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My furnace: sad orphan or eligible bachelor

Member Posts: 184
I have a 30ft 3- sided double flue masonry chimney.  My house insulation is poor and I have stack affect. I am air sealing and getting cellulose blown in soon.  My CO alarms have never gone off. The chimney is looking suspect to me.  The brick has spots on it. I def. think it's seen condensation during the 50 years it's been attached to the house.

I have a NG 100,000 btu input 1968 standing pilot cast iron boiler exhausting with a 60,000 btu furnace through a combination (siamese) fitting.  As well as a 40,000 btu water heater utilizing the second flue. I removed the boiler and capped it's side of the the fitting. I bought the house 5 years ago and kept that boiler alive longer than I should have.  The furnace is next to go. I'm installing more radiant and getting a modcon. Which I wanted to exhaust through the chimney.   The boiler heats an upstairs apt with baseboard. Soon It's also going to heat the main floor and basement which is currently heated by the furnace.

-I might not finish the main floor radiant by the time it gets cold.  Can I have that orphaned furnace by itself running on that flue?   It needs a liner but I don't want to get one since I'm getting rid of the furnace.

Could I combine it with the water heater for better draft?

• Member Posts: 15,976
Measure the inside of the flue tile and find the area. The measure the area of the flue pipe(s) going into the flue. The area of the flue tile is not supposed to be MORE than 49x the area of the connected flue pipes.

Example: 10 x 10 flue tile is 100 sq inches. If you had a 6" flue pipe going in it's area would be 6x6=36x.7854=28.27 sq inches

28.27 x 49=1385 sq inches which is ok as long as the flue is big enough. By todays code your not supposed to vent that into an outside chimney but since it's been that way and is temp it should be ok.

Run the furnace and make sure you have enough draft and your likely fine

• Member Posts: 1,388
Hello @HydroNiCK,
HydroNiCK said:

The furnace is next to go. I'm installing more radiant and getting a modcon. Which I wanted to exhaust through the chimney.

This is not really my thing, but... Are you sure there is two independent 30 foot vertical flues in that chimney ? Or just two thimbles entering one flue ? To me the picture of the chimney it does not look big enough for two separate flues.

With a modcon I am under the impression if a chimney is used it is used just as a chase and the modcon venting piping has to go to the top. If your chimney is actually only one flue that would probably be incompatible with the existing water heater.

I would keep the furnace if only for the A/C in the summer, since it is already there. And maybe a plan 'B' heating source when the modcon starts making codes in the dead of winter, but that is me.

National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System
• Member Posts: 184
edited September 2022
109A_5 said:
Hello @HydroNiCK,
The furnace is next to go. I'm installing more radiant and getting a modcon. Which I wanted to exhaust through the chimney.
This is not really my thing, but... Are you sure there is two independent 30 foot vertical flues in that chimney ? Or just two thimbles entering one flue ? To me the picture of the chimney it does not look big enough for two separate flues. With a modcon I am under the impression if a chimney is used it is used just as a chase and the modcon venting piping has to go to the top. If your chimney is actually only one flue that would probably be incompatible with the existing water heater. I would keep the furnace if only for the A/C in the summer, since it is already there. And maybe a plan 'B' heating source when the modcon starts making codes in the dead of winter, but that is me.
No I'm not 100% sure what they did. I have mini splits installed..don't need the furnace.  I hate forced hot air. Plus the house was expanded without resizing the furnace/AC. ducts.  My 1964 Utica was old but those ducts are older then that boiler.

I looked into the combo fitting and no liner.  I couldn't see the water heater vent on top.  I peaked into the water heater thimble.. I was hoping that one was lined.  The water has draft and the pic I took I think I see a vent poking down.  I didn't see the combo thimble below it but when looking from outside on roof the vents are near and far not side by side.   The boiler flue is in back of the water heater flue. The chimney was built up when they added 2nd floor.  It looks like when they added the second floor they broke through the inside wall and added the water heater flue or maybe added the boiler flue from the outside and went through what was chimney clean out under the water heater vent. I dunno... I have to go on top and measure. I hope there is a wall between the two flues.  I'm getting the chimney cleaned next week and the chimney cleaner is going on vacation the week after that with the money he's going to try to get from this job.   I have a couple of options but not sure which way to go.  I want to get away from the chimney and use it as a chase.if I need the furnace before my radiant is finished and it's currently not safe to run  by itself since it may be too small for that flue and back drafts... the chimney needs to be fixed and lined.  At that point is it even worth it to get the modcon.  I could get a cast iron boiler and plug it back in since I just rebuilt the chimney and pipe it to run radiant.

View looking up from Siamese fitting. I think the water heater is on the right above the panel. Looks like there is separation between the two.

View looking into the water heater thimble.

• Member Posts: 1,388
Hello @HydroNiCK,
HydroNiCK said:

I have mini splits installed..don't need the furnace.  I hate forced hot air.

I need the furnace before my radiant is finished and it's currently not safe to run by itself since it may be too small for that flue and back drafts... the chimney needs to be fixed and lined. At that point is it even worth it to get the modcon. I could get a cast iron boiler and plug it back in since I just rebuilt the chimney and pipe it to run radiant.

I don't blame you for not liking forced air and I did not know you had the mini splits.

What I don't like about modcons is it seems to me that between the greater maintenance needed and the cost of Tech trying to get failed unit back up and running again out weighs any savings of the higher efficiency. Some folks seem to love them. Your mileage may vary.

For equipment that is going to keep me warm in the winter I like it as simple as possible. For me it does not get much simpler than an old Natural Gas fired one pipe Steam Boiler. This house happened to have that when my family bought it many years ago, so I feel lucky.

I think lining the chimney and using it as a chase for a modcon venting is two different things, only one would be needed depending on the heating equipment used (the water heater flue is a separate issue).

I'm assuming there is another flue (maybe uncapped) not visible in the picture that shows the top of the chimney. Maybe the chimney cleaner can draw you a diagram of the internal configuration of the chimney since he should be looking at it from both ends, that way you would have a better idea what you are dealing with, and then you can make more informed decisions.

Per @EBEBRATT-Ed calculations, it seems to me that the chimney flue for the forced air furnace 'only' would have to be almost a square yard (36" x 36") before it is too big for that furnace. 49 times bigger is a lot.

National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System
• Member Posts: 139
There is the seven times rule to consider also I think?
• Member Posts: 8,310
edited September 2022
Sometimes the terms used to describe parts of a heating system can be confusing. The Flue can be defined as the part of a boiler or furnace where the byproducts of the combustion process pass thru to give up the heat to the home. These include the part of the furnace or boiler that are just past the combustion chamber (commonly called the fire box) and include the combustion gas side of the entire heat exchanger all the way to the place where the exhaust pipe is connected to the furnace or boiler. This exhaust pipe is referred to as the Connector or the Flue Connector

Also the OP refers to a Siamese fitting. I believe that is a WYE fitting used to make several appliance connectors enter one chimney. This Siamese WYE is part of the Connector.

There is another definition for the word Flue. when referring to a chimney, the Flue is the part of the vertical hollow area the combustion gasses use to go up the chimney (AKA the part that Santa uses on December 24). As illustrated in the following series of diagrams and photo, the terms used by the OP are confusing when applied to the standard meaning of the words like Double Flue and Siamese fitting.

By the photos provided by the OP, and the Queries posted, I believe the answer would be the furnace is not an orphan because it is already in the same chimney flue as the water heater. If however the furnace is not used, or rarely used, as indicated by the OP, then the water heater is an orphan appliance on the chimney flue. The OP does not have a double Flue chimney as illustrated BELOW

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

• Member Posts: 184
Sometimes the terms used to describe parts of a heating system can be confusing. The Flue can be defined as the part of a boiler or furnace where the byproducts of the combustion process pass thru to give up the heat to the home. These include the part of the furnace or boiler that are just past the combustion chamber (commonly called the fire box) and include the combustion gas side of the entire heat exchanger all the way to the place where the exhaust pipe is connected to the furnace or boiler. This exhaust pipe is referred to as the Connector or the Flue Connector Also the OP refers to a Siamese fitting. I believe that is a WYE fitting used to make several appliance connectors enter one chimney. This Siamese WYE is part of the Connector. There is another definition for the word Flue. when referring to a chimney, the Flue is the part of the vertical hollow area the combustion gasses use to go up the chimney (AKA the part that Santa uses on December 24). As illustrated in the following series of diagrams and photo, the terms used by the OP are confusing when applied to the standard meaning of the words like Double Flue and Siamese fitting. By the photos provided by the OP, and the Queries posted, I believe the answer would be the furnace is not an orphan because it is already in the same chimney flue as the water heater. If however the furnace is not used, or rarely used, as indicated by the OP, then the water heater is an orphan appliance on the chimney flue. The OP does not have a double Flue chimney as illustrated here
No actually I do have a double flu chimney as pictured.  The water heater has its own flue and connector in chimney.  The furnace and boiler are connected through a Siamese fitting(I think Bob Harper called it that) into a separate flue.
• Member Posts: 1,461
If those are standard red bricks those flues must be tiny or something cause that chimney doesn't look any where near big enough for two separate flues.
• Member Posts: 10,192
JakeCK said:

If those are standard red bricks those flues must be tiny or something cause that chimney doesn't look any where near big enough for two separate flues.

Or part of it is in the house. It really looks like those breaches are aligned vertically to each other so i can't see how they could connect to 2 different flues in an acceptable way.
• Member Posts: 1,461
mattmia2 said:
If those are standard red bricks those flues must be tiny or something cause that chimney doesn't look any where near big enough for two separate flues.
Or part of it is in the house. It really looks like those breaches are aligned vertically to each other so i can't see how they could connect to 2 different flues in an acceptable way.
Perhaps. He needs to post a picture of the top from further back.
• Member Posts: 8,310
edited September 2022
HydroNiCK said:

No actually I do have a double flu chimney as pictured.  The water heater has its own flue and connector in chimney.  The furnace and boiler are connected through a Siamese fitting(I think Bob Harper called it that) into a separate flue.

I must disagree
THIS is what you have,

one flue with 2 openings installed correctly. the smaller connector is installed above the larger connector. The larger connector has a WYE fitting

that apparently had 2 appliances connected to it
The byproducts of combustion from the water heater go into the same chimney FLUE as do the byproducts of combustion from the previous boiler and the existing furnace. By the look of the outdoor photo of the chimney, there is not enough room for 2 separate chimney FLUES inside that brick enclosure.

If you look at the double flue photo above, there are 3 bricks from front to back and 5 bricks from side to side. That should make a dimension of approximately 44" x 24"

If you look at the photo of your chimney, it is 2.5 bricks by 2 bricks. That is approximately 20" x 16" unless you have custom made bricks. From my 40+ years of experience working on oil heat and gas heat heating systems that are vented into chimneys, you have a single 8" x 8" terracotta tile liner that makes up one flue, surrounded by standard brick. But I could but wrong. Of course it is your house and you obviously know what you know.

Currently you do not have an orphaned furnace. You don't have an orphaned water heater. You probably do have a deteriorating terracotta liner and should think about getting a chimney liner installed.

The future Mod Con boiler may not be able to be installed with the water heater in the same chimney Flue. Depending on whether the ModCon boiler is a category I vent (unlikely) or a category III vent (more likely) you will need a stainless steel or plastic exhaust pipe that is not compatible with the existing water heater vent in the chimney FLUE. I would be sorry to hear that you installed the venting incorrectly and that there is a Carbon Monoxide event in your home, that your neighbors get to read about in the newspaper

Respectfully Submitted,

Mr. Ed

PS. for clarity, I edited my tutorial post above by changing the last word from here to BELOW
If you know me, as the regulars on this site do, If I post something that is not funny or sarcastic, It is a fact that is supported by documentation as is the tutorial on chimneys above.
Just Ask @109A_5 about Short circuits, he can tell you that if I know a fact to be true, I will persists on that fact until you learn something or just give up. You have two crocks (openings at the chimney base) in one Flue. You do not have two Flues

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

• Member Posts: 184
Hear Ye! Hear Ye! I got a chance to go up on the roof today.  I found a couple of things. One of them being....I'm an idiot.  1 Flue...So Ed I apologize for doubting your professional common sense advice.  My chimney does not defy the laws of logic.  Your brick count was correct.  So I have a 26' 6.5 x 6.5 I.d. clay liner in surprisingly good condition. I am staring at the fuel gas code tables and my eyes are glazing over.  If I thought I saw 2 flues chances are my razor sharp thinking is definitely going to miscalculate my venting configuration.
• Member Posts: 184
I found the manual to the furnace. They also sell a blower to install if it's being vented into a masonry chimney but don't know if I need it.