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cooling, not heating

Tom S._2
Tom S._2 Member Posts: 11
Okay, this is a general question really, but it happens to fall under cooling and not heating. Seeing how many of you all do both, I am just looking for some advice on how best to proceed.

I've got two estimates for unico a/c systems, both around the same price by what seem to be reputable companies. One specs a 3.5 ton unit, the other a 3 ton unit. Both companies came out and measured all the rooms and the windows, both companies did some sort of calculation to come up with their estimates (company "A" did it in about 30 min in their truck after measuring, company "B" went away and got back to me a few days later). Both companies think their sizing is accurate.

Any advice on how I should proceed?


Bob D.


  • Ken Field
    Ken Field Member Posts: 127
    AC sizing

    They are pretty close. If you were going with a 2 stage unit, I would lean toward the larger one. If you are going single stage, I would lean toward the smaller. Especially if you can modify your lifestyle on those few days a year when the heat gets over the normal high temp. Keep the shades drawn or just be prepared to have the inside temp creep up a few degrees. It's no big deal but it can save you from running an otherewise oversized system for the rest of the summer.


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  • Jack
    Jack Member Posts: 1,045
    Another view

    depending upon where you live. If you are in the northern part of the country what you need more than "cooling" is dehumidification. An oversized AC system will give you a cold, wet climate. A system which is not running is not going to dehu. Not knowing the property I would ask if you have any particular "direct gain" areas such as lots of skylights or late pm sun exposure which would require the larger system.

    I represent Fujitsu in NE and must acknoldge my bias prior to going ahead, but I would strongly suggest you look at mini-split air conditioners before going ahead with the hi velocity. Spend some time at You will find very high seer (cooling efficiency) and eer (heating efficiency) ratings, especially compared to hi v. They are full inverter and therefore total variable speed. For single systems the Seer rating is as high as 21 seer. For the "multi type systems the rating are 15 and 16.5 seer.

    Being heat pumps you can also do a good job with heating. Where I live (MA), I would not suggest year round heating but you could do a fine job in the fall and late winter/spring. If you live further south you could run them year round. This way you do not run your single stage furnace/boiler until it has a load against it and can run efficiently. What I am saying there is that it makes no sense firing a 100,000btu appliance to deliver 10,000btu of heat, which on spring/fall days may be all you need.

    On the multi units there are also concealed air handler units which can be used if you object to the exposed high wall evaporator (indoor unit). And, this system will operate more quietly than a hi V system.
  • I suspect...

    Dealer A is bumping it up a notch to be "safe". I'd go with the 3 ton myself given the dehumidification requirements as stated above
  • Tom S.
    Tom S. Member Posts: 94
    Thanks all

    Thanks for the responses - some things to think about. Not a lot of "big sun" exposure, except the back of the house is south-facing without much shade, but just one picture window and some regular double-hung windows. FYI both bids are single stage units, and one is carrier and the other Lennox (I think), don't know if that matters.

    I will look at the Fujitsu site; I am in Massachusetts.

  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    The most comfortable A/C systems I've felt and designed are slightly (10%-15%) undersized based on Manual-J.

    Unico and other high-velocity mfgrs. specifically recommend such sizing due to the inherently high humidity removal ability of such systems. With our brutally humid summers, high-velocity systems really are capable of producing higher comfort at a higher room air temp.

    The only problem I've experienced with such sizing is if the occupant/customer shuts down the system (or uses far too much thermostat "set-up") on hot days and then expects it to cool down fast when they come home from work. One renter of mine in particular had difficulty with this and was forever complaining of poor comfort and high electric bills. I kept telling him to use far less "set-up" during the day, but he refused to listen. Next renter uses a far more sensible set-up schedule and has higher comfort and lower bills...
  • bruce_21
    bruce_21 Member Posts: 236

    I agree. A lot of contractors seem to think that a little more is always better. With A/C a little less is usually better, because as was pointed out you need to get all the extra moisture out of the air. By having the unit slightly undersized it will run all the time and extract the maximum amount of humidity and give you the greatest comfort.
  • heatguy
    heatguy Member Posts: 102
    a cave

    go smaller you go too big and its like living in a cave cold and damp,your better with the longer cycle to remove humidity
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