Replacing outdoor unit of home HVAC system with a used unit

June 8, 2016 2:15 PM

The outside condenser unit for my AC, which provides AC to the second floor of my house, has a bad condenser/motor (it's seized). I've had two techs look at it, and both recommend replacing both the outside unit and the air handler in the attic, since the condenser is old (20 years) and runs on R22. Can I just replace the outside unit with a used R22 condenser without worrying about the air handler?

I've found contradictory information about this on the Internet, so curious if anybody here has any experience with this. I'd like to replace the whole system, but wondering if a used unit might be a reasonable, more affordable stop gap. I would be replacing the outside unit with a different brand (looking at a used Lenox R22 unit), but keeping the air handler. Further, is this something I can reasonably do myself? I've done some HVAC repair in the past, so reasonably handy, but wondering if this is more than I can handle. Is it simply a matter of hooking it up?

posted by drobot to Home & Garden (5 answers total)

I don't have the answer, but in researching a similar concept, learned about potential issues with the control of the condenser unit. In my case I was looking into ductless mini-split units, and wondering if I could use one brand of outdoor condenser unit and a second brand for the inside unit. I remember finding a few pieces of information that suggested the control systems were proprietary by brand. I didn't verify if this was true, and I'm not sure if it's analogous to your whole-house system, but might be a good thread to pull before you buy something.
posted by reeddavid at 2:45 PM on June 8, 2016

20-year old AC units are often terribly inefficient compared to new models, so while replacing with an old unit may save money in the short term, it could cost much more in the long term. Be sure to consider the energy costs/savings in your decision.
posted by soylent00FF00 at 5:12 PM on June 8, 2016

Best answer: If you can find a used condensing unit of the same capacity then yes you can just change out the condensing unit. (In fact if your system wasn't obsolete you could just buy a new condensing unit of the correct size). The controls are simple and mostly universal. At worse you would have to buy a contactor.

drobot: "Further, is this something I can reasonably do myself? I've done some HVAC repair in the past, so reasonably handy, but wondering if this is more than I can handle. Is it simply a matter of hooking it up?"

You need several thousand dollars in specialty tools to do this correctly:

• A recovery unit to reclaim and store your existing R22 before cutting the refrigerant lines (seriously illegal to vent to atmosphere).
• A torch capable of silver soldering and the skills to make a 300psi pressure tight joint.
• A manifold set to charge the system properly after the system is sealed.
• A vacuum pump capable of pulling 29in/Hg to evacuate the system before charging.
• A high pressure nitrogen tank and regulator to purge and pressure test the sealed system after evacuating.

You might also need the ability to buy R22 if your replacement condensing unit happens to use more refrigerant. In my jurisdiction you need a specific license and training to do so.

Also you need the skills to wire a 240V 30-50A circuit and the ability to figure out if your current breaker and wire combination is sized correctly (sizing is specific to hermetic refrigeration systems and can be sort of arcane if not specified by the condensing unit manufacturer). In my jurisdiction you'd technically need to pull an electrical permit.

In Canada R22 equipment hasn't been made since 2009. Honestly unless you have some reliable indication that the used unit has very low hours I wouldn't buy an at least 7 year old used condensing unit. It's past it's expected lifetime and could fail at any time. I'd also give your evaporator unit coils a good inspection to verify they are in good shape; 20 years of use often results in the fins rotting away at least partially. If you decide to go ahead make sure you install a good size filter/dryer.
posted by Mitheral at 5:17 PM on June 8, 2016 [4 favorites]

Response by poster: Thanks everybody! Mitheral - definitely (way) out of my range of skills (and tools)!
posted by drobot at 8:21 AM on June 9, 2016

I agree with Mitheral: you can't do this, and you probably shouldn't try.

My house has two of these pairs, and the air-handler for the second floor is finally d-e-a-d. We've had a pro come out and re-charge it several years in a row, but now we need a whole new system (chiller and air-handler) because of the refrigerant change. It's going to cost like sin, be we are hopeful that 2016 technology is more energy-efficient than 1994-era tech. *gulp*

Interestingly, he suggested that we drag the chiller out behind our shed in case the remaining one dies in the next few years, because we could get someone else to come cannibalize maybe that one part -- but only because we know where it's coming from (i.e., our own house).

(And there was a news story making the rounds last week warning against itinerant a/c techs who will mis-handle R22 -- or a substitute -- and blow up your house. Possibly made-up? Maybe. Scary? Youbetcha!)
posted by wenestvedt at 8:30 AM on June 9, 2016