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How does Reverse Cycle work?

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Hobartian

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Discussion starter · #1 ·
I wish to understand how the reverse cycle is accomplished in a 2.5 kw split system air conditioner.

Obviously, when you change to heating the evaporator becomes the condenser and the condenser becomes the evaporator. However, I don't understand how the expansion device is moved from one component to the other. Split systems are coupled with a suction and liquid line and I assume the expansion device (capillary tube or TX valve) is in the fan coil unit when it is on the cooling cycle. How is this device bypassed when the air conditioner is on a heating cycle and what is used for a expansion device when the condenser becomes the evaporator.

I would appreciate if someone could give me a simple explanation. I am a refrigeration mechanic but have not worked much with small air conditioners.
 
Discussion starter · #2 ·
After posting my question I found this online site which gives a fairly comprehensive explanation on how a box type reverse cycle room air conditioner works. However,
there appears to be no explanation on how a reversing valve works in a split system where the evaporator and condenser may be ten meters apart.

The link is: http://www.refrigerationbasics.com/1024x768/heat_pumps1.htm
 
#3 ·
Does it matter how far the indoor and outdoor coil are apart?

We are reversing the flow of refrigerant through the piping.

10 centimeters or 100 meters, the gas will get there.
 
#4 ·
4-way reversing valve:

Refrigerant pressure within the system is used to move a slide piston inside the reversing valve from one side (cooling) to the other (heating). A 24VAC solenoid controls a magnet that is used to control the refrigerant flow that operates the slide piston through pitot tubes that use the pressure differential on the two sides of the slide piston to move it. It has four tubing connections: true suction, true discharge, to the outdoor coil, and to the indoor coil. In cooling mode the true suction side is routed through the reversing valve to the tubing that goes to the indoor coil (although that is somewhat misleading since the refrigerant is actually coming from the indoor coil) while the true discharge line is routed through the reversing valve to the tubing that goes to the outdoor coil. When the system is switched to heat, once the reversing valve activates the true suction line routes the refrigerant through the reversing valve to the outdoor coil (where the refrigerant is again being brought back to the compressor) while refrigerant in the true discharge side is routed through the reversing valve to the indoor coil.

It sounds complicated, but really it isn't once you are familiar with them.

Common issues with reversing valves:

Failed 24VAC solenoid
Low system charge prevents operation of the slide piston due to inadequate pressure differential
Slide sticks due to physical obstruction (valve is dented, dinged, ect)
Slide sticks due to mechanical obstruction (contaminents in refrigerant, the infamous green sludge that results from not properly flushing line sets when converting from R-22 to R-410a, ect)

I hope this helps

Adam
 
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Discussion starter · #6 ·
How Reverse cycle works on split units

Thanks for your responses.

I understand how the flows are reversed for the suction and discharge lines but I do not understand how the expansion device for the evaporator which may be ten meters away is bypassed when the unit is on a heating cycle
 
#7 ·
Hobartian said:
Thanks for your responses.

I understand how the flows are reversed for the suction and discharge lines but I do not understand how the expansion device for the evaporator which may be ten meters away is bypassed when the unit is on a heating cycle
Click to expand...
If you look at the link in the earlier post, you'll see a check valve that bypasses the expansion device when the flow is reversed.
 
Discussion starter · #8 ·
Reason for my question about reverse cycle valves

Thanks for your earlier responses.

I have bought a brand new 2.5 KW condensing unit from a split system air conditioner for a very reasonable price. (Next to nothing)

It is my intention to use this unit to heat water for my home. I intend to install a stainless steel heat exchanger inside my hot water cylinder. Water will be pumped from the bottom of the tank through a a large capacity Y strainer and then fed through the heat exchanger. Hot refrigerant gas will be passed through the opposite side of the heat exchanger which will function as a condenser.

Originally, I imagined having to remove the reverse cycle valve but with the explanations given it would appear it can stay in place and the only thing I need to do is ensure the reversing valve has power supplied to it.

If this is the case I only have to connect the heat exchanger (condenser) to the service valves on the condensing unit and modify the wiring.

I also intend to fit a flow valve in the water line going through the heat exchanger so that if a flow of water is not detected the compressor will not run.

Can you see anything wrong with this concept?
 
#10 ·
If the heat exchanger in the water is taking the place of an indoor coil, then the condenser would be running in heating mode. In that case, the reversing valve would not be energized. Almost all heat pump condensers require the reversing valve to be energized for cooling mode, not heating.
 
#11 ·
garya505 said:
If the heat exchanger in the water is taking the place of an indoor coil, then the condenser would be running in heating mode. In that case, the reversing valve would not be energized. Almost all heat pump condensers require the reversing valve to be energized for cooling mode, not heating.
Click to expand...
Unless it is a Rheem or Ruud condenser.
 
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#13 ·
I figured you probably knew that, I was adding that more for Hobartian on the off chance he was about to install a Rheem thinking he didnt need to worry about wiring in the changeover valve...
 
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#14 ·
HVACTechNC said:
I figured you probably knew that, I was adding that more for Hobartian on the off chance he was about to install a Rheem thinking he didnt need to worry about wiring in the changeover valve...
Click to expand...
I think there are some older (very old) units from other mfgs that do it that way too. I seem to recall is was York maybe, but I've drunk a beer or two since I read that, so maybe those brain cells are dead.

I think the way Rheem/RUUD does it makes more sense, logically.
 
#15 ·
I agree. In fairness, I sell/install alot of Rheem equipment so I'm a bit biased, but it is the standard configuration in commercial equipment, and it always insures you have both heating and cooling in the event of a reversing valve problem whereas with other equipment you have two stages of heat and 0 stages of cool if there is a reversing valve prob.

Also, I have done a fair amount of York equipment, and have not seen that configuration on any of their residential equipment. Now I have only been installing York for abt two years, so they may have done it "back in tha day" and I just haven't run across it yet....

Adam
 
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Discussion starter · #17 ·
Reversing valve energised in the cooling cycle

Thanks guys for the responses.

I guess I will be able to determine whether the reversing valve should be energized when I get to look at a wiring diagram.

The condensing unit I have bought is a Hisense which I understand is made in? ------ Surprise surprise - China!

In response to jpsmith1cm's question about where the heat will come from.

Where I live the temperature of the air during daytime will mostly be in the range of eight to thirteen degrees celsius and hopefully the unit should be able to extract some heat from that. I will not run the unit at night.
 
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