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Sizing and configuring a draft inducer w barometric damper

I have a steam boiler with a built-in draft hood.  I have a 40' 5" diameter B-vent chimney installed all the way up the side of a 3-story 3-flat.  The boiler co-vents with a water heater.  At 5" diameter, the chimney is undersized quite a bit.  We installed it a couple years ago instead of putting in a chimney liner.  Everything seems to be working ok and no CO Alarms, but haven't had a combustion test performed.  In order to guarantee proper draft conditons, based on some research, my plan is to have a professional install the Draft-Inducer model DI-1 from Field Controls with the 'multi-appliance control' so it can work with both appliances.  I have a 5-7' stretch of lateral B-vent after the Y junction and before it exits the building.  It can be installed into this stretch. After it leaves the building it has a couple more elbows before the 40' riser. The individual venting for the boiler and water heater before the Y-junction are about 8 and 6 ft, respectively. 

My question is will there be turbulence in the system with so many elbows before the riser or will the draft inducer do its job?  The boiler has a 90 and a 22.5 before the Y-fitting----the water heater, is vented with 4" venting..  It has a 90 elbow, then is pretty straight.

Is there a good chance of success if I hire a skilled installer to install it so it creates the right amount of negative pressure up the flue?  My main concern is turbulence going thru the flue elbows.   

I saw another example on this board of a poorly vented multi-appliance co-vented setup but the vertical-to-lateral venting ratio was much more skewed than mine (about 3:2 vs 2:1). 

In terms of sizing the vents based on input BTUs--if I also install a barometric damper,  according to tables even with the draft inducer I should still be using 6" venting, though I'm just barely above the threshold for my existing 5" size.   In order to get within the recommended range, I will eventually replace the water heater with a more efficient brand or power vent it separately.   For now, based on the tables, I'm probably only about 30K BTU/hour undersized, mostly because of the lateral venting and extra elbows adding to the equivalent length of piping.

My thoughts were to install the draft inducer and just pay someone to do combustion tests to see if the setup is actually working as theorized.   If I hear a cacophony of "nos" here, however, I will just scrap the idea.  


  • Zman
    Zman Member Posts: 7,466
    The manufacture of the vent inducer has software that will do all these calcs. Send them a drawing of your vent and the specs for the appliances. This is not something you will find in a chart somewhere and frankly, you don't want the liability if there is an issue.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • George76
    George76 Member Posts: 27
    I sketched it up. See link below. Any suggestions? Comments?? It's 5" pipe, the chimney is 40'. The horizontal is about 15-20 ft. There are about 5-6 elbows, a couple 90s, some 22.5 or 45s.


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,236
    Just to be clear, no draft hood, each appliance gets it's own dual action baro with spill switch. Whether it works or not depends on what the manufacturer says like @Zman stated.
  • Zman
    Zman Member Posts: 7,466
    Fire that off to the manufacture and you will have your answer.
    The manufacturer's engineers are a great resource and you should use them.
    I have never had a code official reject a plan that is approved by the manufacture. Once they bless it, it should be smooth sailing.
    Always ask the engineer for the letter first, they don't much like writing them after the fact.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein