Btu Buddy 121: Heat Pump No Heat Call - Part 2
Now, Bob’s company has promoted him to help train a new employee, right out of a school specializing in HVAC, just like Bob was. Bob is now Tim’s Btu Buddy. Tim is anxious to travel with Bob. Tim realizes that he is right out of school, with the theory and lab work that he accomplished in school, but still needs help. He knows that he worked with many of the components of the systems in the school, under ideal conditions with good light and air conditioning. Now it is into the field, sometimes under the house with poor lighting, or out on the rooftop in the sun, where the real action is. He is naturally and normally reluctant, but he has Bob to help guide him.
Bob and Tim had left this residence a few days ago when they discovered that the four-way valve on the home’s heat pump was stuck in the cooling mode. They tried several things to get the valve to do its normal changeover to the heating mode and couldn’t get it to function. The valve would need to be replaced. It was very cold so they opted to let the system heat the house with the strip heat until more favorable weather. They disconnected the control wire to the outdoor unit so it would not start up. That effectively put the system in emergency heat mode, except the customer could not change it out of this mode at the thermostat.
Bob said, “Since we left the heat on the crankcase heater, the refrigerant will be easier to recover from the unit.”
They connected the recovery unit to the system and started it up. It was a beautiful 40°F day, unlike the 20° day when they discovered the problem.
Tim then asked, “What are the special procedures that we need to use to install the new four-way valve? You mentioned the other day that it was quite a procedure.”
Bob responded, “A four-way valve changeout is more difficult than a compressor changeout because the valve requires much better piping skills. I have seen technicians heat the piping to remove the old valve. That involves heating more than one connection at a time and is very hard to accomplish. Remember, there are four pipes that fasten to the valve; three of them are on the same side and are the largest pipes.”
Tim asked, “Can’t you just heat the connection and pull the pipe back one at a time?”
Bob answered, “Sometimes there is enough flexibility in the piping that you can do that, but sometimes you can’t. Let’s assume that you can pull the piping back after heating the connection. The pipe will have high temperature solder filler material on the end of the pipe (Figure 1). That filler material will have to be filed off of the pipe and it is hard to do without getting filings down in the pipe. Filings are likely to get into the valve and cause it to stick again. Or they may migrate to the compressor and cause problems.”
Tim asked, “What do you suggest?”
Bob said, “The four-way valve has a sliding piston inside that has composite seats for the ports that it covers. Composite seats are usually some type of plastic and excess heat can damage them. These composite seats must be very smooth for the piston to slide over the ports, and heating them too hot will damage the surface. The valve body must not get too hot. Using the wrong technique will easily overheat the valve body and do seat damage.”
Tim asked, “What do you do to prevent this from happening?”
Bob explained, “There are several ways to accomplish this. There are special torches to heat multiple pipes at the same time to remove the old valve. But when you get the old valve off, you still have to deal with the high temperature solder or brazing residue on the pipe stubs. This is very hard to remove.
“I really like the idea of using fresh pipe all the way by cutting the valve piping loose by using very short tubing cutters to cut the pipe as close to the valve as possible.
“After cutting the old valve out of the system, I now have four pipes to connect to the new valve.
“I then measure the distance between the pipe and the valve and cut 4 short pieces of pipe to make up the distance.
“Then I would fasten the new valve in a vice (Figure 2) with the stubs in the new valve.
“Then wrap the new valve in wet rags (Figure 3) and silver solder the stubs into the valve. You now have a valve with stubs that should fit in place of the old valve.”
Tim then said, “It looks like the assembly is just a fit. How will we get the couplings to slide between the piping and the stubs?”
Bob said, “The couplings that go between the pipes normally have stops in them. We can put the three big pipes together easily and then when we try to fasten the discharge line to the other side of the valve, you can see that it will not swing into place with the coupling on either side. Take a rat tail round file and gently file the stop out of the coupling so it will slide up the pipe, then line the pipes together and slide the coupling into place.”
Tim did this and asked, “OK, what is next?”
Bob said, “Wrap the valve in the wet cloth again and solder the valve in place.”
When the valve was in place and permanent, Tim leak checked all of the places they worked on and said, “I don’t see any need to leak check the rest of the system because the system was operating correctly when we arrived and has been operating for several years without any complaints.”
Bob agreed and said, “Get set up to evacuate the system and charge it by weighing in the correct amount of refrigerant. This is a package unit so the charge is printed on the nameplate.”
After charging the unit with the correct charge, they removed the gauges and started the system in the heating mode. The system was heating as it should and Bob said, “Place a jumper between the ‘R’ terminal and the ‘O’ terminal and see if the system will change over to the cooling mode.”
Tim fastened the jumper and asked, “Now what? How long should we run it in the cooling mode?”
Bob said, “Let it run for about a minute and remove the jumper and see that it changes back to heat. This is the same action that it would go through to go into and out of the defrost mode, so we want to make sure that it operates.”
Tim used the jumper several times to change the modes and then said, “I am satisfied that the system is functioning correctly.”
Bob agreed and said, “I think it will give many more years of good service. Let’s tell the owner that the system is good to go.”
Publication date: 4/22/2013