Stack temperature question

ralman

Using information I learned from the wall I determined my existing 50 year old flue needed replaced. Thanks for the help. I hired a chimney pro who confirmed my suspicions and I had him remove a 14 foot horizontal section that was cracked and 60% plugged. I now have an entirely straight flue with a stainless steel/titanium liner. Just finished that 3 days ago and I got a heating pro to fire the boiler and do combustion analysis this morning. I have an oil fired peerless boiler. Prior to the chimney work I had a 450 degree stack temperature. The stack temperature is much higher now at 530 degrees. I am just curious if I am going to use more fuel now than before. Does the higher stack temp mean the water in the boiler will heat quicker and reduce burner run time? Or am I going to have increased standby heat loss which will increase fuel usage? What do you think about these numbers?

old numbers o2=6.5, co2=10.7, stack=450, ea=42,
draft=-0.005

new numbers o2=6.8, co2=10.5, stack=530, ea=44.7,
draft=-0.09

Jim Davis_3

Based on the numbers posted the boiler is less efficient by 2 or 3 percent because the higher flue temperature means less heat is being transferred. The draft over the fire should be checked. Ideally it should be -.01"W.C. The O2 reading is also a little high 4-5% would be more ideal. But if equipment isn't serviced every year the air is usually left higher to compensate for annual deteriation.

Big Ed

Draft

What looks odd is the high draft.The reading is taken over the fire. Peerless wants .02".The draft regulator need to be ajust to curve excess....

A dirty boiler would increase the stack temperature ...

Faulty testing equitment or bad placement .probe needs to be half way in.

Also we use net temperature...You need to subtract the room temperature from reading...

Depends on the temperature of boiler...Should be at normal water temperature.

J.C.A._3

The new draft reading,

Is a bit too high for my liking. The barometric should be adjusted as said.

Is this a center boiler flue? (right out of the top of the boiler) These units tend to run a higher flue temp. as opposed to rear flue models. The new chimney is working too well! Get that draft down to around .03-.04 with an overfire about .01.I also would like to see the excess air down somewhere between 29-35% ideally.

Like Jim said , sometimes a tech will leave the air adjusted a bit on the high side to keep the unit "clean" a bit longer, but it does cost. I would have it set for best effeciency, and make sure it gets serviced 1 time per year. As he can attest to, the higher the O2 number, the more CO you'll be making. If you get the draft down, the stack temp will drop (more heat extracted),the CO will come down and effeciency will come right back up. Chris

ralman

Thanks to all who posted.

I sincerely appreciate all the help I get here. I am a new resident to this town which is very rural and limited on companies that service heating systems. I don't know what these numbers should be during a combustion test. I am a firm believer in servicing equipment and have had my boiler serviced annually since I moved here. If I interpret everyones comments correctly, it seems like I didn't get a properly adjusted burner. I brought my question here because I found it odd that the tech didn't make any adjustments to anything. He set up his bacharach fyrite pro tester, performed the test and said all the numbers were right on the money. I just couldn't believe that a boiler connected to 25 foot tall chimney with a 14 foot horizontal section that was cracked and 2/3 plugged would have the same settings as a boiler with a brand new steel lined, completely straight, 25 foot tall chimney.

S Ebels

Here's what happened

In a nutshell, straightening out your flue and sizing it correctly has increased the draft in the boiler. This has to be compensated for by allowing air into the flue somewhere other than through the boiler, typically this is done with a barometric damper. Your draft has gone up but from the numbers you give, the increase in draft has not been addressed. The burner and your draft control have to be adjusted to match the new operating conditions presented by the new chimney.

To answer your question, yes you have lost a little efficiency.

joel_19

quick check

in most cases if you have that kind of draft and your barametric damper is closed it is adjusted improperly . It would need to be fully open after maybe 5 minutes run time to get that down to a .04 or less. If that damper doesn't open after a few minutes (to get some heat in flue) then there's your problem. Or you don't have a damper at all on the flue pipe which is a distinct possability . i've seen them removed by oil companies on low draft chimneys to cut down on oder problems as opposed to fixing the real issue like you did.

GaryDidier

draft

Ross, Sometimes in high draft situations you need two draft regulators to bring the readings to where it should be. You have too much draft and it is effectively pulling the heat out of the boiler before it can be captured. Adjusting draft requires testing as it will change all other parameters. Good luck.

Gary from Granville