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Category: HVAC
Satisfied Customers: 14604
Experience:  15yrs in the HVAC industry with certificates and degrees.
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I'm no HVAC expert so I need some insight from the experts.

Customer Question

I'm no HVAC expert so I need some insight from the experts. I have a problem with a fairly old. late 1990's model, Trane 50 ton air cooled chiller. The chiller cools the glycol in a closed circuit glycol circulation system through two air handling units to condition a large volume of air inside a building. The chiller has two cooling circuits with two compressors each. It incorporates hot gas bypass (HGBP) in circuit 1 to limit the cooling capacity at low loads. The problem is that the chiller trips intermittently reporting error code 8A, evaporator entry water temperature, when in HGBP mode. It is my understanding from studying the chiller manual and online literature that error code 8A means the evaporator entry water temperature dropped more than 2 deg F below the leaving temperature while in HGBP. Today I observed it's behavior under low load by turning off the air handling units so the chiller would just have to chill the glycol to the set point and then go into HGBP. As far as I can tell, without pressure gauges on the discharge and suction lines, it behaved as expected, no trips. The water temperature differential over the evaporator never exceeded 1 deg F while in HGBP. Tomorrow I plan on hooking up pressure gauges and run it under different load conditions to see whether I can catch it in the act. We had a Trane tech out a month or two ago but the chiller still trips on exactly the same error code. I can see with the hot gas being dumped in the evaporator that it would warm up the water in the evaporator. The question is why this temperature differential would intermittently exceed the 2 deg F limit. I suspect an intermittently misbehaving HGBP metering valve or maybe the valve just needs fine tuning to reduce the amount of hot gas being bypassed. Any suggestion on what I should be looking at or measure to pinpoint where the problem lies. The facility is located in a remote area so it is not easy (expensive) to get a Trane tech, or any HVAC tech, out here to track down a problem that may or may not occur while the tech is on site. Thank you for your help.
Submitted: 11 months ago.
Category: HVAC
Expert:  HVAC Guru replied 11 months ago.

No start a cycle and unplug the interface cable to the main board.

Customer: replied 11 months ago.
No idea what you mean with "No start a cycle" and what interface cable to the main board? This system is so old, all we have is a serial comms link to send it the required temp set point. We don't get any info back from it. But not sure what this has to do with my problem.
Expert:  HVAC Guru replied 11 months ago.

Hello and thanks for using this service! If you are a JA Member I would like to personally thank you and welcome you back! I am HVAC Guru an extremely experienced tech on the site and I try to get to as many customers as I can. NOTE it takes time for me to type research and add links so please wait for me to respond back I can only do this so fast. Secondly, I do not know your skill level so please try and let me know if I am going beyond your knowledge or comfort zone thanks! I strive for a 5 star rating from everyone especially our JA members.

That was not meant for you the site messed up and sent you that sorry. Now for your issue I think you may be on the right track, but I would install a new sensor at the water entry and see if that solves it often the sensors give issue for that problem and it may just be a defective sensor on the unit. Let me know what you find out.

Customer: replied 11 months ago.
Thank you, ***** ***** about the temperature sensors and will certainly check them out. I have not seen any spurious changes in temperature readings while monitoring at the chiller. But then the chiller refuses to trip when i stand there watching it. I read somewhere that the evaporator entering and leaving trmperature sensors are a matched pair. Is that true in which case I will replace both sensors. That may take a week or two to order and install so if you don't mind, can we keep this conversation open that long?
Expert:  HVAC Guru replied 11 months ago.

Yes they are a pair same sensors. If you will not get back to me for a couple weeks and need to just rate my service and you will get a link to get directly back to me so the question will not close itsefl out and then you would have to restart.

All I ask is please use the Star Rating Link at the top of your page before leaving, that is the only way I am paid anything through the site and that will also get you a DIRECT LINK emailed to you to get back to me! are greatly appreciated so thank you very much for using this service. If you have any other questions just post back to me here anytime!

Customer: replied 11 months ago.
Will do. I also read somewhere that in HGBP the goal is to control the suction line pressure so that the temperature of that line remains above freezing with a pressure around 58 psi. I'm not sure whether this is for an HVAC system meant for human comfort. Our chiller is not for human comfort. We want the cooled space cooled to whatever temperature we want to condition everything inside at that temperature, often down to freezing in the winter months. So in the current state of the chiller I do see the suction line freezing just before it goes into HGBP. Does that seem normal? It does not freeze up close to the compressors though, maybe up to a foot or two away from the compressors. If normal then I guess in our case the suction line pressure runs lower than 58 psi. We have R22 in our chiller. What pressure should I expect to see in the suction line when in HGBP?
Expert:  HVAC Guru replied 11 months ago.

Did you check the sensors or replace them?

Customer: replied 11 months ago.
Not yet, my day was a bit hectic today so I did not get any time to work on the chiller issue and I was out of office yesterday. The plan is to check the temp sensors tomorrow and also hook up a pressure gauge to see whether that seems reasonable. I'm still trying to figure out what pressure reading to expect hence my question to you yesterday. To replace the temp sensors we need to identify the correct part number and order them. I have one of the technicians looking into that in between many other tasks we have to accomplish. Thanks for your patience and help so far.
Customer: replied 11 months ago.
Any response to the question above; What pressure should I expect to see in the suction line when in HGBP?