Shared Flues

[vgrund]

On "vacation" at my inlaws' pad (I use the word vacation loosely), out of curiosity, I opened the closet containing their gas fired furnace and hot water heater. The closet is in a finished basement in their townhouse. Both appliances draw room air for combustion but the interesting thing is, both flues join in an upside-down Y configuration (bottom of the Y heads out). Wow. I thought that was a no-no. Their place is old, but it can't be older than the late 80s.

Victor

 

[Mike Wilson]

I think that its only illegal if they use different fuels. I had an oil boiler and an oil hot water heater in the past, and they both used the same flue.

-- Mike

 

[elkimmeg]

If using the same fuel and the chimney liner has enough capacity it is ok.

Explanation the gas hot water heater probably has a 3" flue collar It by its self it never could generate enough heat to draft the 8/8 masonry flue
Say an 8/8 masonry flue has a 48" cross -sectional area. The 6" flue is 26 .25" and a 3 " is 7 " 4" is around 11 /12"
Even combined they do not exceed the cross-sectional are but colectively each enhances the ability of the other appliance
Code wise the smaller flue should enter above the larger one

 

[vgrund]

This forum is a never ending source of enlightenment. Thanks.

 

[Gooserider]

A few years back we replaced our NG furnace with a high efficiency unit that pulls outside air and vents through the side of the house via 4" PVC sewer pipe, leaving the current NG hot water heater the only thing on the gas flue, but the old furnace shared with the hot water heater.

My understanding is that at least until recently (I'm not sure about the latest and greatest code versions - I don't get paid for it) it was OK for an NG furnace and hot water heater to share a flue under certain conditions. I think they had to both be on the same floor and next to each other, I'm not sure what other rules applied. I think it may also be OK for an oil furnace to share with an oil water heater, but I'm not sure.

Sounds like an Elk call...

Gooserider

 

[elkimmeg]

Goose brings up a good point. When replacing you gas furnace with the high efficiency PVC vented one. The furnace installer is obligated by code to address the gas hot water heater as well

A 3 or 4" vent does not draft well alone in an area up to 5 times larger. That is venting the 3" gas hot water is in an 8/8 flue

To satisfy the heaters drat requirements or hot water needs there are 3 options than can be employed

#1 line the exiting chimney with the gas hot water heater with the 3 or 4" gas liner kit

#2 install an in line draft inducer

#3 Remove the gas hot water heater and install an electric one

For those that do not have code books, just read the burner's owner/ installation manual. I sure there is language addressing your hot water heater


Goose is correct, the same arangement can by done withan oil burner and oil fired hor water heater

 

[vgrund]

Interesting. I have an LP water heater that vents 3" into what I must assume is an 8x8 masonry flue. I had a new one installed last January and no such considerations were mentioned. Perhaps it is grandfathered because I changed nothing else. Or perhaps I am wrong about the flue size. It seems to draft well. The water heater is using one of two flues in the chimney, the other being an open fireplace on the 1st floor that is certainly an 8x8 flue (I'm lining it in two weeks and installing a gas insert).

Victor

 

[elkimmeg]

V you do not have to take my word for it. Just read the manual to the gas hot water heater. It is quite obiuos the installer did not open it up.

The danger is the posibility of backdrafting, If the 3' vent does not heat up that liner to draft and looses the battle of reversing the down draft flow, then there is only one way for it to go backwards. That installation is required to be lined. It's not grandfathersed, if installed as recent as 1993. The Cross-sectional codes apply to gas appliance as well. If a 3 inch
vent, one cannot expect a decent drant into 700% larger area. If you do decide to get lined alumium liner are cheaper and code compliant but life expectancy is 8 to 10 years
Stainless steel cost more money, but you will have change the Alumium 3 more times to equal the stainless life expectancy.
I thought I should let form member know of the limitations, of cheap alumium gas liners.

 

[saichele]

This is all well and good, but I have the same set up described (NG furnace with 6" flue; NG DHW with 3") venting into an 8" (ID more like 6") tile chimney. Thng is, the furnace is switched off 320 days a year or thereabouts. so 80% of the time, the water heater is the only heat source in the tile. Even in a 'normal' house, the furnace isn't likely to run often enough to keep the stack warm, certainly not in the summer. It would seem like a dedicated 3" liner would almost be a requirement.

Steve

 

[webbie]

The gas liner thing is one where the utilities and code authorities learned on the job. Millions were installed into clay liners and even unlined chimneys....then, as units became more efficient, the problems started. By the 1990's most utilties were red-tagging any gas installs into masonry chimneys without a liner.

It is quite common, as other mention above, for MANY gas appliances to be vented into a single stack. This is also done in commercial and industrial heating equipment. The code addresses SOLID fuels (wood coal) not being vented into the same chimney as liquid and gas fuels. I also would guess there are issues with forced fuels (like oil) as opposed to natural draft appliances.

 

[Jay H]

My gas water boiler (for heat) and my gas water heater (for hot water) share the same flue, although it is on a T connector and not a Y connector as it's supposed to be. Of course, my water boiler is off so in reality, it's not really shared in working

Jay

 

[Metal]

From the GAMA venting tables:

"The flow of the largest section of vertical vent of chimney shall not exceed 7 times the smallest listed appliance categorized vent area, flue collar area, or draft hood outlet area unless designed in accordance with approved engineering methods."

also:

"Masonry chimneys used to vent Category 1 central furnaces must be either tile-lined or lined with a listed metal lining system or dedicated gas vent. Unlined masonry chimneys are prohibited."

and lastly:

"A Category 1 appliance must never be connected to a chimney that is servicing a solid fuel appliance. If a fireplace chimney flue is used to vent this appliance, the fireplace opening must be permanently sealed."

So, don't vent into a chimney that has an area 7 times larger than the smallest appliance vent, make sure it is tile lined or metal lined, and never share a chimney with a solid fuel appliance. If anyone would like a copy of the GAMA venting tables, let me know and I can send you one.

 

[elkimmeg]

Metal, thanks for the clarification according to the National Gas codes, I do have the international Gas codes here but, not being a gas inspector,
I do not work with them often. Naturally the installer, who is licenced ,should have knowledge of the code, But we find out everyday this is not the case

Also every owners manual is required to detail draft requirements, as part of their listing. Too bad nobody takes the time to read them?

 

[Gooserider]

Goose brings up a good point. When replacing you gas furnace with the high efficiency PVC vented one. The furnace installer is obligated by code to address the gas hot water heater as well

A 3 or 4" vent does not draft well alone in an area up to 5 times larger. That is venting the 3" gas hot water is in an 8/8 flue

To satisfy the heaters drat requirements or hot water needs there are 3 options than can be employed

#1 line the exiting chimney with the gas hot water heater with the 3 or 4" gas liner kit

#2 install an in line draft inducer

#3 Remove the gas hot water heater and install an electric one

For those that do not have code books, just read the burner's owner/ installation manual. I sure there is language addressing your hot water heater


Goose is correct, the same arangement can by done withan oil burner and oil fired hor water heater

Hmmm..... I hadn't thought about the relative areas part of it. I know there weren't any changes made to the stack when they put in our new furnace. (They did move the heater, but only a couple of feet to make everything fit better.) The WH uses 3" OD galvanized pipe coming out of the heater, then T's into a 6 galvanized" pipe going up into a larger (8"? OD) insulated looking metal pipe, I'm not sure what ID, that goes straight up through the roof, on the other side of the house from the woodstove chimney.

Does this sound like it would be a problem? FWIW, I've never seen any indication that it would be.

Goosrider

 

[elkimmeg]

goose part of my post is to make others aware of potential issues that may never occure. Remember our CO detector discussion , well here is a good reason to monitor your situation
I would suggest one some where near these two appliances A potential is there

 

[Gooserider]

goose part of my post is to make others aware of potential issues that may never occure. Remember our CO detector discussion , well here is a good reason to monitor your situation
I would suggest one some where near these two appliances A potential is there

Well, as you may recall from that discussion, since I have hardwired detectors that are sort of in the right place, I want to replace them first. They currently have Firex smoke detectors installed that theoretically aren't due for replacement for another three years. When I wrote to Firex Tech Support for advice as to a replacement combo smoke / CO unit, they said they didn't have anything that met current MA code, but would in a few months.

I don't know if I could swap all three out for a different brand, as I'm not sure what the wiring requirements are for the different units (reccomendations welcome) but I'm inclined not to just to avoid the hassles of changing the wiring.

Looking at the potential CO hazards, I'd list them as:

1. The living room woodstove - probably the greatest potential risk.

2. The kitchen cookstove (gas) - not vented, but AFAIK, gas stoves aren't big risk items if they are operating properly. This one is a fairly new stove (~4yrs old), w/ sealed burners, and does not give any symptoms of not working well.

3. The Hot Water heater, gas - been in that location a long time, never given trouble, don't expect it to, low risk

4. The gas HVAC furnace - Fairly new (~6yrs old), high efficiency unit, (hopefully) never even runs, very low risk

5. The gas clothes drier in the laundry room - Another fairly low risk unit, and if it were to put out CO, it is close enough to the stove that it would probably trigger on anything that the stove would set off.

I do have ONE stand alone, AC powered, CO detector that I used to have plugged into the garage ceiling. I have pulled it from there, and plugged it into an outlet in the living room near the woodstove, since I figure it's the highest risk. The location isn't optimal, but it's the best I can do short term.

My current plan is to wait until the new MA-Code-Compliant Firex units come out, and replace the existing smokes with them. When I do that, or when burning season ends (whichever happens first) I'll move the stand-alone detector to the basement room where the furnace and water-heater are at.

I might get an additional standalone unit for the laundry room, but I'm not sure it's worth it.

This isn't optimal, but I think it is an improvement over what we were doing, and will eventually get us to be compliant w/o spending huge amounts of money.

Gooserider