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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8768
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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I have a business in a small bay with a very old AC unit. I

Customer Question

I have a business in a small bay with a very old AC unit. I don't know the size of the unit with it being on the roof. I can find out if needed. We have on duct running from back to front of bay. There are four 6"x20" registers coming directly off of that duct blowing horizontally from the drop ceiling. Two of those ducts are hidden in small "rooms". The cold/hot air can't get out into the bay to cool it enough. The bay is 20' x 60' but we mostly have walls that don't go to the ceiling except for one of those small rooms that the vent is hidden in. It's hot and humid here now and I'm looking for a quick way to help out in getting it cooler temporarily until we can have the entire system design looked at. I want to tap into that duct and shut off one of those registers. I would run a flex tube about 8-10 feet and attach it to a 2'x2' ceiling diffuser. I'm not sure what size tubing to use. I do know that the AC is probably at least 25 yrs old or older. it was pushing out 56-61 degrees air throughout the day and we had the Freon checked and it is good.
JA: Did you double-check the thermostat's settings? And what about the filters?
Customer: One other thing, the only return is at the back of the office at the unit where the filter is located. Filter is good.
JA: Do you plan on doing the work yourself?
Customer: yes
JA: Anything else we should know to help you best?
Customer: We are a bay which has front door opening directly to outside elements.
Submitted: 3 months ago.
Category: HVAC
Expert:  Phil replied 3 months ago.

Hello, the odds are in the 100% range that the condenser coil (radiator) at the unit on the roof is partially clogged with dirt deep between the fins where it cannot be seen. Cleaning the condenser is a must. If the unit is in workable condition otherwise that will increase the cooling capacity by 30% or more.

Click here for a video.

You can buy the condenser coil cleaning kit at most larger hardware or home fix up type stores.


I am a retired mechanical engineer and HVAC/ electrical/ refrigeration and boiler contractor ... with 56 years in this business. The last 20 of those years consulting for major firms nationally..


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Customer: replied 3 months ago.
We did have a maintenance crew come out and check it out to make sure it was charged and not froze up. I can't say about the coil because I didn't actually see it. I guess I could get on the roof and check it myself. I have been in this bay for 10-12 years and we always have this problem. I do know it was fully inspected last year but I don't think this year.
Expert:  Phil replied 3 months ago.

Hello, these sorts of inspections are normally done in the off season by an apprentice in order keep the crew busy.. the odds that you got any sort of valid service are less than 50%..

The dirt in the condenser coils is deep between the fins where it cannot be seen, after 25 years in needs to be cleaned in the worst way.

Customer: replied 3 months ago.
ok, maybe it will be best to have someone out to clean it and see how it helps. If it doesn't help and I want to move that vent from hidden in a small room to out in the open office, what size of flex tubing would you recommend? 6" or 8"?
Expert:  Phil replied 3 months ago.

8 inch

If they send an apprentice, he will have a compressed air tank with him, that will clean about 30% of the dirt from the coil. He will need to pressure wash it with a hose from both sides to get it 80 to 90% clean... you should go up there to see both sides of the coil and supervise if you want the job done correctly. Its not at all an easy job


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Customer: replied 3 months ago.
I have a pressure washer and an air tank. Maybe just do it myself. I will do it Sunday. I'll let you know the results. Would I need to take the top off to clean from inside out?
Expert:  Phil replied 3 months ago.

Yes, take the top off, and buy an AC coil cleaning kit at the hardware store, you will need it.

Customer: replied 3 months ago.
I'm looking at Lowes, Menards and Home Depot. I don't find an AC coil cleaning kit. What does it include? maybe I have stuff around here already.
Expert:  Phil replied 3 months ago.
Expert:  Phil replied 3 months ago.

the kit comes with a very special brush that you will need also

Click here

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