santa99

I presently have a 81% mid efficient furnace (132k btu/hr input) & a 42k btu/hr draft hood hot water tank common vented up a common 5" B-Vent (approx 30' total length). It is a code installation where i live.
I am hoping to soon start a ~500 ft2 renovation / addition which may require upsizing my furnace or adding an additional furnace. The easy solution is go high efficiency (hence separate pvc vent) but i am curious if it would be possible to vent an additional (say 60k btu/hr max) furnace up the same b-vent?
Dont worry, i will have the work hired out / permitted appropriately. I am just in the cost estimating / scoping phase & figured i would post here for response prior to calling a local qualified contractor.
Note: the addition is incremental 2nd story square footage hence i may be ok with existing furnace (per my own heat loss calculations on local degree heating days on house).

glennjanie

Welcome Santa:
If you go with a larger furnace (if the existing one is over 15 years old) you may need larger vent. You are correct about the PVC vent for the higher efficient furnace which would not affect the present vent.
Furnaces have been sized downward with the new ones; we used to want 'pull down' power but, now the furnace is sized to the cubic feet heated and heat losses factored in. In that case your present furnace may be sufficient.
Glenn

santa99

Sorry, a little more info - exisiting house (and furnace) is only 5 years old. Good furnace, Carrier 58 STA/STX. The other driver here is that i have AC on present furnace and dont want to have to add another AC unit on 2nd furnace (for cost reasons). I will likely just have additional small high effieciency installed to avoid messing w/ existing vent (or any probs / backdrafting that could be associated with adding to existing vent). Will likely put 2 stories above grade on the big mid effecient one (with AC) and run basement on separate zone from smaller high efficient one (without AC). Just wanted to make sure options were evaluated. At present cost of nat gas the payout differential on mid vs high eff is ugly. Perhaps nat gas wont be so cheap forever but i think it will be for a few years.