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On-demand propane boiler backdraft

Hello people, looking for advice. I have installed a propane tankless water heater. It has a 100mm flue through a log wall with 500mm vertical flue outside with a raincap.

First evening below zero it blocked up which I can only explain by backdrafted 2(Celcius)below air coming through the flue into the house at 15C and freezing pipes inside the unit. 

(Ttulpe B6 is the model on a propane tank)

Is there any experience or suggestions on how to address this? Winter gets to 28C below here so I need a solution or a whole new setup! 


  • kcopp
    kcopp Member Posts: 4,443
    Many tankless WH have a backdraft flapper to prevent the cold air from settling back down and freezing the HX. Was this a natural draft type or a power vented type? The natural draft ones are most susceptible to the freeze up.

    HVACNUT Member Posts: 5,865
    Those are some harsh conditions. If venting options aren't defined in the manual then you should consult the manufacturer.
  • Derheatmeister
    Derheatmeister Member Posts: 1,550
    Read especially Page 6,7,8,9 of the installation instructions (I would read the entire thing and make sure to adhere to the recommendations )..https://www.manualslib.com/manual/1637829/Ttulpe-Indoor-B-6-P50-Eco.html?page=9#manual

    This Waterheater Looks to be a natural draft on demand waterheater that is something i would use in a RV during the Spring/Summer/Fall...
    Harsh Winter Environments may not be a good match.
    I would contact the manufacturer for advise.
  • juskin
    juskin Member Posts: 4
    Thanks folks. I have contacted the manufacturer had several emails back but they are not giving me any advice just giving me generic answers to back flush it. It is backdrafting when not running for sure, it is natural draft which makes me a bit unsure how I can fabricate something to reduce backdraft and still be sure it vents. I chose it after reading some reviews of it being used in subzero conditions but now I can't remember where I read them! 
  • juskin
    juskin Member Posts: 4
    And yes all is installed as per the manual and manufacturers specifications. 
  • Jamie Hall
    Jamie Hall Member Posts: 23,445
    Nature can be really mean sometimes. Any opening -- and it doesn't matter whether it's a chimney. a window, a door, whatever -- is going to backdraught if the pressure inside is less than outside and there is no closure. They will also exhaust air to the outside if the pressure inside is greater. That includes differences in density from being warmer or colder.

    There are two choices to keep a natural draught flue or vent from either exhausting warm air or bringing in cold air: adjust the pressure (not easy) or add a damper. The problem is much worse, of course, if the flue or vent is vertical, as that gives the density difference from warm to cold something to really work with.

    I can envision two approaches to the immediate problem. The first is to keep the boiler always somewhat warm. This will waste some energy -- both in keeping it warm and in warm air going out the vent -- but at least you won't have a cold draught. The other is to install an automatic damper, interlocked with the boiler, which is only open when the boiler is called on to fire.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • juskin
    juskin Member Posts: 4
    90 to vert only. An auto damper would good but I'm a bit clueless how to connect it to this unit. When it has a manual water flow trigger to fire. 
    EBEBRATT-Ed Member Posts: 15,629

    I am just throwing out some items to see if any stick.

    Any bathroom exhaust fans or other exhaust fans or clothes driers or fireplaces taking air out of the house and making the house negative in relation to the outdoors?

    Or a boiler or furnace in the area that is "looking " for combustion air.

    Try and figure out if the house is in a negative....tissue paper held near a cracked open door?

    You may want to consider some make up air to the house if it is negative and heat the MU air

    An auto damper would help it would have to be wire to open first and close last and not allow the burner to fire (interlocked) unless it is open