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Fedders 3 ton unit...Low and High pressure valves are not standard
Hi Folks,
We have a Fedders unit model CA-36A7 3 ton 36k btu 3 hp unit for almost 50 years and the unit was probably made in the 60's. The unit is a stand alone unit encompassing the compressor, condenser, evaporator, etc. The powerful squirrel fans are controlled by one 4 - 5 amp motor that circulates air internally and cools the compressor/condensor thru 16" intake/exhaust ducts. Aside from cleaning the evaporator fins, changing the filter, and oiling the fan motor, the unit never failed until now. When starting it up this year, it ran for a few minutes or less and then the low pressure switch would kick in and shut the compressor. When the contacts are manually activated for a short time for testing, the compressor runs and low and high pressure get cold and hot.
We spoke to few commercial/residential HVAC companies about looking to have it tested, but the valves are not standard. There are no other valves. If you zoom in on the attached pics, you will notice that these valves have an inner female thread which is a 1/4" - 28 fine thread. The outer thread had the plastic dust cap on it. There is no schrader valve, just a pin that is flush with the bottom of the valve.
We have had the unit checked out around 15 years ago, so I have to believe there is some kind of adapter out there.
Any insight would be greatly appreciated.
Thanks Alan
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They were standard back then. Somewhere I still have the tool to gain access.
You must love the electric usage on that hog!
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Possible the Low Pressure control lost its calibration if your positive it was the LPC. You mention cleaning the Evaporator coil, but what about the OD coil?
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Hi Joey,
It was the Low Pressure Switch that kicked in. Bypassing the LPC allows the unit to run. The condenser coil...OD coil??...is clean. The evaporator coil was pretty much clogged up this time around. I'll give it a careful cleaning and test it again. Could a clogged evap coil cause the LPC to kick on in as little as 10 seconds and other times over two to four minute or is it more likely to be something else such as a calibration problem with the LPC?
Thanks
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Hi pecmsg,
Good to know you have the adapter.
Yes, it's a hog, but it's been good to us....:-)
Thanks Alan
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Originally Posted by
Alan55
Hi Joey,
It was the Low Pressure Switch that kicked in. Bypassing the LPC allows the unit to run. The condenser coil...OD coil??...is clean. The evaporator coil was pretty much clogged up this time around. I'll give it a careful cleaning and test it again. Could a clogged evap coil cause the LPC to kick on in as little as 10 seconds and other times over two to four minute or is it more likely to be something else such as a calibration problem with the LPC?
Thanks
Clean the inlet side of the coil to see what happens first, could be due to vibration also when the unit starts and not necessarily calibration. Who knows, it’s nearly 50 years old, bypassing the LPC is not the worst thing in life at that age, many OD units nowadays don’t even have any safety pressure switches from the factory. The question is, bypassing the LPC does the system cool, or just lets the system run? Doesn’t those access taps have a brass screw on cap? If so it would be prudent to reinstall them. Is there a gasket inside the cap for a seal?
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The make line taps that would allow you to hook gauges up.
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Originally Posted by
Bazooka Joey
Clean the inlet side of the coil to see what happens first, could be due to vibration also when the unit starts and not necessarily calibration. Who knows, it’s nearly 50 years old, bypassing the LPC is not the worst thing in life at that age, many OD units nowadays don’t even have any safety pressure switches from the factory. The question is, bypassing the LPC does the system cool, or just lets the system run? Doesn’t those access taps have a brass screw on cap? If so it would be prudent to reinstall them. Is there a gasket inside the cap for a seal?
Hi Joey,
I will clean/test the unit this weekend. Surprisingly, the unit came with dust caps made out of plastic that are not rigid enough to securely tighten with a gasket. I should be able to locate brass caps/gasket.
Thanks
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Do Not Run with the Bypass in place.
A good tech can install access ports to recover the charge and then install permanent access.
Is it worth it, doubtful!
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Cleaning the evap coil had no effect as far at the LPC is concerned. I did not run the unit for more than a couple of minutes with the bypass of the LPC in place. The low and high pressure lines were getting cold and hot.
Pecmsg: It would be greatly appreciated if you could upload pics of the adapter tool to access these valves? Do you know the name/manufacturer for the adapter tool? As they were standard back then, when did they change the design?
Thanks
Alan
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Originally Posted by
Alan55
Cleaning the evap coil had no effect as far at the LPC is concerned. I did not run the unit for more than a couple of minutes with the bypass of the LPC in place. The low and high pressure lines were getting cold and hot.
Pecmsg: It would be greatly appreciated if you could upload pics of the adapter tool to access these valves? Do you know the name/manufacturer for the adapter tool? As they were standard back then, when did they change the design?
Thanks
Alan
Haven't seen it for years.
There are other ways of gaining access to that system.
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Treat this as you would a sealed system refrigeration system. Or use pinch off tool at LPC copper to isolarte and install stinger 1/4" tap.
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A tech could put one of those C&D piercing saddle valves in that you braze around the exterior or use a pinch off tool and braze a valve in somewhere. Then you can hook up standard gauges.
They just don’t make them like that anymore
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Hi Joe,
We haven't used the system for a couple of seasons...sorry to bring you to the past. There are no valves on the unit and those valves in the pics were the original service production valves that were sealed permanently with pins. Condensing/Evap coils are all clean. I thought before having a tech install valves, I would try to bypass the low pressure switch and measure the temperatures and maybe that will give a clue if the system is running fine. Results: The temp outside was 60 deg, indoors was 65. After a five minutes the High pressure was 150 deg, Low pressure 97 deg, air passing the evap coils 37 deg. The High pressure slowly rose to around 165 for the next 5 minutes, then started to more quickly rise to 177 while the low pressure started to drop to around 80. At that point around 11 minutes passed and I shut off the system. It had not yet reached steady state.
Any input would be greatly appreciated.
Thanks,
Alan
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Young Techs. Don’t know this but,..if your actually going to install valves ( both high and low ) Temperature pressure chart R22 ( you can download ) shows at 60 degree outdoor temperature you add 30 degree to that and you come out to 90 degree, then read the pressure it says 168 PSI head pressure so your pretty close. Thinking I remember being told decades ago it’s ( head pressure ) around 5 PSI plus or minus of those figures. That was one way how they read those pressure in that time frame. Typically at that low a load condition your low side probably be in the 50’s PSI range. You get the load higher as described your looking at 65+ PSI suction, after system stabilizes. You should feel a good amount of heat rejection coming from the outdoor coil.
https://www.google.com/url?sa=i&url=...AAAAAdAAAAABAD
Your operating it in a low load condition, your taking readings at to cool a outdoor and indoor temperature, better if it was say 75-80 degree indoors and outdoors. Then you let system run 15 minutes to stabilize before taking readings. The discharge air temperature is somewhere around 55 degree at those parameters. Give or take. If your getting 37 guessing your system is doing OK. Recheck again at higher temperatures.
Last edited by Bazooka Joe; 04-12-2021 at 08:23 AM.
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Sorry I misread your post, “ high pressure was 150 degree “ then slowly rising to 165. I was reading it your readings where in pressures,...forgive me for the confusion,...I didn’t even have my 1st cup of coffee when I responded I tried to delete most of all of what I wrote but the time ran out. I was going to also say some young Techs. dont know to use the 30 degree above outdoor temperature how to get head pressures on those old systems using the TP chart, but alias I ran out of time to update...
Just concentrate on the parameters mentioned and if you get around 55 degree discharge air temperature your pretty good.
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Hi Joe,
I will definitely test it at higher temperatures after it stabilizes.
While I will test the temps of the system at different locations, I would like to try to get the saturated temps in the evaporator/condenser. I'm thinking of attaching the thermocouples at the evaporator and condenser coils covered with tubing insulation. I am guessing based on the 30 degrees added to outside temp should give me some idea roughly of what the condensation saturation temp should be.
I will also measure discharge temp after the compressor to make sure the temp does not get too high.
Thanks...greatly appreciate it.
Alan
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Why after 50 years your looking at all those temperature reading,.... the 30 degree is added to the outdoor air temperature then converted to pressures. That was a common practice of how to check the head pressures back in the day....Thinking getting around a 55 degree discharge air temperature and depending if you live in a humid area, a beer can cold sweating suction line right to the compressor will suffice lol. Thinking shoot for at least 75-80 degree outdoor and the same indoor temperatures when taking readings...
Last edited by Bazooka Joe; 04-14-2021 at 09:00 AM.
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You gots a picture of the whole thing?
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Over 50 years old...they don't make em like that anymore.
Thanks
Alan