RE: HVAC tech issue, weird AC behavior

Hi Guys,

I just wanted to run something past you all.

Here in Australia we can cop some pretty brutal summers. I was looking at a LG hi wall split system (running on R410a) for a family friend, the other week, and then returned today.

Their original complaint was that the unit seemed to not be as cool as it used to be.

So, first time I ran the unit flat out, checked the saturated suction pressure, and found it to be pretty stable at 162 psi (14c). The unit has no access for high side pressure (I hate cheap units!) Now, with my limited knowledge in AC (refrigeration is my thing), that would seems a little high for high temp AC applciatons, no? I would have expected it to be around 120psi (5-7c)?

The air on the evap was 33c, and the air off was 16c. So the unit was still working hard, and achieving some cooling.

Now I know the unit is slightly undersized for the room, just from my limited sizing experience. I expect it will need around a 9kw unit in there to do the job properly. The unit that I was working on is only 7kw.

So I returned today, as I wasn’t really happy with the vague results form the first visit. I decided that the suction temp was too high, and either the reversing valve was passing refrigerant or the scroll compressor was not pumping efficiently and the heads were worn. Time to test the reversing valve. I checked suction and discharge temps across the valve, and only found very minimal temp differences on the corresponding pipes (under 1-2c differnce). So I expect that the reversing valve is ok. I also switched the valve to heating, to ensure that it wasn’t stuck. Everything seemed fine.

Today the suction pressure was around the 150 psi mark, with slight variations with the compressor spinning up and down.

I also checked the copper line set temps, and found the temp after the EEV, at the start of the line set,  was 9-10c (corresponds suction pressure) , and the return temp of the line set was colder at 3-5c!?! This blew my mind a bit, because everything I have ever been taught, would tell me that the  start of the copper lines, or evap, should be colder than the end… Heat is picked up, and refrigerant should be super heated….

So this then makes me question the compressor head as well, maybe the scrolls are fine, if we are having return temps around the 3-5c mark.

But why would there be a drop in line temp on the return!?

So……maybe nothing is wrong with it, and it is in my friends head?

Sorry for the noobish question from a 16 year tech LOL!

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8 Answers

The coldest part is going to be where the refrigerant is boiling, not knowing the ambient temp it’s hard to say where the suction pressure “should” be. Maybe cleaning the condenser would help bring it down a bit by allowing the refrigerant to be subcooled better.

Super Tech Answered on March 9, 2018.
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