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Swapping gas orifices

Has anyone ever done anything with the orifices for a gas fired residential steam boiler? One option is to use orifices with smaller openings, another is do replace orifices with plugs that don't have holes drilled in them.

What tests would be needed to ensure proper combustion?

Are there any combustion gas issues that might occur with less energy driving the draft?

Just food for thought.


  • Mad Dog_2
    Mad Dog_2 Member Posts: 4,407
    You MUST have an analyser and know how to use it.. its very serious business..  prepare to spend the days tuning and tweaking it.  Also, be aware that you are modifying original equipment and need to contact the engineers at that boiler ls company or void the warranty  Mad Dog
  • Tim McElwain
    Tim McElwain Member Posts: 4,580
    What is your reason for wanting to do this? I would assume if the equipment has not been altered then the correct orifices are already installed and the equipment is operating at the correct input?

    If you are looking to de rate the boiler then I would contact the manufacturer for their procedure to do anything to the boiler.

    I would also suggest you hire a professional who is familiar with gas equipment and derating.
    Mad Dog_2
  • Mad Dog_2
    Mad Dog_2 Member Posts: 4,407
    Yeah..I've done alot of this work and it's REALLY Cool and gratifying but its not for DIYers, tinkerers, et al. You should be a 
    1) Licensed Master Plumber or under his direct supervision. 
    2) very familiar with boilers and combustion,  flues, draft.
    3) Have a Bacharach or Testo Combustion Analyser and know how to use it
    4) Alot of time & patience .
    5) Liability Insurance.    

    Mad Dog  
  • ethicalpaul
    ethicalpaul Member Posts: 4,445
    edited March 15
    I would never even experiment with changing the size of the orifice. The burner was designed for a specific orifice at a specific pressure. With a smaller orifice, the velocity of the fuel into the burner will be directly affected.

    I have experimented with temporarily blocking an orifice. I used a small piece of aluminum tape. My old oversized boiler had I think 6 burners so I took it down to 5. It seemed fine. I did not have an analyzer. I ran it that way for a month or so but once I got all my venting in order I found I could run without hitting cut-out pressure so I removed the tape.

    Due to the design of the gas valve, the pressure will remain at design spec. Individual burners will not "know" that one of them has been blocked.

    Given the design of atmospheric boilers, it seems to me that there is plenty of "slop" where if there were a missing burner, the input (the area under the boiler-- a wide open hole) and output (the flue hood and flue-- a wide open hole) are not really going to affect combustion noticeably. Happy to be proven wrong, but please include harmful, observed analyzer output caused by a blocked burner before tearing me a new one).

    Here are some other places of "slop" when it comes to flues/exhaust that I believe make the reduction of a burner be insignificant to the combustion:

    1. How much air can get to the boiler? How many installers close up the house, turn on every exhaust fan and test combustion air flow like the manual tells them to?

    2. How big is the chimney flue or liner? Are you going to change it if you go with a larger or smaller boiler? How big is the flue pipe? They only come in 1" increments. Is the 7" pipe perfect for all the boilers it is allowed for? How much tolerance is there?

    3. My boiler (model 63) came with an optional reducer from 6" to 5" right from Peerless that I could choose to use or not. How many installers test the airflow to use the correct one?

    4. The big concern seems to be too little combustion air flow, limited by the house or the room the boiler is in. I believe that in thinking about too much air flow, it is somewhat regulated by the number of burners--fewer burners will demand (and create!) less air flow, somewhat regardless of the size of the draft hood.

    I'd love to hear from anyone who did this and had an analyzer at their disposal. Just thinking about the percentage change, I wouldn't block more than one for a 5-6 burner boiler, nor more than 2 for a boiler with 7-11 burners. I wouldn't block two near each other. I would only consider it after all venting was as good as possible and if the cut-out, cut-in cycle were still ridiculous, like if it starts cycling before the steam even gets to the radiators.

    I have heard people talk about the idea that a missing burner will create a "cold spot" that could harm the boiler. I guess it's possible but it seems unlikely. The boiler doesn't really get that hot--it is moderated by the water which is sloshing around inside it.

    Disclaimer: this is all straight out of my butt for purposes of discussion, and not advice or instructions!
    1 pipe Peerless 63-03L in Cedar Grove, NJ, coal > oil > NG
  • ethicalpaul
    ethicalpaul Member Posts: 4,445
    edited March 15
    Here are the vent sizes for the Peerless 63 series (as an example). Note how boilers with different numbers of burners share the same vent size. For the (63-03, 63-03L) and (63-04 and 63-04L) (the "L" has one fewer burner), you can install an optional reducer if you feel like it (!!) depending on what's handy for you I guess?? So we can see how precisely engineered the combustion airflow is on these.

    1 pipe Peerless 63-03L in Cedar Grove, NJ, coal > oil > NG
  • SteamingatMohawk
    SteamingatMohawk Member Posts: 832
    edited March 19
    Since more than one discussion on HH has to do with oversized boilers, over the 33 years I have had my steam heated building, I have speculated on reducing the gas input so as to reduce the heating capacity. I haven't actually done anything to experiment for just about all of the above mentioned reasons.

    Great feedback.