Natural Gas Flue

[thunter78]

Hello,

I am looking into replacing my old electric water heater with a natural gas tankless water heater. In my water heater closet, there is an old 6-1/2" furnace flue that vents to my roof. Pictures: https://postimg.cc/gallery/kD8s0Jh

I had a contractor tell me I could not convert/re-use this flue for the natural gas water heater and he is now quoting me thousands on re-running water/gas pipes for an external model. This closet has no exterior walls, so we couldn't simply vent through the wall. There is also no other exterior wall that we could relocate a tankless unit to unfortunately (without remodeling).

It seems to me, even if I need a new roof attachment, I could at least run a smaller natural gas flue pipe inside the existing duct? I'm an engineer by trade, I am familiar with some UL and ASME codes, but I don't have any knowledge of building codes.

Can anyone think of a way around this, or know why we can't just use the existing flue? I can't imagine the amount of vented natural gas from a small tankless water heater would be very much.

Any help would be very much appreciated!

 

[TheCommish]

find out the make and model of the unit, read the manufactures instructions for the proper, and allowed methods to install. very doubtful on the reuse of the existing flue

 

[cda]

Hello,

I am looking into replacing my old electric water heater with a natural gas tankless water heater. In my water heater closet, there is an old 6-1/2" furnace flue that vents to my roof. Pictures: https://postimg.cc/gallery/kD8s0Jh

I had a contractor tell me I could not convert/re-use this flue for the natural gas water heater and he is now quoting me thousands on re-running water/gas pipes for an external model. This closet has no exterior walls, so we couldn't simply vent through the wall. There is also no other exterior wall that we could relocate a tankless unit to unfortunately (without remodeling).

It seems to me, even if I need a new roof attachment, I could at least run a smaller natural gas flue pipe inside the existing duct? I'm an engineer by trade, I am familiar with some UL and ASME codes, but I don't have any knowledge of building codes.

Can anyone think of a way around this, or know why we can't just use the existing flue? I can't imagine the amount of vented natural gas from a small tankless water heater would be very much.

Any help would be very much appreciated!

when you say closet

where is the closet located in the house??

A hallway a bedroom where?

What else is in the closet?

Is there natural gas pipe/ shutoff all ready in the closet?

 

[cda]

You might call the city and see if a nice inspector will come look at what you have.

Are you able to see that pipe in the attic?? And the condition of it

 

[north star]

$ ~ $

thunter78,

Welcome to The Building Codes Forum !

Q1): Do you know what type of exhaust pipe you have
[ i.e. - galvanized, stainless steel, transite, other ? ].
Q2): In your existing closet, do you have a natural gas
supply pipe already piped in ?
Q3): Do you have the right size of natural gas piping to
supply the new Tankless WH [ i.e. - enough of gas
volume ], or will another Gas Regulator be required ?
Q4): Where do you plan to install the Drain Line from a
Drain Pan ?.......Terminate inside or outside ?
Q5): Where do you plan to install the Drain Line from
the Temperature & Pressure Relief Valve ?.....Terminate
inside or outside ?
Q6): If you terminate both, Yes, 2 separate Drain Lines,
outside, how do you plan to provide a barrier to insects,
roaches, ants, other varmints and still have fully functioning
drain lines ?

FWIW, ...the amount of exhausted btu's will be a lot !
As an engineer, you are probably aware that the tankless
WH's use a lot of btu's to get the water heated quickly.
On the low side, 200,000 exhausted btu's is typical.
Larger sized tankless WH's have even more exhausted
btu's going up & out the exhaust pipe........That is a lot
thermal activity !

$ ~ $

 

[Robert]

Since you already have an electric water heater, have you considered using an electric tankless instead of gas? Running gas line and flues up there sounds pricey if you already have the 240v for elec. Here in CA, I predict the state will be phasing out ALL gas appliances due to the fire/explosion hazards associated with gas. Berkeley has already started....no gas appliances in new construction.

 

[Paul Sweet]

I think that California is actually banning gas because it produces CO2, which is causing global warming, which they say will kill us all!!!, and not because of fire hazards.

 

[cda]

I am wondering if 78 gave up on us?

 

[Robert]

I think that California is actually banning gas because it produces CO2, which is causing global warming, which they say will kill us all!!!, and not because of fire hazards.

Thanks Paul....I was thinking San Bruno but I think you are right.