Btu Buddy 26: Checking A Heat Pump With Dirty Indoor Coil
The first call of the day is from the owner of a strip mall. There is a post office in the mall that is serviced by a 10-ton heat pump. The owner told the dispatcher that the post office had called and complained about the electric utility bill for the last two months. It seems that it is much higher than usual.
The day is cold for early spring, 40 degrees F, so Bob would get to operate the heat pump under heating conditions without overheating the conditioned space. The indoor coil is in an equipment room with plenty of space around it and the outdoor unit is on the roof.
Bob arrives and goes to the branch manager of the post office to find out what he knows. The manager shows Bob the electric bills for the winter and Bob agrees that a significant rise has occurred. The manager says, "I track the bills here very close. I have been the manager for 10 years and I have never noticed a rise like this in the cost of power for a month. The lights are almost a constant and stay on the same amount of time each month, so the heating system must be the problem."
Bob agrees and tells the manager that he will start looking and see what he can find. He goes to the equipment room where the indoor coil is located. The first thing he does is check the filters; they are clean. He notices a log of filter changes attached to the unit. The filters have been changed frequently and the date noted.
The unit is running when Bob reaches the equipment room and continues to run as Bob looks around. Bob uses his ammeter to check if any of the electric heat is operating; one stage is running. That doesn't look right to Bob. He's sure the heat pump alone should be able to carry the load. Bob begins to think there must be a control problem that caused the electric heat to come on. Then he thinks the heat pump may not be operating to full capacity. He reaches over and touches the hot gas line and it seems too hot. About this time Btu Buddy appears and asks, "What seems to be the problem? You look confused."
Bob says, "This is a funny set of symptoms. The electric heat is operating and it is 40 degrees F outside. The heat pump hot gas line is really hot. I don't seem to get the picture here."
Btu Buddy then says, "How does the liquid line feel?"
Bob touches it and says, "Wow, it is really hot too. That doesn't seem right either. I checked the filters and they are clean. The fan seems to be running normally. This just doesn't add up."
Btu Buddy then asks, "Why is the liquid line so hot? What would cause it?"
Bob answers, "Usually reduced airflow, because it is the condenser in the heating mode."
Btu Buddy then says, "Correct. Is the airflow really up to normal?"
Bob says, "I don't know. I will do some checking around."
Checking AirflowBob goes to his truck and brings out a velometer (Figure 1). He checks the airflow at several registers and remarks, "The air velocity seems a little low. According to the chart, the velocity could be as high as 1,500 fpm (feet per minute) for this application, and it is running about 300 fpm (Figure 2). I think we should look in the coil and fan section for a possible problem. This is a belt drive motor. Maybe the belt is slipping."
Bob shuts off the unit and removes the fan compartment door. He checks the fan belt and it is adjusted correctly. He then starts to examine the coil. Using a flashlight, he looks at the coil from the outlet side and says, "This coil looks really dirty down in the core. I think it needs cleaning."
Btu Buddy agrees and says, "The key to solving this problem was the hot liquid line. You didn't even to have to put gauges on the system to know that the air was not removing enough heat from the coil. Any time the liquid line is much warmer than your hand temperature, there is a problem with a heat pump. The liquid line should not be any warmer than 100 degrees F. Hand temperature is normally about 91 degrees F, so if the line feels really warm or hot to the touch, there is a heat transfer problem with the indoor coil. This is not true for an air-cooled condenser in the cooling mode, even on a heat pump. The air-cooled condenser may have 100 degrees F air or higher passing over it. The return air for a heat pump is normally not warmer than 75 degrees F, a much cooler condensing medium. Now what are you going to do?"
Bob says, "I am going to clean the coil and fan wheel."
Btu Buddy asks, "How are you going to clean them?"
Btu Buddy says, "That is the correct procedure. You are getting good."
Bob removes the fan and motor and takes them outside for cleaning. The fan looks dirty, but not excessively. Then he sprays the fan wheel and coil down with approved detergent. He really gives the coil a good soaking and lets it get wet all the way through. After the detergent has had time to soak through the dirt, Bob uses a pressure washer and cleans the coil and drain pan. Thank goodness there is a good floor drain system in the room. Bob then says, "The dirt is just streaming out of the coil. It was really dirty down in the core. I wonder why it is so dirty when they change the filters so often?"
Btu Buddy comments, "This is a post office and they handle a lot of paper. Paper seems to have a fine dust that will eventually stop up the coil. This type of air filter has a certain amount of bypass air - air that slips through and carries small particles with it. It would be better for the manager to use a little better or finer filter media. You can talk to him about that. He will have to change the filters more often, but the coil will not get dirty nearly as quickly. You may also talk to him about putting a filter alarm system on the fan coil that will measure the air pressure drop through the filter and tell him when the filters should be changed, instead of every so many days. When a coil begins to become plugged with fine particles, it becomes a super filter. Over time the coil becomes so efficient as a filter that it will stop up very quickly. This coil has been getting dirty for a long time, then suddenly it is really stopped up."
Bob finishes the cleaning job and assembles the system. Then he starts it back up. After it runs for a few minutes, he checks the liquid line temperature by touching it and it is cooler than hand temperature. The electric heat is not operating. The system is back to normal.
Bob goes to the manager and tells him what he has found and what he has done about it. He then says, "I will work up an estimate for a filter air pressure alarm and send it to you so you can submit it to your supervisor. That would be a big help to this system, along with a better filter media."
The manager responds, "Thanks for doing a good job with our system and making these recommendations."
While riding away from the job, Btu Buddy comments, "You did a good job back there. Technicians that care enough to do the job right are always in demand."
Bob says, "Thanks. You have been a real inspiration to my work ethics."
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 firstname.lastname@example.org.
Publication date: 05/16/2005