Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Venting Issue- Gas Steam Boiler

steamnewb Member Posts: 5
edited May 2021 in Gas Heating
I recently had an energy audit completed on my home (2 family, built 1931). As part of the audit, the venting for our water heaters and boilers was checked. We have 2 gas fired water heaters (2017 and 2019) and 2 identical Peerless steam gas boilers from ~1970ish. All vent through the same masonry chimney that goes up the middle of the house.

Both water heaters were fine, as was 1 of the boilers. They had negative pressure readings. The second boiler though, had a pressure reading of +3.4Pa and increased CO readings in the basement while it was on. Generally, it seems the first step is to check the chimney. However, given that the other boiler and both water heaters are venting seemingly fine through the chimney, I have my doubts that the chimney is the main issue. First question- does that theory hold up? That if the other three are venting fine, then the likelihood of this being a chimney issue is pretty low.

Next, we had a steam guy (found through this site) out to check it out. His basic message was the following: He did not think the chimney was the issue, that the issue was with the boiler itself. There were scorch marks on the front of the boiler. Troubleshooting would require taking the boiler apart and he would clean it out and make sure everything internally was good. Cost, $. If anything was broken or needed to be replaced, probably going to have trouble finding parts or the parts will be very expensive. He also said that when they take gas fired boilers apart, they usually don't find anything egregious inside that could for the draft issues we're seeing. He did not have a reasonable explanation for what inside the boiler could be causing our draft issues. Mostly chalked it up to... sometimes this just happens and they fail like this. Second question- does this seem legitimate?

We're going to get a chimney person out, to be able to definitively rule in/out the chimney as causing our issues. If the chimney is fine, what would be the next troubleshooting steps to take? It seems to me that the boiler itself is the issue, rather than a venting/ pressure issue since the water heaters and other boiler vent fine. Are there failures within this sort of boiler that could cause venting issues?

Thank you!


  • Jamie Hall
    Jamie Hall Member Posts: 19,954
    Several thoughts here... first one is, I at least don't like venting multiple appliances through one flue, which is what you are implying you have. However, if it's legal in your area... so be it.

    That said, a draught issue confined to one appliance is, as your steam guy has said, something to do with that specific appliance and its venting and breaching. The scorch marks indicate the same thing -- and certainly shouldn't be there. In fact, if it were mine to play with, I'd shut that boiler off and leave it off until the problem was found and resolved. The elevated CO readings when it is running make that -- in my mind -- mandatory.

    And again, your man is quite correct. To diagnose the problem, it will be necessary to thoroughly clean and inspect the entire fire side of the boiler -- and all the breaching, including the connection to the flue.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 12,654
    I don't agree that this just happens.

    There is a reason for everything. Every boiler needs to be cleaned now and then, this is no different. There are no parts needed to clean a boiler that can't be easily purchased for a fairly low price. Cerafelt and furnace cement etc, unless the burners themselves are warped, corroded or damaged.

    And you don't need to take the boiler all apart to check the burners

    It could be as simple as a dead squirle or bird
  • Lance
    Lance Member Posts: 227
    The fuel gas code is very specific on inputs and outputs in this situation. Verify engineering is correct! Height and size of flue determines it's capacity. Sum the Btuh input of all flue attached appliances combined on this flue. Assure all code specifications are complied with. BTUH input must be verified by actual consumption test by clocking the meter. If all good, combustion test each appliance individually and then each with all running. I would suggest test all venting by inches W.C instead of pascals. As rule of thumb for atmospheric combustion appliances the draft overfire should be a -.01"w.c. to a -.02" w.c. The flue draft should be about -.04" W.C. Most code inspections will not fail a chimney unless they have been running for at over 5 minutes with poor draft. Cold chimneys require heat up time to work and ay fail to vent properly until warmed up. Sadly, most modern appliances don't have accommodations for over fire testing. It was helpful in determining restricted heat exchangers. Consult your manufacture's specifications for all settings.