Two appliances connected into single vent

Take a look at the two pics, then:

The furnace is induced draft, the water heater natural. They are connected at the Wye. I know that IRC G2427.3.3 (4) allows these two appliances to join into a common vent connector. The water heater vent is located below the furnace vent (at the Wye). I checked the IRC and it says “where two or more vent connectors enter a common gas vent… the smaller connector shall enter at the highest level”… (IRC G2427.10.4). These seem to be the same diameter. I can’t find anything in the IRC code that mandates the water heater vent must be installed above the furnace in this configuration.

So** my question is:** Which one should be at the top? Why? Where can I find a reference?

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Smaller btu appliance gos on top.

That looks like a bad install.

Not only that. When an additional vent is added on, the vent connector size must increase.

Recommend upgrade…

Did you operate the furnace and check for backdraft out of the water heater flue?

Just guessing from the brick corbel , but is that in an attic area?

(not to mention the insulated supply vent)

How was the insulation and drain pan situation?

Depends on the code.

The first knee-jerk reaction is to assume you must have a larger vent connector that also should require a larger flue. The Canadian oil burning code determines fluepipe/flue sizes needed by (1) the total BTU input into the flue, (2) chimney height, (3) minimum base temperature of flue gases, and (4) insulation value of chimney walls

For example: A flue venting 2 appliances with (1) a total BTU/hr input of 210,000, (2) a chimney height of 28’, (3) minimum flue gas temperature of 320 deg F at fluepipe entry to chimney, and (4) a thermal resistance of less than 6 should have a mimimum inside flue diameter of 5" and a maximum of 7" or the equivalent if clay tile liner.

Just went through this exercise a few weeks ago when a young electrician was buying his first investment property of 4 apartments, each with its own forced air oil-fired furnace. The building only had 2 chimneys with each serving 2 furnaces.

The young fellow’s father (a nervous Nelly) accompanied us for the whole inspection (which added a lot of time and aggravation!!). He kept repeating that he didn’t think 2 furnaces could be vented into the same chimney/flue system. I was concerned with the total BTU input into each chimney flue as well as some other flue safety/oil line protection issues.

Actually talked to the head design engineer for the furnace manufacturer to confirm maximum oil nozzle sizes by design for these furnaces. There was no problem with 2 furnaces being vented by a 5" interior diameter stainless steel liner.

BTW, the call to the engineer earned me an invite to the manufacturer’s design and test labs…an interesting place. They just put a high efficiency (about 95%) condensing oil furnace on the market; they have a wood pellet boiler in the last stages of testing and certification; they’re building the first prototype of a 400,000 BTU commercial wood pellet boiler; they’re are about to start selling a 3 ton outdoor pellet storage hopper with automatic feed auger into the boiler…no more bags of pellets!

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Brian/David thanks for the input. It did not even cross my mind… I guess I was so fixated on the possibility of back draft coming out the water heater vent.
I did check for back draft over the water heater, both with a mirror and CO analyzer. I had the furnace running, water heater off (and on) and the utility door closed behind me. I could not get it to back draft at all. To be honest I was surprised by this, I really thought it would.
As for drain pan install, it was good. – yup it’s in an attic space.

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Exactly. :smiley:

From my earlier post:

“The first knee-jerk reaction is to assume you must have a larger vent connector that also should require a larger flue.” …if that thinking is continued forward!

Here’s a hypothetical situation, Brian:

You have a 6" diameter chimney flue serving a heating appliance. The fluepipe is 6’’ diameter. The homeowner wants to add a hot water heater (same fuel as the heating appliance) since their electric water heater costs are very high. The new water heater has a 5" breech size.

What do you do? Increase the vent connector size to 7" or 8" from the “Y” serving the 2 appliances to the flue and…then have to reduce it down to 6" again where it enters the chimney?

If you know the BTU inputs of each appliance, the flue gas volume output is then known, taking into account the excess air needs of each unit. Then it can be determined if the chimney and vent connector are suitable for the two appliances.

The figures I gave above are from a CSA standard that forms part of our national building code.

excuse me sir…
can I ask why is the natural draft appliance be on the top of the fan assisted one into a common vent?

Vent connectors serving appliances vented by natural draft shall not be connected into any portion of mechanical draft systems operating under positive pressure.

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