Strange Problem Please Help

[daveo661]

Large Package ducted unit evap coil 1 is block of ice... iceing up after tx vlave all the way back to reversing valve.... compressor suction line is sweating.

GAS 407C

suction pressure 30psi
discharge 230psi
adjusted superheat to flood evap no response
defrosted by hard wiring reversing valve
run on cooling again
suction pressure now 70psi
discharge 300psi
Tried adjusting superheat to normal setting but no matter which way i move the valve there is no change.. suction temp is 20 degrees C

[daveo661]

HELP HELP gota fix this tomorroww

[Sandro Baptista]

compressor suction line is sweating.

Tried adjusting superheat to normal setting but no matter which way i move the valve there is no change.. suction temp is 20 degrees C

Is the suction line on a hot and humid ambient air?


Well the first thing that occur me is lack of refrigerant charge.

OR maybe one of the R407C components are gone due to leakages.

[Brian_UK]

Expansion valve shot?

[dougheret0]

Of course you checked air flow across the evaporator - should never be less than 300 cfm/ton.

[daveo661]

it is in queensland very humid environment. im worried about the high discharge pressure so im pretty sure its not short of gas. im thinking Non Condensables, Loss of Charge in Tx bulb, etc but why am i still getting normal compressor current draw...

if the tx is stuck open and evap is getting flooded or there is non condensables in the system would i have high compressor current draw????

what would the suction and discharge pressures look like??? what should the be???

[paul_h]

I have no idea about pressures in PSI, and have no idea why some australians still use it :D
But airflow through the evap is the first thought.
2nd is something wrong with refrigerant, really want to know the systems history before I made that call though. Like if it ever worked, or someone stuffed with it recently. A system a few years old with no previous problems until now and no ones touched it since install, I wouldn't suspect non condensibles in the system for example.
3rd thought is restriction in the suction line somewhere, at the liquid line restriction (TX, capillary etc) or even later on.

[Gary]

Unless/until you have good airflow through both coils, the pressures are meaningless. Insufficient airflow through either coil can drastically change the pressures and alter the behavior of the TXV.

ALWAYS check the airflow first... and adjusting the TXV is the very LAST thing to do, only if absolutely necessary.

[Sandro Baptista]

it is in queensland very humid environment. im worried about the high discharge pressure so im pretty sure its not short of gas. im thinking Non Condensables, Loss of Charge in Tx bulb, etc but why am i still getting normal compressor current draw...

if the tx is stuck open and evap is getting flooded or there is non condensables in the system would i have high compressor current draw????

what would the suction and discharge pressures look like??? what should the be???

Dave,

Why do you say that the compressor is suction liquid from the evaporator?

Non condensables gases promote a higher discharge pressure albeit the partial pressure from the R407C is lower. Higher discharge pressures make the compressor spend more shaft power.

[daveo661]

This is a brand new unit seems to be cooling fine now but strange pressures


anyone know the normal running pressures for r407c in a roof top package unit on a 28 degree day

[Gary]

Brand new doesn't mean it's right, running fine is a vague description and strange pressures doesn't tell us what the pressures are.

I would also like to know the supply air and return air temperatures (measured close to the coil).

[Gideon Beddows]

This is a brand new unit seems to be cooling fine now but strange pressures


anyone know the normal running pressures for r407c in a roof top package unit on a 28 degree day


Assuming an air on of 28, Air off of 16 the Suction pressure should be about 76 psi at a temp of 10 C (6K superheat) - Head pressure of 265 liquid temp of 35 will be 9 K subcooling which is about right, i recon?

Basically check how much subcooling you have? 7-10K is about the right charge, i'd say.
Airflow may well be the key though? Check that no fire - damper has closed? Or some **** has shut a duct because of the draughts!!

[dave hammo]

im with gary on this one i would only be fiddlin around with the tx valve after checking all other possibilities if new install maybe other tradesmen may have crushed ducts in ceiling space(damn electricians) or restriction in liquidline by the way paul h some of us old timers like to reminece about the good old psi days

[paul_h]

by the way paul h some of us old timers like to reminece about the good old psi days
I wouldn't tell an old timer to stop using PSI, just suprised at the younger people I speak to all the time that use PSI.

[DTLarca]

Large Package ducted unit evap coil 1 is block of ice... iceing up after tx vlave all the way back to reversing valve.... compressor suction line is sweating.





GAS 407C

suction pressure 30psi
discharge 230psi
adjusted superheat to flood evap no response
defrosted by hard wiring reversing valve
run on cooling again
suction pressure now 70psi
discharge 300psi
Tried adjusting superheat to normal setting but no matter which way i move the valve there is no change.. suction temp is 20 degrees C

I don't work with pressures - Only temperatures. So I cannot help on the operating parameters, however...

A TEV services one purpose and one purpose only - it controls the evaporator superheat.

A TEV installed and set correctly - as in installed correctly out of the factory box - does not cause frosting all the way through the evaporator or even onto the 4-way valve unless there is a low load condition.

A TEV closed down too much, as in with its static superheat increased too much, can cause the early parts of the evaporator to frost but then it will not also cause the later parts to frost.

If the superheat is right, or just a little low, then the expansion valve is doing its job - the problem is somewhere else.

99.9% of the time when a TEV on an AC system frosts all the way to the compressor it is on account of a poor load condition. Even uncontrolled head pressures in cold ambient temperatures will only cause the early sections of the evaporator to frost.

Unless, as Brian implies, the TEV is now jammed and acting as a fixed orifice device.