Furnace + Water Heater + Chimney/CO2 Safety

[jjp2222]

I was having a conversation with an energy auditor about the safety of our water heater's venting (atmospheric into the chimney). I understand why the venting of a furnace and water heater into the same flue is beneficial and why you don't get the same draw in the chimney when you swap the furnace to a high-efficiency/direct vent. I also get why not drawing properly can be unsafe from a CO2 perspective and bad for the brick due to condensation.

What I didn't ask about or understand is this - the furnace only runs half the year (Oct-Mar in NJ) and at any given time during the heating season there's no guarantee the water heater and furnace are running at the same time. So you're not getting the benefit of the furnace burning to prevent spillage or backdraft from the water heater a lot of the time during the heating season and for half the year, not at all.

Why is it so bad (for water heater venting) to swap out the furnace to a high-efficiency, am I missing something? Thanks!

[BNME8EZ]

With a brick chimney the furnace is keeping the chimney warm every time it runs. In the summer it is not running through a cold attic so it does not cool down like it does in the winter.

When the water heater is orphaned into a brick chimney it often times can't keep the chimney warm enough so the moisture in the flue gases condense of the side of the chimney which runs down inside. This moisture is slightly acidic so it can eat the mortar out of the joints causing the chimney to deteriorate.

The other thing to consider is a cold chimney does not draw as well as a warm one. A cold chimney can have a downdraft which puts the flue gases into the home when the water heater is running.

[timjimbob]

Not enough heat to heat metal flue pipe. from a hi-efficient furnace. moisture condenses on metal and rots it.
Also might be dueto the fact that hi-efficient furnace is power induced burners that adds a positive pressure and may back feed into the draft hood of the water heater

[rundawg]

Why is it so bad (for water heater venting) to swap out the furnace to a high-efficiency, am I missing something? Thanks!

It's not. It is a myth.

When a furnace and water heater (WH) operate together, and the furnace is idle (not running), the flue gas temp is actually lower than when a WH is operating by itself into the flue (no other appliance).

The reason is, when the furnace is idle, you have about 20% dilution air going up the flue from the furnace, cooling the WH flue gas temperature further than it already is.

When the furnace is removed from the flue, the WH flue temp is now 20%+ higher.

I run a boiler and WH in my own home into a chimney, where in the summer the boiler is shut off and it has a vent damper to block any dilution air from going up the flue, and my WH works perfectly all summer long - drafts well and no condensation.

[rundawg]

I also get why not drawing properly can be unsafe from a CO2 perspective................

That would be CO (Carbon Monoxide) and not CO2 (Carbon Dioxide).

You can still have CO issues with a water heater only operation, unless a barometric damper and spill switch are installed in place of the draft hood.

Too much air going up the draft hood, and not through the burner of a water heater, can cause flue gasses to spill out the draft hood and not go safely up the flue.

[icy78]

That would be CO (Carbon Monoxide) and not CO2 (Carbon Dioxide).

You can still have CO issues with a water heater only operation, unless a barometric damper and spill switch are installed in place of the draft hood.

Too much air going up the draft hood, and not through the burner of a water heater, can cause flue gasses to spill out the draft hood and not go safely up the flue.

Good post! I forgot about that "off-cycle" dilution air from the other appliance!

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[BaldLoonie]

You probably won't have condensation in the summer, the chimney is warm. The 7 times rule in the NFGC says the common flue can't be more than 7 times the area of the smallest draft hood appliance. So the 3" outlet of a standard WH is 7 sq in. So if the tile liner of the chimney is more than 49 sq inches, it should be lined. We've had them condense so bad it is running out of the base of the chimney. Even if not, still will be condensing. That 40,000 BTU isn't going to keep the chimney warm and the dillution air isn't either.

[icy78]

You probably won't have condensation in the summer, the chimney is warm. The 7 times rule in the NFGC says the common flue can't be more than 7 times the area of the smallest draft hood appliance. So the 3" outlet of a standard WH is 7 sq in. So if the tile liner of the chimney is more than 49 sq inches, it should be lined. We've had them condense so bad it is running out of the base of the chimney. Even if not, still will be condensing. That 40,000 BTU isn't going to keep the chimney warm and the dillution air isn't either.

I'm going out on a limb here a bit and thinking out loud.

I'm betting that the condensation you see is due to insufficient draft and too-cool flue gases. (Drafthood)
Installation of the liner probably remedied a probable compromised / leaking chimney.

Now as another scenario, if you have a oversize (per NFGC) flue... more than seven times the appliance outlet, and you're drafting through that flue , that is a mass of air leaving the building, and will dilute the flue gases and prevent condensation in its own way.
Imagine a water heater dumping into a 24 in flue with 0.02" draft.
That piddly amount of flue gas will just disappear.



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[rundawg]

I'm going out on a limb here a bit and thinking out loud.

I'm betting that the condensation you see is due to insufficient draft and too-cool flue gases. (Drafthood)
Installation of the liner probably remedied a probable compromised / leaking chimney.

If you have a venting or condensation problem with a water heater that has been orphaned, making the flue smaller, and more restrictive to flow, likely isn’t going to fix the problem.

Ensuring normal flue temperatures, controlling the draft, and having proper building pressures will.