Leak or Partial Restriction?

[gseaquest]

I have been doing a lot of reading lately about restrictions and ran into system that I think may have a partial restriction. It is a lennox 3 ton packaged unit installed in 2009. Maybe you can learn me somethin. I will get the model number an more specifics later.

OAT 88/IDB 77/IWB 64/High 300/Low 90/Comp Amps 7.75/Evap coil clean/Blower operation normal/Speed High/RAT 77/SAT 62

WHen I opened the blower door I observed frost starting to build up on the tubes feeding the evaporator. After I waited a few the frost proceeded to develop on the evap coil. I thought i might be a partial restriction in the metering device but as a first step I added refrigerant to bring the operating pressures up. The pressures were just below manufactures specs after adding 3lbs 410. I checked the super and sub at the service connections. SH 30 SC 20. Hopefully this is enough for you guys to chew on for a few. I am going to grab a bit a will check back later and can provide more specifics.

[heatingman]

You may have both.

If you added 3 # to a packaged unit, you obviously have a leak.

30 deg. SH seems on the high end, but I don't have my super heat requirement chart in front of me, so it may be okay.

Did the frost go away with added 3 # ?

If moisture made it's way into the system, you may also have a restriction, likely in the liquid drier, if it's doing it's job.

What was the initial SH?

[Damien]

Target SH 12?

[beenthere]

R410A at 300 PSIG is a sat temp of 94°F, so with a 30°F SC, that would make your liquid line 64°F. Which is 24 degrees cooler then your ambient temp. So there is a restriction some where before the area you took the line temp.

[JWB]

Piston or TXV? The frost build up is normal as you have no load on the evap with the blower door open. I assume your measurements were with the door closed.....Apply for pro membership.....

[Glennhvac]

Piston or TXV? The frost build up is normal as you have no load on the evap with the blower door open. I assume your measurements were with the door closed.....Apply for pro membership.....

Yep. See this a lot in the spring when people want start ups too early in the year. No sensible heat to remove and no humidity either.

[beenthere]

At 77IDB with 49%RH, at 88ODB, that is enough load to prevent freeze up.