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  1. #1
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    Apr 2013
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    HIGH VELOCITY A/C SYSTEM QUESTION


    I am bidding a high velocity job. The previous contractor was let go and we were referred to finish installing the handler and condenser. The owner has 1700 ft^2. Two 7" round pipes; one feeds the 2nd floor with 13 outlets and the other feeding the 3rd floor with 6 outlets.

    Is there any possible way to zone this system or install two units?

  2. #2
    Join Date
    Dec 2010
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    The Quad-Cities area (midwest).
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    You need 5 full outlets per ton. I try to even the load between the two 7" runs. The 3rd floor with only the 6 supplies can't be that large of an area. Arzel Zoning has zoning for high pressure systems. Give then a call.

  3. #3
    Join Date
    Apr 2013
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    Boston
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    Do you know of a company that makes a one ton system?

    I talked to Spacepak and they make a one ton but it is different. You can't have a lot of duct, you need a chiller, and the representative said it would not work.
    If I install anything higher that a one ton, the system will freeze. That is what Spacepak reps have told me. So, far the only option I have is to install a 3 ton system with no zones.
    You are right the 6 outlet area is only 700 sq. ft.
    Since, you are a professional do you agree the system will freeze if zoned or are the reps being over cautious?
    If I put a 1.5 system for the 6 outlets and a 2 ton for the 13 outlets;in your opinion will the 1.5 ton freeze?

    It would be very hard to even the load, the duct is covered, the home is finished.
    I put a disclaimer in the contract because the duct was suppose to be 9" and I can't chance eating the equipment. This job has been one headache!!

  4. #4
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    Dec 2010
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    Quote Originally Posted by P & J mech View Post
    Do you know of a company that makes a one ton system?

    I talked to Spacepak and they make a one ton but it is different. You can't have a lot of duct, you need a chiller, and the representative said it would not work.
    If I install anything higher that a one ton, the system will freeze. That is what Spacepak reps have told me. So, far the only option I have is to install a 3 ton system with no zones.
    You are right the 6 outlet area is only 700 sq. ft.
    Since, you are a professional do you agree the system will freeze if zoned or are the reps being over cautious?
    If I put a 1.5 system for the 6 outlets and a 2 ton for the 13 outlets;in your opinion will the 1.5 ton freeze?

    It would be very hard to even the load, the duct is covered, the home is finished.
    I put a disclaimer in the contract because the duct was suppose to be 9" and I can't chance eating the equipment. This job has been one headache!!
    You're right, I had forgotten that the the new high velocity systems have gone from the 7" to the 9". I ran into that about 3 years ago on a replacement AH. I had to increase the size of a section of the 7" duct and relocate some of the supplies on to the new 9" duct. If you are not able to make the ductwork corrections, I don't know if you what the headaches. Your reputation is on the line even with a "disclaimer". You'll still be the expert that should have known better.

    Yes, zoning or a 1.5 ton on 6 outlets (and 7" pipe) will probably freeze.

  5. #5
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    Apr 2013
    Location
    Boston
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    Thanks for the advice.
    I would walk from the job but it is a friend of a client of ours that gives us a lot of work.
    I am going to tell the owner I will only install the 3 ton system and explain that he can look for a company he wishes that will zone the system but I don't recommend it.

    I have not dealt with high velocity that much.

  6. #6
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    Dec 2010
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    They work great and they fill a niche but they are very expensive and they don't qualify for the utility rebates. I'm retired now and I don't have my books but, if I remember correctly, the system you're working on is not balanced as SpacePac or Unico would approve. You should call one of the companies and explain the supply layout and mention the 7" pipe.

  7. #7
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    Apr 2013
    Location
    Boston
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    I had a Spacepak representative visit the property. He did the calculations with the 7" duct and said it would work but can not be zoned. We have to bump it up to 9" in order for it to work. We don't have to enlarge the existing 7" but just connect the sevens to nines for as much feet we have to connect to the plenum.

  8. #8
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    So you're saying that you can replace some of the existing 7" pipe with 9" pipe and leave all the supplies where they are because they are covered? Let me know how that works.

    Just an idea, if that is the case, can you add any supplies off the new 9" pipe to help with the 700 sq. ft. area?

  9. #9
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    Apr 2013
    Location
    Boston
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    No, I can not replace any of the existing 7". I am connecting 9" to the existing sevens and pushing the unit as far back as possible in the 3rd floor attic. The 3rd floor is was remodeled. It was just an attic. There is a little room with a door that is unfinished. That is where we are putting the unit and adding the nine inch. I could add supplies off the new nine but now we get into the situation of there being too many outlets.

    How many outlets do you think 700 ft^2 (the finished attic area) can have and work properly?

  10. #10
    Join Date
    Dec 2010
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    The Quad-Cities area (midwest).
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    Quote Originally Posted by P & J mech View Post
    No, I can not replace any of the existing 7". I am connecting 9" to the existing sevens and pushing the unit as far back as possible in the 3rd floor attic. The 3rd floor is was remodeled. It was just an attic. There is a little room with a door that is unfinished. That is where we are putting the unit and adding the nine inch. I could add supplies off the new nine but now we get into the situation of there being too many outlets.

    How many outlets do you think 700 ft^2 (the finished attic area) can have and work properly?
    How well were the existing 6 outlets cooling the attic area? That area is hardest to heat and the first to require cooling. It wouldn't hurt to add 2 or 3 more off of the new 9" to that area. Regarding too many outlets (supplies), there are restrictors (of different balancing percentages) that can be put into the outlets to help balance the system.

  11. #11
    Join Date
    Apr 2013
    Location
    Boston
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    Thanks for the information.
    I think you solved my problem. I am going to contact the Spacepak rep. and ask him if it would work. I think you are right, it will work. It might look a little weird but the owner wants it zoned because it is two apartments.

    Thank you so much!!!
    Take Care,
    Michelle

  12. #12
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    Dec 2010
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    Quote Originally Posted by P & J mech View Post
    Thanks for the information.
    I think you solved my problem. I am going to contact the Spacepak rep. and ask him if it would work. I think you are right, it will work. It might look a little weird but the owner wants it zoned because it is two apartments.

    Thank you so much!!!
    Take Care,
    Michelle
    Oh.....you never mentioned the 2 areas being cooled were being converted to 2 apartments. The system has only 1 return, how are you going to get return from the other apartment? This is sounding more complicated!

  13. #13
    Join Date
    Apr 2013
    Location
    Boston
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    My brother, who knows a lot more about HVAC, said he could put another return.
    I have a B.S in Civil Engineering but am out of work. He has me doing his office work and wants me to learn how to design. He thinks with my background, I should be able to understand and learn design. This is why I joined this site, to educate myself about HVAC. I have been working for him for a year. I probably should take some classes and be on the field more.

    Thanks for the help!!
    I told my brother that I didn't come up with the solution on my own; I had a little help.LOL

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