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Common combustion air piping

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Mynameisyourname
Mynameisyourname Member Posts: 2
in THE MAIN WALL
Is common combustion air piping ok with two heating systems?

I have a Lochinvar Knight WHN110 installed for heating and DHW.  This is currently vented up the chimney via PVC and out the sidewall via PVC for combustion air.  

We’ve added onto the house and our new HVAC tech has installed a separate system for the extension which is a Bosch IDS with gas furnace.  He had the combustion air PVC from the Bosch teed into the PVC for the Knight boiler.  Is that acceptable or does the HVAC tech need to have a separate combustion air intake?

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  • Zman
    Zman Member Posts: 7,569
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    They need to be separate. You don't want both appliances to be competing for the same air.
    If he disagrees, ask him to show you the page in the manual that allows it.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    mattmia2HVACNUTHomerJSmith
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,506
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    ...and it might not be enough air for both appliances.
    steve
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,002
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    Yes, that’s a bit sketchy and the manufacturer wouldn’t like it. But, given my enthusiasm for shortcuts, if the pipe runs are less than 20 feet and the pipe size is increased, I don’t see a problem. 
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • Mynameisyourname
    Mynameisyourname Member Posts: 2
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    Thank you all for the feedback.
  • HomerJSmith
    HomerJSmith Member Posts: 2,441
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    Using one combustion air piping for two appliances like the Lochinvar, would be I think problematic. You would be sucking air from two sources, from the outside and from the other appliance thru the heat exchanger, gas valve, etc and the other appliance's exhaust piping. Can Category IV appliances be common vented? Interesting. I pulled a Lochinvar installation manual and it shows separate intake and exhaust terminations for separate boilers. Call Lochinvar tech and ask. 615-889-8900

    I don't think the Knight gas delivery is shuttered.

    Alan (California Radiant) Forbes

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