Btu Buddy 8: Burned Four-Way Valve Coil On A Heat Pump
In this edition of the Btu Buddy series, the dispatcher contacts Bob with a service call from a customer in a small retail store. She is complaining that the store is too hot; the system seems to be heating instead of cooling. Bob advises the dispatcher to call the store and tell the manager to turn the system off until he can get there. It will be a couple of hours.
Bob is driving over to the job about two hours later and he begins to think about the problem. He is not familiar with the job so he must check the job out from start to finish while becoming familiar with it.
Bob arrives to find a hot and agitated customer.
When Bob drives up, the customer meets him at the truck and tells Bob that she needs some relief in the store. It is a clothing store and customers do not like to look at or try on clothes in a hot, sweaty atmosphere; this is hurting business.
This is one of those times where Bob must maintain his professional manner and get the customer's work done in a timely fashion. There is as much invested in keeping the manager happy as in doing good work. Btu Buddy then appears and says to Bob, "Since you have an aggravated manager, ask for her opinion and she will feel like part of the call and actually help you. This will help get her on your side. There is no need to struggle with an unknown service call and an angry manager at the same time."
Bob goes into the store with the manager and asks her to review what happened. She says, "I arrived at work this morning at about 8:00 a.m. and turned on the system like I usually do. It was warm in the store when I arrived. A few minutes later, I was working at my desk and noticed that my office kept getting hotter and hotter. I reached up toward the ceiling where the cool air normally blows down and it was warm."
Bob says, "Show me what you did."
"Great work," Btu Buddy says to Bob, "she is on your team now. She will show you the thermostat and tell you where the system components are. You did not even know this when you came to the job and she is helping. Sometimes it is hard to listen to the customer, but it oftentimes is a big help."
The manager says, "The system is a heat pump and has worked great for the seven years that I have been here. All we do is change the filters every month. We have not had a service call in the last seven years." She then says, "There are two pieces to the unit; one is in a closet and the other is on the roof. You can access the roof through the ladder in the storeroom. Just push the hood access cover up as you go up the ladder. I learned that from the plumber who had to do some work on the plumbing vents."
Bob says to the manager, "I am going to the truck to get some tools and then we will turn it on and see what it is doing."
Checking Out The UnitWhen Bob comes back with his tool pouch and electrical meter, he turns the thermostat to "cool" and heads for the roof. When he gets to the roof, he notices that the unit is running. When he puts his hand in the air stream leaving the coil, he discovers that it is cool, not warm. He knows this means that the unit is running in the heating mode, not the cooling mode, but why is the big question.
He shuts the unit off using the disconnect switch because he knows that it is very hard on a heat pump to operate when the outdoor air is warm, and it is 85 degrees F. The heat pump will be overworked when it is absorbing heat from an outdoor temperature of 85 degrees. It will soon trip off because of overload. This is hard on the compressor. He will leave the disconnect switch off while investigating the problem.
Bob now starts the thinking process. He knows that he must not run the unit unless absolutely necessary, but he must find the problem. Btu Buddy asks him, "What are the possibilities?"
Bob says, "The four-way valve may be stuck in the heating position. The thermostat or related wiring could be a problem."
Btu Buddy says, "Why don't you look at the wiring diagram and see whether the four-way valve is energized in cooling or heating?"
Bob asks, "What difference could that make?"
"Well," answers Btu Buddy, "the system should not have been switched to the heating mode since it has not been cool enough to cause a call for heat. So whatever happened had to happen in the cooling mode, unless the operator set the thermostat to call for heat. Think about the possibilities."
Bob removes the panel with the wiring diagram and reviews it. "The four-way valve is energized in the cooling mode," he says. See Figure 1 for a simplified wiring diagram of this unit.
"What would happen if the four-way valve coil were to malfunction or the wiring calling for the valve to be energized were to cause no voltage to be applied to the valve coil," asks Btu Buddy?
"Well," said Bob, "it looks to me that the unit would operate in the heating mode."
Btu Buddy explains, "It would be real easy to check the four-way valve coil to see if it has power supplied to it. If not, then check it for continuity."
Bob then checks power to the four-way valve. This could be done without turning on the power at the outdoor unit because the indoor unit contains the power transformer and the four-way valve coil is 24 volts. He found there was power at the four-way valve, but the coil was not warm; it was not pulling current.
Btu Buddy says, "There is one other thing that you can do at this point to see if the coil is functioning. You can hold a screwdriver blade up close to the coil. It will act like a magnet if it is working. When you noticed that it was not warm, you know it is not working, but if it had just turned on, it would not be warm yet and the screwdriver method can be used.
Bob then removes the wires from the four-way valve coil. Since it is only 24 volts, it is not required that he go downstairs to the air handler where the 24-volt transformer is to turn the power off. He then checks it for continuity with an ohmmeter and it has an open circuit.
Bob says, "This is the problem. The valve coil has an open circuit and has caused the unit to change over to heat."
Bob changes the coil and replaces the wires to the coil and hears the valve's pilot valve click. This is a good sign that the unit will now function correctly.
Bob starts the unit up and hears the four-way valve change over to cooling when the pressure inside the unit starts to build up. He then asks, "Why didn't the unit change over immediately?"
How The Four-Way Valve WorksBtu Buddy explains, "The four-way valve is actually two valves in one. The actual solenoid coil controls what is known as a pilot valve. The pilot valve directs the internal gases inside the main valve body piston to position the piston to either heat or cool. (Figure 2 shows the internal workings of a four-way valve.) When discharge pressure is routed to one end of the piston, low pressure will be on the other and the valve will shift positions. For example, suppose high pressure is on the right hand end, the piston will move to the left and discharge gas will be directed to the indoor coil and heating will occur. This is what happened in this system. When the coil is de-energized, discharge gas is routed to the right hand end of the valve. When the coil failed, it was the same as the thermostat calling for heat."
Btu Buddy adds, "Let's make sure the unit is cooling by feeling the large line going downstairs. You can always tell which mode the heat pump is in by touching the large line. When it is cool, it is in cooling; when it is hot, it is in heating. You can also feel the heat in the air leaving the outdoor coil. That heat is coming out of the store."
Bob touches the large line and it is cold. He then goes downstairs to report to the manager.
When he tells her what he has done, she says, "Thanks for the prompt and efficient service call."
Bob has oiled the motors and checked the coils and found that they needed cleaning. He explains to the manager that the heating season is coming and it is very important that the indoor coil in particular should be cleaned for better heating efficiency.
She asks, "Why are the coils dirty? We faithfully change the filters every month."
Bob explains, "Typical filters are only about 5 percent efficient. They do a good job of catching large particles, but a lot of small particles of dust get through. In the summer when the coil is cool and wet, it traps much of the small debris that gets through the filter."
She responds, "Please go ahead and set up a coil cleaning for either early morning or late afternoon so that there will not have to be any interruption in business hours."
Bob sets it up for the following day at closing time where he will have a minimum of overtime and not disrupt the business. The manager thanks Bob for his professional methods.
While driving from the job, Btu Buddy says, "You really did a nice job for that company. Plus you booked extra work by looking around. The manager appreciates the fact that you were looking after the equipment and your company appreciates you bringing in more business. This makes you a valuable asset to both your customer and your service 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 firstname.lastname@example.org.
Publication date: 11/17/2003