Oversized flue liner: min vs max BTU

HomeownerDustin

I need my flue liner replaced for 4 gas appliances (2-unit home) with max total output 264k BTU/hour.

• Current liner diameter is 5" on a 30' run, and the liner doesn't go to the top of the chimney (didn't know when we bought).
  
• Correct liner diameter is 7" for these 4 appliances at max 264k BTU ([https://lifetimechimneysupply.com/chimney-btu-calculator/2-or-more-category-1-appliances/get-liner-diameter.php](https://lifetimechimneysupply.com/chimney-btu-calculator/2-or-more-category-1-appliances/get-liner-diameter.php))
  
• We're debating replacing one boiler (45 years old) and DWH tank with a modcon plus indirect HW tank, so max output up the flue would be 132,000 BTU on the remaining 2 appliances, which corresponds to a 5” liner. The boiler that would remain is 20 years old right now (Burnham series 2).
  

Cheapest thing to do now is just put in a 7" liner and keep all our older appliances. Question: if we were to do a mod-con in the future, how important would it be to replace the liner again (ie go from 7” to 5”) to prevent back draft? In our 1.5 years, we have not ever had a problem with back drafting or carbon monoxide with the current 5" liner. The BTU's are carried up a flue liner even when just 1 is firing, so it makes me think that it won't be that much worse with a bigger liner. That said, a 7" liner has twice cross-sectional area of a 5", and 30% more circumference surface to heat up so that the gasses don't get too cool.

The long-term plan is to take over both units and just run 1 set of appliances.

If we did that, we could potentially just close up flue liner rather than resize it, and bring all the hydronics + hw to a single mod/con and indirect. Any thoughts on whether to convert the 45 year old boiler to a mod-con now are also appreciated.

House stats:

• 125 years old 2-family home, very leaky, balloon framed. We're hopefully getting insulation put in by next winter (contingent on renter getting energy assessment done).
  
• Unit 1
  • 45 years old boiler that burns 100k BTU, heating capacity 78k BTU (78% eff)
    
  • HW is 32k BTU
    
  • monoflow with old fin tube convectors
    
  • Unit 1 heat loss is 33.5k BTU/hr, determined empirically with heat bills per GBA.
    
• Unit 2
  • 20 year old boiler that burns 96K BTU, heating capacity 80k BTU (83.3% eff)
    
  • HW is 36k BTU
    
  • monoflow with mix of old fin tube convectors and baseboard fin.
    
  • unknown heatloss but probably around 40-45k BTU. Based on my unit's heat loss + they have the poorly insulated roof and bigger place.
    

Edit: just to be clear, I have no intention of putting mod con through the flue. If I take 1 units appliances out of the flue by getting a mod con, then the flue would use a 5” liner. If I don’t take 2 appliances out, I need a 7” liner.

Daveinscranton

I love mod cons.  Love them.  But only for really low temperature applications.  If you are out of condensing temperature range, more trouble than they are worth I think.  Maybe they could make sense for the modulating component only I suppose.

I cannot comment on the flue question.

Best wishes 

Bob Harper

The liner must be listed to UL 1738 for CAT IV venting under positive vent pressure with a modcon. There were one or two ss listed so listed but I'm not sure if they're still around. Find out the make and alloy of the liner.

STEVEusaPA

Why would you vent a mod con into the flue when you can direct vent with pipe thru the wall.

HomeownerDustin

STEVEusaPA said:

Why would you vent a mod con into the flue when you can direct vent with pipe thru the wall.

I’m not. Mod con would direct vent, so I’d only have 132k btu up the flue vs current 264 btu. 

HomeownerDustin

Bob Harper said:

The liner must be listed to UL 1738 for CAT IV venting under positive vent pressure with a modcon. There were one or two ss listed so listed but I'm not sure if they're still around. Find out the make and alloy of the liner.

I’d be direct venting the mod con so flue would have 1/2 the current btu (requiring a 5” liner vs the 7” currently required). 

EBEBRATT-Ed

Call the liner mfg and ask them