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  1. #1
    Hello,
    I just want to say that this site is fantastic!

    OK, on to the question....

    I have a 30 year old single Carrier 3 ton split A/C system. I live in a 2000 sqft., 3 level town home of the same age. The AH is located on the middle level. The duct work is all hard line. The first level is just an entrance with stairs and an exit to the garage, but for informative purposes I will count it as a level. It does have a vent going to it.

    I had an estimate from an HVAC tech that came highly recommended from numerous people. When I asked him about doing a Manual J, he said I didn't need it. He said to just replace the old unit with a new unit of the same tonnage. (Is that a word?) He want's to replace the AH too, which is fine.

    Now I've done some fairly extensive changes to my town home. I've knocked out walls, replaced appliances and done some electrical and plumbing. One thing I've learned in doing this is that the hippie contractor that built my place did it as cheap as possible and all appliances and most of the workmanship were bottom of the line CHEAP! So I'm assuming that his choice in A/C systems was inspired by the same line of thought. I know my A/C unit is inefficient. But I also theorize that the A/C unit is under tonned because it was cheaper. My place is never comfortable in the extremes of the seasons.

    I suggested again to the contractor that maybe we should do a Manual J to see if my home is under tonned and he still refused saying that the duct infrastructure is built to handle 3 tons and no more. If I went up in tonnage (there's that word again) then I would have to provide more duct sqft. to accommodate the larger ton capacity. Is this true? Or is it a good thing that I'm asking this question on the forum?

    Thanks so much for your help and advice.
    -Brandon

  2. #2

    Hi...

    First of all, since you have not felt comfortable with your existing unit, AND your contractor is refusing to use Manual J - you should find other contractors.

    Get a few bids from different contractors based on SAME technical requirement:

    1. Heat gain and loss calculations using Manual J.

    2. Use of psychrometrics and Manual D to size your air flow requirement into each room, fresh air and recirculated air mixing ratio, and to size your duct.

    All the best

  3. #3
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    First Man. J to size the system and determine the cfms (air flow)needed per room.

    Second Man. S to select the correct equipment,per the capacity required by Man. J,and the capacity of the equipment at the desired indoor and outdoor design conditions.

    Third Man. d to check the design of the ducts.Some modifications may be needed or not.This part can be a problem as ducts are not all accessable to change sizes if needed,so you may want to live with ,say a minor temperature difference between rooms.


    You can also look at increasing the insulation,etc., to reduce the size system that will be needed.

    With a system as old as yours it may not be producing the full rated "tonnage"(that word again),that it should or could.

  4. #4
    Join Date
    Jul 2005
    Posts
    435
    Being that you have a three level town home, I would opt for a split system. This way you can control the floor to your liking independantly, and probably save on cooling and heating costs since you have the flexibility of control and each unit would be smaller then then a 1 unit system.. Just my opinion.

  5. #5
    mjk_na, dash, kevinmac thanks for the replies....

    I've got a lot of learning to do. I didn't even know there was a Man D and a Man S.

    As far as the split system, I just can't see how the costs could be justified. There is NO room in my attic. It is cross beam construction with 2x4's all over the place! And the third floor is all bedrooms and bathrooms. I don't want a AH in one of the bedroom closets. I know my set-up is not ideal, so I am willing to live with some shortcomings. But not too many!

    I've just replaced all windows with Low E glass and there is a foot of insulation blown in the attic.

    I think I've done all I can to help out the ole' Carrier.

    Thanks for the informative replies!
    -Brandon

  6. #6
    Join Date
    Jun 2006
    Posts
    628
    I am in almost the same boat you are, 1850 ft2 townhome built 27 years ago with a Carrier furnace, but a Day & Night (CHEAPEST) 3 ton compressor in the back yard.

    Get three bids, more information is good.

    Ton or tonnage is just shorthand for 12,000 btu of cooling, so 3 ton is 36,000 btu, but btu is what window air conditioners are rated in, and ton sounds a lot bigger. Ironically the tons seem to also convert pretty well to the cost in pounds sterling.

    Without getting technical, an old cheap AC unit that is running properly with the right amount of freon and clean coils is going to do the same amount of cooling as a new top of line AC unit, its just going to use more power to do it. Chances are good your old unit isn't hitting on all three (running well, freon, clean), so a new same tonnage may have some more cooling to start with, but without checking over your old unit thats just a guess.

    As much as these pros like the manual J etc. I am not sure that some estimates run through a spreadsheet would convince me I don't feel hot. Why you feel hot might be a better approach to solving the problem. Why and how much are you willing to pay to correct it since going up to a 4 ton (1 ton per 500 ft2) may cost some serious cash essentially replacing all or most of the stuff buried in walls and attics.

    If you feel hot because of humidity, there may be other ways to approach the problem.

    I am sticking with the 3 ton size of my old system, but getting a higher efficiency (not more cooling, just lower operating cost) compressor and a more flexible variable speed furnace (that can help lower humidity).

  7. #7
    Join Date
    Oct 2004
    Posts
    167
    By replacing your windows, you've made a tremendous insulation improvement in your house. It could even be possible that with new windows, you could use a 2.5 ton unit.

    Old windows vs. new good windows = night and day!

  8. #8
    Join Date
    Jul 2005
    Posts
    435
    You don't have to have the 2nd airhandler on the third floor. I have seen them on the same floor next to each other. It's all in the ductwork, and where the returns are.

  9. #9
    Join Date
    Jul 2003
    Posts
    2,407
    Without getting a proper man j It's hard to give you an answer. I can say that you CAN'T go down in tonnage (yes, that's a word). Just by what you are saying with the upgrades, doing a 3 ton would work. You origanally had a 3 ton, (old quick rule of thumb-1 ton per 600 sqare ft) which at that time was undersized, from what I get from your post. You could actually probably go up 1/2 ton. It also seems the tech you had out origanally didn't know how to do a Manual J, as a lot of them don't, or he just didn't have time.

    Ultimatly I would have either that company come back out with the knowledge that you want a Manual J done and/or call another company to do the same. Good luck.

  10. #10
    Join Date
    Oct 2003
    Location
    Washington
    Posts
    7,405
    A LOT of mis-information on this thread............


    I'll leave it alone.

  11. #11
    Join Date
    Jul 2003
    Posts
    2,407
    Originally posted by seatonheating
    A LOT of mis-information on this thread............


    I'll leave it alone.
    Don't leave it alone. Post what you think is right, maybe (most likely) I'll learn something.

    This isn't just for the home owner but also for the rest of us too.

  12. #12
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    Here is what I am hearing from the OP, correct me if I am wrong:
    1) The old AC system does not keep me comfortable
    2) Therefore it is undersized.

    Is that too much oversimplification? I am trying to inquire whether there could be *other* reasons for lack of comfort. For example, perhaps the duct system is not delivering air where it is needed in the proper quantity. Too-small ducts are far more common than too-small compressors. I suggest a good place to start would be with someone trained by the National Comfort Institute:
    http://www.nationalcomfortinstitute.com/

    NCI trains techs on tools to measure air flow, building leakage, and indirectly duct leakage. In theory they can take certain measurements and tell you how many BTUs your system is actually putting out. The world of AC techs is a barbaric one IMO, at least connecting with this training will give you someone more systematic than average.


    If you were comfortable then I would be much more comfortable with "present experience" as a substitute for Manual J etc. But since you are not, it may only be prudent to start with Manual J heat load model -- it's the most objective method we have. I predict somewhat of a hassle finding a tech who is comfortable doing it (and it's not free) but when you do, it's probably a guy you should have a long term business relationship with.

    Separately, let me say a variable speed air handler with two-stage furnace can really improve winter comfort. I am a homeowner in S.Texas and we use AC much more than heat, if you told your location I forgot. It would be much more helpful if I could tell you whether two-stage AC would do the same for summer comfort, but I don't know from personal experience. A couple of friends in this climate have two-stage AC and praise it highly. But if you put this top of the line AC on a crappy duct system, even one which is too small, you may not get such great results. In dollar and cents terms, you will do better with a single stage AC. In either case I urge you to get the VS air handler.

    Hope this helps -- Pstu

  13. #13
    Join Date
    Nov 2005
    Location
    Northern VA
    Posts
    512
    Cheaply built condo probably means undersized flex everywhere - quite possibly more problems such as flex runs that have fallen off the boots and are pumping conditioned air into the attic, etc. I would definitely have someone who knows ducts (NCI is a good starting place) check the airflow on your existing duct system vs. ManD.

    If this is part of the problem, and you're not going to spend to correct it (could be VERY expensive to change ducts, depends on your exact situation) a variable speed air handler could help *some*. They're NOT a substitute for properly sized ducts, but they are better at dealing with undersized duct systems than non-variable speed blowers. The air will get there, it will just make a bit more noise at the registers.

    Do have a new heat load done, especially with the changes you've made - balancing the airflow may help even with your existing system, but you should know the correct size for the new one. One other thing to make sure of is that the contractor doing the Man J knows what indoor conditions you want. (If you want 75 inside don't let them assume 78, etc)

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