Draft induced furnace venting with water heater

Hi Guys
My friend recently had an 80% draft induced forced air gas furnace installed. The furnace shares a b-vent with a natural draft gas water heater. The water heaters vent connector is installed correctly above the furnaces vent connector. I read on the nachi site that natural draft appliances should not share a vent because of the pressure that draft induced furnace creates. The furnace and water heater are in enclosed room ,but they do have sufficient air supply. Should he worry about this or are their new guidelines permitting this installation

Jeff Prince
Safe At Home Inspections,Inc.

The way I understand it is like this,

Under category IV forced venting codes:
No natural and forced vent allowed to a common flue.

So, if you have a 80% furnace as you stated it would be allowed.

Some local codes may require separate vents. It is always best to check with local agency.

Don’t follow you. Isn’t jeff saying that he has a power vented furnace and a natural draft water heater on the same flue?

No he stated induced draft.

Doesn’t induced draft mean power vented, as opposed to natural draft.

Power venting is different than induced draft.

A draft inducer is mounted in a furnace. Modern furnace heat exchangers are narrower plus have multiple passes through the heat exchanger and require a draft inducer to pull the vent gases through the heat exchanger. Once the vent gases exit the furnace natural drafting takes over.

Older furnaces that are natural draft, without inducers, have wider passages that allow the natural drafting of vent gases through it.

A power vented system is a system that has a powered drafting fan at the termination of the vent pipe. Properly installed on a call for heat the power venter starts. Draft is proved and then a control signal is sent back to terminal W onthe furnace to start the furnace ignition process.

All of the 80 percent efficiency furnaces that were side walled vented with the recalled HTPV (High Temperature Plastic Vent) were replaced with a barometric damper, double wall B vent and a side wall power venter.

Thanks Gary. I guess some folks use the terms interchangeably.

Yes, but he does not have a cat IV furnace so it would be ok unless some local AHJ decided to have their own interpretation.