inserted and the system can be put back in operation.

Finding A Leak

Bob leak checks around the gauge ports and finds that one of them is leaking. It is the high-pressure port in the heating cycle, which would explain why it had leaked down so quickly.

Btu Buddy says to Bob, "This system could have been low on refrigerant for quite a while and the thermostat would start the auxiliary heat to help it when it needed it and the homeowner may not know the difference. Before we leave, you should explain the auxiliary heat light on the thermostat to her."

Bob then says, "We have found the leak, or at least one leak. I think we should start the heat pump and charge some refrigerant into it."

Btu Buddy says, "That sounds good to me. It is about 35 degrees F outside so what do you think the approximate suction pressure should be for this unit?"

Bob says, "I don't know. I know that the suction pressure should correspond to a temperature much colder than the 35 degree F outdoor air temperature, but I don't know how much colder."

Btu Buddy explains, "The actual suction pressure depends on several things: the outdoor air temperature, how clean the outdoor coil is, and the amount of moisture in the outdoor air passing over it. The suction pressure for this unit will be lower than normal until we get the ice melted off of the coil. I would suggest that we use a water hose to melt the ice rather than multiple defrost cycles. I think it would be faster. Get some plastic to cover the electrical box and the compressor terminal box, and turn off the power, and I will show you what I mean."

Bob brings some plastic sheets from his truck and covers the electrical boxes. He then turns on the hose and lets the water flow over the coil. The ice begins to melt quickly. He then comments, "That ice sure melts fast."

Btu Buddy remarks, "The water from the city water mains is probably about 45 degrees F and will melt ice quickly. The ice that is built up is not actually solid ice. It has a lot of air in it and will melt quickly."

When the ice has melted off, Bob starts the unit with his refrigerant cylinder connected to the system. The unit uses R-22. The suction pressure begins to drop, approaching a vacuum.

Btu Buddy says, "Shut the unit off and let's repair that Schrader valve that is leaking before we go on."

Bob shuts the unit off and asks Btu Buddy what he suggests.

Btu Buddy says, "The Schrader valve service connection has two chances of preventing a leak, the seat in the actual valve and the valve cap or cover. This valve has to be leaking from the seat and the cover. Replace the seat under pressure using that special tool that you have (Figure 1) and then replace the cap and all will be well."

Figure 2. This standard efficiency heat pump outdoor unit has 35 degree F air passing over the coil and the refrigerant is boiling at 10 degrees F creating a suction pressure of 32.8 psig. The coil is boiling at 25 degrees lower than the outdoor air. (From Refrigeration & Air Conditioning Technology, 5th Edition, by William Whitman, William Johnson, and John Tomczyk, published by Delmar Publishers.)
Btu Buddy then explains the answer to a previous question about suction pressure. "This unit seems to be a standard efficiency heat pump. I believe the suction pressure should correspond to a coil boiling temperature about 25 degrees F lower than the air temperature. That would be 10 degrees F (35 degrees F air temp - 25 degrees F = 10 degrees F). When we look at the pressure-temperature chart for R-22 at 10 degrees F, we find that the suction pressure would be about 33 psig (Figure 2). This is only an approximation; we will need to go to a charging chart to fine tune the charge. Use the chart that is inside the panel on the unit."

Bob asks, "Why does the chart call for the outdoor wet bulb temperature as one of the pieces to the puzzle?"

Btu Buddy answers, "The outdoor wet bulb temperature is an indication of the total heat that is being absorbed into the unit. Remember, the outdoor coil is the evaporator during the heating cycle. The moisture on this coil is usually in the form of ice in the winter cycle and the load on the coil can vary by a considerable amount due to the differences in the moisture in the air."

Bob uses the chart and wet bulb thermometer along with his gauges to bring the charge up to the correct operating conditions.

The homeowner comes out of the house about this time and asks how things are going.

Bob shows her where the leak was and tells her that he has repaired the leak and everything is normal now.

She then says, "I noticed that the system was really putting out a lot of heat and hoped all was well. Thanks for the prompt and professional service. It was really getting cool in the house."

Bob then says, "You have a small blue light on your room thermostat that will light up from time to time. Have you ever noticed that light burning?"

She says, "Yes, but I do not know what it means."

Bob explains, "When that light is on, it tells you that the auxiliary heat is helping your heat pump heat the house. Auxiliary heat is expensive to operate and should only be on in weather of about 30 degrees F and lower. If you ever see that light burning a great deal when the weather is 30 degrees F or higher, it is a sign that the heat pump is not carrying the load and there may be a problem."

She responds, "I have noticed it burning a great deal the last few weeks. I will know what to do next time, thanks."

Bob gathers his tools and, as they are driving off, Btu Buddy says to him, "That went smooth. You did a good job with that call."

Bob asks, "Why didn't you suggest that we recover the remaining refrigerant and charge the system from a deep vacuum like many manufacturers recommend?"

Btu Buddy explains, "This was a much shorter process that did not involve the use of a recovery setup and evacuation along with scales to recharge the unit. It was less expensive and frees you up to go on to another call."

Bob agrees, "Saving time is really important because of the workload that we have. And, time saved for the customer is money saved for the customer."

Btu Buddy says, "Good point, and believe me, the customer notices an efficient service technician, and that will lead to more business for you and the company."

Bill Johnson has been active in the HVACR industry since the 1950s. He graduated in gas fuel technology and refrigeration from the Southern Technical Institute, a branch of Georgia Tech (now known as Southern Polytechnic Institute). He taught HVAC classes at Coosa Valley Vocational & Technical Institute for four years. He moved on to become service manager for Layne Trane, Charlotte, N.C. He taught for 15 years at Central Piedmont Community College, part of this time as program director. He had his own business for five years doing installation and service work. Now retired, he is the author of Practical Heating Technology and Practical Cooling Technology, and continues as a co-author of Refrigeration & Air Conditioning Technology, 5th Edition, all published by Delmar Publishers. For more information, he can be reached at 704-553-0087, 704-643-3928 (fax), or bmj@myexcel.com.

Publication date: 02/21/2005

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