A Teacher in Training

Reversing valves are an important part of a typical residential heat pump, as it reverses the flow of refrigerant depending on whether the heating or cooling cycle is selected. It's not uncommon to see reversing valves fail, but when they do, they can be a bit difficult to properly diagnose and repair.

As you complete your HVAC training, you'll want to understand how to properly and accurately troubleshoot reversing valve problems. The following offers a few pointers for taking care of this critical HVAC problem.

Why Reversing Valves Fail

The most common reason for a reversing valve failure is often because the valve itself becomes stuck, leaving the heat pump locked in the heating or cooling mode until the valve can be freed again. A lack of lubrication or an accumulation of debris can cause the valve to stick in one position.

Another common cause of reversing valve failures is when the valve itself leaks refrigerant. A valve leak can be a bit troublesome to diagnose, as the same symptoms (high suction, low head pressure issues, decreasing cooling or heating capacity) are often shared with a failing compressor.

Diagnosing Stuck Reversing Valves

The first step of diagnosing a stuck reversing valve involves checking the reversing valve solenoid or pilot valve, which should be energized when the heat pump is in heating mode or cooling mode, depending on the manufacturer. Use a multimeter to check for 24-volt power at the solenoid's wiring connections and transformer.

If power is present, grab a flat screwdriver and check for magnetic pull at the solenoid coil. If you don't feel any pull, you'll want to make sure the solenoid resistance is around 10 to 60 Ohms. A solenoid that receives no power or whose resistance is out of range should be replaced.

If the solenoid is in good condition, your next step should be to knock the reversing valve free. You can do this by tapping one side of the valve with a rubber screwdriver handle or similarly soft object. It's possible for the valve to get stuck due to a lack of lubrication, which is also brought on by a low refrigerant charge. If the valve doesn't budge, it'll have to be replaced.

Diagnosing Reversing Valve Leaks

The first step towards diagnosing a leaking reversing valve is making sure that there are no other issues that could possibly mimic reversing valve or compressor problems. One such issue is a liquid bypass, which occurs when the evaporator coil becomes flooded with liquid refrigerant.

Once the possibility of a liquid bypass has been ruled out, your next step should be to verify whether the reversing valve is causing problems or if the compressor itself requires replacement:

• Set up the heat pump so that it runs without the condenser motor. This will allow the refrigerant head pressure to build up. Use your gauges to keep an eye on the pressure, as you'll want it to top out as high as it can go, up to a maximum of 475 pounds.
• Once enough head pressure has built up, turn the system off. Once you do, you'll likely hear the hot gases escaping either through the leak in the reversing valve or the compressor. As an alternative, you can position one hand on the suction line at the compressor and another on the same line near the reversing valve. See which one gets hottest the fastest. If the latter gets hotter faster than the former, then the reversing valve is likely leaking.

Replacing the Reversing Valve

Removal and installation of the reversing valve can be a bit tricky, given the limited amount of space available. Since the connections are brazed, great care should be taken to heat the fittings without exposing the valve itself to large amounts of heat.

Applying ample amounts of heat-shielding gel on the areas surrounding the fittings can help reduce heat transfer while removing and installing the valve. Don't forget to handle the reversing valve with care, as any major dents can cause the slide to stick.

For more information on training courses for HVAC maintenance, visit a site like http://hvac-tech.com.