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exhaust vent run for 95% furnace

jjc155's picture

I'm likely gonna replace my 80afue furnace with a new 95% furnace that vents out the wall using the PVC (I know different but cant remeber what it is right now) pipe. It will have fresh air intake from outside to and will be a sealed combustion type furnace, variable speed fan, etc.


Question is: How far can the exhaust and intake runs be? Where my furnace currently is if I go one way the run will be like 6 feet but would need atleast 4 elbows to make it around and through my floor joists and rim joist. Joists are the engineered i-bean type and I know your not supposed to punch big holes through them. I have a floor joist sitting about 4 inches from the rim joist (flange of the joist is only about 3 inches from foundation) on that side and don't even know if I could get everthing through (under the floor joist between floor joist and top of foundation and then 90deg through the rim and out the brick)


If I go the other way it will run parallel to the joists and only need 1 elbow, but the run would be about 15feet.


Does the distance really matter or is there a limit?


Thanks


J-

(post #115437, reply #1 of 39)

Either should work. The install instructions for the furnace should give formulas for allowable flue runs. Often you can find the install instructions on the web, to look at them before you get the furnace.

Punching holes in the web of I-beams is generally OK, so long as it's not too close to the ends. Others can give the precise rules, or again you can probably look them up on the web.


Corporation: n. An ingenious device for obtaining individual profit without individual responsibility. --Ambrose Bierce


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #115437, reply #2 of 39)

    Oh, there's definitely a limit! Your furnace manufacturer ought to have that information available for you.


     The resistance if the intake and exhaust runs becomes far more critical the more efficient the furnace is. Remember, it is that 'waste' heat that makes the air move ... if there's not enough of a temperature differential, then the air won't flow.


   While a fan can help with the exhaust, the free flow of fresh air in is just as important.


   A service call last summer brought this point home. A commercial water heater was malfunctioning. The exhause was a short run, with two elbows giving the pipe an offest about equal to the diameter of the pipe. The heater was itself located about six feet to the side of an door open to the outside, with no obstructions. Yet, circumstances were such that this water heater was starved for fresh air, to the point that the burner would not stay lit.


     The proof this was the cause of the problems? Providing the heater with another source for fresh air solved the problem.


   Never underestimate the need for make-up/ combustion air .... or the need for good venting.

(post #115437, reply #4 of 39)

Huhhhh?


 


OP is talking about a 95% furnace.  "waste heat" ain't got nuttin to do with it.  The length and size of the pipe is the governing factor.


 


The furnace already has an exhaust  fan.


Edited 10/26/2008 3:54 pm ET by rich1

(post #115437, reply #8 of 39)

Perhaps you misunderstood what I was trying to say.


The heat that is not used to heat the home, and goes out the exhaust, is often described as 'wasted heat.' There are any number of devices on the market that promise to catch this heat, and put it to some use.


My point is that some of this heat is absolutely essential, as hot air won't rise, convection won't take place, unless there is a temperature difference.


How much of this heat is necessary to make things work is determined by the same factors that determine fan size: how much air must overcome how much resistance to get out. Or, in the case of fresh air, in.


The resistance is determined, in turn, by the length, pitch, and shape of the duct / flue.


   To help make the point, I went on to describe a situation where  a 'can't fail' arrangement for a water heater did, in fact, fail.

(post #115437, reply #9 of 39)

There is an induction fan!! That moves the air in and out through the combustion chamber. The intake and flue are at the same level (on the exterior of the building), so there can be no "heat rising" effect.

Have you ever actually SEEN a high-efficiency furnace??


Corporation: n. An ingenious device for obtaining individual profit without individual responsibility. --Ambrose Bierce


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #115437, reply #10 of 39)

I fail to see your point.


The only thing that matters here is that, as a 'high efficiency' design. there's less room for error. Think of it as a disadvantage to being too efficient.


Fan or no fan ... the length of the duct is crucial to how much air flows. The furnace instructions ought to give the guy the answers he needs.


Though I must confess my ignorance ... what is an induction fan?

(post #115437, reply #11 of 39)

Yes, you have confessed your ignorance.


Corporation: n. An ingenious device for obtaining individual profit without individual responsibility. --Ambrose Bierce


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #115437, reply #12 of 39)

The ONLY way the exhaust leaves a high efficient furnace is by the use of a fan.  There is NO natural draft.


Edited 10/27/2008 12:54 am ET by rich1

(post #115437, reply #13 of 39)

I am a bit puzzled by this insistance that there is no natural draft, etc. Are we claiming that the air that exhausts goes out at exactly the same temperature that it entered? If so, then there is no draft, no convection.


Likewise, just how is the flame established? One of the problems discovered in our space program is that in a 'zero gravity' environment, where the lack of weight results in there being no convective currents, is that a flam will extinguish itself with it's own smoke.


Now ...  if someone wants to say that there is so little heat remaining that a fan is needed to ensure proper flow ..... that's different.


Which, of course, is completely irrelevant to the original question: how long can the duct be? Fan or no fan, the math is the same : you need enough of a 'push' (from whatever source) to overcome resistance. The original question only addressed length, ignoring other factors (duct size, bends in the line, etc.)


Do fans have limitations? They sure do. Even the common bath fan often moves little air, simply because of resistance in the duct line. Less obvious is the limitation caused by inadequate fresh air being available; again, the common bath fan often fails to move much air, simply because the bathroom door is closed - blocking the source of fresh air.


All the OP can do is refer to the materials that came with his furnace. There's not an engineer in the world that can give a definitive answer without the details of every element of the installation.


An alternative approach would be to instal, and then measure. The air flow measurements would provide the answers.


Anything else is pure guesswork.

(post #115437, reply #14 of 39)

Forced induction furnaces have been around for about 20-25 years. The fact that you're ignorant of them is no reason that we should have to waste time educating you.

Look it up on the internet.


Corporation: n. An ingenious device for obtaining individual profit without individual responsibility. --Ambrose Bierce


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #115437, reply #15 of 39)

Well, as I see it ... that's at least twice you've felt the need to make personal attacks ... and thrice you've failed to answer anyone's questions ... including the original post.


If you're not here to answer questions .... why are you here?


 

(post #115437, reply #16 of 39)

Ok.  Hope you don't take this personally. 


The OP asked about venting for a  furnace.  He was given good advice to check with the manufacturer.


You started giving advice that had NO application to the OP.


You have no idea how a high efficient furnace works. 


OK. 


A high efficient furnace is a sealed combustion gas fired appliance.  Generally, the flow of exhaust through the furnace is from top to bottom, which means that natural draft won't work.  So, there is a fan that provides the needed pressure to vent the furnace.  To confuse the issue slightly, most can also be installed on their sides or upside down.  A fan is still needed.


This fan also provides a positive pressure in the venting system, so it needs to be a sealed system, which means either plastic pipe or special metal venting.  Class A,B or C venting cannot be used.


The air intake and the exhaust usually need to be within about 2 feet of each other.  This also negates any draft since there will not be any noticeable difference in pressure.  If fact, most manufactures want both to be in the same pressure plane.  Which means you can't take combustion air from one side of the house and exhaust on the other.


There is a usually a pressure switch in the furnace which references both the exhaust and the combustion air pipes to insure that there is the correct amount of air flowing.


This flow of air through the furnace is confirmed before any heat is produced due to the possibility of carbon monoxide problems from a blocked vent.


Clear as mud so far?   Any questions?


Edited 10/27/2008 9:34 pm ET by rich1

(post #115437, reply #17 of 39)

Very good ... and I thank you. I seem to have mentioned checking the manufacturer's instructions in my first post - and repeated it a few times. I also seem to recal having stated that the nature of the pipe run is important.


I don't think those comments are 'ignorant' at all.  Indeed, they seem pretty close to what you are calling good advice.  Looks like we've been on the same page all along.


As for not knowing about "high efficiency furnaces," you are surely correct. Part of my problem is the vast number of designs, accessories, etc., that all claim to make your furnace/ boiler / stove highly efficient. If the term has somehow become reserved for a particular design, I must have missed that meeting.


Likewise, as far as I know, every furnace maker claims that their designs are unique, and superior to, everyone elses'.


One of the things I do is help teach apprentices. An amazing transformation takes place between the second and final year: the students manage to lose their absolute certainty, and begin to appreciate just how much there is left to learn. I find that this pattern continues ..... the Masters are always willing to think things through, while it is the handymen who rant about how ignorant the other guy is.


 

(post #115437, reply #18 of 39)

Yep, you mentioned the manufacturer's instructions -- right after I did. And then you launched into a tirade about length being more critical for higher efficiencies, not having the foggiest how a high-efficiency furnace works.

The basic design of high-efficiency gas forced-air furnaces has been the same for the past 25 years or so -- they're all basically the same, with one or two minor variations. The only really different design was the original Lennox "Pulse", which worked like an old German buzz bomb, and one other early brand I can't remember that circulated hot water internally. Otherwise they all use an induction fan and primary and secondary heat exchangers in pretty much the same configuration. The main difference between brands is in the design and materials used in the secondary heat exchanger, since that's where corrosion is a problem.


Corporation: n. An ingenious device for obtaining individual profit without individual responsibility. --Ambrose Bierce


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #115437, reply #19 of 39)

Thank you.


Now, again, for terminology's sake ... what are you calling an 'induction' fan?


Thanks

(post #115437, reply #20 of 39)

There's two different terms, and I can't remember the other one. But it's a fan that induces a draft in the burner. Think of a blacksmith's forge -- the bellows (or, in more modern setups, a hand-cranked blower). That the induction fan -- it induces a draft.

The fan can be either on the input side of the burner, blowing air, or on the output side (though after both heat exchangers), sucking fumes. Since the burner is tightly sealed, it doesn't really make any difference (though obviously there are engineering considerations for one vs the other). And since the exhaust gasses are at a temperature below 212 (boiling point), the fan doesn't have to stand high temperatures if it's on the exhaust side.


Corporation: n. An ingenious device for obtaining individual profit without individual responsibility. --Ambrose Bierce


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #115437, reply #21 of 39)

Thank you.


Being electrically inclined, I think other thinggs when I use the term 'induction.'

(post #115437, reply #22 of 39)

Ok, I'm gonna bust your chops now.  :)


 


A draft inducer does just that, it starts a draft.  On a mid eff furnace, you have a draft inducer fan, but you still have a negative pressure in the chimney.  Which is why you can use class B venting.


Sealed 90%+ units have a combustion fan.  Positive pressure, no draft, sealed venting. 


Draft is dependant on temp difference, chimney size, vent connector length and height and a few other issues.


 


Let those who live in glass houses..........


and yeah, I know I'm looking in a mirror  :D

(post #115437, reply #24 of 39)

Like I said, I've heard several terms for the fan, in different contexts.

I'm actually going to defend Reno here a bit (as well as myself) in that I went online trying to find a writeup or (better) diagram of a HE furnace and found none, after a good 10-15 minutes of searching. I'm sure there's one out there, but it's hard to find.

When the HE units first came out the sales lit always included diagrams and descriptions (which is why I know as much as I do), but that was 20-odd years ago, and now all that's treated as a black box, so no good info, just platitudes in the literature.


Corporation: n. An ingenious device for obtaining individual profit without individual responsibility. --Ambrose Bierce


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #115437, reply #25 of 39)

Looks like it's time for a group hug :D:D

(post #115437, reply #26 of 39)

WOW, looks like I'll just check the manufactures info that comes with the furnace!! LMAO.


Sorry about starting a thread that went alittle awry. I was looking for general info as one route is straight and lone and the other is short and with lots of bends (also more of a pain to run with the way it is framed).


Thanks for the input though guys, it is appreciated.


J-

(post #115437, reply #27 of 39)

I'll chime in a little here, as I am currently installing a 92% unit in my wife's studio bldg.


Your intake and exhaust pipe sizes are dependent on the BTU rating of the furnace and the distance you need to run the pipe(s). Up to about 80,000 Btuh you can run 40' of 2" pipe with 5 elbows. For 70' of pipe with the same number of elbows the pipe size increases to 3". At 100,000 Btuh the pipe size starts at 3" for the three models listed in my installation manual.


For the 95% models there is a similar table, so your manual will give you all the information you need.


The manual will also give you termination clearances for location of both pipes with respect to fixed or operative windows, doors, soffits, building corners, porches, decks, service regulator, dryer vent, or hot water vent. All things you need to consider when planing your pipe routing. 


You will also need to plan for the condensation. Either a direct drain to a floor drain or  a separate condensation pump to lift it to a drain line. These puppies put out a lot of condensate so you need to plan carefully how you are going to get it out.


Even though the exhaust gas is cool enough to run through pvc, it will still have some moisture remaining in it, so the exhaust pipe needs to be pitched slightly to drian back to the furnace.

(post #115437, reply #28 of 39)

My gut feel, given the installations I've seen and the install instructions I've read, is that you'd probably be OK with either configuration.

But of course you need to follow the manufacturer's specs in all cases.


Corporation: n. An ingenious device for obtaining individual profit without individual responsibility. --Ambrose Bierce


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #115437, reply #29 of 39)

For you and DanH


http://www.patentstorm.us/patents/5406933/description.html


The small blower motor on the hi-eff. furnaces I have installed have always been on the exhaust side. They are used to extract the latent heat in the secondary heat exchanger, thus cooling the exhaust gas temperature to a low enough point that pvc can be used.


Now on power generation boilers there are both supply and exhaust induction fans.

(post #115437, reply #30 of 39)

Of course, the primary reason for low exhaust temp is not to permit PVC flues, but rather to extract as much heat as possible, thereby increasing efficiency. In a closed system (where no air is mixed with the flue gasses) efficiency goes up as exhaust temp goes down.


Corporation: n. An ingenious device for obtaining individual profit without individual responsibility. --Ambrose Bierce


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

(post #115437, reply #23 of 39)

Oh yeah, on a lot of mids, the fan is on the exhaust side and has to withstand high temps.

Right--he's talking about a (post #115437, reply #36 of 39)

Right--he's talking about a gravity vent appliance (Category I) that operates with a negative pressure in the vent, but the 95%'ers are Category IV appliances, and operate with positive vent pressure that's created by an exhaust blower, as you say.

The manufacturers' instructions on venting allow surprisingly long runs of pipe--up to 60' for some, although it sometimes means that you have to increase the vent pipe size up to 3".

The OP will likely find the either way he wants to run will be OK with only 2" pipe. As DanH says, the manufacturer will have tables that will answer this.

Remember, it is that 'waste' (post #115437, reply #34 of 39)

Remember, it is that 'waste' heat that makes the air move ... if there's not enough of a temperature differential, then the air won't flow.

??  On a 95% furnace the draft is forced which is unlike natural draft furnaces that I think you are referring to. That is why the high efficiency furnaces can vent directly horizontal ... or in almost any direction for that matter.

There ain't NO free lunch. Not no how, not no where!

The heater was itself located (post #115437, reply #37 of 39)

The heater was itself located about six feet to the side of an door open to the outside, with no obstructions. Yet, circumstances were such that this water heater was starved for fresh air, to the point that the burner would not stay lit.

I'm curious--was this in a restaurant?

(post #115437, reply #3 of 39)

My HVAC guy told me a 90 elbow has as much resistance to air flow as 15 foot of stright pipe, in your case 15' + one ell would be the best. Check the install instructions for details of whats best.