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  1. #1
    Join Date
    May 2009
    Posts
    15

    3.5 ton HP versus 4 ton 2-stage HP

    Hi folks. I am replacing my heat pump/air handler for a house with 2000 sq ft on the first floor and 800 sq ft on the second floor including a large loft and smaller bonus room. The house has vaulted ceilings. We were unable to adequately heat or cool the second floor with the original system (3.5 ton HP, 4 ton blower, and electric heat backup) installed in 1996 when the house was built. We live in Indianapolis, IN.
    We have had seven estimates and all the proposals are for either a 3.5 ton HP or 4.0 two-stage HP with variable speed air handlers. A couple people have proposed a 5 ton blower. No one has mentioned a Manual J calculation although one person did measure the perimeter of the house after I told him several estimates were for 4.0 ton HPs. He said it was borderline between a 3.5 and 4.0 ton HP.
    Here are my questions.
    1. In my situation, is there much difference between the 3.5 HP and 4.0 two-stage HP? I like the idea of having a two-stage heat pump, but I do not want to oversize.
    2. Should I consider zones for the house? If I decide to create two zones (for the first and second floors) with zone dampers, will it make a difference which size heat pump I purchase?
    3. Is it reasonable for me to sign a contract to purchase contingent on a Manual J and/or Manual D calculation? Should I pay for the calculations? How much should they cost?
    Thanks!

  2. #2
    Join Date
    May 2007
    Location
    DC Metro Area (MD)
    Posts
    3,368
    Well, your heating and cooling problems of upstairs are likely to do inadequate airflow/ductwork to that portion of the house rather than equipment sizing. Bottom line is you need a load calc and to have your ductwork evaluated.

    1. Generally speaking if 3.5 tons is what the load calls for, since it's a 2-stage unit, no problem going to 4 tons if the ductwork is sized right. It's not oversized--it's what is needed.
    2. Zoning can be great if done right, but the key is finding an experienced contractor who will evaluate your ductwork and run tests afterward to make sure the static pressure is not to high with one or both zones calling. If you went with zoning, you'd want a 2-stage unit. Shouldn't change your load requirements since you'll still need full capacity on the hottest days where both zones are calling.
    3. Some dealers will do the load calc after the contract is signed to prevent other dealers from outbidding them using their sizing information. Some companies will charge, but I think if you look around you'll find at least one reputable contractor who considers this part of a good installation. Costs vary. You can do your own for $50 by clicking the HVAC-Calc link above, if you have all the information available.

  3. #3
    Join Date
    May 2009
    Posts
    15
    Quote Originally Posted by RyanHughes View Post
    Well, your heating and cooling problems of upstairs are likely to do inadequate airflow/ductwork to that portion of the house rather than equipment sizing. Bottom line is you need a load calc and to have your ductwork evaluated.

    1. Generally speaking if 3.5 tons is what the load calls for, since it's a 2-stage unit, no problem going to 4 tons if the ductwork is sized right. It's not oversized--it's what is needed.
    2. Zoning can be great if done right, but the key is finding an experienced contractor who will evaluate your ductwork and run tests afterward to make sure the static pressure is not to high with one or both zones calling. If you went with zoning, you'd want a 2-stage unit. Shouldn't change your load requirements since you'll still need full capacity on the hottest days where both zones are calling.
    3. Some dealers will do the load calc after the contract is signed to prevent other dealers from outbidding them using their sizing information. Some companies will charge, but I think if you look around you'll find at least one reputable contractor who considers this part of a good installation. Costs vary. You can do your own for $50 by clicking the HVAC-Calc link above, if you have all the information available.
    Thanks so much for your comments! I will definitely ask for the load calcuation. Below is what the companies said in their proposals about the ductwork. Do you or other folks have any thoughts on these solutions? Do they sound practical?

    - Three companies did not mention anything about the ductwork.
    - One company said it would inspect the ductwork during installation.
    - Another company proposed to install a scoop system for the ductwork in the crawlspace (although he never looked in the crawl).
    - One company actually looked at the ductwork in the crawlspace and said it was adequate. He proposed to (1) inspect the plenum and takeoff for leaks, (2) build a bigger box (?) for the ret. and sup. plenum transitions and drop it in the floor (I think that is what he has written), (3) increase the size of two returns on the second floor, (4) add a 5 ton air handler to improve circulation.
    - The last company proposed to (1)seal and fix all ductwork, (2) add high velocity registers to the second floor, (3) add a return for a first floor bedroom on the far side of the house, (4) increase the size of the heat run and install new floor registers in the first floor master on the far side of the house. This company also proposed zoning the entire first floor from the second floor (as an additional option).

    Finally, should I ask for a Manual D calcuation or can contractors just look at the duckwork and tell if it is big enough? I don't want to be a pushy/difficult customer, but I do want the job done correctly according to reasonable industry standards.

    Thanks again!

  4. #4
    Join Date
    Nov 2008
    Location
    Chapel Hill, NC
    Posts
    1,070
    Given your existing problem conditioning the 2nd floor, it does sound like you have ductwork issues. Increasing the returns on the second floor would help up there but who knows what it would do to the downstairs. As you will likely need some ductwork changes, whatever the final decision, have you considered two separate systems for upstairs and downstairs (a larger unit for downstairs and a smaller unit for upstairs)?

  5. #5
    Join Date
    Oct 2008
    Location
    Indiana
    Posts
    78
    I would lean toward the 4-ton 2-stage, of the choices you mentioned. Even if that's a bit more cooling capacity than you need, you won't regret having it on the hottest days, and the system will stay in low-speed mode most of the time.

    Also, you will get the benefit of more heating capacity from the heat pump, which should reduce the operating time for the backup heat.

  6. #6
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    JG from what I have read on this board, about the best most people do is to get a contract for the business with a promise to do Manual J before they do the install. If they can do Manual D for duct sizing, well you probably are talking to the best HVAC pro in the neighborhood (that's the way it is in my TX region). When you find such a skilled pro I think you ought to pay him handsomely because part of his work is actually consulting and engineering design.

    It wouldn't hurt to go to the Hvac-Calc Manual J program at the top of the page and do a Manual J calc of your own. At the least you will have something to compare notes with the pro. It will be easier than you think, most of the work is measuring rooms and windows and keying that data in. I've done it two times and can promise the program is easy to use.

    Best of luck -- Pstu

  7. #7
    Join Date
    May 2009
    Posts
    15
    Quote Originally Posted by sktn77a View Post
    Given your existing problem conditioning the 2nd floor, it does sound like you have ductwork issues. Increasing the returns on the second floor would help up there but who knows what it would do to the downstairs. As you will likely need some ductwork changes, whatever the final decision, have you considered two separate systems for upstairs and downstairs (a larger unit for downstairs and a smaller unit for upstairs)?
    Thanks for the suggestion. We did talk to a contractor about two units when we bought the house several years ago, but the layout of the house makes it hard to find a acceptable place for a second air handler. Also, the extra cost and need to maintain two separate systems made that option less attractive.

    I noticed in your location that you live in Chapel Hill. I lived in Durham for about seven years in the late 1990s. We love the area and miss living there.

  8. #8
    Join Date
    May 2009
    Posts
    15
    Quote Originally Posted by jstjohnz View Post
    I would lean toward the 4-ton 2-stage, of the choices you mentioned. Even if that's a bit more cooling capacity than you need, you won't regret having it on the hottest days, and the system will stay in low-speed mode most of the time.

    Also, you will get the benefit of more heating capacity from the heat pump, which should reduce the operating time for the backup heat.
    Yes, I am leaning towards the 4 ton 2-stage. My only concern is oversizing. I know nothing about heat pumps, but have read that oversizing is a problem. One contractor claimed that going to a 4 ton 2-stage from a 3.5 ton was not a big deal because the 4 ton will often be running on low and the unit will, therefore, have a long running time. That made sense to me, but I'm not the expert. It sounds like you agree with the guy. I just want to make sure that the contractor was giving me good advice about the 3.5 ton versus 4 ton 2-stage.

  9. #9
    Join Date
    May 2009
    Posts
    15
    Quote Originally Posted by pstu View Post
    JG from what I have read on this board, about the best most people do is to get a contract for the business with a promise to do Manual J before they do the install. If they can do Manual D for duct sizing, well you probably are talking to the best HVAC pro in the neighborhood (that's the way it is in my TX region). When you find such a skilled pro I think you ought to pay him handsomely because part of his work is actually consulting and engineering design.

    It wouldn't hurt to go to the Hvac-Calc Manual J program at the top of the page and do a Manual J calc of your own. At the least you will have something to compare notes with the pro. It will be easier than you think, most of the work is measuring rooms and windows and keying that data in. I've done it two times and can promise the program is easy to use.

    Best of luck -- Pstu
    I have no problem paying a premium price for premium work. The problem is finding a premium company! Of course, when I visit the websites of all the companies, they all assure me that they do top quality work. I may look into the Manual J program if I can carve out some free time. Thanks!

  10. #10
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304
    I too have had a hard time getting the ACCA methods from pros who otherwise seem to be good people. FWIW I went from a 3.5 ton AC to a 3.0 ton 2-stage and am very content with the results. Less stress on the ducts and the 3.5 evidently was oversized. With a 2-stage you don't necessarily have to upsize.

    Best wishes -- Pstu

  11. #11
    Join Date
    Nov 2008
    Posts
    226
    Quote Originally Posted by pstu View Post
    With a 2-stage you don't necessarily have to upsize.
    Best wishes -- Pstu
    Or downsize, either. It seems to me that having the extra capacity is desireable. As in all I really need is a 2.5 ton but I am going to get a three ton. And I'd like that extra capacity to be in reserve. That extra capacity would handle several people in the house on some hot, humid summer day and not cost me much extra, up front. Makes sense to me.

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