Combustion test questions

dav

Hello to all and thank you for this good site.
Just a couple questions about combustion testing.
I have read that you're supposed to put the probe above the breach 6in or so and in the center of the flue pipe, which is supposed to be the hotspot. But on my particular set up though it is not a hotspot there. a hotspot is more towards the outside of the flue pipe maybe an inch away from the outside of the flue pipe and it is about a hundred degrees hotter. Now should I put it in the hotspot or should I keep it in the center of the flue? Btw co2 is 13.2 and o2 is 2.8 In the center it is co2 11.7. o2 4.8

That would be question 1.

The second question is. When I do a smoke test, I I get just a trace of smoke and then when I adjust my air with the combustion analyzer I get zero smoke but I get a real damp spot on the smoke test paper. Is this normal? Now i can add a touch more air and the vapor goes away.

My set up is new Yorker boiler 990-1.
190k btu
Retro fitted with beckett afg.
Boiler calls for 1.75 gph 80 a
I burn a 1.25 gph @ 140 psi which is 1.48 gph
Burner has f3 head.
It also is steel tube vertical with baffles.
Flue temp is 519 outside pipe area. 425 in center of flue.

Thanks in advance for your help.

Dave

Grallert

You get a damp spot at 425? What are your combustion numbers with the damp spot? what are those numbers at zero smoke and what are they with a trace?

STEVEusaPA

I'd like to see a picture of the entire flue pipe. I've never seen numbers vary that differently depending on the insertion depth.
I think you need to leave the probe in the center. I'd also check combustion with another analyzer to check.
Regarding a combustion test:
https://www.beckettcorp.com/support/tech-bulletins/proper-burner-adjustment/

dav

L just changed the blast tube t a 7 1/4. Was a 4 or 4 1/2. Still the same with probe about an inch off of outside wall of 8 inch pipe 520 f. My CO2 was 13. 2 and I upped the air a bit to bring it to 12.8. no smoke. In center I run about 450 f. 11.5 CO2.. I am assuming that the damp spot is water vapor from good combustion. I think I should be in the hot spot to do the measurements.i think it is just the way the air moves through the boiler

dav

It is a simple 8in vent off the top of boiler. About 18 inches up into an elboe then a tee with a barometric damper 4 jnches into an outside.masonry tile lined I inch chimney. Nothing special

dav

8 inch pipe I meant

dav

Sorry 8 inch tiled lined chminey .draft is spot on between .01 and .02 over fire with digital manometer

HVACNUT

Measurements should be taken in the center of the flue pipe, at least 12" from any turns or offsets.
I wouldn't go over 12% CO2.
O2, between 4.5 to 6.5%
Excess air about 20%
A steel tube boiler will have a higher stack temp.

dav

Why would the center of the flue be about 80 degrees cooler though. Can an elbow cause that? I am about to inches below the elbow no choice

STEVEusaPA

I'd check it with another analyzer. After your first post, I checked it on 3 different heaters (2 furnaces, 1 boiler) with my Bacharach Insight, and virtually no change in flue temperature based on being dead center or an inch or two off.

dav

Well, I do not have access to a other analyzer, but I do have access to another themocouple and dvm. I checked the temps with 2 different thermocouples on the same dvm and they indeed did show a temp difference of about 80 f from center to outside of pipe. Now that makes 3 different thermocouples that all said the same +/- 10 degrees. The analyzer reads 20.9 in open air so I am going with the analyzer is probably pretty close to accurate. I opened air shutter and set CO 2 for 12.8 at excess air of 19, flue temp at 480 F . Probe inserted about 6 inches into an 8 inch pipe. At center of flue reads about 430 F, 12.0 CO2. 22 excess air.
I am going with these numbers but I just don't get why the outside of pipe is hotter than center. Thank you for all of the replies.

ratio

The 90 is throwing the flow in the flue off, just like it would for air in a round duct. Is it towards the outside of the 90?

I dunno which set of numbers you should be using, but at a guess I'd say that you were right to take it from the hottest spot. That's got to be the area of largest mass flow, & so should be the best sample point for a good approximation of the total. The alternative is to traverse it like you would a duct, but I have no clue how you'd average out the readings.

captainco

Combustion tests should be taken where the O2 is the lowest or the CO2 is the highest. Flue temp is never the right place.

ratio

Thanks, I'll remember that.

Why that spot, and not an average across the whole flue? Is it because it's not going to be a significant difference, or because the limits are what drives the show?

Jamie Hall

It's because that's the spot that is most representative of the actual exhaust gas. Cooler/lower spots are likely to be in eddies and not representative -- such as the OPs!

ratio

I though the OP said he was taking it in the hottest spot. I understand avoiding cooler spots, but would hot spots coincide with lowest O₂/highest CO₂?

I don't really know as much about combustion as I'd like, I'm hoping to be schooled by my betters!

captainco

The lowest O2 or highest CO2 will have the highest CO and the lowest Flue T. O2 represents how fast the gases are moving through the heat exchanger. The lower the O2 the slower they are moving therefore more heat is exchanged and the Flue T is lower.

ratio

Ok. I was thinking that the highest temperature was the 'best' representation of the flue gas, but we're actually looking for the lowest that's undiluted with excess air, correct? And lowest O₂ is that location.

captainco

That is correct and also the highest CO.