I have a condensing gas boiler for tap-water and central heating. Not very far from the inlet there is a downpipe outlet for the sewer. Since this smelly pipe is partially filled with the same (expensive) gases that I'm using to heat up the water, how much would I gain (if any) by directing it to the inlet of the boiler?

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There are a few factors that I was considering, but have no answers for:

  1. The air in the downpipe may be more damp, potentially negatively effecting efficiency?
  2. The extra high concentration of gas could bring the optimal gas/air mixture for combustion out of balance, will any average condensing boiler detect and compensate for this? (how sophisticated are condensing boilers these days?)
  3. It could be potentially dangerous if the downpipe contains an explosive mixture of air and gas, I don't want to use a flashback arrestor for the inlet.
  4. The pressure fluctuations in the downpipe (when flushing a toilet for example) could cause problems, potentially chocking the flame.

In order to find an answer for point 2, I opened my boiler to see if I could find any sign of a CO2 meter which would indicate that the boiler could regulate the air/gas ratio dynamically, but it doesn't look like there are any electronics or wires up there.

Since gas-bills are well over €500,- yearly, and the cost of a small peace of pipe with compound materials is negligible (guess I've already got it all in my shed), even a small margin would be worthwhile.

Surely I'm not the first to come up with this "quite obvious" idea? However I could not find anything on the net regarding a study of the pro's/con's of this idea. Any pointers to studies are welcome!

Thoughts on a sidetrack (related, not this question, and not seriously considering):

Stretching this idea farther, if I'm the only one doing this in my neighborhood, I might have a relatively high gas concentration, and maybe I could build an installation that would extract gas and eliminate the need of the expensive energy-company's gas? Off course I would have to be careful not to exaggerate, if the pressure in the sewer gets too low, then all the siphon's and water seals would get emptied in the whole neighborhood. I'm also wondering if there would be any legal issues involved with "stealing sewer gas". If too many people in one neighborhood would do this, the effectiveness would diminish quite rapidly.

Edit: Accomplished so far...

Adressing the existing points:

  1. No one has thoughts so far on if air dampness will affect the efficiency. In a condensing boiler, the last part of the warm exhaust is lead through a condenser where the coldest water is lead through to warm up, and water vapor in the exhaust condenses, and flows into a drain. This means that heat is extracted from water vapor, however so-far it is not clear if the boiler will be more or less efficient with more or less damp air. I think that most of the water is coming from the provided gas, and not from the air intake. The reason I think so is because when I put a pot of water on the stove and switch it on, it takes just a few seconds for condense to form on the outside of the pot (up to the level where it is filled with water) and it takes some time before the water reaches a temperature that the condense evaporates again. Any way, no progress with this one yet.

  2. It looks like my condensing boiler does not automatically adjust air intake by measuring CO2. The manual says that it should be adjusted yearly by a mechanic (I've never seen any service mechanic do it ever...). I guess it has a limited amount of fixed strengths that follow some curve with a bias that can be set manually.

  3. The ideas are divided, Fiasco Labs thinks an explosion is quite likely while Shirlock Homes thinks there would be almost no inflammable gasses present, not even in Europe . I guess that even if it would be explosive, mixing it with the standard air inlet would diminish the concentration quite a bit. A search on "Sewer explosion" does come up with some interesting video's, but as a child I've also thrown fireworks into manholes and nothing spectacular ever happened. I'm inclined to believe Shirlock more than Fiasco. Any way, I guess the real question here is "is the oxygen ratio at least equal to the normal inlet?". Any other gas would probably be more beneficial than nitrogen, and as long as it does not react with the inner metals of my boiler.

  4. The way I've drawn the picture, it would not be a problem, the boiler would always have the standard inlet, and I'm not planning to block it in order to get more sewer gas.

So the status so far is:

  1. Indecisive
  2. Negative
  3. Unknown
  4. Irrelevant

The sum of these is a bit negative so far. Maybe this question should be transferred to physics?

  • 3
    Are you really serious? The concept is just that, a concept. The gas would have to be extracted, isolated, and metered through the fuel feed nozzle. Don't experiment, you are toying with potential for explosion, introducing poison into your house and on extreme, blow up the whole neighborhood by backfeeding natural gas into the sewer. Nothing you are thinking of doing would meet code and maybe get you fined, or arrested. Feb 15, 2014 at 21:05
  • Perhaps if there are cows or sheep in the neighborhood, you could rig flexible hoses to each of their nether-ends to further advance your calorie conservation efforts. Of course it would only work if they are not too free-range, or the energy lost from manufacturing of overly long tubing would negate the eco-friendliness of the endeavor.
    – bib
    Feb 15, 2014 at 21:14
  • @shirlockhomes ok, leave the stretched idea out then and go back to version 1 of the idea, we're (mis-)using the air inlet for a minimal amount of additional gas, so we're not fiddling with the fuel feed nozzle and there's no potential whatsoever of feeding natural gas into the sewer (if gas was feeding out of the air inlet I would fix the boiler or get a working one). I am seriously considering to give it a shot if I can find an affordable 2nd hand CO2 meter, or can otherwise assure myself that the boiler is adjusting air flow to fluctuating gas/air ratio. Feb 15, 2014 at 21:42
  • 3
    Here is a little education on public sewers. The contents of raw sewerage is apx 97% water. The methane gas you hope to harvest is actually a very low percentage of the gas you smell in effluent fluids. The organics have not had time to ferment in most modern systems like one might see in a septic system. The waste is drained to a lift station and exit the area in a matter of an hour or so. What you smell is human waste bacteria, hydrogen sulfide, not methane, dissolved in water I strongly suggest you have a conversation with the sewer district before you experiment and get their take on it. Feb 15, 2014 at 22:07
  • @bib You can purchase such an installation here (use Google translate if your Dutch isn't sufficient). This is not a joke, these systems really work. No hoses off course, but the concept is a success! Feb 15, 2014 at 22:08

1 Answer 1


You would gain... Sewer gas explosion.

Never should sewer vents be exposed to any ignition source...

In mines it's known as firedamp and it kills indiscriminately. All it takes is the proper ratio of flammable gas to oxygen and you have a fuel-air explosive.

Sewer gas is vented to the atmosphere for several reasons:

  • it displaces oxygen

  • sulphide gasses are toxic

  • if allowed to accumulate, it is explosive

For more information on what you're playing with, NACHI (National Association of Certified Home Inspectors) has this nice page.

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