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Combustion Air is the air that is required to provide complete combustion for a fuel source. Enough oxygen is required so that there is not an issue with carbon monoxide. This is an issue in houses and anywhere that has a furnace, water heater, boiler, etc. that isn't electric.

All of the codes and references (1, 2, 3) that I have seen use either the volume of the room that contains the equipment or an area of free-flowing vent from the outside in a ratio to the BTUs or Watts of the equipment.

Neither of these measures seem to be directly related to the chemical equation of combustion.

  • Comparing the volume of the room doesn't seem to take into consideration a replenishment of the oxygen from anywhere.
  • Specifying an area of vent seems to ignore the speed of air through the vent. It also doesn't seem to relate to the size of the chimney.

What is the logic behind either specifying a room volume or vent area for combustion air?

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These are things that installers and builders can control and understand. They aren't interested in running the equations themselves, they just have a customer who is asking for a product.

So the industry has settled on some basic standards, and as long as everyone builds according to code and the devices is rated for indoor use, then they only need either a big enough room, or a big enough vent.

The big enough room falls under safe working environment. Assuming a specific length of work period, and sealed room starting with the average air mixture, can a person complete their work with the device running and at the end of the period still have a low enough CO2 and a high enough O2 that they are still well within the EPA limits. This is long before headaches start and other physical or mental problems manifest from high levels of CO2 or low levels of O2 become apparent. Combined with an oxygen sensor (required in such devices) the gases in this size room will generally not exceed a limit unless a door is never opened and the room is truly sealed. And generally most rooms built to code have plenty of air migration, few are truly sealed. Still, it's possible to cause problems with this setup, which is why the room size requirement is so very large, to the point where it'd be odd to have such a large room without multiple windows or doors. Even then, I'd still recommend venting.

A big enough vent simply assures adequate CO2 and O2 exchange with an area of standard air mixture. This exchange will occur even if the vent isn't powered, and without a pressure differential - similar to osmosis. An area of high CO2 in air contact with an area of low CO2 will cause CO2 migration from the high concentration area to the low. The rate of diffusion through the vent is directly related to the area of the vent. Thus continuous operation with the proper vent area is possible without ever reaching dangerous levels. There is a tradeoff between vent size and heat loss, though, so it's worthwhile keeping the vent closer to the smallest acceptable size than the largest if the point is to heat the room.

Note that these devices are (almost?) never rated for sleeping areas, and they advise users to leave the area if they develop headaches, one of the first signs of CO2 poisoning.

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  • $\begingroup$ It seems like you are describing non-vented heaters. I was interested equipment that is vented to the outside through a chimney, etc. This also helps by creating a pressure differential that would draw air into the room. $\endgroup$ – hazzey Mar 3 '15 at 19:04
  • $\begingroup$ @hazzey Yes, but the safety standards and reasons are very similar. $\endgroup$ – Adam Davis Mar 3 '15 at 19:07
  • $\begingroup$ @Air I may be missing the intricacies of the definitions. I did not think that "direct vent" would cover draft-type furnaces and water heaters, where the combustion air comes from the room, but goes up a chimney naturally. This answer seems to make the most sense when talking about a standalone propane/kerosene room heater without a chimney. Both of these are also separate from what I have always called "direct vent" equipment that have their own powered intake and exhaust to the outside. $\endgroup$ – hazzey Apr 30 '15 at 21:43
  • $\begingroup$ @hazzey I had to look up the definitions for the documents, but you're correct. I'm accustomed to slightly different terminology. $\endgroup$ – Air Apr 30 '15 at 21:51
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I don't know the US situation. Codes over here (Germany) demand, among other things, that a room with a funrace or boiler in it is not at an underpressure re. the outside, which in many cases larger than a domestic water heater mandates active ventilation. This serves two goals I think:

  • underpressure would make it hard to open doors (have to open outwards, because escape route)
  • Makes sure enough combustion air is supplied

I've only ever seen one sizing calculation for a ventilation system for a boiler room and the combustion air was definitly included.

OTOH, th furnace/boiler whatever has it's own emission control scheme - be it an active lambda sensor or the tunig of a nozzle - that has to meet, among other things , CO requirements.

So you, as the civil engineer planning or delegating the ventilation system have to make sure that the ventilation satisfies certain conditions and the manufacturer of the furnace has to make sure the furnace satisfies emission limits in 'normal' conditions. Over here, both (ventilation & safety of boiler room, actual emissions) are controlled by the chimney sweep.

Looking at the two ways to specify it you mentioned:

volume of the room that contains the equipment

would work if (big if!) the room is ventilated to achieve certain airchanges per hour, then the flow is proportional to the volume

an area of free-flowing vent from the outside in a ratio to the BTUs or Watts

Air consumption is proportional to Watts or BTU, just not exactly the same for each fuel. So sizing a free vent to wattage makes sense, if the code contains a large enough margin of safety.

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