HVAC hi side flutter

T77911S · 09-11-2019, 11:02 AM

a friend was working a r410 unit here at work.
the discharge needle was fluctuating so fast you could not see the needle. it looked to be fluctuating about 50-100 psi but it was so fast it was hard to tell.
low side was 155 but then the room temp was 80degrees so the load was pretty hi. this is on circuit 2

the unit is a carrier and about 10ton with 2 circuits and its only about 4 years old.
circuit 1 looks to have a bad compressor already.

billybek · billybek's Garage

Might be non condensable gas in the system.

What was the temperature drop of the air across the coil?

Is this a roof top unit (self contained) or a split system (condenser outside and air handler inside)?

What would make you think compressor 1 is bad?

T77911S · 09-12-2019, 02:51 AM

roof top package.
I would not think it would be a non condensable issue with it only 4yrs old and I don't think anyone has done any work on it other than him connecting gauges

comp 1 had a charge, contactor was pulled in and comp was extremely hot and not running.
he told me it was short cycling (run very very short time) the other day.
it was freezing up going into the evap as soon as it came on but the low side pressure was above 150. (not sure why it was short cycling because the pressures were not causing it).
I thought maybe a valve problem on the comp with the low side that hi but then I think the heat load was pretty hi. (I was not there), maybe 80 degrees inside I assume.

billybek · billybek's Garage

Carrier has a 4 year warranty on compressors for their commercial RTU product.

It is possible that the system had lost refrigerant.
R410 is a zeotropic blend. It will fractionate when leaking or if you try to vapour charge from the cylinder.
Simply put, you don't know what part of the blend leaked out first (smallest molecule) and if you vapour charge, you don't know what part of the blend you are installing. This will result in goofy things happening as far as the pressure temperature relationship of the saturated refrigerant.
The short cycling could have been due to the temperature of the compressor cutting out on high temperature protection, high amps, high discharge pressure if the switch is auto resetting.
Information at hand is a bit short to diagnose but hope this helps.

Edit:
Example of Carrier Warranty for RTU products.
https://dms.hvacpartners.com/docs/1009/public/0c/49004dp70.pdf

Zeke · 09-12-2019, 05:07 AM

Billy, good info. What about the TXV (valve) ?

T77911S · 09-12-2019, 05:25 AM

bad thing is the contractor that put it in subd it out to another contractor which never got paid so we have warranty.

I don't think there is a leak. both circuits "look" to be charged based on what little I have seen/been told is going on.

circuit 1(bad comp) was equalized at around 200psi, also contactor was pulled in but it was not running., yes extremely hot.

im going up there to look at it now.
maybe it has cooled enough it will come back on.
will let you know.

still curious what would make the hi side flutter so fast I cant see the needle

T77911S · 09-12-2019, 09:58 AM

ok got to look at it for a few minutes, had something else I needed to do.

the comp is not bad on C1.
it is very low on 410.
about 50psi on low side and 300 on hi.
it was off due too over temp and that is why it was cycling when they were working on it before.

C2 was not fluttering on the hi side but the pressure was around 500 by the time we got gauges on it. (side cover was off so no cond airflow until gauges on).

john70t · 11:40 AM

Quote:

Originally Posted by T77911S

still curious what would make the hi side flutter so fast I cant see the needle

WAG:
A blockage in the condenser right after the gauge?
(The expansion valve maybe or a crushed coil or fitting.)

Turbulence. Each new pulse from the compressor would hit the blockage and bounce back.

billybek · billybek's Garage

Pretty low for sure.
I would leak check with what is left inside. Recover and dispose of the remaining. Repair the leak.
Evacuate and charge circuit 1.
500# is pretty up there too but looks as though the side panel was off.
When I worked on reciprocating equipment, I would on occasion come across a system that would give the high side gauge a good shaking as you described.
I would sometimes run a bit of refrigerant from the low side of the machine through the charging hose (purge the hose), then seal the hose to the gauge set and close the low side gauge valve and open the sealed charging hose to the high side of the compressor. Sometimes the extra volume on that the hose offered would stabilize the high side gauge and stop the fluctuation.
That being said, I have seen systems with non condensibles inside behave in a similar fashion and also overcharged systems.
Again, only trying to offer some options and it is tough to troubleshoot from the keyboard.

T77911S · 09-13-2019, 03:09 AM

i appreciate the help.
I have a good understanding of AC and been to a good school (quite a while ago) but I don't work on it very much.

yes I thought 500 was hi, the other day the low side was at 155 which I though was hi but then it did have a pretty good heat load in it. it was 90 outside and 75 to 80 inside.

thanks for the help. its nice to have someone to run things past,

I am getting a new set of digital gauges, hopefully today. then I can get subcooling on both circuits. I had a tool to pull the shredder valves and put a thermometer in to check the temp but all my thermometers are broken.

T77911S · 09-23-2019, 09:16 AM

update:

put new gauges on today.
C2 that had the flutter:
hi side was 500. low around 150
I said this the first time I was up there, the fans don't sound like they are blowing that hard and the air coming off them seems very warm. so the other guy had a board for the "variable fan speed" (well duh). so we put that on and fan speed still the same. well duh again, its connected to C1 which is not on. so I move the sensor to C2 on the cond. WALLLLLA! we have proper fan speed!!! the hi and low sides come down, SH and SC are about 14!!!! mystery solved for C2.

C1-
low side still low, low SC and hi SH (all this before fixing the cond fan speed issue).
so I put some 410 in.
same thing except now hi side is really hi. (then we fixed the cond fan).
hi side still hi.. low side still low and SH and SC very hi.

well duhh! it must be a bad TXV valve.
now I am sure we have too much in there.

going to replace the head on the TXV and I suspect I need to pull some 410 out.
oh well.
I got caught up on the other 2 saying it had a leak and was low. but I take the blame for not seeing a bad TXV sooner. I was a bit leery of it being low with the hi side as hi as it was, but my fault for not following my instinct.
my lack of experience. I just don't get to work on AC very much.

T77911S · 09-23-2019, 09:23 AM

this system is really messed up as far as air supply and return air.
there is about 2ft between the drop ceiling and the roof. its about 60 degrees in there. we have to go through this space to get to the roof.
I was telling the other guy I think you have air supply leak leaking into the space above the ceiling. so we get to looking at it, the air supply pumps air INTO that space then I think is come out around the edges of the walls. what I thought were the supply ducts in the ceiling are actually the returns.
we pulled the cover off the return so it was pulling straight from the main duct, not the flex dcut and within about 5 minutes the air temp came down 1 degree.
we did have the attic door down but then it was down the entire time we were on the roof.

the building I work in is opposite of that. the return pulls from above the drop ceiling and there is flex duct for the supply.