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Flue liner size for new gas water heater ?

heatheadheathead Member Posts: 59
Installing new water heater .62 natural gas 40,000 btu 50 gallon in masonry chimney that is 30 feet high, only water heater now. Washington, D.C. Area. The chimney is on the outside. Going to line with 3 or 4 inch pipe which size is better and should one wrap the liner in insulation even if chimney is already clay lined. Corrugated aluminum liner thinking pipe is corigeted that will be derated by 20 percent. I am leaning toward 4 inch. The water heater has 4 and 3 inch outlet. What size should i use and how important is it to insulate the whole run or more important just at top. Thanks


  • SpenceSpence Member Posts: 316
    Liner Sizing

    Never insulate a vent connector or a common vent section! Use the GAMA Tables for a single appliance to size your vent system. This is a life-safety issue and should not be taken lightly. Choose a liner that is stainless steel; the aluminum liners have a very short lifespan.
  • icesailoricesailor Member Posts: 7,265
    Flue Sizing:

    I don't know how they do it in DC but that wouldn't fly in Massachusetts. Especially on a outside (3 sided?) chimney.

    It might be a good idea to run that by the AHJ and get their input on the subject. Chimney flues can be too big AND too small.
  • M LaneM Lane Member Posts: 123
    edited February 2014

    Upgrading to a more efficient power-vented unit and using PVC? Would save headaches.
  • Steve WhitbeckSteve Whitbeck Member Posts: 669
    power vented

    Just because the water heater is vented in PVC pipe does not mean it is higher efficient than a standard natural draft heater.

    The power vented model uses electric where the natural draft doesn't.

    The power vented model uses a LOT more house air ( heated by furnace ) to dilute the exhaust gasses of the water heater to lower the temperature down to the point that it can be safely vented out the PVC pipe.

    So in reality the power vented model is probably much less efficient after all of the other factors are included.

    OH one other thing - If the power goes out the natural draft model will still give you a hot shower as long as you don't have a well.
  • SpenceSpence Member Posts: 316
    Power Vent

    You certainly got a bucketful of fabulous comments from Steve. Taking all of that into consideration, and not discounting his input in any way, there still may be a nod toward the PV. Venting is a serious business in which many contractors and building inspectors are not on top of their game. With the PV, the venting instructions are in the IOM, so you have pipe sizes, equivalency, pitch, connection requirements, termination configurations, and termination locations. As long as you follow the IOM to the letter you can't get hurt. With the atmospheric model, the maker assumes that you understand the GAMA tables and will vent accordingly.

    The efficiency difference Steve mentions would indeed be the vent motor's consumption, yet his comment about combustion air (another life-safety issue most disregard) is more important than a few watts of power, assuming both atmospheric and PV tanks have the same 1st hour rating (how you size your tank) and energy factor (the higher the EF, the more "MPG").
  • M LaneM Lane Member Posts: 123

    Being from the West I am not accustomed to having to consider day after day without power due to ice storms. If you are in an area with a history of that, I'd install a generator. Meanwhile, the PV water heater is a more effective option, IMHO, than lining a chimney for a 40MBH 50 gallon water heater. If you are going to go through all that trouble, I'd suggest at least investing into a 55 or 65MBH unit and get faster recovery.
  • SWEISWEI Member Posts: 7,356
    Is there a boiler in this building?

    If so, an indirect might make more sense.
  • HenryHenry Member Posts: 753
    Vent size

    Use only S/S corrugated liner. The aluminium ones can tear during installation. According to Z-Flex, 5 feet horinzontal and 30 ft vertical: 42,000 BTU : 3 ich. A 4inch liner is good for 85,000 BTU.

    Sorry Steve, but a PV hot water tank is about 15% more combustion efficient than an atmospheric. The main reason is that there is no pilot and no pilot enduced draft through the tank to cool it down. I have personally run the tests in the lab. You might also look up the ratings of the tanks at a manufacturer's website.

  • heatheadheathead Member Posts: 59
    Many thanks

    I want to thank all of you who have replied.  The power vent would be the easy way out, but I feel it has a lot more moving parts to break. With the water conditions in the area we can normally get 20 + years out of water heater.  I looked at some tables that say the 8 x8 clay liner which have inside of 7 x7 should work based upon flue vent area.  I don't agree with that  and am worried about drafting and condensing exhaust gases.  If one  was to line chimney with a liner if it's to big it is harder to draft?  Insulating the liner should keep it hotter and drafting better?  I am leaning to the 4 inch. I have read articles about the problem but I can find any  clear answers.  Any NFPA charts or Gamma charts that I should be looking or articles.  Thanks Again. 
  • HenryHenry Member Posts: 753
    Vent sizing

    NFPA 54 Table /3.1 (e) 30 feet, 5 Ft lateral : 4 inch with maximum 78,000 BTU
  • SpenceSpence Member Posts: 316

    I would be happy to send you a copy of the GAMA tables.
  • JackJack Member Posts: 946
    Insulating an external chimney

    Is no guarantee that the flue gases will not condense. If you choose to go this way I would suggest running B-vent from the draft hood to the chimney and I think I'd go B-vent all the way up the flue. Years ago when Rich Krawjeski and John Strasser of Brookhaven ran the numbers for venting that became Appendix E in NFPA 31 I was talking with Rich and he was surprised at what an insulated vent connector could do for a flue. He said that in all their modeling, inside, outside flue, lined or unlined, no matter what, the thing that had the most positive impact was an insulated vent connector.

    That said, for the cost of the liner/labor you are in the neighborhood of a tankless water heater. Masonry chimneys are excellent architectural devices but very poor mechanical devices with today's equipment.

    Btw, going into that larger flue exacerbates the problem. Basic physics. When a gas expands, it cools. Take a nice 3" vent connector at about 7.1" cross sectional area with all those hot molecules and throw it into an 8x8 tile...approx 56 sq" nominal, if memory serves me correctly(which is happening less and less it seems) and your heat dissipates quickly, along with your draft.
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