Concentric Venting questions

[skinned_knuckles]

I'm trying to understand concentric venting and I have a few questions.

1. In addition to cooling out-going exhaust gases it seems to me that one benefit of concentric venting is to warm up the incoming combustion air. If this is the case, why is the inner pipe of the system double walled? That drastically cuts down on the heat transferred. If that is a code requirement, could you explain the reasoning behind the requirement?
   
2. I see many horizontal concentric venting systems. Should I assume that concentric venting cannot be used with vertical pipes? If so, please explain why.
   
3. If I understand Class B double walled pipe it is a 5" stainless steel or aluminum inner pipe surrounded by a 5.75" galvanized outer pipe. So if that arrangement meets code requirements, it seems to me that a 5" inner pipe surrounded by a 7" outer pipe would be even better. Am I missing something?
   
4. Is there a website that explains all of this? I've looked and haven't found one.

 

[fitter30]

I've only had experience with pvc flue systems where the concentric fitting is used for a single hole for both exhaust and combustion air for roof or sidewall penetration. Heres a company that makes both pvc and metal. Heating equipment manufacturers usually will sell the recommended fitting.
https://www.centrotherm.us.com/Products.aspx

 

[danpik]

The answer to #1 is this... concentric venting is not designed to cool or heat the gasses flowing thru them. It is done simply to provide one path out of the building for incoming and outgoing gas/air. The reason for the double wall center is so that you do not cool the flue gas to much. In a nat gas system it is critical that the flue gas temp stays above ~150 deg F. This is so the water in the gas does not condense out. Nat gas exhaust gas contains a large amount of water vapor and will stay as that vapor until the temp drops below ~150 deg. This is why high efficiency furnaces require PVC venting and a water drain.

#2 I have installed both horizontal and vertical concentric. Mostly PVC though.

#3 I have installed 6" B-vent. Never seen it in a concentric though. Never looked for it either.

Dan

 

[Jackfre]

Direct venting does a couple things. Any warmingmof incoming air is incidental. Concentric DV units reduce the clearances at terminations. DV appliances less than 50kbtu have a 9” clearance to a door or window. Above 50kbtu it is 12”. You can have zero clearance to combustibles. Single hole thru wall.

 

[ripperd]

In category 3 stainless venting, the inner pipe is NOT double wall.

http://www.littlegreenhouse.com/accessory/heaters4-sterling-gg.shtml#concentric

 

[yeldogt]

It makes for easier and in my opinion better looking install -- except vertical through the roof. The ones I have done are pipe in pipe -- not double pipe with another pipe. The exhaust in the middle on the cabinet heaters allows for .. again, a simple single hole install .. mine have always been small metal.

I don't like concentric though the roof -- it's too big. On a side wall it looks better vs the common up and down pipe

 

[Jackfre]

I don't like concentric though the roof -- it's too big. On a side wall it looks better vs the common up and down pipe

To large? 2x4 or 3x5” OD. What equipment takes larger than that?

 

[yeldogt]

To large? 2x4 or 3x5” OD. What equipment takes larger than that?

Jack -- I don't like the big bulge .. too noticeable.

But, I'm neurotic ........ This is a poor picture of my current project

pipe on the right is my boiler exhaust .. the left is plumbing. Zinc roof - centered over a bump out. I'm using a wall inlet

This is the Rheem that I have previously used ...on a roof they need quite a bit of height to eliminate problems with updraft.

 

[Jackfre]

Well, everyone gets to make the call. It’s your place. Is this for the furnace or water heater? Also, should you choose to do so, you could paint that pvc any color you want to fade it out.