Oversized Water Heater Flue?

The furnace and water heater were originally connected to the same flue. 10 years ago they installed a high efficiency furnace so now the flue is solely for the water heater. But in the attic it looks like they just used the same size flue all the way up. Does that create a draft problem?

Also, this is a single-wall flue in the attic correct? I’m planning to note that a double-walled should be used in the attic and should have a 6" clearance from combustible materials.

Just double-checking to see what you guys think about the larger flue diameter being connected to just the water heater. In the attic the diameter measured around 7" and above the roof measured around 6". I didn’t get a good picture of the adapter from the smaller water heater flue to the larger one. But you can kind of see it in the second picture.


Yes ,an issue.

Should not reduce in size as it goes up.

I’m also interested to know, for future reference, is it okay to remove the furnace and still keep the same size for just the water heater?

The need to be sized
There are charts all over the internet based on among other things,BTU,flue length,number of elbows,etc.

Here is one type…Vent sizing.jpg

When the original natural draft furnace is removed and changed to a sealed combustion high efficiency furnace, the term for the water heater left connected to the shared flue is typically called an “orphaned water heater”. Do a search on “orphaned water heater” for more info.

This can become a problem as the water heater may not be able to draft properly causing it to backdraft.

When I see this, I call it out, explain that an orphaned water heater exists, explain what that means, why it’s a problem and tell them to have a licensed qualified plumbing/heating and cooling/chimney professional evaluate and repair as necessary. I would also have the professional evaluate and repair the entire flue vent.

I should not forget to mention that in the above case a larger opening is fine as far as everything I have read goes.

Better to big than to small…:slight_smile:

Can you explain what you mean by this? I’m not following…

Thanks Andrew, that’s kind of what I thought. However, Bob you just confused us. Since a flue should be sized according to multiple factors, if a piece of equipment was removed, it would seem it would no longer be appropriately sized…too big. But if too big is no problem, it would seem to be okay to remove a piece of equipment and not change the flue size?

I read that in one of the Docs.
I will find it later this morning when back.

No time to search the PDF but here is a link to the site that should have it ,and it is a very good site from what I can tell.

I have the PDF from that site but it is to large a file.

OK I found the page link and I hope this helps.
Great PDF!

I believe the science is that if the flue is over-sized, the gas velocity is not adequate particularly around the walls. Gases along the walls can cool and fall, while the hot gases travel up through the middle of the cooled and falling gases. In this case, the cooled gases would likely only be able to flow down to the adapter. At that point, they could either choke the hot rising gases or cool them, further exacerbating the problem. I can see where a column of cooled gases could accumulate in the flue and then backflow once the water heater burner is turned off.

Makes sense but how do we explain Manifold systems in multi unit buildings.

Not everyone will use the Furnace on a given day and no issues exist , if you follow me.
B vents are branched into a main larger vertical flue and on some days only one unit will be on.

The problem that will happen with oversized chimney flues in cool/cold climates is that the velocity of the flue gases is slowed down.

For example: a 5" fluepipe exits into a 7, 8 or 9 inch flue, especially if the chimney is totally outside the heated building envelope. (the wrong place for chimneys)

(1) the chimney was sized for a much larger volume of hot flue gases to be exhausted…which will keep the interior of the flue warm/hot

(2) the smaller volume of heated flue gas contains less heat to heat the liners and keep them warm

(3) the velocity of the gases reduce after entering into the chimney, allowing them to wash the sides of the flue, giving up heat to the exterior and becoming cooler and less buoyant…therefore losing draft

(4) losing draft can affect the combustion efficiency by not drawing enough air into the fire chamber to get complete, clean combustion of the gas

Also, if the chimney gets cool enough and can’t be maintained in a somewhat warm state, you can get condensation of exhaust gases on the interior of the flue liner. These gases contain sulpher and nitrogen oxides that form acids that can corrode the metal or even the mortars in clay tiles.

The worst case of an oversized flue that I ever consulted on was for an EPA certified wood stove served by an outside masonry chimney with a 12"x12" clay tile liner on the north side of a house. It was January and the flue would not draw strongly enough to keep the fire in the wood stove going. I was in the rural area giving public talks in the evenings so had the AM off and went to visit the home.

After I arrived, I learned that this was the third wood stove that they had installed on this chimney with each one being larger than the previous…but still with no success of having the flue warm up enough to keep a strong fire going. The wood stove vendor had taken back the first 2 stoves and supplied the current one installed at cost in an effort to have happy customers. Everyone thought it was stove problem!!

Even with the new larger BTU output stove, upon lighting the fire, they would have to keep the loading door open (overfire the stove) to get the flue warmed up with lots of hot gases. When the door was shut, the supplied combustion air, and hence fire, was much reduced so the flue would cool off within an hour and the fire would start to die out…lack of combustion air caused by lack of draft.

The local thinking that was that a bigger flue was better!! The stove had a 6" diameter fluepipe (28 sq in) that vented into the 12x12 flue (10x10 interior dimensions). The flue x-sectional area was almost 4 times the size of the stove outlet and fluepipe. My suggestion was to reline the 7-8 month old chimney with 6" diameter stainless steel…problem solved, I guess, as I never had a call back from the homeowners.

Venting tables also list minimum rise from appliance as 1 foot. One elbow is not one foot.

Link to 1990 GAMA vent tables http://icpindexing.mqgroup.com/documents/003046/gama%20vent%20tab.pdf

This is a copy of the instructions that Duravent developed http://www.duravent.com/pdf/shbook.pdf

Hello all,

Looking for 411 on oversized water heater flues and discovered this post on your excellent site. Signed up and look forward to accessing the wisdom of the group. I’m a retired EE and somewhat of a go-to guy in our community and when queried about home issues often say “I 'll get back to you on that :-)”.

From now on, guess where I will often be informed.

A neighbor called me about lack of of hot water from her 10 year old, 40 gallon, gas unit. Preliminary inspection revealed the pilot was out, but following a relight, flames crept out of the combustion chamber (!). Throttling the shut off valve helped, but obviously not a solution. Cleaned accumulated detritus off the burner head, but no joy.

Met with the plumber next day and based on serious corrosion around inlet/outlet lines, main water shut-off and pressure relief valve, he recommended R&R (hmmm, how did i miss all that :frowning: ?). Here’s where my “search” and visit here comes in. He also observed that the 4" flue on the WH expanded twice on the way up, going through the roof at about 6" (Hi-efficiency, condensing furnace on its own in/out plumbing). He commented, as did someone here earlier, that condensation in the “too large” flue may have allowed liquids to drip onto the burner, causing or contributing to the problems.

I was skeptical when he suggested R&R of the entire flue, but based on what I’ve learned here, looks like I owe him an apology. Whodathunk too big was bad? Amateurs!

My WH is the same as hers, but I use the flush line for an occasional car wash (after the minerals stop coming out of the hose) and my system appears to be in good condition. Wondering if I should consider an automatic flue damper?

OK, I’ll stop rambling. Pleased to have discovered the site!

Best wishes and Happy New Year all!