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Heat Pumps Part 1: Reversing Valves

When the Arab nations cut back on oil exports in 1973, making it apparent that fossil fuels would eventually be depleted, energy costs soared.  Energy saving devices would have to be made available as soon as possible.

An energy-saving machine termed a “heat pump” had been around for some time, but sales had been poor, and consequently, development of heat pump technology stagnated.  The energy crisis changed that.  There was now a demand for such a device.  Manufacturers of heat pumps experienced growth of 200% to 300% per year. 

Much mystery surrounded the operation of a heat pump.  As a result, proper installation, operation, and maintenance of heat pumps suffered.  Knowing what a “heat pump” is and how it works, was and still is, a needed skill in today’s marketplace.

A heat pump is a machine that pumps heat!  Every air conditioner is, technically, a heat pump.  Heat is pumped from the evaporator to the condenser.  If the air conditioner was constructed so that the evaporator (the indoor coil) became the condenser (the outdoor coil) and vice-versa, the air conditioner could be used to both heat and cool an enclosed space.  Early attempts to do this were very primitive.  One of the first heat pumps simply rotated the whole unit!  Another used a duct and damper system to direct the heated and cooled air to the conditioned space.  All of the methods left a lot to be desired.

Soon, manufacturers found that by adding valves and by-pass piping to the refrigerant circuits, it was possible to make the indoor and outdoor coils behave so as to have either space heating or cooling.

The first efforts used four hand valves to direct refrigerant flow.  Then, these valves were combined with two valves, which became solenoid operated.  Finally, the four-way “reversing valve” was developed, and is in use today.  In a heat pump, the compressor may be the heart of the system, but the reversing valve is the nerve center.

Two types of reversing valves were developed, a poppet-type and a slide-type.  The slide-type proved to be the better of the two, and poppet-type valves have been obsolete for many years.

While there are many manufacturers of slide-type reversing valves, their basic construction and operation is the same.

Figure 1 shows a typical slide-type valve without its piloting valve.  The two connections labeled “C” next to the suction connection would connect to either the indoor or outdoor coil, depending on the configuration of a pilot valve.

Figure 1.

In Figures 2 and 3, a solenoid operated pilot valve has been added to the slide valve.  The pilot valve is usually mounted directly to the slide valve, so a complete reversing valve consists of a slide valve and pilot valve.


Figure 2.                                                                      Figure 3.

In Figures 2 and 3, the pilot valve has been configured so that when the pilot valve is de-energized, the system is in the heating mode, and energizing the pilot valve the system is in cooling mode.  Reversing valves can easily be configured for opposite operation.  Manufacturers of heat pumps vary the configuration of the valves, depending on whether they feel the system should “fail” to the heating mode or “fail” to the cooling mode.

This depends on how the reversing valve is piped.  In Figures 2 and 3, to fail to cooling mode the indoor coil would be connected to valve port “C1” and the outdoor coil to port “C2.”

In Figure 2, the system is on the heating cycle with discharge gas flowing through reversing valve ports “D” to “C2” making the indoor coil the condenser.  The suction gas is flowing from the outdoor coil (evaporator) through reversing valve ports “C1 to “S” and back to the compressor.

With the 4-way solenoid pilot de-energized, the slide is positioned so as to connect ports “D1” with “B”, and “A” with “S1.”  When the pilot is de-energized, high-pressure discharge gas builds up on the end of the main slide.  The other end of the main slide is isolated from the high pressure by a cup seal and exposed to low-pressure suction gas.  Thus, the unbalanced force, due to the difference between discharge and suction pressures acting on the full end area of the main slide, holds the slide in position as shown in figure 2.

When the coil is energized, the slide in the pilot solenoid valve shifts, now connecting pilot ports “D1” with “A”, and “B” with “S1”.  With the pilot solenoid so positioned, the discharge pressure imposed on the other end of the main slide will flow through the pilot solenoid valve to the suction side of the system.  At the right end of the main slide, high-pressure discharge gas will accumulate so as to increase the pressure.  An unbalanced force in that direction is again due to the difference between discharge and suction pressures acting on opposite ends of the main slide.

This unbalanced force moves the main slide to the position as shown in Figure 3 and the force unbalance across the area of the main slide holds the slide in the new position.

The system has now changed over to the cooling cycle with the discharge gas flowing through reversing valve ports “D” to “C1” making the outdoor coil the condenser with the suction gas flowing through reversing valve port “C2” to “S” . . . making the indoor coil the evaporator.

Figures 2 and 3 show a modern 4-way pilot valve.  Many reversing valves are made with 3-way pilot valves, as shown in Figure 4.  Note that the slide valve is the same whether piloted by a 3-way or 4-way valve.  Therefore, one can replace the other.

Figure 4.

Four-way pilot valves are being used more and more, as evidenced by Alco using 4-way pilot valves on their new series of reversing valves.  Older style 401 Alco valves used 3-way pilot valves.  Four-way pilot valves are more expensive than 3-way, but they last longer and have more spacious cavities in them to allow system debris to pass rather than clog the pilot.  They also assure full system DP across the slide during shifting for reliable operation. Reversing valves are expensive.  Contractor prices range from about $90.00 to over $300.00, depending on capacity.  They are delicate!  They are easily damaged in shipping and handling.  A dent, even a tiny dimple can make the slide stick.  Handle any reversing valve with extra care!

As we will see later, when describing heat pump systems, you cannot simply add a reversing valve to an air conditioner and make a heat pump. 

The most common cause requiring reversing valve replacement is compressor burnout.  When you have a compressor burnout, REPLACE THE REVERSING VALVE!  You cannot clean up a reversing valve!  It is first in line from the compressor discharge, and as the compressor is cremated, combustion products leave the compressor.  Carbon particles, tar, resins, acids, a wide assortment of burnout products are generated by the breakdown of Freon, oil, and electrical insulation.  These vaporized substances find a resting-place in the nearest cooler object, the reversing valve, where they condense.  Trying to clean a reversing valve is a total waste of time.  Actually, the valve has made system clean up after a burnout easier.  It contains most of the debris that contaminates the system.

The discharge line connection is always the single port connection on one side of the slide valve.  The suction line connection is always the center port on the other side of the slide valve, where there are three connections.  The two connections, one on each side of the suction connection, go either to the indoor or outdoor coil, depending on how the system is configured when the pilot valve is energized and de-energized.

Other than replacing a burned out coil on the pilot valve, there is no other field service for a reversing valve.  An Alco RV or 401RD will replace any other brand of reversing valve.  To replace the valve, one needs to know:

1.  The voltage of the pilot valve coil.

2.  What refrigerant is in the system? (99% of the time it will be R-22)

3.  Nominal line sizes.

4.  Capacity, in tons, of the heat pump.  Capacity tables in the Alco catalog are based on a 2-psi

DP across the valve suction ports.  The 2-psi DP is a standard used for capacity ratings.

Undersizing the capacity rating will result in too high a pressure drop, which will cause a loss of BTU capacity of the system.  Oversizing may result in poor or no operation of the reversing valve.  A larger than necessary capacity rating will result in a very low pressure drop, possibly so low that the slide will not move, may chatter, or not seat well when the pilot valve is either energized or de-energized.  It is the pressure difference across the slide that moves the slide.

Chances are a reversing valve in the selected capacity will have line sizes available to match the valve being replaced.  It is not crucial that line sizes match up perfectly.  Fittings may be used to increase or decrease the connections to fit the existing tubing.  If the replacement valve has grossly mismatched line sizes, you’ve probably selected the wrong capacity valve.

Reversing valves can be mounted in any conceivable position.  The replacement valve will usually be mounted in the same position as the valve being replaced.  Today’s reversing valves are very reliable, long-lasting devices.  Except when the compressor burns out, they almost never have to be replaced.

In Part II, we’ll examine the various heat pump systems in regard to refrigerant circuits.

Go to Part II

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