Rear AC Evaporator Cools only the 2 left vents

[rassy]

I apologize in advance if this question has been asked...

I have owned this bus for a year, last summer the Rear AC worked great after a charge.

I have a ProAir EZ5 Evaporator in a 2010 Express Eldorado bus. Initially the rear A/C Compressor wouldn't kick on. We ruled out Freon, as we made sure with an AC machine it was at the 6LBS. The compressor will kick on when 12V put to it, so I am working on the back unit at this point.

I jumpered the low pressure switch and it activated the compressor. When I did this it got cold for the first vent and gradually got warmer from left to right.

I replaced the Low pressure switch, and it still didnt fix it. My next step after doing research is to replace the TXV Valve. My question though, is would a faulty TXV valve cause the evaporator to not cool across? Or do you think i am dealing with a blockage in the evaporator?

If you have any other thoughts to my system, please feel free to share. Thank you in advance!

[cadillackid]

yep a faulty TxV will cause that issue.. I replaced one of mine.. ive also seen dirt lodge in the TxV and partially block it.. the condition you'l likely notice is low suction pressure.. most people will then try to charge the freon more which results in an overcharge and the system performance degrades the more freon you put in and the suction pressure wont come up..

[Simplicity]

In trying to edumacate myself, I watched this video a week or so ago. It may be helpful.

https://www.youtube.com/watch?v=oSLOHCOw3yg

I'm not sure if I understand how the valve will make part of the evaporator cold and not the other part, yet I think it's because the valve isn't letting the proper amount of compressed refrigerant through, this only allows a certain amount of cooling to happen, so only part of the evaporator gets cold???

Also, as the video states, if the evaporator doesn't vaporize the condensed refrigerant, and sends a more liquefied refrigerant to the compressor, it can wreck the compressor.

[cadillackid]

correct.. the TxV is a metering device.. it sprays a fine mist of liquid freon int othe evaporator coil.. the pressure in the coil is much lower than the liquid side.. the freon is warm and will be above the temperature that its chemical properties allow it to be a liquid so it evaporates..



think of it as water and steam.. at normal atmospheric pressure and low altitude water turns to steam at 212 degrees.. if you pour the water into a vessel, seal it up and start heating it up above 212 it builds pressure.. if you suddenly hooked a hose up to that water thats now say 220 degrees and you sprayed it into the air its going to evaporate to steam instantly...


every substance has a boiling point and a freeze point.. and a published pressure temperature chart.. shows what its pressure will be in a vessel at a given temperature ..



freon is the same way.. it boils and condenses and if you got it cold enough the freon could freeze.. (not in an A/C though)..



back to the A/C.. ther compressor compresses the freon and it gets hot (ever notice how when you pump up a tire the air hose gets warm?).. the hot freon goes into the condenser.. blowing a fan pver the coil cools the freon.. it cools it enough that it condenses back to a liquid (remember that pressure temperature thing)..



the chemical properties of a substance says that in a liquid state it can hold 'X' amount of "heat" .. as a gas it can hold 'Y' amount of heat.. when you change states from gas to liquid the substance is going to need to get rid of a bunch of heat it can no longer hold.. so it sends it to the surroundings.. which are the pipes of the condenser.. the fan takes the heat away.. the result is a nice warm liquid which travels up to the expansion valve..



the expansion valve is thernal it meters the amnount of freon going into the evaporator.. since it is a gate of sorts. . the pressure at its inlet is much higher than the pressure at its outlet.. it sprays a fine mist of freon into the evaporator.. as mentioned earlier it evaporates into a gas.. remember all that heat I gave away in the condenser as I changed from a gas to a liquid? yeah I need my heat back.. as a gas the freon wants back all of it.. so it gets it from the pipes of the evaporator coil.. making the pipes cold.. the fan blowing over the coil is adding heat to the coil for the freon to pick up... the air is giving up its heat to the coil.. and what you get is the resulting cold air...



inside the TxV is a bulb filled with refrigerant of its own.. as that control bulb's substance heats and cools its pressure varies.. coupled with a spring this causes the valve to open and close and meter the amount of freon going into the evaporator..



the lower the bulb temperature the more tendency to close the valve... based on the bulb pressure.. so lets say that control bulb gets a leak.. and its internal charge begins to lower.. now even as the evaporator temperature rises , the control bulb pressure cant get high enough to properly overcome the spring.. the valve fails to open enough for proper freon flow..



the small amount of freon flow that is allowed will evaporate instantly into the first part of the coil making it cold but since there is a flow deficiency the fan will have warmed up the refrigerant by the time it gets to the end of the coil and you end up with warm air on that part of the system..



a small blockage in the valve will cause the same results.. as will a misadjusted TxV..



the front A/C in my DEV bus has 2 evaporators.. balancing the 2 TxVs was important as you want the same rate of flow on both to maintain a common low-side pressure.. the valves have an adjustment screw where you can adjust the spring tension effectively controlling the flow.. I balanced both of mine so that my pressures matched the PT chart perfectly..



my middle unit failed to balance properly.. I could never get enough flow through that coil.. I was always freezing it up in the first half of the coil while my suction temperature was high enough I shouldve had high pressure.. I ended up calling ProAir and ordering a new TxV for that evaporator and it solved my issue.. after I adjusted it a little further open the freezing issue was solved and the whole coil gets cold.. my thoughts are that my control bulb pressure was low, even with me opening it up lessening the spring tension i still had trouble getting enough flow..



with as proper TxV you should never send liquid down to the compressor.. if the fan turns off bu the compressor stays on a TxV will keep lowering the flow down so any flow still evaporates..



remember the cars with the big silver canisters under the hood? those canisters were accumulators.. there was no TxV.. instead there was just an orifice tube.. essentially a fixed opening for the freon to spray through... if it didnt all evaporate i nthe evaporator it got caught in the silver canister where it could finish evaporating.. the compressor sucked the gas off the top so it didnt return liquid back in..



the design was such that as the freon temperature drops so does the low-side pressure.. the low pressure switch would shut off the compressor when the freon got near 32f.. the flaw of those was that if you got a leak even just a little low and the compressor would cycle out prematurely and then kick right back in.. you'd hear the cars going down the road constantly.. Click--clack...click-clack..



-Christopher

[Simplicity]

Originally Posted by cadillackid

correct.. the TxV is a metering device.. it sprays a fine mist of liquid freon int othe evaporator coil.. the pressure in the coil is much lower than the liquid side.. the freon is warm and will be above the temperature that its chemical properties allow it to be a liquid so it evaporates..



think of it as water and steam.. at normal atmospheric pressure and low altitude water turns to steam at 212 degrees.. if you pour the water into a vessel, seal it up and start heating it up above 212 it builds pressure.. if you suddenly hooked a hose up to that water thats now say 220 degrees and you sprayed it into the air its going to evaporate to steam instantly...


every substance has a boiling point and a freeze point.. and a published pressure temperature chart.. shows what its pressure will be in a vessel at a given temperature ..



freon is the same way.. it boils and condenses and if you got it cold enough the freon could freeze.. (not in an A/C though)..



back to the A/C.. ther compressor compresses the freon and it gets hot (ever notice how when you pump up a tire the air hose gets warm?).. the hot freon goes into the condenser.. blowing a fan pver the coil cools the freon.. it cools it enough that it condenses back to a liquid (remember that pressure temperature thing)..



the chemical properties of a substance says that in a liquid state it can hold 'X' amount of "heat" .. as a gas it can hold 'Y' amount of heat.. when you change states from gas to liquid the substance is going to need to get rid of a bunch of heat it can no longer hold.. so it sends it to the surroundings.. which are the pipes of the condenser.. the fan takes the heat away.. the result is a nice warm liquid which travels up to the expansion valve..



the expansion valve is thernal it meters the amnount of freon going into the evaporator.. since it is a gate of sorts. . the pressure at its inlet is much higher than the pressure at its outlet.. it sprays a fine mist of freon into the evaporator.. as mentioned earlier it evaporates into a gas.. remember all that heat I gave away in the condenser as I changed from a gas to a liquid? yeah I need my heat back.. as a gas the freon wants back all of it.. so it gets it from the pipes of the evaporator coil.. making the pipes cold.. the fan blowing over the coil is adding heat to the coil for the freon to pick up... the air is giving up its heat to the coil.. and what you get is the resulting cold air...



inside the TxV is a bulb filled with refrigerant of its own.. as that control bulb's substance heats and cools its pressure varies.. coupled with a spring this causes the valve to open and close and meter the amount of freon going into the evaporator..



the lower the bulb temperature the more tendency to close the valve... based on the bulb pressure.. so lets say that control bulb gets a leak.. and its internal charge begins to lower.. now even as the evaporator temperature rises , the control bulb pressure cant get high enough to properly overcome the spring.. the valve fails to open enough for proper freon flow..



the small amount of freon flow that is allowed will evaporate instantly into the first part of the coil making it cold but since there is a flow deficiency the fan will have warmed up the refrigerant by the time it gets to the end of the coil and you end up with warm air on that part of the system..



a small blockage in the valve will cause the same results.. as will a misadjusted TxV..



the front A/C in my DEV bus has 2 evaporators.. balancing the 2 TxVs was important as you want the same rate of flow on both to maintain a common low-side pressure.. the valves have an adjustment screw where you can adjust the spring tension effectively controlling the flow.. I balanced both of mine so that my pressures matched the PT chart perfectly..



my middle unit failed to balance properly.. I could never get enough flow through that coil.. I was always freezing it up in the first half of the coil while my suction temperature was high enough I shouldve had high pressure.. I ended up calling ProAir and ordering a new TxV for that evaporator and it solved my issue.. after I adjusted it a little further open the freezing issue was solved and the whole coil gets cold.. my thoughts are that my control bulb pressure was low, even with me opening it up lessening the spring tension i still had trouble getting enough flow..



with as proper TxV you should never send liquid down to the compressor.. if the fan turns off bu the compressor stays on a TxV will keep lowering the flow down so any flow still evaporates..



remember the cars with the big silver canisters under the hood? those canisters were accumulators.. there was no TxV.. instead there was just an orifice tube.. essentially a fixed opening for the freon to spray through... if it didnt all evaporate i nthe evaporator it got caught in the silver canister where it could finish evaporating.. the compressor sucked the gas off the top so it didnt return liquid back in..



the design was such that as the freon temperature drops so does the low-side pressure.. the low pressure switch would shut off the compressor when the freon got near 32f.. the flaw of those was that if you got a leak even just a little low and the compressor would cycle out prematurely and then kick right back in.. you'd hear the cars going down the road constantly.. Click--clack...click-clack..



-Christopher

Yay! Great write up!

I think the thing that was causing me confusion was talking about an evaporator creating cold air.

Once I got that what it was doing is evaporating the refrigerant by taking the heat from the air.

I think the other thing I was not getting is how effectivly the atmospheric pressure and the properties of the chemical (refrigerant) allows for rapid and extreme conversion from liquid to gas and back to liquid.

[cadillackid]

its common for people to get the condenser and evaporator mixed up.. as they hear the water on the ground called condensate so they autmotically thing the cold coil is the condenser

[rassy]

Fellas, Thank you for all of the help and clarification on the subject. The TXV valve replaced fixed the issue. Blowing cold now. Thanks again for the help!

[rossvtaylor]

Originally Posted by rassy

Fellas, Thank you for all of the help and clarification on the subject. The TXV valve replaced fixed the issue. Blowing cold now. Thanks again for the help!

Cadillackid for the win! Nicely diagnosed, sir.