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  • Piping Size

    I looked at a duplex this weekend for sale I am considering. Both units had newer furnaces, less than five years old I believe. They are different brands and appear to have been installed at different times, the second floor unit is a Goodwin that was spotless and is only, I think, two years old. Both units were piped with the original box and 8 inch piping, cutting down the box size to fit the newer furnaces. The second floor lines were run up through closed fireplace flues, with vents cut into the covers for two rooms.
    The question is, with these newer higher flow furnaces, shouldn't the pipes be 5 inch to improve efficiency? I believe I could run 5 inch inside the 8 inch in the existing piping to the second floor, and replace the rest in the basement. Would this improve the air temperature and flow to the second floor?

  • #2
    Re: Piping Size

    Without knowing the btu output of the furnace no one can tell you what size you need. Also, the dimensions and quantities of existing supplies and returns, as well as the type of venting for the furnace (natural, induced 80%, or high efficiency (pvc vent)) are needed.

    If they are using flues (brick? steel?) with no sleeving that is a violation of code. Walk away...walk away...
    Last edited by Plumber Punky; 06-17-2013, 10:03 AM.
    ~~

    ... it was plumbed by Ray Charles and his helper Stevie Wonder

    Comment


    • #3
      Re: Piping Size

      THe second floor is a Goodwin 80 % 45,000 btu furnace. The second floor apartment is about 600 sq. ft., plaster on brick, so no wall insulation. They did run 8 inch pipe up the flue, slightly bowed to make it fit, probably 30 years ago. They 90'd it and somehow found an 8 inch round to 9x9 square adaptor for the vent cover. The area is nice for the building price, and the house is solid and square. I'm at this point wondering if this will require a complete HVAC survey to see if the system will work wihtout spending thousands to re-plumb the piping, prior to making an offer.

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      • #4
        Re: Piping Size

        So if the heating pipe runs up the chimney/flue....where does the furnace vent to?

        For 80% 45,000 btu furnace you need about 600 cfm. That equates to a minimum of a 12" round pipe. 8" round only supports up to 15,600 BTU. Your furnace is severely choked and 15,600 btu will not heat an apartment.
        ~~

        ... it was plumbed by Ray Charles and his helper Stevie Wonder

        Comment


        • #5
          Re: Piping Size

          THere are three flues in the house, two of them that had fireplaces were covered up, these have the vent pipes in them, the third one connects to a fireplace on the first floor, which also has a vent flue within it. The furnaces go through the flue vent side, and they have been fixed with stainless liners. THere are three vents to the second floor, all three are 8 inch, one runs up the corner of the first floor wall, the other two are in the old chimney flues. So, that gives a potential of 46,800 cfm total, correct. THe other thing that is interesting is there is only one visible return, an 8x16 cutout in the wall on the second floor. There is a grate in the entrance hall, I'm betting when the house was first furnaced that was the main return.

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          • #6
            Re: Piping Size

            Originally posted by nkyrental View Post
            the third one connects to a fireplace on the first floor, which also has a vent flue within it.
            This is a violation of code. Gas and wood flues cannot be combined. Power vented gas flues should not pipe into fireplace flues, even if the fireplace is gas. I'm not sure about gas logs and natural draft furnaces in the same flue, i think it's ok from what i recall.

            The furnaces go through the flue vent side, and they have been fixed with stainless liners. THere are three vents to the second floor, all three are 8 inch, one runs up the corner of the first floor wall, the other two are in the old chimney flues. So, that gives a potential of 46,800 cfm total, correct.
            yes, multiply by the number of 8" flues for total btu load carrying.

            THe other thing that is interesting is there is only one visible return, an 8x16 cutout in the wall on the second floor. There is a grate in the entrance hall, I'm betting when the house was first furnaced that was the main return.
            8x16 will only allow for a capacity of 48,900 btu at 660 cfm so you should be fine provided the filter is kept immaculately clean and there are not any tight obstructions.
            ~~

            ... it was plumbed by Ray Charles and his helper Stevie Wonder

            Comment


            • #7
              Re: Piping Size

              I put in a bid that took into account that it could result in a complete replumb of the HVAc piping and the new ceilings that would need to be dropped to put everything in correctly. The third flue is closed off as well for the fireplace, so the only thing in it are two stainless liners for the two furnaces. We will see oif they counter or not. Thanks for the information!

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              • #8
                Re: Piping Size

                no problem, good luck.
                ~~

                ... it was plumbed by Ray Charles and his helper Stevie Wonder

                Comment

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