Thread: natural and draft induced common vent

Hi guys, does anyone have a reference pdf regarding venting natural draft water tanks and draft induced 80+ furnaces into the same B-vent line?
I always call it out for various obvious reasons, i.e. b-vent too small for both units, configuration bad, etc.
Builder is fighting this one. I thought I had something but don't and can't find a specific reference in our Code.
Thanks, Markus

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That is very common in my area..... I don't believe it is wrong as long as everything is properly sized.

Typical furnace with power assisted venting may not be on same B-vent with a gravity draft appliance (typical water heater) G2427.3.3
IRC 2003
Check this out from a quick google search, I don't have my IRC handy.

2427.3.3 #4 states:

Vent connectors serving appliances vented by natural draft shall not be connected into any portion of mechanical draft systems operating under positive pressure.


#3 states: Forced draft systems and all portions of induced draft systems under positive pressure during operation shall be designed and installed so as to prevent leakage of flue or vent gases into a building.



95% of all homes in this area have a water heater tied in with the 80+ furnaces.

HVAC forum discussing this topic: Venting a furnace and water heater? - HVAC-Talk: Heating, Air & Refrigeration Discussion

Good link Brandon. I think the first two paragraphs of post #5 explain it.

I think your installer may be confused. Many people make the mistake of thinking the draft inducer in 80% furnaces is actually blowing the combustion products up the vent pipe, it is not.
The fan just delivers the flue gases to the vent system, natural draft takes over from there.

If the fan was actually forcing all the combustion products up the vent pipe, it would be a different category of appliance, would use vent pipe that doesn't leak when it is pressurized, and couldn't connect into the same common chimney with your water heaters.

#3 states: Forced draft systems and all portions of induced draft systems under positive pressure during operation shall be designed and installed so as to prevent leakage of flue or vent gases into a building.

I think the only "portion" under positive pressure is from the inducer fan to the heat exchager itself. Appendix B of the International Fuel Gas Code has many examples of natural and "fan-assisted" (read "induced") draft appliances venting into a common flue. As you say, it's by far the norm. Still has to be configured right, of course.

Re-reading the original post here, we may have jumped the gun a bit on this. I don't think Marcus is calling them simply because they share a common flue, but because of other factors like size, etc.

The codes do allow common venting of an 80% fan assisted furnace with a natural draft WH if the venting is properly sized, the vent connectors are properly sized and routed and,..........it works!

Now for the bad news:
If your chimney is blocked, that furnace will continue to vent out the WH draft hood rather happily without tripping any of the 3-4 safeties in the furnace. This can be corrected by removing the draft hoods, installing bullhead tees with double acting barometric dampers with spill switches. Then you add a post purge timer on the inducer to reduce condensation.

The lower vent may be under positive pressure for less than a minute once firing but is not considered a "positive pressure vent" system. If someone installs a draft inducer in the vent connector, it will pressurize the connector and chimney and is thus illegal. I have investigated CO cases that were due to these "inducers" on water heaters with single walled snaplock pipe, which is not intended or approved for positive vent pressure.

Thanks for the very helpful responses guys. I really appreciate it.
I've gotten a bit frustrated over time with this set-up. I don't like it because the majority of the time it is installed wrong. Often times also installed, in what should be an obvious, hazardous configuration.
Usually writing these is a slam dunk. In this case the builder did size it and configure it properly. I guess I was hoping that something definitive had developed that I could use.
Part of my concern is based on having seen Mr. DIY reconfigure the flues in the utility rooms so that he can hang his shelves, etc. I wish they would just dis-allow this all together.
Jim and Brandon, thanks for the IRC ref I will check it out.
Richard I would love to write them for sharing the common flue, instead I try to pick at the details to get them to change it.
Thanks, Markus

It has to be the single most dangerous installation allowed by code.

The practice of common venting should have been banned years ago.

I attended a seminar on this subject last summer. It is perfectly acceptable to combine a fan assisted and natural vent appliance together. There are some rules of thumbs:

1) (at least in colder weather) If you replace a natural draft appliance with a fan assisted one and it's connected to an exterior chimney; the chimney will need to be relined.

2) If a natural draft (water heater) is connected to a vent that had a natural draft furnace but now the furnace is fan assisted, the WH vent connector needs to be increased in size.

Sizing vents and connectors using the tables is a pain in the _ _ _; watch this video from a company that sells software sizing vents and connectors.

They also sell software for just about everything mechanical.

http://www.elitesoft.com/pub/videos/gasvent/gasvent.htm

Originally Posted by DavidR

It has to be the single most dangerous installation allowed by code.

The practice of common venting should have been banned years ago.

David is a lot smarter about this stuff than most of us, so if code "allows" something and David said that it needs to be tested to make sure it works, then I'm going to go with David.

Being as David has seen and test so many which *do not work*, I'm going to go with David on his post above.

We all see many things which are "allowed", however, we also see that they do not work as intended in all instances.

Originally Posted by Darren Miller

I attended a seminar on this subject last summer. It is perfectly acceptable to combine a fan assisted and natural vent appliance together. There are some rules of thumbs:

1) (at least in colder weather) If you replace a natural draft appliance with a fan assisted one and it's connected to an exterior chimney; the chimney will need to be relined.

2) If a natural draft (water heater) is connected to a vent that had a natural draft furnace but now the furnace is fan assisted, the WH vent connector needs to be increased in size.

Sizing vents and connectors using the tables is a pain in the _ _ _; watch this video from a company that sells software sizing vents and connectors.

They also sell software for just about everything mechanical.

http://www.elitesoft.com/pub/videos/gasvent/gasvent.htm

All the information they fed you in that seminar was also based off of computer simulations Darren.
They are based in no way off of field conditions and the variables that occur in it.
GAMA venting tables are considered the Gospel of venting, read the disclaimers in the front of that document if you want to see how flawed it really is.

Common venting never has worked and it never will, Mother Nature and basic physics will win every time regardless of what a computer program spits out.

Originally Posted by Jerry Peck

David is a lot smarter about this stuff than most of us, so if code "allows" something and David said that it needs to be tested to make sure it works, then I'm going to go with David.

Being as David has seen and test so many which *do not work*, I'm going to go with David on his post above.

We all see many things which are "allowed", however, we also see that they do not work as intended in all instances.

Thanks Jerry, smarter I doubt that.
I've just been very fortunate and lucky to be able to go to the right classes and talk with some really sharp people.

I'm not a code basher by any means but I have to call out something if it doesn't work in real world situations.
Passive combustion air follows the same principles of not always working but few question it because it is code approved.

Thank goodness for guys like Jim Davis who questioned those things and helped me to learn the way things really work.

I just try to help where I can and talk to anyone who will listen to my ramblings.

Interesting....

The seminar was offered by the NJDCA; the licensing division for code inspectors. The presenter is no smuck; his name is Julius Ballanco P.E.

Mr. Ballanco's resume speaks for itself; among other affiliations:

He currently is president of The American Society of Plumbing Engineers.

Past Chairman of ASSE Code committee.

11 1/2 years Head of Plumbers and Mechanical Engineers, Building Officials and Code Administration International Inc.

He's also a monthly columnist for "Engineering Insights" & "Plumbing Primer".

I dunno, god forbid I ever get hauled into court, I'd rather have Mr. Ballanco and his engineering background on my side of the table than ANYONE else.

Originally Posted by Darren Miller

I dunno, god forbid I ever get hauled into court, I'd rather have Mr. Ballanco and his engineering background on my side of the table than ANYONE else.

Darren,

Let's say you went to court and Mr. Ballanco was on your side stating all the testing which has been done to arrive at what the charts, tables, and computer programs all say *should work*.

Let's say the other side has someone quite competent, such as David R., who has done *actual testing of the system in place as installed*.

Question for you:
- Who you think the judge will put more credence in:
- - a) someone stating how it *should* work based on standard principles, or,
- - b) someone stating how it *did not* work based on actual testing as installed?

Not sure about you, but I'm going with b).

Originally Posted by Darren Miller

Interesting....

The seminar was offered by the NJDCA; the licensing division for code inspectors. The presenter is no smuck; his name is Julius Ballanco P.E.

Mr. Ballanco's resume speaks for itself; among other affiliations:

He currently is president of The American Society of Plumbing Engineers.

Past Chairman of ASSE Code committee.

11 1/2 years Head of Plumbers and Mechanical Engineers, Building Officials and Code Administration International Inc.

He's also a monthly columnist for "Engineering Insights" & "Plumbing Primer".

I dunno, god forbid I ever get hauled into court, I'd rather have Mr. Ballanco and his engineering background on my side of the table than ANYONE else.

I've read Mr. Ballanco's material for years, he's a very intelligent man.

I do not agree with him on his stance regarding venting and combustion air though.

Learned a long time ago, just because someone is a PE doesn't mean they are infallible.
It's even worse when you're just an average HVAC guy.

David R; with all due respect...

I don't know you or what your credentials are; so this is in no way at shot at you or your occupation.

A home inspector has a duty to his/her client (the homebuyer). There are good and bad home inspectors all across our great country.
A good home inspector will inform their client about potential & existing problems.

If there is anything I've learned over the 12 years I've been in business, it's one simple rule:
If you call out an problem (potential or existing), you'd better have a credible source to back up your findings. In this case, my 'source' is the written codes along with approved seminars and published materials presented by my state code enforcement agency.

David R, you have written here:
"Thank goodness for guys like Jim Davis who questioned those things and helped me to learn the way things really work."
"Common venting never has worked and it never will..."
"The practice of common venting should have been banned years ago."

These may be your opinions, and everyone is entitled to their opinion, but do YOU have ANY credible source to back up your opinions?
Do you know of any state, county or township in the country that has banned the set-up we are talking about?

Gee Jerry, I'm very surprise about what you said in your last post here...

"Question for you:
- Who you think the judge will put more credence in:
- - a) someone stating how it *should* work based on standard principles, or,
- - b) someone stating how it *did not* work based on actual testing as installed?"

I have seen no proof from David, you or anyone else to back-up you claim that 'it did not work'.

This should help explain:

Column - Plumbing and Mechanical

Column - Plumbing and Mechanical

Originally Posted by Darren Miller

David R; with all due respect...

I don't know you or what your credentials are; so this is in no way at shot at you or your occupation.

A home inspector has a duty to his/her client (the homebuyer). There are good and bad home inspectors all across our great country.
A good home inspector will inform their client about potential & existing problems.

If there is anything I've learned over the 12 years I've been in business, it's one simple rule:
If you call out an problem (potential or existing), you'd better have a credible source to back up your findings. In this case, my 'source' is the written codes along with approved seminars and published materials presented by my state code enforcement agency.

David R, you have written here:
"Thank goodness for guys like Jim Davis who questioned those things and helped me to learn the way things really work."
"Common venting never has worked and it never will..."
"The practice of common venting should have been banned years ago."

These may be your opinions, and everyone is entitled to their opinion, but do YOU have ANY credible source to back up your opinions?
Do you know of any state, county or township in the country that has banned the set-up we are talking about?

Gee Jerry, I'm very surprise about what you said in your last post here...

"Question for you:
- Who you think the judge will put more credence in:
- - a) someone stating how it *should* work based on standard principles, or,
- - b) someone stating how it *did not* work based on actual testing as installed?"

I have seen no proof from David, you or anyone else to back-up you claim that 'it did not work'.

A lot of what I say is based on field experience and what I've found testing for the past eight years.
In addition to that I have the backup of thousands of guys across this country doing the exact same testing and finding the exact same thing on a daily basis.
It's not that I'm a loon, just a little more vocal than many.

I'm not going to toot my horn about acronyms behind my name, my credentials, or the company I teach this topic for.
There are a couple of guys here who can vouch for my credentials though if that is really needed.

As you know Darren codes are slow to change, in addition to that they are based on a consensus opinion not actual field conditions.
I think you will eventually see codes change to allow things to be addressed like this but till real world scenarios are addressed it won't happen.
Keep in mind modern day combustion testing that has revealed many of these issues is only a few decades old.
Trying to find the right people to get in touch with about getting on these committees has proved to be a challenge also.
I have a strong feeling if the right people got the right information things would change for the better.
You guys might be able to help me with this, any contact info for getting on code committees would be appreciated.

If you are interested in a basic draft interference test that will show you very quickly why I say "common venting doesn't work and never has" I would be glad to share it with anyone here that's interested.

Don't just take my word for anything that I say go out and test to try and prove it right or wrong.
I was given that challenge eight years ago and by trying to prove Jim Davis wrong I became one of his biggest proponents.

I try not to deal in opinions, there's way to many of them going around.
I try to stick with the facts and what I'm typing can be backed up as fact not opinion.

Sorry for not being around for a thread I started.
As should be obvious from my earlier post, I have to side with David on this.
I can agree with Darren, anyone else or a computer model that it 'should' work, BUT in the real world it usually doesn't. I don't think the computer can account for all the variables that occur in the real world.
- undersized, bad flue pipe configurations
- Joe DIY changing the flue pipe config to suit what he is doing
- temperature, weather and humidity conditions
- positive / negative house pressures
- confined space installs
- open basement during HI, 6 months from now remodeled and closed up
One of my little tests I do during an HI is hold a match (I know) at the water tank draft hood when the 80+ furnace is OFF and then ON. With the 80+ ON, when the pipe configuration is bad, the match almost always back drafts. Even when the pipe config looks good, the match backdrafts at least half the time or more.
I don't know why for sure and don't really care all that much. Years in the field have shown me it often doesn't work SAFELY or consistently. With the unit on, its backdrafting. I highly recommend you change it, blah, blah.
I can't count how many smarta**, know it all realtors and builders that little test has shut-up.
Sometimes I 'know' there's any 80+ and a regular water tank in the basement before I get to the bottom of the stairs. I can smell it.
Along what David mentioned, I think it is one of those great Code flaws.

I remember a thread quite awhile back that I started about the exact issue of water heaters spilling exhaust when the induced draft furnace first fired up.

I think I was told that since it's such a short time, typically just between the time the furance fires and gets warm, that it's accepted as okay. I agree it sounds a little too close for comfort. Is this the basic concept, at least as far as the code and manufacturers see it? That it's okay since it's just temporary.

I've done the match test many times and finally stopped doing it because pretty every one backdrafts under those circumstances.

Originally Posted by Matt Fellman

I remember a thread quite awhile back that I started about the exact issue of water heaters spilling exhaust when the induced draft furnace first fired up.

I think I was told that since it's such a short time, typically just between the time the furance fires and gets warm, that it's accepted as okay. I agree it sounds a little too close for comfort. Is this the basic concept, at least as far as the code and manufacturers see it? That it's okay since it's just temporary.

I've done the match test many times and finally stopped doing it because pretty every one backdrafts under those circumstances.

You're right it's way too close for comfort Matt.
For some reason the officials who wrote that part of the code thought it would be okay if people got poisoned intermittently?
I don't get the reasoning behind that and I don't guess I ever will.

That match test was telling you something Matt.

Originally Posted by Darren Miller

Gee Jerry, I'm very surprise about what you said in your last post here...

"Question for you:
- Who you think the judge will put more credence in:
- - a) someone stating how it *should* work based on standard principles, or,
- - b) someone stating how it *did not* work based on actual testing as installed?"

I have seen no proof from David, you or anyone else to back-up you claim that 'it did not work'.

Darren,

In the cases where it does work, it would not be in court, would it?

In the cases where it does not work, and ends up in court, and is the tested and found that it does not work as planned ... that's what is being referenced.

If it works, why would it even be in court?

The ANSI test for Cat. I B-vented appliances calls for blocking the flue then allowing it to backdraft 100% for TEN (10) minutes before the spill switch trips. That's a full cycle on a WH or furnace. That means if each appliances cycles for 9 minutes each time, then legally, it does not have to trip and lock out. We call that a ventfree appliance.

Houses were built a lot differently when the present codse were written. The guys who sit on these Cmtes. grew up in old leaky houses with 50% efficient gas boilers and furnaces that vented into unlined chimneys and for a large part, they worked. As Ballanco pointed out, at ~75% efficiency, your typical 40MBH WH will dump 10MBH at the chimney. It takes about 50MBH to heat up your std 8x8 flue to 120*F so where's the extra heat coming from if your stack loss from the WH is only 10? It doesn't. Since you have a setback T-stat, your furnace will cycle less often for large parts of the day leaving longer wet times. When it finally does cycle on, it now has to dry the flue then warm it up above dewpoint before you really get any draft. The ~330*F stack temp of a fan assisted furnace will not dry that chimney, heat the walls of the flue and generate sufficient draft, which is why you need a thin-walled liner or B-vent, which only require a few hundred BTUs to warm up.
Now, review my earlier post---what happens when a bird/ squirrel builds a nest in the chimney? The furnace and WH both continue to vent into the home. You bypass three safety features on the furnace--the WH has none or maybe an ECO at the bottom.

What DavidR is saying is that he works on these setups all the time, as do I. We and thousands of other pros find every day these things don't vent in the first place, and often backdraft severely with very little help. You put this setup in a CAZ with even marginal MUA or add an exhaust such as a clothes dryer and you have a CO production and distribution system.

Changing codes takes a lot of time because the old guard don't like to admit they're wrong. They like being comfortable. They resent change. Do you know how long it took to get the code just to recognize the problem with fan assisted furnaces in cold exterior chimneys? How about the pressure from GAMA because they were worried it would kill sales if you said they needed liners.

Codes do not guarantee performance. As I said, the current codes do allow this setup. They also allow other configurations such as separate venting or converting all the appliances to Cat. IV and power venting them. You have options. You now are informed as to the problems with this setup. My question is, on the next case, what will you do? Do you wait to see signs of condensation such as rusted out vent connectors, stains on the units scorched draft hoods and rotting walls?

I think Mr. Peck made a good point. If you have DavidR or an NCI Certified pro make the corrections and test to confirm it works, how are you going to have an incident in the first place? Left alone, you are much more likely to have problems but the problem is, you now know there is a potential hazard, which now puts you at more risk since you know better.

How about something along these lines for your report:
"The furnace and water heater are common vented. While allowed by code, this configuration has been found through field investigations to present hazards to the home and unacceptable performance. The configuration effectivly disables the primary safety controls on the furnace allowing the possibility of backdrafting through the water heater into the home. This could result in exposure to deadly carbon monoxide and other noxious gases. I recommend a qualified professional consult on alternatives that allow for proper venting with the addition of safety controls to the water heater." or some such similar verbiage.

Bob
NCI Certified Carbon Monoxide and Combustion Analyst