Undersized Flue Liner Replacements

Labenaqui

Here in small town, rural New Hampshire we have encountered our third undersized Flue Relining in as many years.
Customers & Contractors alike must refer to their Heating Appliance Manufacturer's Chimney Size Requirement prior to Chimney Relining!
It seems that just dropping in a 6" Dia. Liner with a Chimney Cap fits all ..... it doesn't.
We now have three (3) pulsating with initial blow-back Weil-McLain 2-Pass Gold Boilers.
One we did correct by upgrading to a Weil-McLain UO 3-Pass with a Beckett NX Pressure-Fired Burner. The other two are on our "Will Not Service List".

ChrisJ

Labenaqui said:

Here in small town, rural New Hampshire we have encountered our third undersized Flue Relining in as many years.
Customers & Contractors alike must refer to their Heating Appliance Manufacturer's Chimney Size Requirement prior to Chimney Relining!
It seems that just dropping in a 6" Dia. Liner with a Chimney Cap fits all ..... it doesn't.
We now have three (3) pulsating with initial blow-back Weil-McLain 2-Pass Gold Boilers.
One we did correct by upgrading to a Weil-McLain UO 3-Pass with a Beckett NX Pressure-Fired Burner. The other two are on our "Will Not Service List".

I'm just a homeowner, not a contractor or pro.

But it's my understanding you would need to look at the charts supplied by the liner manufacturer and look at the height vs diameter to see what btu/h you can use with that liner. There should also be a separate chart for if multiple appliances are being vented into it.


@Bob Harper Seems to be our expert on the subject.

Bob Harper

You need to do the sizing according to the code. Then you need to consider the nature of the unit. Some boilers are just more finicky than others and vent better with a little larger diameter liner. If you need a liner bigger than 6" smoothwall in a nominal 8"x8" flue tile, one option is to remove the flue tile. Not fun in an 8x8 but then again flue tile removal is never fun. If you have a liner size on the bubble, a smoothwall liner will exempt you from the 20% derating for a corrugated liner. You'll still need to derate for elbows and offsets. The bottom line is to run combustion analysis to verify its working properly.