mitsubishi puh6

[stevelaird]

Hi All

I have a problem with a mitsi pumping down on cooling mode and runing into a vacum. I changed the four way valve and it works fine on heating but as soon as i put it onto cooling it pumps down into a vacum the head pressure stays at 150 psig i was thinking its a choke does any one have experiance of this the discharge temp is very high through the oil seperator but he condensor remains cold.

[1torr]

Capillary or drier choked?

[get the gauges]

The small copper spun internal filter could be blocked, if it produces frost on the filter when running then thats the problem.

[get the gauges]

yeah why change the valve?

[stevelaird]

The valve was changed as it was ordered by another engineer guess what last on job gets the problem.
where is the small copper spun internal filter located in the pipe system

[1torr]

Outlet of condenser before capillary

[stevelaird]

Must be me but I cannot think of how these symptoms match a faulty 4wv in any way shape or form...

The reason the 4 way valve was changed was that it would originally not heat because the valve was stuck and would not change over from heating to cooling thus the valve needed replacing but after it was put on there looks like a choke in the system but not at 4 way valve.

[Thermatech]

If the unit is R22 then there is an expansion capillary tube on liquid line between outdoor coil header & liquid line service valve.It may be encased in that hated black gungy stuf.
There may also be a strainer which is between the capillary tube & the service valve which could be blocked.
On some Mitsubishi units both service valves have 1/4 gauge connection so you can see what the pressure is after the expansion capillary.
Best idea is put two sets of gauges on the outdoor unit. One set on hp & lp service connections which give compressor suction & discharge pressure. Then put 2nd set of gauges on service valves. This can help you to locate any blockage.
If the condenser coil is cold & outdoor unit pulls vac on suctionin cooling mode then you may have blockage at outdoor liquid line expansion cappliary because all system refrigerant has been pumped into outdoor coil but cannot flow out of the outdoor coil due to blockage.
If the liquid line service valve has gauge connection point why not create a compressor discharge to liquid line bypass with your gauges.
Be carefull & just let some hot high pressure gas into liquid line & then watch the suction to see if it come back from the indoor unit. If it does then that would indicate that the secondary expansion / non return valve at the indoor unit is ok in cooling mode.

If the unit is R407c or R410a then the pre expansion of liquid is acheived with LEV valves.

[Brian_UK]

One other place to check is the strainer in the flare connections at the evaporator unit.

Been known on R22 units to wax up in cooling; unit can run for some hours before dropping into a vacuum.

Bear in mind that the reversing valves needs a pressure differential to operate, if the pressures are low or unusual then it may not change over.

[stevelaird]

One other place to check is the strainer in the flare connections at the evaporator unit.

Been known on R22 units to wax up in cooling; unit can run for some hours before dropping into a vacuum.

Bear in mind that the reversing valves needs a pressure differential to operate, if the pressures are low or unusual then it may not change over.

Cheers brian i disconected the liquid and suction at the condensing unit today brazed in a shreder and blew ofn through the pipes it didnt look like the choke was at the evaperator side would the waxing only occur during running conditions.

[benny_bronko]

It sounds like your orfice is a prime suspect, I would check subcooling right before the orfice. does the system equalize when shut down? if yes then it should be flashing at the point of restriction leaving ample evidence as to where the restriction lies. if the strainer was the issue you would see it in your heating performance, did you blow through it in both directions? remember the flow of refrigerant is reversed during heating operation alowing it to bypass the potentially plugged orfice. with subcooling checks you should be able to find the restriction. check the orfice located in the condensing unit itself, look for the restriction while unit is equalizing "flashing"

[stevelaird]

It sounds like your orfice is a prime suspect, I would check subcooling right before the orfice. does the system equalize when shut down? if yes then it should be flashing at the point of restriction leaving ample evidence as to where the restriction lies. well thats my thoughts anyways

well even though the system dont have a orfice it doesnt equalize out when system is idle. thanks for the input any way.

[Brian_UK]

Cheers brian i disconected the liquid and suction at the condensing unit today brazed in a shreder and blew ofn through the pipes it didnt look like the choke was at the evaperator side would the waxing only occur during running conditions.

It did with mine.

Had a unit do it one day, another guy did all sorts of things and we thought it was sorted.

Next night it went down but restarted fine in the morning when I got to site. First day it ran ok so after about five hours I said OK. That night it went out again.

Reset in the morning and sat and watched the gauges, eventually the pressure started dropping and it dawned on me that I'd had a similar problem some years ago.

Legged it downstairs, 84 of them!, to the indoor unit and whipped off the side panel and the insulation around the connection. Yep, ice on the pipework around the flare joint.

The only good thing about it is the system is in a half pumped down mode so I shut the liquid valve opened the gauges up and let the compressor pull the rest back into the unit.

Back downstairs again, remember those 84 steps go in both directions - jeeez, to crack open the joint. Use small probe of some sort to clean out the gunk and give it a blow with OFN down both pipes if you can.

You'll see a small wire mesh strainer inside the evaporator side.

Good luck with it whatever it is.

[Thermatech]

If the pressure does not equalize at compressor off then it must be a complete blockage & not just a restriction.
If you can connect gauges to liquid line with compressor running then you should be able to see if refrigerant is passing through the outdoor unit pre expansion capillary & strainer or not.
If you have already blown dry nitrogen through indoor unit flow in at liquid line & it came through the indoor coil to suction line then that would indicate no complete blockage at indoor unit.
Slugy oil blockage at indoor unit strainer can take time to develop as Brian advised. So does system run ok for a while & then gradually reduce to a suction vacume or does it always pull a hard vac staight away.
No pressure equalisation at compressor stop tends to indicate permanant & total blockage.