measuring oil boiler stack temperature properly ?

ron

for an oil boiler working with hydronic heat, and indirect water heater, where the boiler heats water up to 180° - 200°F ...

1. if there are multiple zones should all zones be calling to get a correct stack temperature reading?
   
2. does outside temperature and heat load on the house... baseboard... have a significant impact? If the house is already 70°F and you set the stat to 75°F on a 40°F day versus letting the house get to 55°F with the outside temp being around 20°F... does temperatures like this have an impact on measuring stack temperature? Should this be accounted for, is there a best time to measure stack temperature and how can it be done wrong?
   
3. it's stated stack temp should be between 350° - 450° F, above that efficiency goes down and a cleaning is needed. What if the boiler is clean and you can't get stack temp below ~500°F, what does that mean (with CO2 % measurements and various excess air).
   

https://www.facilitiesnet.com/energyefficiency/article/Avoiding-Stack-Losses-and-Excess-Air--9154

Avoiding Stack Losses and Excess Air... Rising energy costs are forcing organizations and management to look in all areas of facilities to improve the efficiency of energy-using systems. Because boiler plants use and waste huge amounts of fuel, managers need to seriously consider maintenance and operations procedures that optimize boiler performance and maximize the organization’s energy investment. Instead of operating a building by the philosophy of, “If it ain’t broke, don’t fix it,” managers can adopt a more prudent approach: “If it ain’t fixed, you might go broke.”

EdTheHeaterMan

Stack temp is not dependent on the load the boiler is connected to. It is measured at the stack after the burner has operated for enough time to reach steady state operating condition

Think of it this way. If the flame temperature is 2500° and the temperature of the gas leaving the appliance is 500° then 2000° was used by the heat exchanger to heat the home. The 500° that leaves the heating appliance is what is wasted. so the appliance used 80% of available heat and 20% was used to exhaust the unwanted byproducts of combustion to the outdoors where they will do no harm... To the people in the home, but will devastate the environment for all time and cause the ultimate destruction of the planet (California disclaimer added 12:27 PM EDST)

EdTheHeaterMan

So you could purchase a very expensive laboratory thermometer to measure the hottest point of the flame in the combustion chamber and compare it to the stack temperature to get your efficiency. In the early 1900s someone at a laboratory did all that measuring of flame temperature and found out that the amount of excess air added to a fire had an effect on the flame temperature. Then they were further able to measure this excess air by testing the exhaust gas for the percentage of carbon dioxide. Eventually they made a chart of what the flame temperature would be based on the excess air by measuring the CO2.

So to get the CO2 or the O2 reading with a modern combustion analyzer is a roundabout way of telling you the flame temperature. More importantly it tells you if the fuel/air ratio is at its most efficient and safe adjustment. As far as the flame temperature is concerned, the stack reading of the CO2 or O2 assumes that there is no air leaking into the appliance from anywhere except the combustion air inlet to the flame.

Now how to lower the stack temperature from 500° to 375°. That may not be possible based on the design of the heat exchanger. But if you could add some baffles in the heat exchanger to slow down the travel of flue gas through the heat exchanger to allow more time for the water or air to absorb the heat, then the stack temperature will drop.

You could reduce the size of the fire by using a smaller GPH nozzle. That can go in the wrong direction by lowering the flame temperature. If you start with a 1700° flame and have a 375° stack temperature the efficiency will actually be 79% even though you lowered the stack temperature. So smaller nozzle can help, but too small could hurt.

HVACNUT

Whatever it takes to get to steady state. Where the stack temperature doesn't rise any more. Run all zones with the burner off to lower the boiler temperature as low as possible. Run a bathtub full hot if needed to keep the burner running without making limit.
It doesn't matter if it's August or February. 

ron

EdTheHeaterMan said:

to slow down the travel of flue gas through the heat exchanger to allow more time for the water or air to absorb the heat, then the stack temperature will drop.

where is "stack temperature" measured?

is it before or after the water heat exchanger in the boiler?

Jamie Hall

um... in the stack (sometimes referred to as breeching), after the flue gas exits the boiler...

ron

so that would be after the heat exchanger...

through the heat exchanger to allow more time for the water or air to absorb the heat, then the stack temperature will drop

so what i was trying to get at is... take time out of the equation... you just said stack temperature will drop as the water [heat exchanger] absorbs the heat, and I agree with that. But then if the boiler [return] water is colder is there not a higher delta-T resulting in more heat transfer in the boiler? And then would that not affect the measured stack temperature?

Flip side... Stack temp is not dependent on the load the boiler is connected to... ok then zero water in heat exchanger for arguments sake, what will stack temperature be then after running for 5-10 or however many minutes "steady state" ?

so then the heat exchanger load would have some affect on exit temperature, right? enough to be significant to ever worry about?

EBEBRATT-Ed

Once the water in the boiler is up to operating temp (212 for steam 180-190 for water) the stack temp won't change much.

The stack temp is taken where the flue pipe leaves the boiler but upstream (before) the draft regulator.

ron

Once the water in the boiler is up to operating temp (212 for steam 180-190 for water) the stack temp won't change much.

if the boiler [water] is not up to temperature, then is the stack temperature significantly lower?

reading on here however long ago someone mentioned an oil fired swimming pool heater... maybe not a traditional house boiler and more of a glorified hot water tank... but that would be a constant boiler water of ~60°F. A hundred degree difference vs the typical 160°F, that has to affect stack temperature wouldn't it?

Or take a newly filled indirect water tank at 50 gallons at 50°F water that needs to be heated the first time and that's all the boiler will be doing... the boiler runs for ? minutes.
Is there ever a scenario where you don't want to measure stack temperature because it would give a lower reading... versus a normal call for heat where boiler water is in the 160°F range?

EdTheHeaterMan

ron said:

EdTheHeaterMan said:

to slow down the travel of flue gas through the heat exchanger to allow more time for the water or air to absorb the heat, then the stack temperature will drop.

where is "stack temperature" measured?

is it before or after the water heat exchanger in the boiler?

After. Think about it... if the flame is at the bottom and the heat exchanger is just above it, then the exhaust pipe is at the top, the only place that can absorb the heat to distribute to the home is in between the flame and the vent at the top. You want to know how much of the heat is going into the water in a boiler or the air in the furnace, that is NOT going up the chimney. So measure the temperature before and after the heat exchanger.

Here are 3 different heaters with 3 different vent configurations.
On the left is an oil burner where the flame is at the bottom. The vent is at the top. Position B looks ideal to measure the Stack Temperature. Position A is actually better. As long as position A is 6" before the Barometric Draft Tee fitting. Why? In the original directions that came with the Bacharach Fyrite Test Equipment, the instructions mentioned that the thermometer probe should be ac close to the heat exchanger as possible (like Position B ) but to avoid direct line of sight to the heat exchanger to reduce radiant heat from the heat exchanger from increasing the thermometer temperature.

In the center photo the first fitting in the vent pipe is a 90° elbow. So the hole C just above that elbow is ideal for stack temperature measurement because the heat exchanger is around the corner from the thermometer probe location.

In the photo on the right, the first elbow after the draft hood D would not be a good location. That location is measuring both the flue gas from the boiler and the dilution air from the draft hood. That is not an accurate representation of the flue gas from the boiler. The only location to take a temperature reading is position E. Even though there is radiant heat from the heat exchanger and possibly from the actual flame itself, that is the only location where you will measure only combustion flue gas temperature. If you want to be more accurate you could construct a baffle just below the probe location, to keep the thermometer probe from the line of site of the heat exchanger. It all depends on how accurate you want to be.

I hope this answers your question.

EdTheHeaterMan

ron said:

Once the water in the boiler is up to operating temp (212 for steam 180-190 for water) the stack temp won't change much.
if the boiler [water] is not up to temperature, then is the stack temperature significantly lower? Lower, by about the temperature difference of the water in the boiler. So 70° boiler water will absorb more heat that 180° boiler water. The boiler is more efficient the lower the water temperature in the boiler is. That is why you should wait for the boiler to reach steady state. the condition that is normal for that usage.

reading on here however long ago someone mentioned an oil fired swimming pool heater... maybe not a traditional house boiler and more of a glorified hot water tank... but that would be a constant boiler water of ~60°F. A hundred degree difference vs the typical 160°F, that has to affect stack temperature wouldn't it?
In this case the steady state or normal operating range is lower temperature water. so that boiler may operate at 140° normally (most pool heater use heat exchangers). That same model boiler in a space heating application might have a steady state or normal operating range of 170°

Or take a newly filled indirect water tank at 50 gallons at 50°F water that needs to be heated the first time and that's all the boiler will be doing... the boiler runs for ? minutes.
Is there ever a scenario where you don't want to measure stack temperature because it would give a lower reading... versus a normal call for heat where boiler water is in the 160°F range? common sense needs to take over here... what is normal? Don't take the reading during abnormal operation... take the test during normal operation.