Instant high head pressure.

[kman60130]

One of my tech's was on a call for a rooftop unit last night, actually it was a split system with a TXV,he told me that as soon as he fired up the compressor the head pressure would sky rocket to 450 psig and trip out the hps. I told him to close off the liquid line service valve and try to see if the unit would pump down, but it would not, it would run the head pressure up but not pull the low side down. any help here would be appreciated.

Keith

[Dowadudda]

Service valve stopper broke off stem and is blocking passage of discharge gas. Just seen something almost identical a few weeks ago. Or somehwere before condenser you got a total blockage.

[hvac-tech-lane]

I saw one like this once, it was so overcharged the condenser was full (yes full) of liquid the system had a plugged screen before the TXV and the tech kept charging it trying to get the suction up until it tripped. Try purging your hoses or dump some liquid from the liquid line service port into a recovery cylinder and see where you are.

Dow says "Service valve stopper broke off stem and is blocking passage of discharge gas. Just seen something almost identical a few weeks ago. Or somewhere before condenser you got a total blockage. "

If this is a split I would assume it has a can and wouldn't have a service valve at the compressor, and the access port would be at the liquid line service port, but if it does and the service port is at the compressor discharge you could have a blockage between the compressor discharge and the condenser

[will 2]

Sent out for a 'no cool' on a sandwich van that had same symptoms.
Thought I had some kind of restriction before condenser somewhere.

Not even close.

Another tech was sent to this job the day before but SM didn't hear back from him and figured he ran out of time and never made it.
Turns out, he did.

He found system empty, 200# nitrogen, leak check, Nextel goes off...

[Green Mountain]

One other thing not mentioned. If it is a split system make sure that there is not a check valve installed in the discharge line. Carrier is good for this. A check valve in the discharge line prevents liquid migration back to the head of the compressor. If this is not working you could get the systoms as posted.

[kman60130]

Thanks for the replies, It turned out to be a plugged liquid line filter drier between the final run of the condenser coil and the L.L. service valve, better trust my instincts next time!

[hvac-tech-lane]

How long and what size of line set did this system have?