Exhaust flow for the chimney design on a custom boiler

  • HVAC
  • Thread starter parkland
  • Start date
  • #1
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Summary:

Exhaust flow in chimney
Hi all 😁 Hopefully it's ok to ask this question here. Ok so my project is a little bit redneck as opposed to being engineered science. I have been experimenting with burning waste oil in my oil furnace in my garage. It is an average house oil furnace with Beckett burner. ( burner has electric motor, fan for combustion air, pump to pump fuel, spray nozzle, and igniter). So far I've been able to get it to run waste restaurant oil, engine oil, anything I can find, and burns so clean that all you see is heat waves. So I'm very impressed, however, I rarely use my garage, so there isn't much savings with free waste oil.
I could put the furnace in the house, but then insurance would be expensive, plus if insurance found out I'm burning anything other than heating oil I'm sure they would freak.

So, that is when I decided to build a boiler of sorts. It is a metal box , 12 inches wide, 42 inches deep, and 72 inches high.
Near the bottom the fire box sits, it it 8x8 inches and reaches from front to back. It has 6 x 1.5x1.5 square tubing flues welded into the back which extend to the top, for heating the fluid from the exhaust. I plan to fill the boiler with oil, so I heat oil, then a copper heat exchanger hangs inside the tank for water to pump through. This way I dont worry about water rotting the tank out.

What I'm wondering about, as I swap the oil burner assembly from furnace to boiler, the furnace 5 inch chimney vent has about 20 square inches of cross section, while the 6 flues of the boiler only add up to about 10 square inches cross section.
Because the fire box in boiler and flues are surrounded by liquid, the combustion gasses may cool off a lot faster than the furnace. May.

So in this case, it seems I may need to do something to increase air flow 🤔.
The burner combustion fan uses about 34 cubic feet per minute of air. If this boiler causes flow restrictions, the little fan might not be enough.
So I thought of some options....

Increase chimney height for more draft.
Put some type of fan in chimney causing suction.
Push air into combustion burner to help it.

So far I think this is the best idea to push extra air into the combustion. I can easily build a box around the burner assembly, and i found a kitchen range centrifugal blower that is rated for 80 cubic feet per minute.
 

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  • #2
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I thought I would add a few details...

The metal used in this boiler is 1/8 inch plate, the flues are 1/8 inch wall, and fire box is 1/4 inch thick. So it it pretty heavy built.

As for safety devices, the burner assembly has an optical flame sensor so if the flame goes out, it will shut off and need manual reset to start again.

I am going to use 3 digital thermostats to control the burner on/off cycles. 2 will be wired in series so that they both will be set to turn on at 180*f and off at 200*f . This way if one were to fail, the burner wouldn't keep running and overheat.

The 3rd sensor will be in the exhaust collector (which I havent made yet) to measure the exhaust temperature so it will shut off if the exhaust is too hot.
I think between these 3 sensors it should be impossible for an overheat situation that can start a fire or cause oil to start vaporizing in the tank.

The hole in the top of the tank I have a plate that fits over, and I will drill holes for the heat exchanger so it sits inside tank in the hot oil. Also I will put a 2 inch fill line on it which will route outside for fumes to vent when the oil gets hot.
I will use engine high temp gasket maker to seal this plate to the top of the boiler.

The fire box and flues were leak tested before welding the box around, but the box has about 5 or 6 pinhole leaks that need fixing. I tried filling the tank with water but then I couldn't weld over the leaks. So next I will try pumping air in and finding leaks with soap spray and welding. Also I got a metal punch to punch the pinhole before welding.

Once this is running and the oil is hot, I have 120 feet of insulated 1/2 inch hoses running to house where there is a radiator and water pump. So in the house I will have a thermostat and when it gets cold enough, the fan will turn on and blow through the rad, and the water pump will turn on and circulate water to the boiler in garage through the copper sitting in the hot oil in the boiler tank. I'm excited to get it all working.
 
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  • #3
jrmichler
Mentor
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1,403
A couple suggestions:

1) The combustion air flow must be matched to the burner. Too much or too little air will cause multiple bad things to happen. Study burners until you understand why.

2) Calculate the proper water flow rate to the house, then the line loss from your 1/2 inch hoses. Do not be surprised if your water pump is not powerful enough to push the proper amount of water through those little hoses. The proper water flow rate is calculated from the temperature difference from boiler inlet to outlet. You decide the temperature difference, after making sure that you understand the reasons why too much or too little flow is bad.

3) Be aware that there is an ideal range of stack temperatures, and why too high or too low is bad.

4) Be advised that all sensors will fail sooner or later. They can fail open, or fail closed. Carefully consider the consequences of the failure both failure modes of each sensor. For example, one possible failure mode has the burner flowing oil with no flame, and oil running out on the ground. You decide if that failure mode is catastrophic or merely messy.

5) Definitely an interesting project. Go for it and have fun.
 
  • #5
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This is the pump I have

https://www.amazon.ca/dp/B0002APPBM/


It seems to pump pretty decent, I've pumped through the 2 way 120 ft length and it seems to flow pretty good.
The pump is only good for 150*f anx the system may be closer to 200*f but the pump will be after the radiator in the house so it shouldn't be an issue.

The radiator is just a car radiator adapted down to 1/2 inch fittings. I know it ain't pretty but it should work.
 

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  • #6
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Yes my concern is with stack temps and airflow. There isn't much to compare with on the boiler I made since it's an anomaly, like it may cool the combustion gases more than a furnace because of all the contact to liquid. The exhaust velocity may be higher though. It's hard to say exactly. Theoretically with half the exhaust cross section, the exhaust should move twice as fast. That's why I was thinking maybe a stronger fan helping to push air through the burner if it isn't pushing enough and burns with black smoke.

As far as the sensors, that's why I plan on using 3. Let's say for conversation sake that the boiler turns on at 180*f and off at 200*f, and the exhaust is 350*f ......
So each sensor has a controller with high and low setting, and a relay.
So I have 2 sensor modules on the boiler body, let's say the cold cut on is 180f and hot cut off is 200f. So I will wire the thermostat trigger wire to 1 sensor module then the other, so for the thermostat wire to be triggered on, both modules have to be between 180f and 200f. So if either one fails, the thermostat wire will stay off.

Also, I will have a 3rd sensor, in the exhaust from the boiler, so I can run it for a while, see what is normal temperature when running, then set a limit. If it normally runs 350*f I could set it to say 380*f so this would also detect a temperature problem and stop the burner.

If the sensors fail it's supposed to show a temperature of -40 so I will have the minimum temperatures above that so a failed sensor won't allow operation.

I know it's not a great idea having 500 liters of 200*f oil, kind of a fire hazard, but really for it to get hot enough to catch fire, all 3 sensors would have to fail. I'm thinking the chances of that happening are smaller than the electric heaters I use sometimes starting a fire
 
  • #7
Tom.G
Science Advisor
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You mention sensors in series to allow burner operation. How will you determine when one sensor fails?
 
  • #8
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I will wire the thermostat trigger wire through the relays of all 3 digital thermostats, this way all 3 thermostats have to be in the operating range to complete the circuit.

The 2 on boiler might be 180 to 200*f, and the exhaust maybe 0 to 400.
So when the temperature gets low to 180*f, the 2 on the boiler should turn on, and the one for the exhaust should turn on too, completing the circuit to run the burner.
If either of the boiler sensors or chimney exhaust sensor goes out of limits, the circuit is disconnected for the thermostat so it will stop the burner.
 
  • #9
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1,033
So far I think this is the best idea to push extra air into the combustion.
That's a NO. There are fans available for exhaust/chimney-side usage: pick one of those, so any leakage (DIY fire stuff has always chance for leakage: that's a rule) will just suck in more air and won't spew exhaust gas around.
Negative pressure is a wonderful thing to prevent leakage.

Also, those water pipes... With some care you could do something better. Check on 'counter flow heat exchanger'.
By the way, what about using some real heat exchanger, salvaged from a retired gas heater?
 
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