In the case of heat pumps, there may be two TXVs to be concerned with, one indoors and one outdoors. Many that are returned to the manufacturer have nothing wrong with them. This fact eventually leads to callbacks for the service contractor because the problem that caused the customer to call in the first place will recur.
"On heat pumps, a lot of the OEMs have been using TXVs on the outdoor coil and a cap tube or orifice valve on the indoor side," said Al Maier, vice president of application engineering, Flow Controls, Emerson. "Now there are TXVs on both the outdoor and the indoor coils."
"On the indoor unit, in heating mode, there is a check valve in the valve that opens," explained Jan Holm, application engineering manager, Danfoss. "It's only operating as a pipe, not as a valve. In the cooling mode only, it works as a valve."
"We get valves back from the field where somebody thought it was defective, but more than half are not defective," Maier said. "People do not properly troubleshoot expansion valves. It's an area that needs to be focused on."
THE TXV'S OPERATION
In order to understand how misdiagnoses happen, it is important to understand how a TXV operates.According to Sporlan, "A sensing valve is connected to the TEV by a length of capillary tubing which transmits bulb pressure to the top of the valve's diaphragm." Liquid refrigerant applies pressure to the diaphragm; this, in turn, actuates a mechanism that causes the valve to open to a preset degree. When the pressure backs off, the valve closes.
If there isn't enough pressure in that suction line leading up to the diaphragm, the valve either won't open enough, or may not open at all. To a technician in a hurry, it could look like the TXV is malfunctioning, but it isn't. It's a symptom of a problem somewhere else in the system.
"In order for a TXV to work properly, you've got to have liquid going to the valve," Maier said. If the line contains too much vapor, that could be causing the valve's malfunction. "The only way to properly evaluate it is to calculate superheat.
"Look at the pressure and temperature of liquid going to the valve," he said. "You take the pressure and convert it to a temperature, then subtract; 5°F of subcooling is safe for R-410A - you need solid liquid going to the valve, not a lot of vapor."
"Of course, make sure that the valve has solid liquid in front of it," said Holm. "You need subcooled refrigerant in front of the TXV. If it's vapor, or there's not enough subcooling, or flashing, the valves don't have any possibility of opening enough to supply the evaporator with enough refrigerant."
That lack of refrigerant in the evaporator is typically what prompts customers to call a contractor. Their house isn't being cooled down.
THE SERVICE CALL
When there isn't enough refrigerant flowing to the evaporator, it can't carry away enough heat from the system. The system may keep running without satisfying the thermostat ... or the homeowner."Say the heat pump isn't cooling sufficiently," Maier said. "If the technician is busy, he may only measure the suction pressure, which will not give him a complete picture. If I were servicing the system, I would put a set of gauges on it.
"The TXV could be starving. It could be an undercharge or some sort of restriction in the liquid line, like a kink in the refrigerant tubing or a clogged filter-drier.
"You measure the suction pressure at the outlet of the evaporator coil, and the temperature at the same point," he reiterated. "Look up that corresponding pressure on a pressure-temperature chart. The difference between that and the actual pressure is the superheat."
"People do not properly troubleshoot expansion valves," agreed Holm. "The real problem could be a leak in the system. As more refrigerant goes out of the system, when you reach a certain point, you don't have solid liquid in front of it [the TXV] because you can't subcool the refrigerant. It could also be that if there is a filter installed in front of the TXV; if that is plugged up, it may not allow enough refrigerant to the valve."
Holm continued, "Many times when they [service technicians] analyze the system, the conclusion is that the TXV is the problem. But many times it's due to the charge. So when they replace the valve, they have to recharge the system so that it has enough refrigerant. It works, so it looks like you fixed the problem by replacing the TXV."
Unfortunately, the problem still exists out in the system. And it may lead to a callback, or a call to your competitor.
TXV TROUBLESHOOTING TIPS
In addition to checking superheat and subcooling, Maier and Holm agreed that technicians must verify that the valve is properly attached to the suction line. "The problem could be the bolt, which must have good contact to the suction line," Holm said."The temperature is sensed by the bulb, which is strapped to the suction line attached to the coil," said Maier. "That's what senses the temperature, transmits the temperature to the power element, which opens the valve. Making sure that the bulb is tightly attached at the proper position on the suction line is something that could be easily overlooked."
Maier also said that the bulb can be "a handy tool to verify that the valve is working properly. For example, in a case where somebody thinks the valve isn't feeding properly, you would detach the bulb and hold it in your hand. If your hand is warmer than the bulb, you should see the suction pressure go up." This is simply a good check to see if the valve is working or not.
"If the valve does not respond to holding it in your hand, it's a good indication that the valve has lost its charge," he said. The power element of the valve, which opens and closes it, has its own charge of nitrogen or another inert gas. "If it has escaped, the valve will not function."
Losing the charge can be due to many things, Holm said, "like dirt in the system making its way into the diaphragm of the valve body. It makes a hole in the diaphragm and it leaks." It doesn't take a very big hole for the valve to lose its entire charge, he said.
"When we do have valves that come back and they aren't working properly, we find that they're full of dirt and gunk and garbage," said Maier. "You need to have a good filter-drier on the system and perform proper evacuation."
INSTALLATION TIPS
This is especially true for the POE oils used with R-410A. "POEs are a much better solvent than mineral oil," Maier said."You need to do a better job of evacuating, getting moisture out, and making sure the system is clean and free of contaminants. Put in a filter-drier, not just a drier. On R-410A systems, it becomes mandatory that technicians replace the filter-drier when they break into that system for any reason at all."
When it comes to installing a new TXV, "There are things that you need to be aware of," said Holm. "Normally when technicians install a valve, they cut the pipes, put the valve in, and put everything together. With TXVs, they must place a wet rag on the valve when they weld the pipes. If they're not wet ragging, too much heat around the power head and brass body can cause failure.
"Let's say they sweat it in without wet ragging," he said; "the orifice may be incapable of changing the stroke, and the system won't perform as it should.
"Weld the cover pipe on the inlet, and then the outlet, depending on what valve you have," said Holm. "If the contractor is using our TU stainless steel valve, he doesn't need to wet rag. It saves a little time, makes the job a lot easier, and it prevents him from putting water into the system."
Like Superman, TXVs do have one weakness, Maier said. "It's where the cap tube attaches to the top of the power element. The braze where the capillary tube attaches is a common area where a crack or leak can develop. Take care in handling the capillary tube. Don't apply force where the cap tube joins the power element."
Publication date: 05/22/2006
