# I never seen that before



## Sylvain

Hello,









Why some boilers have two air vents ? What's the advantages ?

Overhere boilers have just one concentric air vent


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## user2090

Maybe I am missing something obvious, but isn't that just a standard two pipe condensing unit?

One pipe is make up air.

Other pipe is for exhaust.


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## greenscoutII

Indie said:


> Maybe I am missing something obvious, but isn't that just a standard two pipe condensing unit?
> 
> One pipe is make up air.
> 
> Other pipe is for exhaust.


This^^^^^^


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## plumber666

You can get a concentric vent adapter that slips right in to the top of those boilers.


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## rocksteady

Indie said:


> Maybe I am missing something obvious, but isn't that just a standard two pipe condensing unit?
> 
> One pipe is make up air.
> 
> Other pipe is for exhaust.


I think he understands what that is a picture of but wants to know why some are separate pipes and the ones he's familiar with are concentric.






Paul


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## OldSchool

Indie said:


> Maybe I am missing something obvious, but isn't that just a standard two pipe condensing unit?
> 
> One pipe is make up air.
> 
> Other pipe is for exhaust.


Actually technically

one is combustion Air 

Other is pipe is for exhaust


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## ironandfire

We have all kinds of options over here in the states. I'd be interested to see some heating pics from France.


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## ironandfire

Sylvain said:


> Hello,
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Why some boilers have two air vents ? What's the advantages ?
> 
> Overhere boilers have just one concentric air vent


I think it boils down to a manufacturing thing.


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## Sylvain

ironandfire said:


> We have all kinds of options over here in the states. I'd be interested to see some heating pics from France.


Ok, 

It is not pictures of my works.

Condensing combi boiler










Oil combi boiler


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## Sylvain

Indie said:


> Maybe I am missing something obvious, but isn't that just a standard two pipe condensing unit?
> 
> One pipe is make up air.
> 
> Other pipe is for exhaust.



Actually I don't understand why manufacturer make a boiler with two pipes while one concentric pipe works, and it faster to fit as well.


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## Scott K

Sell post below - EDIT.


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## Protech

That's some fancy pipe work there frenchy :thumbsup:



Sylvain said:


> Ok,
> 
> It is not pictures of my works.
> 
> Condensing combi boiler
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> Oil combi boiler


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## Scott K

*Concentric Venting*

First, for those who are unfamiliar with concentric venting, it is essentially a pipe inside a pipe utilized to exhaust the producst of combustion, as well as bringing in the combustion air. The Flue (exhaust) pipe is typically inside pipe, the intake (combustion air) is the external pipe. 

The chief advantage of concentric/coaxial type systems is if the piping is long enough, the concentric vent acts as sort of a "heat exchanger" between the warm exhaust products going to outside in the internal pipe, and the incoming cooler combustion air. By increasing the temperature of the incoming combustion air, this helps combustion efficiency because the now slightly warmer intake air does not chill the flame in the burner as much. 

Typically, most European Modulating Condensing boilers (think Viessmann) have a semi sealed cabinet where this combustion air is pulled into by the fan, and then this intake fan sucks it in (out of the boiler's semi-sealed cabinet) and premixs it with gas and spits this precise flammable mixture into the burner to be ignited. This semi sealed cabinet is "technically" essentially just a large spot in the combustion intake piping and could be considered part of the concentic system. Because the heat exchanger is also in the cabinet, and because the heat exchanger is also warm bordering on hot (obviously,the burner and warm water piping is inside the heat exchanger) this large spot (the cabinet) is another area where the intake air travels and also becomes prewarmed by the heat exchanger. By doing it this way, you further reduce the effect of cold air chilling the flamem, on top of the heat exchanger will no longer be sending a bunch of heat into the mechanical room where it is usually not needed. Also, because the boiler cabinet is a slow spot in the combustion air travel, it also provides an oppurtunity for most particulate to drop out of suspension to the bottom of the boiler cabinet instead of getting ingested into the combustion chamber where impurities could build up overtime causing all sorts of bad things. 

The other advantage of concentric systems is if they are piped right to a sealed or semi sealed boiler cabinet, if the internal exhaust pipe (in the concentric system) ever failed for some reason, the exhaust would just get reingested into the intake air instead of spilled into the living quarters. 

One disadvantage to concentric systems is you must have a very well placed and proper termination. Because the intake and exhaust pipe are one, you must ensure there are provisions at the termination to reingest as little exhaust air into the combustion intake as possible. If this occurs on a sustained basis, the reingested products of combustion will raise the acidity of the condensate in the condensing boiler and this could cause premature wear or failure of the heat exchanger or other components. The other disdvantage to true concentric systems is the lack of versatility. This is mostly related to Retrofits however.

Sometimes when you're doing a retrofit, I know myself personally if I'm using a boiler with a 2 pipe option, I'll use the existing B-vent from the Atomspheric boiler we are replacing as a chase for the new plastic pipe exhaust and vent the exhaust through the roof, and I'll pull the intake air from outside just outside the mechanical room in a sidewall termination (if allowed by the boiler manufacturer) or in some cases, use inside air (I try and direct vent whenever I can however), depending on the application. I try and explicitly avoid sidewall exhaust penetrations due to the all sorts of code rules and issues that tend to conflict with them these days (and more to come too!). 

Most concentric vent systems in North America are not in my opinion true concentric vent systems like I described above. To do a concentric vent properly requires a semi sealed to sealed boiler cabinet, and piping directly into the boiler cabinet. The boiler manufacturers here in North America tend to offer a concentric venting option that involves tying the 2 pipes together that come out of the boiler (intake/exhaust) downstream of the boiler which means they do not have a true sealed cabinet. I think what has hindered some boiler manufacturers in North America from doing this is competitiveness & cost. To offer a coaxial system, which is a special venting system, requries research and cost to have available. It also requires extra R & D to better seal up the boiler cabinet, as well as develop a concentric system to fit your boiler, which may require special certificatin which also costs more $$$, especially if very few people buy your special concentric vent piping, the cost goes up further, and people do not see the benefits in it and just buy the CPVC or PVC. The other problem is we are hung on PVC and CPVC for vent materials, which often have sch. 80 pipe walls which do not transmit heat as well. So they don't make as much sense in a concentric system as polypropolene cocentric systems that you find in Europe (and on Viessmann Boilers as an option) where the exhaust pipe is quite thin and more readily transfers heat to the incoming air on the outside. 
Some companies offer a bit of a b*stard option" where they have 2 pipes coming out of the boiler cabinet and a sealed or semi-sealed boiler cabinet. Buderus, Triangle Tube, and IBC's new SL20-115 boiler offer this option. In my mind this is sort of the best of both worlds for those who want to run 2 pipes, but you also have some prewarming of intake air by having the combustion air in the boiler cabinet picking up heat off of the heat exchanger before being ingested into the fan to the burner.


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## ZL700

Sylvain said:


> Actually I don't understand why manufacturer make a boiler with two pipes while one concentric pipe works, and it faster to fit as well.


Your boilers overseas use polypropylene concentric vent kits. While some are used here too, many of our European boiler designs have been Americanized allowing us to use sch 40 dwv pipe which is about 20% of the vent cost of your polypropylene.


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## Scott K

ZL700 said:


> Your boilers overseas use polypropylene concentric vent kits. While some are used here too, many of our European boiler designs have been Americanized allowing us to use sch 40 dwv pipe which is about 20% of the vent cost of your polypropylene.


Polypropolene is a superior vent pipe in every way compared to PVC, and CPVC. There are also manufacturers here (Centrotherm) that produce a single pipe polypropolene with a slip fit system that is actually slightly cheaper than CPVC options (personally, I will NEVER use PVC for any appliance venting, period).


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## jim connolly

Sylvain said:


> Ok,


Flat-panel rads! I've been trying to get a few customers interested in these. Unfortunately, no luck yet.


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## plumber666

Sometimes concentric's the way to go, sometimes not. Some manufacturers say the vet and combustion pipes must be within 20' of being the same length. Provided the combustion air duct in a mech room is sized big enough, I've just goosenecked the comb. air and only shot the exhaust outside.


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## plumber666

"Some companies offer a bit of a b*stard option" where they have 2 pipes coming out of the boiler cabinet and a sealed or semi-sealed boiler cabinet. Buderus, Triangle Tube, and IBC's new SL20-115 boiler offer this option. In my mind this is sort of the best of both worlds for those who want to run 2 pipes, but you also have some prewarming of intake air by having the combustion air in the boiler cabinet picking up heat off of the heat exchanger before being ingested into the fan to the burner."

That's what gets the efficiency higher, right Scott?


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## Catlin987987

Thats very nice work, no one up here ever bends copper anymore


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## Scott K

plumber666 said:


> "Some companies offer a bit of a b*stard option" where they have 2 pipes coming out of the boiler cabinet and a sealed or semi-sealed boiler cabinet. Buderus, Triangle Tube, and IBC's new SL20-115 boiler offer this option. In my mind this is sort of the best of both worlds for those who want to run 2 pipes, but you also have some prewarming of intake air by having the combustion air in the boiler cabinet picking up heat off of the heat exchanger before being ingested into the fan to the burner."
> 
> That's what gets the efficiency higher, right Scott?


It helps efficiency because it helps to prewarm the intake air, which does not chill the flame as much. I thought I already said that though?


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## tk03

Here in the states we do much of our venting in PVC which is quick. As I was told Europe cannot vent in PVC. The whole chloride issue and brittle piping over time.
We do offer concentric in Polypropylene also.


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## Fireman

*Check your pricing on Poly again...*



ZL700 said:


> Your boilers overseas use polypropylene concentric vent kits. While some are used here too, many of our European boiler designs have been Americanized allowing us to use sch 40 dwv pipe which is about 20% of the vent cost of your polypropylene.


Maybe have a look at Duravent`s PolyPro product. Last I checked, it is a whole lot less then CPVC. While a little more than PVC, it has a much higher operating temp, and installs much quicker. No glues. No solvents. :thumbsup:


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## hasbean

wow a plumber with a bending machine,


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## bikefitter0

all the talk i hear is europe is way ahead of the us in heating systems .dont realy know


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## 907plumber

id like to know more about those installs. they look so bare. No zone valves, pumps, fittings, backflow/fastfill, pressure guages, purge points, air elimination...

There is nothing there but pipe and a boiler!


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## gtmechanic

*PVC on boiler exhaust*

using PVC on boiler exhaust is very questionable. PVC manufacturers state maximum temperature of 140f , and in non-condensing mode it is higher.


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## OldSchool

You guys using 636 venting in the USA yet?


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## ZL700

OldSchool said:


> You guys using 636 venting in the USA yet?


You mean the same sch 40 PVC & sch 80 CPVC grade pipe and fittings only yours is made by ipex and carries orange approval stamps and costs 5x more?

http://www.ipexinc.com/Content/Products/Product.aspx?IsDownload=true&FileId=3653


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## ZL700

gtmechanic said:


> using PVC on boiler exhaust is very questionable. PVC manufacturers state maximum temperature of 140f , and in non-condensing mode it is higher.


It's 149 degrees

Generally with flue temps 20 to 25 degrees F lower than supply water temps, sch 40 PVC shouldn't be used with set point temps over 160. 

Higher temp ops works well with CPVC which is rated to about 195. Many manufactures requires 5,6 or 10' of CPVC sch 80 then you may adapt to PVC.


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