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John Goodin
09-01-2020, 8:50 PM
It would take a long time to explain so I will skip the background story. Basically, HVAC repair company recommends replacing two capacitors, one inside, one outside, and the plenum. They did not say anything about the 1980s gray ductwork in the attic. The Ingersoll Rand unit -- condenser/compressor, evaporator and furnace -- were manufactured in 2013. It is housed in a closet with the plenum mainly in the attic.

The questions…
I was always under the impression that capacitors either work or don’t work, like an incandescent light bulb. The AC cools although the temperature differential is a little low. Can capacitors need replacement if the AC works? Is there a reason to replace a working capacitor?

There were air leaks at the joint between the evaporator and plenum but the duct board plenum appeared to be satisfactory. Is there a reason to replace a plenum that was structurally sound and square? It may not have been an exact fit to the replacement HVAC unit but not off by much.

Finally, the duct system is the old early 1980s gray flex duct. Should/Must this be removed and replaced or is it probably to still be fairly leak free? A limited inspection did not reveal obvious issues. I know the only way to tell is by testing it but was surprised the HVAC company didn’t say anything about it.

This is not my house and know nothing about the HVAC company and do not access to the house anymore. Any advice would be appreciated.

Stan Coryell
09-01-2020, 10:06 PM
Capacitors can degrade. Highly unlikely that yours are causing issues. Easy enough to test.

If the plenum leaks, do you're best, and caulk the rest. Or, use foil tape. Some codes require foil tape to be fire rated.

If there is any duct board in the house, I would get rid of it. It's nasty stuff when it breaks down. It sounds like the plenum is duct board.

As far as leak testing, the flex will fail. So will the air handler and coil. We only leak test duct.

Mike Soaper
09-01-2020, 10:33 PM
Electrolytic capacitors can also start to leak or swell.

I'm not a hvac tech but I suspect the inside cap is for the blower motor. The outside cap could be for the ac/heat pump compressor run circuit, maybe the compressor start circuit , or the fan run circuit/motor.

Sometimes there are 2 capacitors in one can i.e. compressor run and fan.

Bruce Wrenn
09-02-2020, 8:39 AM
The newer duct board is 1.5" thick, instead of 1.0" thick. Also the insulation value on the flex duct has been increased. FYI, our house, built in 1980, has duct board in it. It is totally contained within the heated/cooled envelope. Replacing capacitors is a HIGH PROFIT item. Caps fifteen bucks each, plus a $100 service call. Sell them for $50 each, plus service call and suddenly you have a $200 bill. You could replace the caps yourself and seal the plenum. As a side note, our neighbor taught HVAC at local community college for several years. Later went to work doing service work for a major nationally owned company. After about eight months was let go. Reason, NOT SELLING ENOUGH!

Mike Soaper
09-02-2020, 3:43 PM
About 3 yrs ago the run, start and fan caps (all about 18yrs old, 1999 ish ) were replaced on my outside AC condensor when the fan cap went bad (that "can"ister also contained the run cap). The replacement caps were not factory replacements and were about $10-20? ea from a local hvac supply house.

The next year that new run cap failed, i went looking online for info on cap quality.

I found comments that many caps arn't made as good as they used to be (after 2000?), some gave the life span of inexpensive caps to be 2-3 years, better caps 5-7.

I replaced the failed run cap with one from AmRad, made in USA, about $30-35 on line, if I recall correctly. It had the same farad and voltage ratings, but larger in diam that the failed less expensive replacement run cap.

This year the sytem was intermttently starting and the 3 yr old start cap was leaking a small bit. I replaced that start cap with a BMI-HD cap, made in USA, about $35 on line. The 1st replacement cap was smaller in dia than the orginal, the bmi-hd cap the same dia as the original. In fairness to the cap I later found a connector looser than I like that may have been the root cause of the intermittent starts.

So, the question is why do they want to replace the caps, did the caps test bad?, leaking?, preventative maintenance? boat payment due?


FWIW in general I'm now inclined first consider useing OEM caps, Amrad, BMI-HD or Gentaq (previously GE) before low cost caps . YMMV

Bruce Wrenn
09-02-2020, 8:37 PM
I use Titian brand caps, as that's what local supply house stocks.

Mike Soaper
09-03-2020, 6:43 PM
Thanks, I had forgotten that I seen some good reviews of titan hd caps.