Reversing Valves (RSES NATE Prep)

In this installment of RSES NATE Prep, Bryan covers reversing valves on heat pumps. He shows illustrations and diagrams to help you understand how they work.
Reversing valves, also known as four-way or changeover valves, allow heat pumps to switch from heating to cooling. Most systems will energize the reversing valve in cooling mode (usually via the orange wire and O terminal), though some manufacturers will energize the valve in heating mode. In doing so, the indoor evaporator becomes the condenser and vice versa. Instead of Y being the "cooling call," it merely energizes the contactor coil; O or B usually dictates heating or cooling.
A reversing valve consists of two pistons, a ported sliding block, and a pilot valve. The 24v solenoid activates a pilot valve that relies on discharge and suction pressure to shift the valve back and forth. A sliding block that looks like a canoe reroutes the refrigerant based on where it has been shifted. The common suction will always be at the bottom center of the reversing valve, and the common discharge will always be at the top of the reversing valve. Depending on the block position, the two lines on either side of the common suction could be suction or discharge lines. The reversing valve allows the direction of refrigerant to reverse while maintaining the same refrigerant piping.
You can only shift a reversing valve when the system is running and when the compressor is pumping sufficiently. Otherwise, the valve won't shift; it needs a pressure differential.
In cooling mode, the discharge gas goes to the outdoor unit. In heating mode, the discharge gas goes to the indoor unit. Even though the refrigerant moves in both directions, the liquid line remains the liquid line. As a result, we need to use a bi-flow filter-drier to filter the refrigerant in either direction. When we switch from cooling mode to heating mode, we want the refrigerant flow to bypass the cooling mode metering device and vice versa. Most heat pumps have two metering devices with check valves to allow refrigerants to bypass the metering device, though some also have TXVs with internal check valves, allowing flow in both directions. Some heat pumps also have pistons that can seat or unseat depending on the direction of flow.
Due to the increased likelihood of compressor flooding or slugging, some heat pumps have an accumulator to intercept liquid refrigerant before it reaches the compressor.
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[@JuanTodoli]

Very illustrative, thanks for share 👍

[@pabloacosta1703]

You are amazing, I have been using your videos to teach my technicians my guys love it and they understand everything from it, you have taken my techs to anoher level. God Bless.

[@blakewells1599]

Just recently received the air conditioning and heat pump book by RSES. Thanks for the great videos! Helps me understand better while being a young gun learning in the field.

[@soloch69]

Great video and info for my preparation for the Nate exam air conditioning and heat pump , I just got your Book on Review of vacuum for service Engineers love , Big shout out from Orlando Florida keep them Nate preparation videos coming

[@astefchrist]

good video, thanks for the notes aswell

[@MG19100]

Im actually doing one tomorrow! Thanks for the video!

[@deltat5775]

Thanks again. Learned something again.

[@kemonbannister3405]

Thanks for the great info.

[@spelunkerd]

At 7:49 you write that the accumulator is between the condenser and metering device. I thought an accumulator is between the evaporator and the compressor, is it different in a heat pump?

[@realSamAndrew]

It says "receivers" in that paragraph. It's confusing because it is written on the slide with Accumulators at the top.

[@edilsonserafim4784]

Thank you Bryan

[@zekenzy6486]

Great Video, Thank you for sharing ^_^

[@MrChipBryant]

Always great..

[@abdulqadirsalik387]

Good work sir

[@javiersimon1285]

Chingon good teacher

[@pblack.]

Thanks

[@understandingmentalillness5018]

How do I sign up as an individual for one of your HVAC beginner classes? I work for a company but it’s a father and son team in the training just isn’t the greatest.

[@ParabellumX]

Did he say, "Rubber meets the road..."? (@ 5:09) 😂

[@InsideOfMyOwnMind]

So what would cause a reversing valve to give off a loud howling noise for a few seconds after a heat call?

[@bryanswanson5007]

defrost

[@InsideOfMyOwnMind]

@@bryanswanson5007 Maybe I worded the question badly. I should have asked "at the beginning of a heat call."

[@bryanswanson5007]

@@InsideOfMyOwnMind gotya, typically the valve will make a loud wooshing/howling noise in heat mode when it initiates a defrost and the reversing valve switches under pressure. Its normal as annoying as it may be. Ive personally never heard one make any noise at the startup of a heat call though. Does it happen every startup? Could be that it starts up in heat mode and immediately goes into defrost but ive just never been around one at the time it does that.

[@InsideOfMyOwnMind]

@@bryanswanson5007 Ever since the TXV was replaced on this 2T split Goodman it would do this intermittently at the start of a heat call. It lasts a few seconds and mostly does it if the system is fully equalized when it starts up, like several hours between calls.