Vents close together

[Mark P]

The metal exhaust vent is being used by the water heater (it was previously used for a furnace) the concentric vent is for a new furnace. It seems they are to close together, but I could not find any reference in my search that says they are too close.  I already sent out my report and I explained my concern that the water heater's exhaust could be drawn into the concentric vent and to have the venting details looked at by a HVAC contractor and corrected if needed.  What do you folks say?

    

    

[Trent Tarter]

I agree it's a concern that should be looked at by an HVAC contractor. 

[Jim Katen]

I think it would be nice to have a rule to refer to, but I'm also unable to find one. 

Without a rule or other reference, the HVAC contractor isn't going to know what to do. After all, he or someone like him installed it in the first place. 

For things like this, in the past, I've asked the furnace manufacturer for a written opinion, and used that. 

 

 

 

[Tom Raymond]

Your concern for which appliance is going to draw exhaust is backwards. 

[Rob Amaral]

Well... chimney (AKA 'vent') terminations should be 3' above where they leave the roof and 2' above anything within 10' of the roof... the problem here is the water heater vent is simply too short..even if it was by itself.. . it's also probably an over-sized chimney now that the furnace was removed from it...   It's apparently an 'orphaned' water heater and that can make the water heater draft poorer and exacerbate the blah blah blah blah blah.. and lead to corrosion.. If the WH is older, I'd say replace WH with direct-vent and remove the old metal chimney...it's rusting pretty good ..   Does that make sense? (Especially the blah blah blah)?  As for the water heater exhaust getting entrained into the furnace intake there.. I doubt it but I guess it's possible..  

[Jim Katen]

7 hours ago, Rob Amaral said:

Well... chimney (AKA 'vent') terminations should be 3' above where they leave the roof and 2' above anything within 10' of the roof... the problem here is the water heater vent is simply too short..even if it was by itself.. . 

If only it were so simple. The rule you're quoting is for masonry chimneys that serve fireplaces.  Alas, gas B-vents don't follow the rules for "chimneys." Their minimum height above the roof depends on the slope of the roof and the diameter of the vent. 

 

7 hours ago, Rob Amaral said:

it's also probably an over-sized chimney now that the furnace was removed from it...   It's apparently an 'orphaned' water heater and that can make the water heater draft poorer and exacerbate the blah blah blah blah blah.. and lead to corrosion.

Maybe, or maybe not. There's quite a bit of leeway when you actually do the calculations.