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jailbird371 05-29-2009 12:04 PM

Clearance for hot water heater flue.
I'm building a room in my basement and one of the walls will be right next to my gas water heater. What type of clearance do I need to keep combustible material (studs) from the metal flue. It gets pretty hot to the touch.


Termite 05-29-2009 01:36 PM

Type B (double wall) gas vent requires minimum 1" to combustibles. Single wall flues (what comes up from the water heater in most cases) require more....Can't remember exactly because it rarely comes up but for some reason I'm thinking 4".

If you're enclosing the fuel-burning appliance(s) in a small room then you also probably need to provide combustion air ventilation as well. It is a definite BIG safety concern and a lot of people don't think about it when finishing basement space. For every 1000 btu/h's of input rating on the water heater and the furnace (unless the furnace pulls air from outdoors) you need 50 cubic feet of space in the mechanical room.
For example, a 40,000 btu/h water heater alone needs 2000 cubic feet of room space just to provide it enough air for combustion. That's 50cf x 40 (btuh's x 1000)= 2000cf.

jailbird371 07-05-2009 09:04 PM

Right now there is a single walled vent leading up to the floor and then goes to a double wall up through the house. Sounds like it would be best to use double for the whole thing.

Anyone else know if it is indeed 4".

Gary in WA 07-05-2009 10:04 PM

If only the one gas unit: Size--minimum or 3"diameter...........IRC T2427.2(1) UMC 808.0
No single wall in attic or cold or concealed space2426.7.6 815.1
Sheet metal screws to appl. vent collar 2426.10.7 815.1
(three sheet metal screws at connection to vent 2426.10.12 815.1

Minimum clearance to combustible for single wall is 6 inches T2426.7.7 T3-3
Single wall may not pass through floor or wall 2426.10.15 815.2.2.6
3in.-dia. connector OK to 4in.-dia.flue collar if fan assisted appliance and vent height is >10 ft. 2427.2.2
Connector not >two sizes larger than flue collar 2427.2.10

Be safe, G

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