u/h's sharing same venting

[iceage]

Came across a Lennox U/H with ventor motor sharing common vent with ICP U/H with natural draft. Besides code, why is this a wrong situation?

[hvac69]

Venting into a chimney, B Vent stack or like a side wall? If its a properly lined chimney or B Vent stack and its sized properly why couldn't they share?

[superfittertech]

natural draft water heaters with a draft induced furnace is installed on millions of residential homes without any problems, provided closest one to the main stack is "saddled in"

[RuralSpaceman]

No code issue in BC, as long as the vent and both connectors are correctly sized.

[hvacpope]

Ideally the power vented unit should be tied into the chimney by itself, above the natural draft unit.
Recently I responded to a CO spill coming from a water heater sharing a common vent pipe with a power vented boiler, when both appliances ran at the same time the positive pressure created by the boiler limited the water heaters combustion air and venting resulting in poor combustion,soot and CO spilling.

[RuralSpaceman]

Have to clairify myself. No problem so long as both units are a Cat I appliance - nameplate would say so. Cannot connect atmospheric unit into postive pressure vent for obvious reasons.

[billygoat22]

Have to clairify myself. No problem so long as both units are a Cat I appliance - nameplate would say so. Cannot connect atmospheric unit into postive pressure vent for obvious reasons.

That hasn't stopped people from trying. Saw a positive pressure furnace and wh on same pvc flue one time. hx in furnace got the smackdown from wh backflow