Fan assisted and power vent category 1 furnace

[Jon mackay]

This is a configuration that I have not seen before. Can this be done?
It's a category 1 furnace with fan assist and then power vented to the exterior.

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[Scott Patterson]

Well the photo is proof that it can be done!

But it is wrong! For starters it needs to be 3' of more above the roof in most cases (depends on size of pipe and slope). Then we can talk about the basement window and the window above the vent termination.

[Bob Harper]

The venting is under positive pressure. Therefore, the venting must be tested and approved for positive pressure. Single walled vent connector and B-vent, whether gooped or not is Not approved for positive vent pressure.

I see signs of condensation corroding the joints. Also, inside the cabinet to the left of the inducer appears to be dusty from blowback.

The unit must be approved for horizontal venting.

The vent termination is too close to the windows.

I would want to see the installation manual to see how they propose to vent this unit.
Bob

[Richard Drennan]

Allow me to introduce myself, Rick Drennan, Technical Support for HVAc Equipment and Instructor, former technician with 25+ years experience in the field..... Actually, B-vent and single wall vent pipe is allowed for positive pressure venting and has been used for many years. All furnaces these days are draft induced wether they be 80% or 90%+. If they are 80% they would have a draft inducer and would use single wall or B-vent if they are 90+ they also have a inducer and would use PVC or ABS pipe. The reason for the condensation is due to the flue gas temp. being reduce to the point below the condensation temperature. Yes. the termination is definatly wrong and a safety hazard and should be taken above the roof line. As for the reason that somebody ADDED the additional inducer to the flue is beyond me, possibly a tech that actually had no idea what he was doing in the first place and the one that originaly installed the unit put it in trying to resolve some unknown issue that he thought he had rather than actually repair the problem which was a firing or pressure switch issue.

[DavidR]

Didn't know that B-vent and single wall pipe were allowed for positive pressure applications in regards to flue gas.

Judging by the look of those stains from condensation running down the B-vent in the pictures it didn't do a very good job in that application.

[Richard Drennan]

Yes, but if you measured the pressure it would only measure about .01 to .oo1 inches wc maximum, So, is that actually considered positive pressure, yes. is it negative pressure catagory I, NO. If you look at the rust stains you can see that the unit is probably underfiring so the flue gasses are cold or the secondary inducer in moving the combusted gasses far to fast there again super cooling the flue gasses. Both of which would cause condensation in the flue pipe and thus you have rust coming from the joints.

[DavidR]

Kinda missed my point, if it will leak condensate it will leak flue gas.

Never seen double wall pipe that was a sealed system when put together.
Thus the need for the flue to be negative.

Furnace underfired? Probably
I would guess that the condensation is due to a lack of postpurge more than anything.

[Bob Harper]

Richard, I'd like to know where you are getting your information from about B-vent. To re-state for the record: B-vent is NOT listed or approved for positive vent pressure! The mfrs. put that in the manuals. It is in the UL441 listing. I know of positive pressure units that were pulled off the market because they tried to list them for use with B-vent that had the joints siliconed. B-vent is Not tested for positive vent pressure and there is nothing in UL 441 about a positive pressure test. The only venting standard that does test for positive vent pressure right now is UL1738 for cat. IV condensing gas heaters. That std. tests to 3 inches w.c. positive vent pressure. I have made proposals to UL to investigate new standards for pellet venting but that is just being explored.

A fan assisted cat. I unit uses a fan to draw air through the combustion chamber but the exhaust gases are expected to generate natural draft in the vent. Also, to vent B-vent horizontally would throw off the 1" clearance to combustibles in addition to voiding the warranty and listing.

If this unit was tested and approved for horizontal venting, you could put a fan termination on that sucks the flue gases out in which case single walled pipe or B-vent would be ok.

Richard, unlisted single walled pipe is NOT approved for positive pressure because the IRC states any positive pressure venting must be tested and listed for that use and it is not. NFPA 211 echoes this requirement.

HTH,
Bob Harper
UL 103 Standards Technical Panel, which includes UL 441 for B-vent

[Richard Drennan]

No, you miss the point, the pressure exerted on the pipe is less than 1/100th of an inch wc of pressure, the weight of water exerts more pressure than that and a properly firing 80% furnace does not have condensation in the flue gasses. B-vent and single wall has ALWAYS been used on furnaces less than 90%, there is NO other vent used in this application. Name one substitute! There is none, I have been in this trade now for almost 29 years, 25 in the field and almost 4 now as Technicle support and teaching. Trust me...

Post purge is NOT for condensation reasons, it is to cool the exchanger to keep it from cracking. If a hot exchanger sits without properly cooling it can crack and fail. Then you also have Pre-purge, this is to insure that there is sufficient combustion air and NO non-combusted gasses in the exchanger that could cause an unwanted explosion. The combution process in this application must maintain a flue gas temp of at least 275 degrees, any lower and you risk condensation. Since the addition of the external draft inducer it increased the air flow thru the exchanger without increase of the flame, decreasing the flue gas temp and increased the flue pipe pressure causing condensation and leakage of the condensation and flue gasses by increasing the pressure exerted on the interior of the flue pipe. Do I need to continue? Might I remind you, This IS what I do every day for a living and I am the best at what I do. I presently hold 6 NATE certifications, 2 of which are in Heating that include venting. I am also a proctor for Rheem/Ruud, York, ICP (Carrier), Trane, Amana, ACCA, ESCO, NATE, GE, Trackpipe, Wardflex, Fujitsu & Freus. All I can say is Yes the install was incorrectly done and yes there is a problem and yes you CAN use B-vent and single wall pipe on a 80% forced draft furnace because there is NO other means to vent it. And, finally, this unit can be corrected to operate safly and efficiently simply by venting it directly the same root WITHOUT the second draft inducer and taking it COMPLETLY to the beyond the roof line aprox 2 feet using double wall B-vent which is air tight when properly installed and operating correctly at the CORRECT pressures.

[Richard Drennan]

Richard, I'd like to know where you are getting your information from about B-vent. To re-state for the record: B-vent is NOT listed or approved for positive vent pressure! The mfrs. put that in the manuals. It is in the UL441 listing. I know of positive pressure units that were pulled off the market because they tried to list them for use with B-vent that had the joints siliconed. B-vent is Not tested for positive vent pressure and there is nothing in UL 441 about a positive pressure test. The only venting standard that does test for positive vent pressure right now is UL1738 for cat. IV condensing gas heaters. That std. tests to 3 inches w.c. positive vent pressure. I have made proposals to UL to investigate new standards for pellet venting but that is just being explored.

A fan assisted cat. I unit uses a fan to draw air through the combustion chamber but the exhaust gases are expected to generate natural draft in the vent. Also, to vent B-vent horizontally would throw off the 1" clearance to combustibles in addition to voiding the warranty and listing.

If this unit was tested and approved for horizontal venting, you could put a fan termination on that sucks the flue gases out in which case single walled pipe or B-vent would be ok.

Richard, unlisted single walled pipe is NOT approved for positive pressure because the IRC states any positive pressure venting must be tested and listed for that use and it is not. NFPA 211 echoes this requirement.

HTH,
Bob Harper
UL 103 Standards Technical Panel, which includes UL 441 for B-vent

Then show me what is on the market today to vent ANY 80% furnace....

[Richard Drennan]

There are ONLY 2 types of vent pipe sold in the HVAC market, that for 90% plus and that for below 90%.

90% and above use PVC and ABS

80% up to but not including 90% uses "metel sectional pipe" also known as and sold as "double wall" "B-vent" "amerivent" and other names also single wall pipe. This has been an industry standard for years. City and County inspector know this and have passes these pipe as "acceptable" for what you are calling "Positive Pressure" when I have been trying to tell you the pressure is less than 1/100 of an inch of pressure inside the pipe. That is not even enough th cause a match flame to flicker. Check your information.

[Bob Harper]

Richard, category I venting only goes up to 83%AFUE. Above that, you need special venting such as AL29-4C listed to 1738 until you get up to Cat. IV when you can use plastic.

You seem to be bouncing all over. One minute we're talking about an aftermarket power venter used on an 80% furnace power vented out horizontally with single walled leaky pipe under positive vent pressure and now you're talking about vertical venting with single walled pipe to B-vent, which IS the normal, accepted, listed means of venting a Cat. I furnace under negative vent pressure.

IMC 801.9 Positive Pressure "Where an appliance equipped with a forced or induced draft system creates a positive pressure in the venting system, the venting system shall be designed and listed for positive pressure applications"


FYI, some techs will put a longer post purge on just to clear the exhaust gases so there're less chance of spillage or condensation at standby. In the case in point, once that power vent cycles off, you have warm moist gases trapped in the horizontal vent with no draft to evacuate them so they cool and condense as seen.

" Since the addition of the external draft inducer it increased the air flow thru the exchanger without increase of the flame, decreasing the flue gas temp and increased the flue pipe pressure causing condensation and leakage of the condensation and flue gasses by increasing the pressure exerted on the interior of the flue pipe. Do I need to continue?" --- Huh? I'm sorry but does anyone else see this as a cohesive thought?


"Might I remind you, This IS what I do every day for a living and I am the best at what I do." ---Well, at least you're humble.....

Have a good day, ;-)
Bob

[Richard Drennan]

Richard, category I venting only goes up to 83%AFUE. Above that, you need special venting such as AL29-4C listed to 1738 until you get up to Cat. IV when you can use plastic.

You seem to be bouncing all over. One minute we're talking about an aftermarket power venter used on an 80% furnace power vented out horizontally with single walled leaky pipe under positive vent pressure and now you're talking about vertical venting with single walled pipe to B-vent, which IS the normal, accepted, listed means of venting a Cat. I furnace under negative vent pressure.

IMC 801.9 Positive Pressure "Where an appliance equipped with a forced or induced draft system creates a positive pressure in the venting system, the venting system shall be designed and listed for positive pressure applications"


FYI, some techs will put a longer post purge on just to clear the exhaust gases so there're less chance of spillage or condensation at standby. In the case in point, once that power vent cycles off, you have warm moist gases trapped in the horizontal vent with no draft to evacuate them so they cool and condense as seen.

" Since the addition of the external draft inducer it increased the air flow thru the exchanger without increase of the flame, decreasing the flue gas temp and increased the flue pipe pressure causing condensation and leakage of the condensation and flue gasses by increasing the pressure exerted on the interior of the flue pipe. Do I need to continue?" --- Huh? I'm sorry but does anyone else see this as a cohesive thought?


"Might I remind you, This IS what I do every day for a living and I am the best at what I do." ---Well, at least you're humble.....

Have a good day, ;-)
Bob

I never was talking about a negative pressure vent, I was always talking about 80% furnaces and they have always in all applications used B-vent and single wall pipe verticaly and horizontally. I was refering to the addition of the second indercer. As for ajusting the post purge, on mondern furnaces thhe post-purge is factory set and is NON ajustable. as for the AFUE rating residential furnaces go as high as 84% and still use B-vent. But, keep your chin up, if you don't understand the combustion process and how you need the proper air to flame mixture to prevent condesation and keep the flue temp up I am sure I can direct you to the person or persons that can explain it to you, maybe in crayon.

In short I did not sign on to this web site to argue with somebody that thinks they know something that they have no clue and by saying you can not use B-vent and single wall flue pipe on a standard modern day residential furnace and saying that they can adjust the post purge time prooves they have no clue to the equipment or it's venting. According to you signature you work with fireplaces, why don't you stick to that and leave the furnaces to us. Goodbye for I am canceling my account. I do not need this garbage. I tried to help by answering a concern and all I got was critisized by somebody that only thought he knew the equipment before he knew the facts. Yes B-vent is not considered positive pressure. But we are only talking about 1/100 of an inch. Check it out...

[Bob Harper]

Richard, no offense intended but have you been drinking tonight? I think you'd better sleep this one off.
Have a good evening,
Bob

[Jerry Peck]

In short I did not sign on to this web site to argue with somebody that thinks they know something that they have no clue ...

Richard,

Before you go tooting your horn too loudly, here or anywhere, I would first suggest getting to know Bob H. and David R., and find out what they do ... EVERY DAY ... and what they find ... EVERY DAY ... and you three can compare qualifications, education, technical training, etc.

I believe you will find that they are VERY KNOWLEDGEABLE is what they know and do. As I am sure you are.

We (all of us) welcome you here to teach us what is correct, not because you say so because you teach the stuff, ... we all see examples of things done wrong every day by contractors who were taught wrong by people who knew one thing and taught something else, either that or the contractors never learned anything from anyone except themselves.

You said "In short I did not sign on to this web site ", we are glad you signed on, but if you think you know it all - and your posts said as much, to wit "and I am the best at what I do" - you will find someone who knows enough to show you do not know it all.

In short, we are "home inspectors", but we are not "dummies", which you seem to think we are. I should add that we were many different things before becoming home inspectors. Many highly skilled and knowledge people here, all contributing to a common goal of learning.

Bob H. and David R. are not home inspectors, they are knowledgeable professionals in their field.

[DavidR]

I think I missed something?????

This could have been a great discussion.