Return Air Temperature Industry Standards 3

[toolbag]

I have Lennox roof top HVAC units. Most are 10 to 20 years old. They range from 5 ton to 10 ton with economizer inlets.
Are there set standards for the return air temperature coming back into the unit? I have approx 58 to 63 degree's F leaving the different units entering the building.

For some strange reason some of the units freeze up and need to be manually defrosted for about two hours. The refrigerant pressures are in the ball park and the evaporator coils and filters and condenser's are clean.

Any help would be great.
Thanks toolbag

 

[willard3]

I will guess that these machines are taking outside air?

Older controls and equipment do not cope well with high latent loads and freeze up. You could do a hot gas bypass on the evaporator.

 

[IRstuff]

Doesn't that point to a problem with the blower? If the air flow isn't high enough, wouldn't the moisture in the return air will freeze onto the exchanger in the plenum?

Maybe there's been a change in the usage profile as well. A/C salesmen are notorious for selling overcapacity. If the user decides to cut back on usage because of cooling costs, the system might wind up grossly oversized.

TTFN

FAQ731-376

 

[berkshire]

toolbag (Industrial)
Can you drill a hole, or find an existing check hole, and get a pitot /static tube in there, to measure the velocity at the face of the coil? That would confirm or eliminate low velocity as a source of the problem. although it is more likely that you have a leak in a duct.
B.E.

 

[imok2]

We could use more information like what is your standerd room temperature and RH
Are your OSA Dampers working correctly
Is The CFM correct for tonnage and application (static pressure)(velocity pressure)
What are the entering & leaving WB and DB temp.
Is the compressor amperage in the ball park and if you have similer units (tonnage) are amps about the same
When have the evaporators been cleaned? You may have to cut access holes to get a good look; or take a series of static pressure drops across the coil and find out from mfg. what it was originally
It all comes down to a process of elimanation.

 

[simsd]

When did this start to occur? I would agree with IRstuff and a flow issue. Have you upgraded the filters by any chance? If so, that might be creating a problem. Berkshire and imok2 have the right idea in their analysis.

 

[toolbag]

Thanks for the reply: My roof top units do have some intake of FA through the economizer's that are closed during the summer operation. They are not air tight and some air passes by and is mixed with the RA. My return air is about 64 degree's F and the inlet is 58 degree's out of the trunk line. Do these numbers fall within the ball park.
Question: Is there a rule of thumb for RA temperature and actual temperature coming out of the trunk line.
Question: What is the temperature difference in the winter months?
Thanks toolbag

 

[berkshire]

toolbag (Industrial)
Your return air should be coming back to the unit at a higher temperature than 64 degrees. Are you sure that is not an exit temp off the coil?
What is the average room temperature in the buildings the units are serving?
B.E.

 

[krb]

You need to look at the design basis to determine what the actual return air temp should be, but in general it is tha same at the setpoint on the thermostat. I would expect the return to be around 72 deg F, plus or minus a few degrees depending on what system and conditions you have. RUle of thumb would be a 20 deg F temp drop across the coil, so if the return air temp was 72, I would expect to see 52 to 55 leaving the coil. The return air temp you mention seems low. I would follow the advice given above to get an idea of the flow, and verifiy you are taking readings in the correct place. It from the data you have provided, it seems you may have more than one problem.