Venting options

PS101

I am about to replace my boiler and am studying my options.  A high efficiency condensing boiler is both attractive to me and has serious downsides.  Specifically, I have an old house.  The best location for the boiler is where it currently sits, and where the pipes terminate.  Unfortunately, it is below the fireplace so the first floor wall is stone and is very thick.  Direct venting is a challenge.  One option is to run the vent to the side and out a basement window in the outside stairwell and up the side of the house.  This isn't very attractive because 1) it will obstruct part of the stairwell and 2) a stairwell is not a good place for condensation.   I am also not crazy about corrosive exhaust coming out anywhere at a low level on the house.  There are not other aesthetically acceptable direct venting options I can think of. 

For those reasons I am leaning towards getting my chimney relined and buying an 82% efficient boiler. 

I am also thinking about getting two small boilers instead of one larger one (3,400 square foot house.)  Most of the time only one will be running.   I think I can gain efficiency that way. 

I have a question whether both boilers can be vented into the same flue.  I have heard "yes" and to size the flue for 50% above the requirements for he largest appliance.  My gas water heater also vents into this flue.

Does anyone have an opinion on this?  FYI, the cost of the condensing boiler and 82% boilers are the same considering the tax break and cost of a chimney relining.

Thanks for any feedback.

Tim McElwain

It is not an opinion I

have on this but it is code. The rule of thumb is to size the common vent to the area of the largest plus 50% of the additional. Better however to size according to the charts in NFPA 54 National Fuel Gas Code. Yes they can be vented together into a common vent. If however you are installing a liner then the liner has 20% less capacity than a regular double wall "B" vent.



If you get hold a professional contractor they can look at your home and give you the venting options along with equipment that can be used to meet your needs.

binskibok

Vitodens 200 venting

I'd like to use the Viessmann concentric regarding my newly acquired boiler but the outside termination from my basement is only 18" above the ground level. This does not seem to be above the recommended 1 foot minimum snow line for New England. My next best option would  be to use  the room dependant air with the outdoor snorkel. What would be the pros and cons  of using this seperate venting option? Since my basement (24'x34') is unfinished do I need to have a combustion air opening to the outside?       

Tim McElwain

Assuming the 24 x 34 is unrestricted

and wide open you can have up to 130,000 BTU's in that space before you would have to bring in outside air.



Your space 24 x 34 x 8 (height) is 6,528 feet at 50 BTU's per cubic foot = 130.56 times 1,000 BTU content of a cubic foot of gas = 130,000 BTU's allowed in that area without bringing in outside air.