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Came across a one family dwelling (house) today that was built in 1990. The furnace was located in the crawlspace and the water heater on the first floor. The 2003 International Residential Code book clearly states that two gas appliances cannot be installed in a common natural daft venting system when those two gas appliances are on separate floors. I came across a house that was built in 1990 where the 80 plus gas furnace is located in the crawlspace and the standard 40 gal. water heater is located on the first floor. The house was clearly built this way so I'm wondering if this code existed when the house was built. Again, I am referencing the 2003 IRC M1801.11.

Another question. If the type of venting noted above is not allowed, why not. What is the difference if the water heater and furnace are both vented on the first floor or on different floors. I've been told in the past the reason is in case of back drafts. But if the unit is going to back draft, I don't see the difference if the water heater back drafts with the furnace on the same floor or on a lower floor than the water heater.

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Originally posted by qhinspect

Came across a one family dwelling (house) today that was built in 1990. The furnace was located in the crawlspace and the water heater on the first floor. The 2003 International Residential Code book clearly states that two gas appliances cannot be installed in a common natural daft venting system when those two gas appliances are on separate floors. I came across a house that was built in 1990 where the 80 plus gas furnace is located in the crawlspace and the standard 40 gal. water heater is located on the first floor. The house was clearly built this way so I'm wondering if this code existed when the house was built. Again, I am referencing the 2003 IRC M1801.11.

Another question. If the type of venting noted above is not allowed, why not. What is the difference if the water heater and furnace are both vented on the first floor or on different floors. I've been told in the past the reason is in case of back drafts. But if the unit is going to back draft, I don't see the difference if the water heater back drafts with the furnace on the same floor or on a lower floor than the water heater.

Chapter 18 doesn't apply to gas appliances. You should be looking in Chapter 24.

I'm not aware of any restriction on gas appliances on different floors sharing a common vent.

- Jim Katen, Oregon

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I'm trying to understand what M1801.11 is refering to if it doesn't refer to multiply-appliances on different floors. Why doesn't M1801.11 apply? Chapter 18 is titled Chimney and vents and M1801.11 talks about multiple appliance venting systems. A furnace and a water heater by the definition in the same book is considered an appliance. Now there are exceptions when it comes to M1801.11 but from quickly going over them, I don't see how that applies to this situation.

When it comes to chapter 24, I'll have to look at that a lot closer.

M1801.11 Multiple-appliance venting systems. Two or more listed and labeled appliances conected to a common natural draft venting system shall comply with the following requirements:

1. Appliances that are connected to common venting systems shall be lcoated on the same floor on the dwelling.

Exception: Engineered systems as provided for in Section G2427.

2. Inlets to common venting systems shall be offset such that no portion of an inlet is opposite another inlet.

3. Connectors serving appliances operating under a natural draft shall not be connected to any portion of a mechanical draft system operating under positive pressure.

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Originally posted by qhinspect

I'm trying to understand M1801.11. Why doesn't M1801.11 apply? Chapter 18 is titled Chimney and vents and M1801.11 talks about multiple appliance venting systems. A furnace and a water heater by the definition in the same book is considered an appliance. Now there are exceptions when it comes to M1801.11 but from quickly going over them, I don't see how that applies to this situation.

When it comes to chapter 24, I'll have to look at that a lot closer.

It's a common and understandable error.

Chapter 18 really applies to oil & solid-fuel appliances. It defers all gas appliance venting to Chapter 24. Look at the first paragraph of Chapter 18. Toward the end it says, "Gas-fired appliances shall be vented in accordance with Chapter 24."

The only places where Chapter 18 mentions gas appliance venting is where it has an impact on oil or solid-fuel appliance venting.

- Jim Katen, Oregon

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