Oversized Chimney Liner?

dannymk2

I currently have a masonry chimney on the outside of the my house that i need to have a liner put in. It's 20' tall. i have a natural gas furnace and hot water heater venting into the chimney. they are 75000 and 40000 btu respectively. Ive read the sizing charts and see that a 5" stainless liner should be suitable for my situation. my only concern is that per the chart, i am close to the limit of the 5" liner. my hot water heater is new but my furnace is quite a few years old. i dont want to potentially limit myself if i need to put in a larger furnace. i understand that a significantly larger chimney will not vent correctly, but is going with a 6" liner over a 5" going to cause any issues? for reference, the chimney guy recommended the 5" for the my current situation but said he would install 6" if i wanted it. this would lead me to believe i should be fine but i just want to confirm.

bburd

If your existing furnace heats the house well, the only reason you might need a larger one is if you plan to add on. How old is the place?

random12345

I'm not a pro. Is your vent connector B-vent or single wall, and how long? My understanding is that an insulated vent connector, especially for an external chimney can have a significant impact in terms of preventing condensation. If it's very short, might not make a difference though. According to NFPA 54, single wall and 20' chimney with 3' connector rise capacity is 113. You are at 115. A smaller vent keeps the flue gases hotter all else equal.

dannymk2

bburd said:

If your existing furnace heats the house well, the only reason you might need a larger one is if you plan to add on. How old is the place?

i dont have an issue with it, but i certainly wouldn't want a smaller one. doing a quick search, im not even seeing a 75000 btu replacement. i see a few 80000 and quite a few 88000 out there, the later would be too large for the 5" liner. im probably over thinking this, it was just my thought that if going 6" would have no negative effect on my current set up, id rather have room for more then sell myself short.

random12345 said:

I'm not a pro. Is your vent connector B-vent or single wall, and how long? My understanding is that an insulated vent connector, especially for an external chimney can have a significant impact in terms of preventing condensation. If it's very short, might not make a difference though. According to NFPA 54, single wall and 20' chimney with 3' connector rise capacity is 113. You are at 115. A smaller vent keeps the flue gases hotter all else equal.

single wall vent connectors, each about 4' long, give or take.

Bob Harper

You can install a 5.5" liner if you feel better. Note that corrugated liners require a 20% derating plus additional derating for offsets. If you're really, REALLY worried about it just install a 5.5" smoothwall ss liner and don't worry about it.

captainco

Bigger is always better. Chimneys are just ducts. Things flow through them better when they are bigger. Draft is created by cold air displacing hot air. The hotter the flue gases and the cooler the flue, the higher the draft. A warm flue does not produce higher draft. That can be easily verified by checking draft in a water heater flue in the summer, when it is 90 degrees outside and in the winter when it is 30 degrees outside.

random12345

@captainco I think we both know that is a contrarian position to take. I am not disagreeing with you, but I am curious about this topic. My understanding is that the simulations in the back of NFPA 31 suggest that a smaller liner keeps the flue gases hotter and lowers the probability of them reaching dew point inside the liner and condensing, which is the whole point of putting a liner in there in the first place. Are you saying that a higher draft vents the flue gases faster, which doesn't give them time to condense inside the flue? Is that the rationale, plus your experience obviously?

captainco

random12345 - Not contrarian just a fact. Did you know that nothing cools flue gases down faster than B-Vent? Flues are chases not heat exchangers. How many water heaters vent in oversized flues when a furnace is not operating. I have tested equipment vented into 48" flues that were 20 times the size of its flue but they were common vented with other equipment that was idle. Simulating in a lab and actually testing in the field are two different worlds. The less contact the flue gases make with the surface of a flue the less heat is tranferred and the flue gases stay hotter. If you ever read some of the disclaimers in these so called studies it makes you wonder why did they even do them? I have tested in the field for 42 years and have read just about everything I can. When I see the recommended fixes to a problem make the problem worse, it is best to avoid them.

random12345

Ok, thank you for your response. Why did the guys who wrote Appendix E in NFPA 31 claim that an insulated vent connector can make the biggest difference in eliminating condensation? Once the B-vent warms, it has that cushion of air to insulate and stays warmer than single wall right? What you're saying might make sense for a perfectly straight flue with a short vent connector, but what about when the vent connector is long and there are offsets in the flue itself?

Why do people have condensation problems at all if not because the flue is too big? What is the solution? I am always interested in hearing new perspectives from experienced pros.

captainco

Condensation in flue pipes is caused by improper venting which can be caused by combustion air problems, underfiring, undersized or restricted flues. In the case of all induced draft equipment it is lack of post-purge. The current venting tables are based on computer generated programming. I have never met a computer programmer that vented furnaces.

One additional thought. Why do the vent pipes in the warm basement rot out when this is where the flue gases are the hottest and the vent pipes the warmest??

The National Fool People Association (NFPA) is professionals protecting professionals so they can blame us.

MikeAmann

@captainco makes a lot of sense.
And I will take real-world common sense over what some "engineer" wrote without ever being out in the real world ANY DAY.

ChrisJ

captainco said:

random12345 - Not contrarian just a fact. Did you know that nothing cools flue gases down faster than B-Vent? Flues are chases not heat exchangers. How many water heaters vent in oversized flues when a furnace is not operating. I have tested equipment vented into 48" flues that were 20 times the size of its flue but they were common vented with other equipment that was idle. Simulating in a lab and actually testing in the field are two different worlds. The less contact the flue gases make with the surface of a flue the less heat is tranferred and the flue gases stay hotter. If you ever read some of the disclaimers in these so called studies it makes you wonder why did they even do them? I have tested in the field for 42 years and have read just about everything I can. When I see the recommended fixes to a problem make the problem worse, it is best to avoid them.

And yet, if you take a long straight "chase" and wind it into a coil so it fits in a small space you then have what we often call a condenser or evaporator which is very much a heat exchanger. Throw some fins on the outside of it and it works even better.

ethicalpaul

i dont have an issue with it, but i certainly wouldn't want a smaller one.

Why not? How many hours does yours run on the coldest day of the year?

it seems like you are willing to keep the flue too small and your furnace possibly too big?

DJD775

ChrisJ said:

captainco said:

random12345 - Not contrarian just a fact. Did you know that nothing cools flue gases down faster than B-Vent? Flues are chases not heat exchangers. How many water heaters vent in oversized flues when a furnace is not operating. I have tested equipment vented into 48" flues that were 20 times the size of its flue but they were common vented with other equipment that was idle. Simulating in a lab and actually testing in the field are two different worlds. The less contact the flue gases make with the surface of a flue the less heat is tranferred and the flue gases stay hotter. If you ever read some of the disclaimers in these so called studies it makes you wonder why did they even do them? I have tested in the field for 42 years and have read just about everything I can. When I see the recommended fixes to a problem make the problem worse, it is best to avoid them.

And yet, if you take a long straight "chase" and wind it into a coil so it fits in a small space you then have what we often call a condenser or evaporator which is very much a heat exchanger. Throw some fins on the outside of it and it works even better.

To be fair I would not choose a thermal insulator like clay if I was going to build a heat exchanger.

ChrisJ

DJD775 said:

ChrisJ said:

captainco said:

random12345 - Not contrarian just a fact. Did you know that nothing cools flue gases down faster than B-Vent? Flues are chases not heat exchangers. How many water heaters vent in oversized flues when a furnace is not operating. I have tested equipment vented into 48" flues that were 20 times the size of its flue but they were common vented with other equipment that was idle. Simulating in a lab and actually testing in the field are two different worlds. The less contact the flue gases make with the surface of a flue the less heat is tranferred and the flue gases stay hotter. If you ever read some of the disclaimers in these so called studies it makes you wonder why did they even do them? I have tested in the field for 42 years and have read just about everything I can. When I see the recommended fixes to a problem make the problem worse, it is best to avoid them.

And yet, if you take a long straight "chase" and wind it into a coil so it fits in a small space you then have what we often call a condenser or evaporator which is very much a heat exchanger. Throw some fins on the outside of it and it works even better.

To be fair I would not choose a thermal insulator like clay if I was going to build a heat exchanger.

That's true.
But if you were trying to insulate something you wouldn't choose clay either.

DJD775

ChrisJ said:

DJD775 said:

ChrisJ said:

captainco said:

random12345 - Not contrarian just a fact. Did you know that nothing cools flue gases down faster than B-Vent? Flues are chases not heat exchangers. How many water heaters vent in oversized flues when a furnace is not operating. I have tested equipment vented into 48" flues that were 20 times the size of its flue but they were common vented with other equipment that was idle. Simulating in a lab and actually testing in the field are two different worlds. The less contact the flue gases make with the surface of a flue the less heat is tranferred and the flue gases stay hotter. If you ever read some of the disclaimers in these so called studies it makes you wonder why did they even do them? I have tested in the field for 42 years and have read just about everything I can. When I see the recommended fixes to a problem make the problem worse, it is best to avoid them.

And yet, if you take a long straight "chase" and wind it into a coil so it fits in a small space you then have what we often call a condenser or evaporator which is very much a heat exchanger. Throw some fins on the outside of it and it works even better.

To be fair I would not choose a thermal insulator like clay if I was going to build a heat exchanger.

That's true.
But if you were trying to insulate something you wouldn't choose clay either.

Would not be my first choice.

dannymk2

ethicalpaul said:

i dont have an issue with it, but i certainly wouldn't want a smaller one.

Why not? How many hours does yours run on the coldest day of the year?

it seems like you are willing to keep the flue too small and your furnace possibly too big?

your totally missing the question at hand.

either way, i got my answer. thanks guys.

ChrisJ

dannymk2 said:

ethicalpaul said:

i dont have an issue with it, but i certainly wouldn't want a smaller one.

Why not? How many hours does yours run on the coldest day of the year?

it seems like you are willing to keep the flue too small and your furnace possibly too big?

your totally missing the question at hand.

either way, i got my answer. thanks guys.

I'm guessing you went with the 6"?