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Thread: Coil sizing

  1. #1
    Join Date
    Jan 2005
    Location
    Bakersfield, CA
    Posts
    209
    I am getting bids for a split system for a new house. I spoke to an American Standard contractor (highly regarded) today and asked them for recommendation. I don't have the bid yet but we discussed issues like sizing and SEER and AFUE ratings. House if 3139 sq ft and architect plans provides for a spilt system (4 ton and 2.5 ton). I am located in central California (hot, relatively dry summers and mild winters).

    AC Recommendation

    His preliminary recommendation was to go with a 12 or 14 SEER AC (14 SEER with a 2 speed fan) and use an oversized coil to increase efficiency. I asked if this would create a problem with warranty and he assured me it would not and that using an over sized coil was a common method to increase efficiency. He said this would give me reasonable efficiency without the extra cost of an 18 SEER unit.

    Heating Recommendation

    He recommended using an 80 AFUE system with a variable speed fan (or two stage valve? Sorry, can't remember). He said for your area the cost of a 90+ AFUE system would not be cost effective.

    Sizing

    Contractor said he would do a calculation but said 6.5 tons for our area for the house size sounded about right.

    Any comments from the professional members? Any other questions I should ask.

    Thanks very much.

  2. #2
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,635
    Pay him for a 14 and ask him to put the 12 in right... for a change. Excuse the cynicism. HVAC is done right so infrequently that I can't help myself. Part of the problem is that doing it right takes man hours that customers don't want to pay for. If he's underbidding as so many contractors do, his price for the 14 is probably what he should charge for a 12 done right.

    But you wanted helpful comments? Oh, sorry.

    A 90+ will take 5 to 10 years (or more) for payback. It has its pros and cons. The high efficiency zealots don't appreciate how mild our winters are. And just one extra failure caused by the added complexity of the 90+ will instantly cause their "guaranteed" five year payback to become 10 or more. The real reason to get 90+ is features. If you want the added quietness and safety then it might be worth it.

    Dry climate air conditioning means a BIG evaporator coil and LOTS of airflow. MORE than 400 cubic feet per minute of airflow per ton is a VERY good thing in our climate. Personally I would demand in writing that the AC deliver to the diffusers a minimum of 400 CFM of air.

    Make sure he does a real Manual J and not some goofball short form. The Manual J will determine how much air each room should have. Then ask that the airflow be balanced to a reasonable percentage of the calculations. That means they'll actually have to MEASURE the output once everything is done. You can't believe how often 10 or 20 grand worth of work will get done on a house and the end result isn't even quantified! I bet the contractor runs away screaming like a little girl after you ask for all that.

  3. #3
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    nathan,

    6.5 tons is a lot for a well insulated,3100sq. ft., "new" home.

    You can do the load calc. on this site,see the bullseye above!

  4. #4
    Join Date
    Jan 2005
    Location
    Bakersfield, CA
    Posts
    209
    Thanks for the information. I will try to do the loan calc.
    As to the coil size - please tell me if using a larger coil is a problem. If the consensus is that this is definitely not an appropriate thing to do then I'll most likely disregard this contractor and try to locate one I can trust.
    Thanks very much.

  5. #5
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Originally posted by nathan9999
    Thanks for the information. I will try to do the loan calc.
    As to the coil size - please tell me if using a larger coil is a problem. If the consensus is that this is definitely not an appropriate thing to do then I'll most likely disregard this contractor and try to locate one I can trust.
    Thanks very much.

    Lots of opinions there.

    Dry climate in summer ,not a problem,wet in summer ,some think it is .

    We find(wet climate here),that with a variable speed fan,350 cfm per ton,larger coil is no problem.

    Take a look at Carriers new Infinity control system,lots of comfort benefits.

    Search this site for homeowners comments,"Infinity".

  6. #6
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,613
    A-S lists quite a few different coil possibilities for each outdoor unit. Proper way to select is have an accurate heat gain done and it will estimate latent & sensible loads. A good dealer would choose which coil according to that.

    I agree with the guess that 6.5 tons is a lot. You may get hot but a well built new home will keep that heat out. But only the measurement & calcs will tell you.

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