Cocncentric Vent Preheat

Mike T., Swampeast MO

Has anyone else monitored the air intake temperature of their concentric vented mod-con?

I'm not datalogging it (like other system temps) but have been watching it carefully for a few days.

At around freezing outside, incoming air is about 67F; in the low 20s it's still around 63F.

From the boiler the flue consists of: a vertical inspection coupling, a 90 degree L, a slide coupling, and about 4' of concentric pipe and the termination. Actual length of the flue to the inside edge of the 12" thick foundation is about 5 1/2' measured on the center line.

Viessmann concentric flue for a Vitodens 200 6-24. Polypropylene inner (exhaust); aluminum or hard plastic outer (inlet).

Even though the exuast vapor trail shows zero sign of such, I thought that I must be getting some exhaust recirculation. So I:

1) Opened the slide coupling cover and temporarily stuffed insulation on the foundation side as I could feel the air pulling quite nicely across the gap. Temp went down rapidly and stabilized at about 59F (ambient temp in the basement about 55F). After about 20 minutes, I removed the insulation, reinstalled the slide coupling cover. Temp first dropped rapidly, then began a very slow rise. After about 30 minutes it was back to 67F. This is exactly what I would expect if the inner exhaust tube were significantly heating the incoming air. If there was much (or any) recirculation of the exhaust, I would not have expected the temp to drop so rapidly when the slide coupling cover was reinstalled nor would I have expected it to take so long for the inlet temp to return to the previous measurement.

2) Last night, with temp steady in the low 20s, I measured the surface temp of the horizontal portion of the outer aluminum jacket in three places. Immediately next to the foundation, in the middle and at the boiler side of the slide coupling very close to the elbow turning down into the boiler. Incoming air temp read 63F. Right next to the foundation it was 26F; in the center is was 43F; next to the el it was 56F. (Again, ambient temp in the basement around 55F). Again, this is what I would expect if the inner tube were significantly heating the incoming air.

Yesterday was sunny by afternoon and the boiler shut down for a while. Of course air stopped coming into the boiler. Temp at the inlet dropped rapidly and stabilized right above ambient air temp. The boiler "pulsed" a few times before solar influence diminished and the boiler went back into constant firing at minimum input at about freezing outside. The incoming air temp nosedived once the boiler began firing. (This digital thermometer is a bit slow reading.) It dropped to about 45F, stayed there for a while and then took nearly 2 hours to again stabilize near 67F!

I had always assumed that the concentric vent (particularly a short, horizontal one like mine) might provide a <I>bit</I> of preheating, but nothing like what I seem to be seeing---<B>especially at the low flue temps I measure (90F-92F) for this outside temp range</B>.

Can something else be happening, or is there really this much preheating? What about the long time lag after the boiler had stopped firing? Does this mean it takes a significant amount of constant firing before the boiler is operating at its' peak of efficiency?

I'm nearly certain that Viessmann bases its efficiency ratings (actual and theoretical) on 60F incoming air temp. I always considered this a "laboratory" condition that wouldn't be replicated in the real world. To my utter surprise, it would [seem] that my measurements show even higher temps even at outside temps low enough to be down near the 10th percentile!

Would anyone else like to measure similar on their mod-con (with concentric flue)?

hot_rod

stick an analyzer in that other port on the top

of the boiler. it's designed to read the recycled exhaust gases, if there is any.

That flue inside combustion air pipe is really a HX. I suppose knowing all the variables you could calc the heat exchange rate.

hr