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I'm designing a system which will heat up water by burning natural gas which is commonly available. I would like to have a slow flame because water will need to reach $\mathrm{70^\circ C}$ and stay there. Having in mind that even weak winds can extinguish the flame as well as the system will be put together with other objects and systems in a tight space I decided it would be a good idea to build a chamber around the flame nozzle thats directly attached to the water container. I will build a small opening in the chamber to which I will attach a tube which will feed atmospheric air into the chamber through some kind of a filter (probably a piece of cloth) to keep it clean. I read that the byproducts of burning natural gas are $\ce{CO2}$ and water vapor which raises 2 questions:

  1. Do I have to worry about humidity increasing in the chamber and eventually water forming on the inside, in extreme conditions water in the system can be as low as $\mathrm{-20^\circ C}$ (I call it water but it will be actually antifreeze)

  2. Do I have to worry about the chamber filling up with $\ce{CO2}$ and eventually putting the flame out which will then be unable to be lit again. Is there anything I can do such that the byproduct will exit the chamber naturally without me having to do any work for it.

Edit:

So far from the comments it was made clear that it would be better to use a condensing boiler design efficiency wise, however due to construction complications that I'd like to avoid I'll stick with only the hot half of the design in question where the water will pass through a tubing inside the chamber where it will be heated, instead of applying heat to the container directly. There will be an exhaust that will utilize natural convection to expel the exhaust gasses in addition to the inlet.

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    $\begingroup$ For better efficiency, you might want to consider a design like this: de.wikipedia.org/wiki/Brennwertkessel#/media/… $\endgroup$ – Loong Aug 26 '19 at 11:09
  • $\begingroup$ @Loong thank you very much for the reference. I'm definitely going to be using this design which is probably well known but for a guy like me with 0 experience is a gem. I'm curious how exhaust gasses leave the system on their own but it's probably well explained on the internet. $\endgroup$ – php_nub_qq Aug 26 '19 at 11:18
  • $\begingroup$ Well turns out I will need a fan as well as it adds some complexity to the build so I'm probably going to sacrifice some efficiency and stick with the initial plan. $\endgroup$ – php_nub_qq Aug 26 '19 at 11:44
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    $\begingroup$ @php_nub_qq Yes. Exhaust gases are hot and, therefore, natural circulation can be exploited to make them leave the space because to gases will rise. You only need a fan if you need stronger forced circulation (which you might do if you want to achieve a more powerful flame than natural circulation allows.) That's why domestic gas boilers have fans. $\endgroup$ – matt_black Aug 26 '19 at 11:57
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    $\begingroup$ @matt_black Gas boilers need a fan if their exhaust temperature is very low, i.e. if they are built to be very efficient. See the link in loongs comment. $\endgroup$ – Karl Aug 26 '19 at 20:11
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Rather than design something that offers danger of overheating, carbon monoxide poisoning, gas and over-pressure explosion, etc., why not use an off-the-shelf "instant" tankless water heater, available from sources such as Home Depot, Amazon or Lowes?

Find one certified for your area. Making your own is a tankless task!

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    $\begingroup$ Its not much different than heating water on a gas stove. Obviously I left out some details such as there will be temperature monitoring, which are not related to the question, however I'm really interested if you could dig deeper on the carbon monoxide and explosion aspects of your answer. Anyway to answer your question you can simply look at the prices of those and consider the $20 I will have to pay for my 5 liter gas bottle + change for tubing and a spare motor for valve control (the bottle comes with a valve which requires very little effort). For the ignition I will use a \$5 spark plug. $\endgroup$ – php_nub_qq Aug 28 '19 at 23:04
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    $\begingroup$ Additionally I can not afford to waste electric power for heating because my battery is not powerful enough and they are still quite expensive for my pocket, hence the whole gas thing going on. $\endgroup$ – php_nub_qq Aug 28 '19 at 23:53

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