What is this device

Ok guys school is on how many have seen this device in the liquid line to the A/C A-coil what is it called, what does it do and what controls it. It use to be pretty common in the day. It was more commonly used in commercial applications.

2551 Homestead Enid 4-5-12 054.jpg

Ok I will start with looks like some sort of temp gauge.

Looks can be deceiving it is not a temp device:D Don’t let Larson beat you he will be along directly

Looks like a solenoid valve.

Installation link at: http://icpindexing.toddsit.com/documents/003350/48201517000.pdf

Ya got that part correct how about the rest of the question what does it do and what controls it

It’s called a Liquid Line Solenoid Valve and it holds refrigerant in the line when the sytem is not running.

The liquid line solenoid valve closes when the thermostat demand is satisfied to prevent liquid refrigerant migration. This device is for use in long−line applications. The liquid−line solenoid valve is open any time there is a demand signal from the thermostat.

Isn’t Google great :wink:

Why would ya want to hold the refrigerant in the line which it does when the valve closes. BTW when the liquid line solenoid first closes the exterior condensing unit still operates for a short time. I read part of your link and I do not agree with all that is stated it states a start capacitor must be used in conjunction with this valve I don’t agree and will explain later

Sure is but one does not always have Google in his back pocket it kinda helps to know a little about what you are trying to operate

This device is for use in long−line applications.

As for Google, I don’t produce reports on site so they are sometimes handy.

It’s a pump down system, used to prevent slugging on startup.

Solenoid closes, compressor continues to run until suction pressure is low enough to ensure there is no liquid refrigerant in the oil. All liquid is now on the high side, so liquid can’t get sucked into the compressor.

It don’t know if I’ve explained it very well or not - Google “pump down system” for more.

Not a bad explanation and yes the system is called a pump down system. The solenoid is the tell tale device when identifying this type of system but another tell tale sign is no thermostat wire to the outside condensing unit. The Solenoid valve is a normally closed valve controlled by the thermostat. The exterior condensing unit is controlled on and off by a normally open low pressure switch located within the unit close to the compressor which closes on a pressure rise when the solenoid valve opens.

When the solenoid valve 24 volt coil fails the exterior condensing unit will not operate. When the solenoid valve leaks bye its seat you will see frost on the outlet side of the valve and the exterior condensing unit will be short cycling on and off

We also used them in staging on commercial units

Would ya explain a little more in detail please and thank you

Charley, what is this one, its on the liquid line next to the attic coil on a small townhome.

The A/C worked good for about 20 minutes then started blowing warmer air.
The outside unit was running and the suction line was same as ambient.

The line between this device and the inside coil started getting frosty.


I’m not Charley, but it’s still a TXV or Thermostatic Expansion Valve. It changes the high pressure liquid refrigerant into a low pressure vapor by a variable restriction. Turn a can of air (computer duster) upside down - it’s exactly the same thing.

It throttles based on the measured superheat at the end of the evaporator coil. That’s what the capillary tube is for - the temperature bulb.

Sounds like the system was low on refrigerant, but there are other causes as well.

Why do some have these exterior to the unit when most are inside the cabinet? Looks like its asking for damage to the capillary tube.

I’ve seen it both ways as well. What I’ve seen is that if the TXV and coils are a matched set with the valve installed in the factory, the valve is usually inside the case. If purchased seperately and assembled in the field, the valve is on the exterior.

Either way, function is the same. I like to insulate the tubing after the TXV as well, for efficiency and keep condensate from dripping all over the place.

Been a while we used it with 2 stage cooling basically shuts the flow to the second coil. when full cooling was not needed. 20 ton unit. when it is was cold we did not need full 20 tons and certain times of the year free cooling was not enough.
Was it right way? lol not sure but it worked for many years

A Solenoid valve is frequently use in air conditioning applications where the condensing unit is higher than the evaporator coil (specifically when a TXV is not the metering device). This prevents the refrigerant in the condenser (on the roof or higher location) to flow by gravity and overload the evaporator and subsequently the compressor on startup.

It is also used in commercial freezer applications if a defrost heater is utilized on the evaporator. This increases the efficiency of the defrost and prevents overloading the compressor and requiring a crankcase pressure regulator to prevent over amperage draw on the compressor during initial start up.

I didn’t read that link, but I will agree with Charley that a start capacitor and potential relay are not necessary just because the solenoid is utilized.

Thanks Charley, thanks everyone for the lessons everyone.
I truly must get caught up on HVAC.