Oversized gas exhaust flue for boiler and water heater

[Magicalpig]

I'm looking for some opinions concerning size requirements of a flue service an atmospherically vented water heater and a fan-assisted boiler. I tried to have the flue certified while applying for a permit to replace my water heater and the chimney company would not give me the certification. They say that the IFGC charts indicate my flue is oversized for the BTUs going through it. I'll just assume that's true for the purpose of posting this question.

I contacted the company that installed the flue liner and they never heard of failing a chimney inspection in a residential application for the reason that the flue was oversized. I'm wondering if the company I hired for the certification was trying to get me to pay them to install a new flue liner. Further, if an oversized flue is really an issue in residential settings, why is it that the code does not take into account the fact that during the warmer months of the year only the water heater will be used. That effectively sets up an orphaned appliance situation during those times.

What do you all think?

 

[hj]

Merging an atmospheric vent and a power vent in the same chimney is asking for trouble.

 

[Sponsor]

 

[Magicalpig]

So, hj, you don't recommend putting them on the same vent even if done to code?

 

[hj]

I don't think connecting them to the same chimney could be "done to code", because the power vent COULD overpower/backdraft into the atmostpheric vented one and cause CO in the building. Power vents almost always have to be individually vented to the exterior.

 

[Magicalpig]

I've been reading up on this a lot lately, and my interpretation of the code is that it is allowed because both are category I appliances. The power vented boiler does not create positive pressure in the flue. There's a good write-up on the situation here: http://www.ashireporter.org/HomeInspection/Articles/The-Venting-in-Common-of-Multiple-Gas-Appliances/2067