Heat scavenging from condensing boiler exhaust.

[orange]

Hi

Just had a 96% efficient IBC modulating condensing boiler SL-20-155 installed and was thinking if I could scavenge even more heat by using the exhaust gas to preheat the intake air. I would need a compact air to heat exchanger in tubular form. Minimum duct size is 2" I can fabricate with stainless so custom is an option.

I wonder how much more I can squeeze out of it this way. Is it worth the trouble? Any thoughts?

[Piwoslaw]

My condensing boiler uses a tube-in-tube setup that goes through the wall. It is made out of thick plastic, about 1 meter long, the outer intake tube's diameter is 10 cm, the inside exhaust tube is ~6 cm. This gives a heat transfer area of under 0.19 m2, so not much.

Also, most newer models probably take note of intake air temperature when calculating combustion parameters (mainly because cooler air has more oxygen), so this should be accounted for.

On the other hand, if you would like to do some A-B-A-B testing, then I'd love to see the results.

[Ryland]

So 4% of your fuel is being wasted, how much fuel do you use per year?
Average person spends something like $800 to $1,200 on heat per year if I remember correctly so 4% of $800 is $32, seems like a worth while amount of money to try to save, seeing as how it will be saved every year for the life of the system.
You are lucky that you can get stainless steel because natural gas is rough on copper so I'd assume that natural gas exhaust could be hard on copper too, altho if it's burning right it should be pretty clean.
I'd put the cold water line that you want to heat right in the exhaust that is venting out, make it as long as possible with as much surface area as possible.

[Mikesolar]

Highly illegal to mess with the inside of the venting. On the outside you can do pretty well what you want. Most condensing boilers are running exhaust temps of 30- 50C most of the time and higher when doing DHW. There is some heat that can be used. Did they use PVC or Stainless steel for the vent?

[orange]

It's PVC.

Anything I do will be on the outside. Even if I could mess with the inside venting, there's no room to do it short of running a legal vent to the opposite side of the building through conditioned space. Probably not legal either.

[Mikesolar]

Originally Posted by orange

It's PVC.

Anything I do will be on the outside. Even if I could mess with the inside venting, there's no room to do it short of running a legal vent to the opposite side of the building through conditioned space. Probably not legal either.

if you use the right materials, you can do it legally (if you are a gas fitter). It depends on the diameter of the venting and length. The boiler has a max run with 2" CPVC and a longer one with 3", typically sloped back to the boiler for condensate.

[BradC]

Originally Posted by orange

I can fabricate with stainless so custom is an option.

Stainless has a _lousy_ heat transfer characteristic.

Have a look at large shell & tube oil coolers or intercoolers from diesel engines. They will have the sort of characteristics you are looking for and are relatively cheap as scrap.