Exhaust Backflow Prevention - Necessary?

[Paul Somlo]

I'm replacing a Takagi T-KJr tankless with, most likely, another Takagi, their smallest condenser. Current one is not a condenser and I'm using stainless steel Z-Vent (4"). The first Z-Vent section above the heater is a combination backflow preventer/condensate drain. Do I need a backflow preventer on the new condenser? I've got a 5 to 6 ft. vertical run through the roof, plan is for indoor intake air. I would just reuse the current venting, but the exhaust on the new heater is farther out from the mounting surface. Which means, I may ditch my Z-Vent and go with PVC. Of course, I don't even know if backflow prevention is offered in PVC, maybe it's not. So, not sure where to go with this - I could go with some sort of Z-Vent offset, I suppose, expensive stuff. Not sure if there's compatibility between Z-Vent and PVC, size-wise. If so, could do the offset in PVC. And moving the vent pipe is not an option - I just had a $20,000 flat roof put on last winter.

Hoping some of you can throw out ideas on this. Climate is Colorado.

Paul

 

[Bannerman]

What does Tekagi specify is the appropriate vent configuration for your new model?

Since it is a condensing model, the exhaust temperature will be substantially cooler than the non-condensing model. PVC or CPVC is commonly specified with cooler exhaust and if the new model is lower BTU, the exhaust pipe diameter needed will likely be substantially smaller than the old SS vent.

Depending on the diameter needed for the new vent, the old SS one might possibly be utilized as a chase for the new pipe to run inside to above the roof.

 

[Sponsor]

 

[Paul Somlo]

Takagi specifies 2", 3", or 4" PVC, Polypropylene, or SS. They list a 4" SS backflow preventer as an option for cold climates - I have a feeling that almost nobody ever uses these, as they're expensive, and I'm not sure they can interface with any other piping material than SS. Which makes it difficult to get my offset to reach the exhaust vent on the new heater.

Sure, I can run 3" PVC up through the 4" Z-Vent. That gives me two "chases", since I'm already running the Z-Vent through 6" single wall duct with two reducers, to get minimum clearance to combustibles. I could just tear out the Z-Vent and run the PVC through my single wall chase. Still doesn't answer whether I need the backflow preventer.

 

[Bannerman]

The backflow preventer will be typically needed when configured as a 1 pipe system. If using the interior air for combustion, cold air entering the home through the exhaust vent while the unit is not operating, would not usually be welcomed and may result in freezing within the WH.

When using outside air for combustion, a backflow preventer will not be typically used when the exhaust and combustion air terminations are exposed to the same building exposure such as when exhausting through an exterior wall with combustion air obtained through the same wall. If the two pipes are terminated on opposite exterior walls, winter wind forces on one side would cause excessive airflow through the WH while exiting through the pipe terminating through the opposite wall. Depending on the WH's distance to the exterior wall, the winter air entering may remain cold enough to freeze the WH's water passages while not operating. If you plan to exhaust through the roof but obtain combustion air through an exterior wall, ensure you comply with the WH manufacturer's specifications.

While you mention clearance to combustibles, as the new WH is a condensing type, the exhaust temperature will usually be cool enough that combustibles clearance may not be required for the exhaust pipe.

While various size exhaust pipe may be utilized, I suspect the appropriate size will be dependant on your total length of run including elbows and other fittings. Any horizontal runs of both the exhaust and combustion air pipe, are to be installed with sufficient slope so any water entry or condensation within either pipe, will drain back toward the WH's condensate drain connection drain trap or a Tee installed with a drain line.

 

[Paul Somlo]

This will be a one pipe system, most likely, unless I decide to cough up the dough for a concentric add on.

I'll need to keep my 6" chase, because the flat roof is already flashed to it.

Exhaust is a straight run up, no elbows, 6 ft. max. 2", 3", 4" - it won't matter.