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Dunkirk modulating / combi gas smell at exhaust at start

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Hi, I have a 3 year old Dunkirk DCC 205 combi boiler and was hoping somebody could answer this question. It has always had a what I would consider a wasteful mode of operation where on every start up cycle, both DHW and central heating it runs the propane pressure from 11-13 WC to about 4 or so then the fan stops / pauses then the sequence starts over and then lights. Hard to explain buts it's almost as if the boiler wants to purge out the gas line first. This process seems to emit raw propane out the exhaust. I observe this using a digital manometer.

Thanks,
Mario

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  • GGross
    GGross Member Posts: 1,069
    edited August 2023
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    "11-13 WC to about 4"

    Undersized gas line, this is not a wasteful mode of operation, the unit has no way to lower the gas pressure, it is just trying to fire and you are getting an extreme pressure drop, which is symptom of an undersized gas line. The unit will purge out the gas from a failed ignition, which as you noted can be smelled at the exhaust, again this is not wasteful, it is to protect you. The gas line is horribly undersized in this case, or your gas pressure, or regulator has something off. This is not a boiler problem, it is an install problem

    you should never get more than a 0.5" wc pressure drop on ignition, any great than that and you have an undersized gas line, or issue with the regulator
  • marsfromrexford
    marsfromrexford Member Posts: 21
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    Thanks! After this first cycle where what I call 'purge' the fan stops then a new or 2nd part of start up sequence seems normal, fan starts back up, no significant drop of WC, fires up no problem. I have all 1inch throughout.
  • GGross
    GGross Member Posts: 1,069
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    Sounds like a slow acting regulator, after the first attempt it likely opens up more quickly, I would replace the regulator. You don't have a regulator right next to the unit by chance?
  • GGross
    GGross Member Posts: 1,069
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    For reference the unit itself has no way to lower the pressure in that pipe except to open the gas valve at full fire, so the drop you see is not the boiler, it is the piping/regulator/fuel source reacting to the gas valve opening
  • marsfromrexford
    marsfromrexford Member Posts: 21
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    Actually now that I think about it that severe drop only happens once per instance of hot water demand - let's say my wife is taking a shower it will do the drop in WC once but then once it fires, satisfies the demand, the flame icon cycles on and off to maintain 120 degree DHW without the drop in WC for how ever long the shower lasts. Then 10 minutes later someone washes there hands, I get the raw fuel smell outside and the drop in WC . so it seems like it does that drop once for every demand instance but not every flame on / off cycle. that's why I almost think its in the programming. I did have a service call from my supplier and they replaced the duel regulator at the tank but didn't seem to change anything, they said it's fine.
  • GGross
    GGross Member Posts: 1,069
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    The unit has no way beyond opening the gas valve to lower that pressure though. If the gas valve were wide open, as wide as it can go, you should still not see more than half inch drop (1" absolute max for properly sized gas line), unless there is an issue with the piping/regulator/fuel source. Possibly air in the tank? air in the line? maybe a regulator located at the unit itself? Has this happened every single year or only since the last time the LP tank was filled? I had issues in my neighborhood with improperly purged LP tanks a few years ago. Beyond that if you can figure out the total piping for your gas system, and the load from the equipment hooked to it you could double check sizing, but if it were undersized fully you would more than likely get repeated failed attempts and the unit should eventually lock out
  • marsfromrexford
    marsfromrexford Member Posts: 21
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    Gotcha, there is a duel stage regulator at the tank, then to 1/2 inch copper that runs under my porch for about 10ft then to my exterior wall that goes to 1" steel pipe. I brought up this issue before on this forum with the duel stage regulator before and the consensus was that I should have a secondary regulator where the piping goes from 1/2 inch to 1". I brought this up with my supplier and they won't budge, thats when they put the higher out put duel stage regulator at the tank but that doesn't fix the 1/2 inch pipe outside that's a potential restriction. When I had my inside 1inch pipe inspected by the gas company the guy said that it was 'way overkill'
  • GGross
    GGross Member Posts: 1,069
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    what is the total load for the gas equipment in the house? 10ft of 1/2" copper limits you to 291,000 btu/hr total, the dunkirk is already stealing 205,000 of that. It does not explain the pressure drop on the boiler though, unless timing is just right and other equipment has already dropped the pressure, the 1" pipe should allow for ample volume of gas at ignition.
  • pecmsg
    pecmsg Member Posts: 4,883
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    What model Dunkirk
    What size gas line?
  • GGross
    GGross Member Posts: 1,069
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    pecmsg said:

    What model Dunkirk

    What size gas line?
    info is in the thread already

    Dunkirk DCC 205

    the gas line is 1" but it starts with at least 10' 1/2" coper which would be their limiting factor
    LP gas
  • JUGHNE
    JUGHNE Member Posts: 11,075
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    As noted in your other posting, you need the little red hi pressure reg at the tank, that would send about maybe 10 PSI to the second stage reg at the house. That is usually green and drops the pressure to 11-13 inches for your 1" piping.

    Or run the 1" to the tank 2 stage reg.

    I would guess the installer does not understand the double regulator theory and pressure drop and pipe sizing. And why learn, they have been paid and gone.
  • marsfromrexford
    marsfromrexford Member Posts: 21
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    I'm almost at the point where I will buy a primary / secondary regulator myself. I see on amazon 'Marshall Regulator Home Propane Supply Kit LP 1122H-AAJ 1622-BCF 1/2" Outlet' @ about 160 dollars, do think that is worth a try?



  • JUGHNE
    JUGHNE Member Posts: 11,075
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    Sometimes the 2 stage at the tank is in two pieces.

    Do you have a picture of the tank reg?
  • marsfromrexford
    marsfromrexford Member Posts: 21
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    https://tinypic.host/image/vKzHO
    https://tinypic.host/image/vKtuk
    https://tinypic.host/image/vKLyM

    Not sure if these will show as pictures but please take a look and see if this set up is proper for my combi boiler

    Thanks!
    Mario