Customer says space is not cold enough.

[Defrost]

I went to do a quarterly maintenance, change filters, clean cond. coils, etc.
When I showed up a woman started complaing her space wasn't cold enough. I checked the thermostat and it read 76f in the space. It also read that the fan was in auto and the system was in auto. I set the desired temp to 70f and went to the air-handler. By the way this is a 3/4 ton split system, air-handler in a closet on fourth floor and cond. coil on roof top. The unit was running when I approached it. The fan any way. I checked the suction and discharge lines by hand, they felt normal. I opened the unit, the evap. coil was clean, all wiring seemed to be in place. I went to the roof top and the compressor and cond. fan were running. So I checked the Delta T at the air handler. I had a return air temp of 72-74f and a supply air temp of 59-63f. But the space wasn't cooling. The temp at the supply grill was 72f. I then cleaned the cond. coil with nu-brite coil cleaner and changed the return air filter with a pleated filter. I checked the space one more time immediately after I did this and still no change. Unfortunately I had to leave but will be returning tomorrow to finish the maintenance. I did not put my gauges on the unit or check for super heat and subcooling, will do tomorrow. Any suggestions?, sorry for the long explanation, thought the more info I gave the less questions I would receive instead of answers.

[51fitter]

I'm assuming you mean 3-4 ton and not a 3/4 ton split, and its big enough for the space. First thing I would do if the airflow is ok is check the charge because I would think the unit would/should pull more then 10 degrees across the coil. Is your stat ok? check that because you can't get 74 return if its 76 at the stat. 72 supply air will never cool anything. You seem to be loosing a lot of temp before it hits the grill. Might have a long run or its in a hot attic area but if you can drop that discharge you can make up for it a little. Check last years check paperwork and see if any temps were recorded or refrigerant/repairs done. good luck :)

[Defrost]

I didn't see any service records hanging around the unit. I'd have to call the office to get the history. The duct work is run above a drop ceiling. It has a main trunk fabricated of sheet metal and flex duct going to the supply grilles. There are two identical units serving one half of the fourth floor. Don't know the exact square footage.

[51fitter]

You have it easy with two units doing the space in question. CHEAT, check the stat and the discharge temp on the other unit...If it's colder then you are on to something. Check the hi and low psi on that unit and compare it.
Also, with any unit in question when doing pm work BUG THE HELL out of your office for background information on the unit. I do :D History is just as important of a tool as anything else you can pull out of your truck.

[Defrost]

Yes normally with two identical units I do compare them to each other. If one's working and ones not. But like I said I had to leave early but will be back tomorrow morning. My only thing with comparing two units to each other is that in the back of my head, I'm always thinking, would if this units got something wrong with it and giving off false readings even though it's cooling the space. Then I'm using bad readings to try and repair a unit thats not cooling.

[Defrost]

I guess I could swap T-Stats and see if that makes a differance. That would help isolate the problem.

[Defrost]

Also, the air handler has a direct drive blower. It's possible it's not wired in high mode. I'm assuming that would effect the temp in the space if the unit isn't pushing the proper air flow to the supply grilles.

[ChilledBeam]

If the supply temp at the AHU is 59-63F and at the diffuser its 72F something is happening in the duct to heat the air that much...

Is there a heating coil in the ductwork that may be on???

[Defrost]

I have not checked for that yet, but it's a good idea. Although I don't recall seeing any wiring for a duct heater at the unit.

[madhat]

Check your flex ducts to see if one blew off, especially if its the top floor, "roof is above that ceiling deck." Also if it's the top floor, did they get a reroof over the winter? Had a building that went from built up with gravel to black membrane, and they took one layer of insulation off and never replaced it.

[toy501]

How many feet rise from air handler to condensing unit. I would check delta t of evap. for efficient heat transfer (possible oil logged evaporator). Superheat & Subcool readings would be very helpful as well.

[Defrost]

I went back this morning. The space was still 76f. I put my gauges on the unit: 90 SP and 190 HP?????????? My compressor was pulling 9 amps when it should have been pulling 17. Delta T was 76 return and 67 supply. The suction line was frozen from the king valve through the accumilator to the compressor, not from the evap coil to the condensing unit. So I checked the king valve. Someone had left it half open. Basicaly turning it into a metering device, which was causing my 90 Sp. So I opened it all the way.....unit is now operating normal.

[Defrost]

Thank you everyone for your input.

[acam027]

The most important thing to do is check your pressures, superheat and sub cooling. if all pressures and temps are good you probably don't have a airflow issue at the evap. so figure out where your air is going or what's stopping it. If your unit is calling for cooling, the compressor is running and you are reading 72 at your supply register I wouldn't waste time by changing the stat.

[acam027]

I guess I was a little late on the advice:D

[Defrost]

No advice is too late. That's just one more thing I will take into consideration the next time I come across a system with a similar problem. Thanks for your time.