You need galvanized piping for vertical venting. PVC will melt. You also need to ensure you have at least 250sq inches of free air space into the room for proper combustion.

Code here for horizontal discharge is stainless.

It did sound too good to be true. As the discharge is horizontal I'll bite the bullet and go with stainless.

Thanks for the reminder on the 250 sq inches of free air space.

For those who wonder why stainless on horizontal, it is because the process of venting that way is pushing the flue gas and this creates friction and the heated gas condensates. The condensate is acidic and therefore, the stainless vent pipe (very expensive) and sealed joints are needed. This process is called positive pressure venting.

Standard vertical venting is called negative pressure venting as heat naturally rises. This is where galvanized, non-sealed venting can be used.

For the free air requirement, it's always best to be over the requirement in order to reduce sooting. If you install a screen or grate in a door or wall, those metal cans usually have an imprint on them that indicates the free air available on each device.

"For those who wonder why stainless on horizontal, it is because the process of venting that way is pushing the flue gas and this creates friction and the heated gas condensates."

Is it the friction, or the cooling of the gas that causes this? Many positive pressure systems such as the minimax nt could have been vented in single wall c vent. The nt did run at a lower efficiency which could also add to the condensation.

Any time an appliance approaches 83% efficiency it becomes a condensing boiler. Consumers want the higher efficiency numbers but they are not paying for a true condensing boiler with condensate traps etc. Stainless venting (very pricy) is just one important part of a condensing boiler system. The fact that these high efficiency pool boilers are only heating the water to 104* max is also very hard on them, for the same reasons that pool boilers are prone to heat exchanger sooting/blockage I personally think these newer boilers will eat themselves up a lot quicker than the older ones.

Thanks for the explanations. My vent will go vertically 3 feet then 2.5 feet horizontally to the outside. Does that make a difference? Or is this still is a 'positive' pressure system.

racket,

The MasterTemp/Max E Therm achieve 84% through the sealed burner system. Per the Pentair book, "the friction created inside the pipe by the hot combustion air creates condensate" as mentioned in the other post.

A better example of this is the Laars HI-E2. It is 98% efficient and condensates like a 5 month old cutting his first tooth. That heater can vent vertically with PVC and also can use PVC for the air intake as it is generally in a "boiler" room. Awesome heater but at 6k, it is generally only worth it on a commercial setting. I put them in on backyard situations but the only ones who buy them are the doctors who heat their own pools all year.

"Thanks for the explanations. My vent will go vertically 3 feet then 2.5 feet horizontally to the outside. Does that make a difference? Or is this still is a 'positive' pressure syste"

No, the vent length does not matter. Unless it's an atmospheric vent (vertical vent). Sidewall venting will require the same pipe under a certain ammount of pipe.

The hi-e2 is 95% efficient. But I am splitting hairs.

Oddly on a Lochinvar e-rite, and copperfin 84% heaters if you use a power vent on them you can use b-vent. While direct vent requires stainless steel. Maybe it has to do w/ the gases properties while under pressure, but friction happens both under a vacuum, and under positive pressure. When you increase the vent size you also increase the amount of condensation, but reduces the friction.

I am not denying that is what the book says, I just refutes most of what I have learned about venting. I understand why large HVAC companies have engineers on staff that do nothing but venting calcs.

"to 104* max is also very hard on them, for the same reasons that pool boilers are prone to heat exchanger sooting/blockage I personally think these newer boilers will eat themselves up a lot quicker than the older ones"

That is a lot of the reason that the unitherm governors are essential on these heaters.