Suction / Discharge Pressures On RC

[gbc.heat]

Hi Guys ,

I'm new to this game .

I went out to service a split Ducted system Fujitsu Inverter AOT45LJBYL . After thoroughly cleaning the coils, filters etc I stuck my gauges on the unit . Standing pressure was about 1200 kpa . I turned on the unit and noticed that the suction was around 1900 kpa and the discharge was around 2000 kpa . ( only 100 kpa difference ) Is this normal on the reverse cycle with an inverter , Scroll compressor and a TXV ? I was lead to believe the TXV isn't used on the RC and I was maybe thinking the compressor was passing .

I checked the 4 way valve to make sure nothing was crossing over and my true suction line back to the compressor was nice and cold and sweating .

I checked the duct outlets and there was a 14 degrees C difference between return air and outlets. The customer never said he had had any issues with regards to lack of heat or cooling in the summer.

With regards to checking the superheat and sub cool to see if it's undergassed do I check the pressure /temp on the gauges , then put my thermometer on the pipe going to the evap & true suction line as it's in heat mode ?

Thanks Gordon

[passandscore]

If your main suction line is sweating perhaps the internal bi-metal over-temperature protection disk is leaking by internally in the compressor causing the pressures that you are seeing?

http://www.emersonclimate.com/asia/en-AP/Products/Compressors/Scroll_Compressors/Commercial/Documents/astp_overview.pdf

http://www.youtube.com/watch?v=_D1OlE1yWoY

As for checking the superheat here is some info.

http://www.emersonclimate.com/Documents/FlowControls/PDF%20resources/2006FC-174R1TXVre-article.pdf

[The Viking]

First thing first...

Where did you attach your gauge lines?

To the two valves on the outside of the outdoor unit? If so you didn't see a discharge pressure...

:cool:

[moondawn]

First thing first...

Where did you attach your gauge lines?

To the two valves on the outside of the outdoor unit? If so you didn't see a discharge pressure...

:cool:

hi viking

are you thinking that he has put gauges to the suction and the liquid line?
if so dont they say that the liquid line pressure should be the same or near abouts as the discharge pressure as its all on the high side.

be nice to know as that is my understanding.

thanks in advance

[Brian_UK]

Also, being an inverter system the pressures do not mean anything unless you know exactly what the compressor is doing.

[The Viking]

No Moondawn,

On a modern split the two interconnecting lines (and therefore the service valves) are normally intermediate pressure and suction pressure. True high pressure only exists inside the outdoor unit.

All assuming the system is in cooling mode.

:cool:

[Mick13]

gbc,

u live in australia, like me, where its cold. u had the unit on the heating cycle. did u check the pressures at the service ports on the exit of the unit? on heating the big pipe becomes the discharge, feeding discharge gas to the indoor unit, where it is condensed into liquid, and returns to the outdoor unit through the small pipe, (liquid line). most fujitsu inverters i have worked on have an electronic expansion valve (not a txv) located on the liquid line just inside where it enters the unit at the bottom right.

so the pressures you are reading at the service ports are actually discharge pressure and liquid line pressure, which are both on the high side of the system. the 100kpa difference is simply pressure drop, through the interconnecting pipes, indoor coil and distributor.

correct me if im wrong and u took your pressures somewhere else? but i reckon both your guages are on the high side....

especially if u say the suction was nice and cold and sweating.... (which is an indication and ONLY an indication of a good superheat, and normal operation of the unit), and u had a normal air on/air off td....

[gbc.heat]

Yeah your right enough , it was a electronic expansion valve on the bottom right at the back .

I put the gauges on the 3 way service valves at the outdoor unit , and I do know the lines reverse when in heating mode and the larger pipe becomes the high pressure side .

Yeah I should have maybe checked it in cooling mode to get a better feel of system pressures.

Makes sense now that that would be a pressure drop . What's the air on air off td ?

How would I get the true discharge on a system then , by measuring the temperature of the discharge pipe then cross referencing with my pressure / temp chart ? Most systems I have worked on only have the service port on the suction side .

I need to get this superheat / sub cool thing right so I know when systems are under or over gassed as you said with inverters it's harder to pin point pressures as the compressor is ramping up and down

thanks

[Mick13]

u put your guages on the service ports? where the outdoor flares connect? on heating, they are both the high side.

mate, a modern split system can be VERY hard to acturately diagnose refrigeration faults at times.... as u said, alot only have one service port, on the gas pipe (big pipe), especially the smaller ones. in this case, on heating that port is discharge pressure, and if there are no other ports u wont be able to get a suction. on cooling, this is reversed, with that port being suction, and you are unable to get a discharge pressure.

from memory u will most likely find that particular fujitsu unit will have a schroeder port or 2 inside the unit, its just a matter of tracing pipes to find out what pressures they will be. most split ducted systems will have more guage ports inside the unit, u may just need to take the cover off and have a look ​around to find them.

discharge and suction are constant ONLY to the reversing valve, after that they change depending on what mode it is set on....

and no, u cannot take the temperature of the discharge pipe and use a pt chart to get the pressure.... discharge gas is highly superheated, pressure/temperature relationships only apply when a refrigerant is saturated (a mixture of liquid and vapour refrigerant) which occurs within the evaporator and the condenser.

by air on/air off td, i was referring to the temperature difference between the air going onto the indoor coil and the air coming off the indoor coil, which you stated was 14k, which is good....

[gbc.heat]

Thanks mate ! :)