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Can I use a 2.5 ton coil with a 1.5 ton condenser?

My client has a new, barely month old 1.5 ton R-22 condenser I salvaged for him from a remodel of his house. He has another house on the property with an oil furnace with an old, never used 2.5 ton probably 8 or 10 Seer coil perched on top of it. I told him this condenser was undersized for the situation but he wants go ahead and use the 1.5 ton condenser anyway. From the age of the coil I'm certain it has a piston type metering device. Will it work without getting a smaller piston to properly control the freon flow? Any other problems possible here? Thanks in advance for your help and advice.


  • Techman
    Techman Member Posts: 2,144
    Make funeral arrangements

    Assuming the oil furnace/cfm/air flow/btu is proper(2.5t), then the undersized comp(1.5t) will never pull down and will run and die, shortly.
  • Spence
    Spence Member Posts: 316
    edited August 2014
    Oversized ID Coil

    The answer lies in the AHRI directory. If your chosen combination has been tested and is assigned a SEER and an EER value, then you know you have an approved combination.

    In many cases, over sizing the evaporator will raise the efficiency of the unit, making this a good choice. However, it can bite you, too. If you do not have an approved match you can end up with humidity issues. With older equipment from the same maker that is not on the directory, call them for guidance. If the OD and ID units are different makes, don't do it, as you cannot guarantee performance.
  • bruce_21
    bruce_21 Member Posts: 237
    What happens

    if I adapt the furnace plenum and use the 1.5 ton coil that came with the condenser?

    Won't cool the space completely so if occupant turns the stat down it will run forever. Aren't these things designed to run all summer? Or only cycle on and off, on and off, etc. I'd thought that continuous operation was better for equipment with less stop/start wear and tear?
  • Spence
    Spence Member Posts: 316
    AC at 1.5-tons

    If you're going to use the matched coil and condenser then you have solved a lot of operational issues regarding the refrigerant circuit. However, that doesn't address the capacity of the unit and it's ability to satisfy the thermostat. Installing a unit without a load and distribution system calculation is a huge gamble.
  • bruce_21
    bruce_21 Member Posts: 237
    We know it is way undersized

    The load for the house is 26,000 btu and the original installer put a 2.5 ton coil in the furnace plenum. Will this just not keep up with the load and have a shorter than expected life due to greater operating time or will some other (worse) scenario unfold?

    The client just wants to get some use out of this equipment that didn't fit the other space.
  • Techman
    Techman Member Posts: 2,144
    edited August 2014
    2.5 vs 1.5

    Have you ever started up an orifice type system on a hot day and watched the lo & hi side pressurers, for a couple of hours! Well, on your 2.5t load/cfm system ,that 1.5t comp will NEVER pull down to temp. The humidity level  will stay high, the amp draw will stay hi, the comp will run 24/7 and the comfort level will be hi on the "I am uncomfortable" list' . On a "proper everything" system , the comp will run all the time when outdoor temps are in the 90* or higher range ,on  this set up you might be comfy on a 76-78* day. Now, there is nothing wrong doing the job because the customer "insisted" on it. CYA first , then take notes of all temps/press/humidity/amps/ and see how one of these systems run and run and run. Then you can recognize the symptom's again, down the pike!

    You have a 2.5t space/load/indoor fan/cfm/ductwork with 1.5t of work. That's 18,000 against 30,000.
  • Spence
    Spence Member Posts: 316

    We really don't know how short you are, since we only have your total load, and not a breakdown of the sensible and latent loads. Without these numbers and your weather station we can only guess. What probably will occur is adequate dehumidification on part load days due to the long run time. Yet you will have a hot house potentially; we can plot the expected performance on a simple graph for you so you will know exactly what to expect.
  • Ironman
    Ironman Member Posts: 7,086
    Penny Wise and Pound Foolish

    Without being technical for a moment, let's look at the situation rationally. You client wants to use a piece of equipment with an installed system that it's not matched with and is insufficient to carry the load because he already has this unit. Is that reasonable and rational? I think the answer is obvious.

    I have a Ram dually and my wife has a Honda. If the dually had a flat tire, would it be reasonable to try and adapt the spare tire of the Honda to the dually because I had it and didn't want to spend the money for the correct tire?

    There are several good technical reasons that have been posted which tell you this is a situation to avoid. Let me add two more:

    1. What refrigerant is the coil rated for? What refrigerant is the condensing unit? If it's an 8 - 10 seer coil, then it's R22. Is the C/U a dry R22 unit? If not, then you need go no further; they don't match.

    2. If the coil is an ARI match, then the piston should be changed to match the O.D. unit. But you'll only have a 1.5 ton system where a 2.5 ton is needed to meet the load.

    You will do yourself, your customer and the trade a great favor by putting a correctly matched system in rather than trying to cob something together because he THINKS what is proposed will save him $$.

    The laws of physics will out weigh the laws of economics - every time.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • unclejohn
    unclejohn Member Posts: 1,778
    The problem here is the assumed load needed to cool the space. Not the small mismatch in coil size. Just be sure to tell the guy it's his money and your not going to warranty any thing. I would leave the piston that came with the coil in, the more refig into the coil the better. I can guaranty you it will work, weather it will work well....
  • Bob Bona_4
    Bob Bona_4 Member Posts: 2,083
    Yikes. 1/2 ton over on the evap is realistic. A ton over? I would walk away.