How to check stack temp on oil boiler

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  #1  
Old 11-01-20, 10:59 AM
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How to check stack temp on oil boiler

I have an oil boiler and am trying to check the flu gas temperature. I have a General laser temperature gun. Will aiming this gun at the flu pipe near the boiler give an accurate reading?

With the boiler hot and running for a while, this gun gives a temp of 175degree f right at the boiler on the cast iron piece that the flu pipe attaches too.
if I aim the gun a few inches farther from the boiler on the flu pipe it reads closer to 90 degrees.

All of these numbers seem low from the 326 degree gross temp from the spec in the book.

Buderus G115/ws 4 chimney

Thanks !
 
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Old 11-01-20, 11:31 AM
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An inferred thermometer isn’t a usable means of checking stack temperature.
You’ll need a stack temperature thermometer.
If your setting your burner up you’ll also need a smoke test and CO meter.
 
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Old 11-01-20, 11:37 AM
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I have a technician coming tomorrow to do the set up of all the specs. I just couldn't resist checking the stack temp with my temp gun! I hoped it wasn't a good method with the temp so low.

Thanks !
 
  #4  
Old 11-01-20, 01:46 PM
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Your tech will have a combustion analyzer with a probe that is inserted into the flue gas. That will measure flue gas temperature and CO - which, in turn, gives O2 and efficiency. Ideally, he will be able to give you a printout of the results. Without a combustion analyzer, there is not much you can do. With a small Dwyer air meter, you can manually measure draft.
 
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Old 11-02-20, 07:24 AM
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Stack temperature is of flow through it, not vent surface temp. Temp probe should go to middle of vent flow 2, 3 or 4 inches in from wall of vent.

I use ordinary $15 digital thermometer probe on system that is typically 350F.

For DIYer, it is easy, cheaper, faster and less hassle way than messing with a professional. Using combustion analyzer is very high priced way to measure temperature.

Note, many stack vent based efficiency charts use “net stack temp” which is measured temp, minus room temp. Measure 400F then subtract 70F room for 330F net
 

Last edited by doughess; 11-02-20 at 10:11 AM.
  #6  
Old 11-02-20, 10:36 AM
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Lower stack temperature than spec is more efficient. Manufacturers data is for rated output, not most efficient.

On any boiler setup the lower stack temp, the higher efficiency. An easy way to reduce fuel costs is to lower firing rate.

One source says 80% of USA boilers are twice size needed. My 60 year old boiler is fired at 60% of rated and runs only 8 hours on 0 F design days. Stack temp efficiency is 86%
 
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Old 11-02-20, 01:06 PM
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Despite all the advice you have been given in the last 2 posts in my opinion the best thing you can do is to get a professional with the proper equipment and knows how to use it.

Second, you have a Buderus and you must use the nozzle and set things the way they require. It is not like a Beckett or Carlin where you can make some adjustments and still be OK. Anybody can fudge efficency results. One way is by not letting the stack temp reach its maximum output. A boiler should run at least 10 minutes or until the stack temp equilizes and stops climbing. Running the boiler for a few minutes to keep the stack temp low will make the numbers look good but you are only cheating yourself. By the way the stack temp is only one part of the efficency test procedure. A trick of some techs is to reduce the air which will give you a smokey fire which most people will not observe but it will give you a higher co2 reading and look good on paper.
 
  #8  
Old 11-03-20, 09:01 AM
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Buderus 115 specs do not show stack temp, but do list oil fired efficiency as 86.9%

On efficiency charts that is net 300 F degrees measured at center of flue.

As some posts mention, professionals play "tricks" with stack temps.

DIYer's temperature probe reading is more reliable, lower cost, less of a hassle and reduces virus risk from exposure to outside professionals.
 

Last edited by doughess; 11-03-20 at 09:23 AM.
  #9  
Old 11-03-20, 03:18 PM
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Temp probe for Fluke multimeter

Fluke's 80BK-A temperature probe plugs right into my Fluke 179 multimeter. It goes up to 500 deg F. The temperature probe goes for about $40 after a 10% discount. My Fluke 179 multimeter is True RMS and is a Cadillac - I think I paid over $300 several years ago. There may be other, less expensive ways to go, either with Fluke or other multimeter companies.
 
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Old 11-03-20, 05:54 PM
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Many digital thermometers use similar thermocouple probes. Here is $15 ebay one. A common male adapter plug can be used for Fluke and other brands.

https://www.ebay.com/itm/6ft-PT100-RTD-0-1-degree-Sensor-Probe-good-for-PID-temperature-controller/350618683930?hash=item51a2808a1a:g2AAAMXQskFRHuW6

Genereally a net stack temp under 300F is good on oil boilers. Higher temps means lower efficiency.

Lowering firing rate with smaller nozzles is easy way to cut fuel costs.
 

Last edited by doughess; 11-03-20 at 06:18 PM.
 

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