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BillyHvac, Journeyman HVAC Tech
Category: HVAC
Satisfied Customers: 22674
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OK...Heres the scoop. Im going to need your A game (just

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OK...Here's the scoop. I'm going to need your A game (just playing...somewhat :-)! I have a 44 year old home which is a rancher that 2652 sq feet home and still has the original furnace. The furnace is a 200,000 BTU (Commercial type) which at the time had an 80% efficiency rating. Let's assume today that rating is like 65%. The furnace is vented with a roughly 6 to 7 inch metal pipe into our masonry chimney which has a regular terra cotta type chimney liner. The gas water heater is also vented into this chimeny about a foot away from where the furnace pipe enters. I have a company coming tomorrow that is going to put a 97000 BTU 96% efficient Carrier furnace in. The company is going to replace the large pipe that goes into the chimney with like a standard 4 in pipe.
However, my HVAC friend of the family said caution. The company doing the work should not just be venting a new high efficiency furnace into a chimney like that because it's not rated correctly. It is to big and the condensation from the furnace, will over time, tear down the terra cotta liner wall. He said that the only proper solutions that he knew of are to either line the terra cotta wall with some chimney liner or run a separate 4 in PVC pipe out the chimney. He asked if the work was going to be inspected afterwards and I said yes. He said he didn't think that the current set-up that the guy are attempting would pass inspection but that he wasn't sure, it might. I really appreciate your sound advice! What say you?!



My name is XXXXX XXXXX I will glad to offer assistance today.


First - the furnace should be vented out a sidewall and not the chimney. As your friend mentioned the only way this would work is if you ran the pvc vent pipes all the way to top out chimney.


BUT, then where do you vent the water heater?


They will need to run intake and exhaust for new furnace out a side wall. Then they must run a chimney liner down the chimney for the water heater. Then the water heater will attach to this liner with 4" galvanized.


Also make sure they use BOTH the intake pipe and vent pipe for the furnace. That way you are not sucking in house air for the burner. It will come from outside..through the burners...out the furnace and to the outside via the vent pipe.


DO NOT vent into chimney. Please let me know if you have any concerns at all. I carry a code book with me.





Customer: replied 4 years ago.

The water heater is vented through the chimney just like the furnace.


I assume if they run a liner in the chimney then both the water heater and the chimney could continue to be vented through the chimney correct?


The furnace is located in the center of the house with no easy access to the outside wall in this rancher. The side wall piece venting is not essential right? The roof is just as good?


Is it against code to run this high efficiency furnace just into the chimney?

It is not against code to go through roof. The chimney is ok IF the pvc ran all the way up and out...


But I do not see how you will be able to fit a chimney liner and 2 pvc pipes through chimney.


If you have the model number I can supply the install directions to you



Customer: replied 4 years ago.

Billy, it's the Carrier 59TPA Data you sent me this three days when I had other questions. Can you check that if the HVAC guy just replaces the 6 in with a 4 inch pipe and just connect it into into the chimney if that is a code violation?

Connecting to chimney with any pipe = code violation.


The ONLY acceptable way is if they run a 1 pipe system up and out the top. But you still have the water heater concern needing a liner.


The days of venting in an unlined chimney are gone. And 90%+ furnaces cannot be vented into the chimney.


Here is the installation guide


I can tell you with 100% certainty that the chimney cannot be used as it is to vent either the furnace OR water heater.


Feel free to ask anything you need



Customer: replied 4 years ago.

Let me know if this makes sense. I am guessing 43 years ago when the system was put in that this was not a violation of code. I am guessing that just because I am getting a new furnace that I am not required to go back and vent the water heater differently. What do you think, maybe I should just have a chimney inspection and see if 43 years of a hot water heater being vented that way has done damage to the chimney liner, no?





Once you touch this the system must be brought to code. It was no big deal before because you had so much heat going out chimney. Now that we milk the heat out we have a warm vapor going out. This would rot your chimney out.


Also we have a big chimney with just the water heater left...not enough heat to keep it is code to install chimney liner (flexible metal liner) in the chimney whenever changing furnace or water heater.


None of this will be grandfathered. And actually it is just for the good of your property and safety.


By code you will have to vent the furnace properly (not into chimney) and run a liner for the water heater.





Customer: replied 4 years ago.

Last question and I will pay...if the HVAC guy adds the flexible metal liner to the chimney can they then run both the water heater and the furnace through the chimney like is currently being done. Thanks for all your help.


no, the furnace is power vented with a blower...if it were added to the water heater then this could happen:


Chimney gets plugged..tile falls, nest etc...the furnace would then pump the carbon monoxide back through the water heater and into your home.


This furnace must be piped individually until it exits the home.



BillyHvac and other HVAC Specialists are ready to help you
Customer: replied 4 years ago.

Billy, when I look on my roof, beside the chimney which (has been) the exhaust output. Are you saying when they install the new system tomorrow there should definitely be a new pipe from the outside for the intake? Currently, intake would just be your return registers no? Thanks again.

We have. Different is the return air in the home recirculating through the unit.

The other is for the flame...intake combustion air. On the old system the flame needed air...which came from your house and sucked in cold air through cracks ...flame needs oxygen and it anything going up the chimney is compensated by drawing air in anywhere it can. People think cold drafts blow in...when they are actually being sucked in.

The combustion air intake goes to the outside and draws burner air from there. That way your house doesn't go into a negative pressure
Please feel free to keep posted along the way.
This is an interesting situation.

Customer: replied 4 years ago.

So the guys just got here (there's a team because new furnace and new a/c) and I got good news and questionable news. I was told we will be running two new pipes through your roof for intake and outtake for the new system but we are not doing anything with your hot water heater that currently vents into the chimney. Is there any code that I can show them? Thanks, Andy

They do not have a choice per code.

Once they remove the furnace from the chimney a liner must be installed for the water heater.

What state are you in?

Customer: replied 4 years ago.


Sorry I was trying to copy and the document will not let me copy it.

This is in the International Code (all states) section 801.

Any time an appliance is removed from chimney, the chimney must be resized to remaining appliance to provide proper removal of exhaust and to prohibit condensation within chimney.

In other words...whoever removes that furnace vent pipe is supposed to install a liner for the water heater.

This is nothing new and is standard practice. If your contractor is not going to do this or is unaware...simply call the local mechanical code inspector. You will get a liner.

DO NOT let them vent the water heater into the chimney as it is.

Excerpt from manufacturers:

Watch out: venting a small, low-BTUH appliance into a chimney of any type (masonry, metal, insulated metal, B-vent, etc) that is too large can be dangerous - the small appliance may be unable to heat the chimney to sufficient temperature to generate sufficient draft to vent the appliance safely. We find this problem in older homes where a residential gas-fired water heater is the sole appliance vented into a large, tall masonry chimney.
Customer: replied 4 years ago.

Billy, I just spoke to the guy and he said because the chimney is an interior chimney, right through the center of the house that it does not need a metal liner but that the current clay liner is sufficient. Does that sound right to you? Thanks.

Does it run through an attic space that is not heated?

See note 3

WHEN TO LINE: The National Fuel Gas Code is clear when liners are needed. 1 If an 80% furnace is alone in any chimney. 2 If an 80% furnace and water heater are using a chimney with 1 or more sides exposed to outdoors below the roofline. 3 If the installation doesn’t meet the “7 times” rule on sizing. That rule says “the flow area of the chimney cannot be more than 7 times the area of the smallest draft hood outlet. “ Since most water heater draft hoods are 3” diameter (7 sq inches times 7 = 49 sq inches) and most chimneys have interior areas larger than 49 sq. inches, most fail to pass this test. Note: the rule looks at the outlet of the draft hood, not the size of the flue pipe used. Few water heater draft hood outlets are larger than 3” even if 4” flue pipe is used. This test applies when a water heater shares an inside chimney with an 80% furnace or if the water heater is alone in a chimney.

Customer: replied 4 years ago.

The chimney runs through about 3 1/2 feet of unheated attic space. Does that change the equation?

see note 3 above
Customer: replied 4 years ago.

Ok Billy maybe I'm a little slow today Embarassed...for a liner...point 1 and 2 don't apply that leaves us with point 3...and I don't understand point 3 well enough to understand whether a gas water heater alone can vent without a metal liner through an interior chimney. Sorry.


what is the size of the inside of the chimney?

I amnot trying to drag this out....this is a basic thing for a contractor and is practiced by all.
I do not see why they are hesitant.
Tell me the inside diameter aprox of chimney.

Customer: replied 4 years ago.

Ok...I just went and took some measurements, let me get the soot off my fingers... here's my best's a one story house, center chimney, 8 foot walls, 14 feet high chimney. At the base, and for the first say 6 feet the chimney opening is like 3 1/2 feet by 1 1/2 feet and then like at 6 feet to 9 feet it narrows to the point where at like 9 feet to 14 feet there is that clay liner and it's 1 1/4 feet to 1 1/4 feet.

You are well above the ruling limit.

You are allowed a chimney area of 87.92 sq inches

You are at 256 sq inches.

Customer: replied 4 years ago.

net/net...we need the liner because the area, the space is just too big for the water/condensation to properly vent leading to possible erosion of the chimney?

That is correct.

And that is per the International Fuel Gas Code

Customer: replied 4 years ago.

Thank you. You can close out this line of questioning. Great job! Your help has been outstanding!

You are very welcome and I thank you


Customer: replied 4 years ago.

My bad, one last question, calculation. I don't realize that my chimney had a double output, like two chimney's in one. The measurement of smaller "exhaust" chimney is 6.5 inches by 12 inches. Can they just use that?

If the 6.5 x 12 is available and does not open to any other appliance (fireplace etc) and the chimney is inside (not outside wall) then yes.

Just make sure the 6.5 x 12 is dedicated to just the water heater.

It does not bother me at all for follow up questions so feel free to ask as you go through your project.


Customer: replied 4 years ago.

Great. Thanks!