The Values of Coaxial Venting

Perry_5

Its a balmy -11 right now and I wondered how cold the intake air to the actual boiler itself was, and how it would affect it from both efficiency and operating life.

I should point out that right now (below area design temp) that the boiler is no longer operating in condensing mode as I need warmer water temps for my heating system. The boiler output is at 165 F and exhaust gases are probably in the 165 F range as well (exhaust gasses from the boiler HX should be about 10 F above the boiler inlet temperature).

But, that does not mean I'm not extracting the heat from the exhaust gases or missing out on the energy associated with condensation... as I have a 10 ft long coaxial vent system (with 3 90 deg elbows).

Just hand touch indicates that the the actual intake air to the boiler is above room temperature; while the nearest point I can access close to the side of the house is very cold to the touch (about 3 ft from the vent termination as my vent system runs up into a closet and then out the side of the house above the snow line: I have boxed around the vent pipe in the closet).

Thus, I am notably warming the incoming air by at least 80 F, which can only occur if I am cooling the exhaust gases by a similar amount (say down to about 80 F at the vent termination). Hence - I am recovering the latent heat of condensation by warming my incoming air and keeping my overall boiler efficiency high as I do not have to burn additional fuel to warm the incoming air.

At the same time I am not stressing the airside boiler components by exposing them to an -11 F air environment (how much more wear and tear would occur if the gas regulator, air fan, etc, were operating at -11 F).

How much corrosion could occur if the boiler airbox was at a temperature below the dew point in a house such that condensation was occurring.

By touch it appears that the point where the intake air is at about room temperature is at about 8 Ft from the vent termination.

Could it be that the "expensive" coaxial vent systems will actually pay for themselves in energy efficiency and avoided boiler maintenance?

Of course - you do have to make them long enough to gain all of these benefits. I am aware that this is not always possible or practical. But something to keep in mind if you have the option to make it more than just a foot or two.

Perry