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Every conventional (i.e., non-direct-vent) gas water heater I have seen has a roughly 1" vertical gap between its exhaust port and the exhaust vent (or "chimney"). See photo below. What function does that gap serve?

How does the gap not pose a danger? I assume that we never want combustion byproducts potentially venting into indoor spaces. But if the chimney were to become blocked, or during any air pressure imbalance between the exterior and interior, this gap seems guaranteed to vent CO2 into the living space. (And if the burner is not functioning properly, the exhaust could include carbon monoxide.)

Water heaters with gap before chimney

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    I've always thought it was to suck in cool air to lower the temp of the exhaust, but I'm not sure, and that doesn't address the "safety" concerns. – JPhi1618 Oct 28 at 16:09
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    @isherwood I see you edited out the normally redundant/incorrect "hot" from the title - after all, a water heater heats cold water. But this looks like two linked together, so one of them is a cold water heater and the other is effectively a hot water heater! – manassehkatz-Reinstate Monica Oct 28 at 17:46
  • If there were no gap above the water heater, and the exhaust became blocked, do you really expect the combustion products would go nowhere? If this were so, the gases would escape to somewhere, wherever the weakest link in the exhaust system was. – Jennifer Oct 30 at 9:21
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This is a draft diverter.

If the heater was connected directly to the chimney, the hot flue gas would rise, creating draft in the heater. While that is a good thing, the amount of draft would depend on the particular chimney configuration in that installation, amongst other things.

Excessive draft can cause problems, such as too much air being drawn through the heater by a tall chimney, which would lead to dilute combustion causing lower efficiency. In extreme cases it could even blow the flame out.

The air gap avoids such problems, because the pressure at the outlet of the heater only depends on the heater itself, and is independent of the particular chimney configuration. So long as the chimney is at least a couple of times larger than it needs to be, the system will find a natural balance, where amount of air drawn in increases until the frictional pressure drop matches the bouyancy draft of the chimney.

It also allows for an occasional downdraft, which could occur if a gust of wind hits the chimney. Yes, you'd get a small puff of combustion products into the room, but a cleanly-burning heater doesn't emit much if any CO, and that's better than blowing the flame out.

https://www.rinnai-uk.co.uk/hotechnology/system-design-considerations/flueing/

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    "The draft diverter, which consists of a baffle plate positioned directly above the primary flue, regulates the direction and amount of air flowing in and out of the burner." -"A draft diverter ensures optimum burner performance by producing a nearly constant draft and a stable air supply regardless of external conditions." – hunker.com – Mazura Oct 30 at 3:25
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    Worth stating with extra clarity (assuming I've got it right) to address the OP's concern: under normal conditions, the flow of exhaust gas from the burner causes room air to be "sucked in" and up the chimney. The "gap" isn't venting exhaust gas into the room because it's too busy moving air in exactly the opposite direction. – hobbs Oct 30 at 18:18
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    @hobbs - the stack effect itself will keep flow going, as long as (as it is under normal conditions) T1 is greater than T0 (indoor temp. vs. outside). - Nothing with a diverter is above 80% efficiency afaik (that's why I call them '80s'), and not only do HW tanks have jacket losses, you're loosing climate controlled air up the chimney 24/7. – Mazura Oct 30 at 22:55
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The actual amount of combustion air - i.e., air flowing past the burner to provide oxygen and leave with less oxygen but more CO2 and possibly other combustion byproducts - is very little. The gap allows additional air to flow in and the combined air, if the ductwork is designed and installed properly, flows through the ductwork and out of the house.

If you did not have the gap, the amount of air naturally flowing through would often be too little to flow at a decent speed and would instead be more likely to sit in the ductwork. Hard to explain, but it just wouldn't work right.

A few additional points:

  • CO2 is not the biggest enemy - CO (Carbon Monoxide) is.

    Yes, if you have too much CO2 you will have problems (think Apollo 13). But any house, even without any combustion equipment, needs to get makeup air with more oxygen to offset CO2 produced by people. But CO is incredibly dangerous and even low levels can cause a problem. CO is primarily produced by incomplete combustion which can happen easily in any combustion equipment if air flow is too low or there are impurities in the fuel or if a burner is clogged or many other problems. Therefore, you should have CO detectors near any area with combustion equipment.

  • Temperature of the output is actually a different problem than it used to be.

    It used to be that furnaces and hot water heaters were relatively inefficient and the exhaust would be hot and you would want to make sure it all got out so you wouldn't burn your house down. But since it was hot, and hot air rises, that was easy to do.

Now furnaces and hot water heaters are so efficient that there is a different problem - the exhaust air, which you still want to get rid of because of CO2 and other potential problems, is too cold! So it doesn't rise as well on its own. That places a lot more rules on how to vent the exhaust, to the extent that where in the good 'ol days you would replace a furnace or hot water heater and reuse almost all the exhaust ductwork, now there is often a lot of work involved. But I digress.

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    Good CO and CO2 explanation. + – JACK Oct 28 at 16:44
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    Your first point reminds me of another problem, IIRC, this solves: The primary combustion product is water. Without enough flow rate through steel ducts the water vapor will condense onto them, potentially rusting them, and also trickling back down when the burner isn't running. – feetwet Oct 28 at 17:54
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    +1. Essentially, the air gap allows separate management of the amount of air needed for combustion (very little) from the amount of air needed to get enough velocity through the vent for the vent to function. – dwizum Oct 28 at 20:23
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    @dwizum - That would be 'Q' : "stack effect draft (draught in British English) flow rate, m3/s" – Mazura Oct 29 at 3:43
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    @Robert First, there should be no CO produced. Second, there should be constant small air flow through the vent, which will pull the CO with it. But if those fail, CO will get into the house even without the gap through the air intake of the furnace, or other small gaps in the construction. – jpa Oct 29 at 7:45
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manassehkatz covered a lot, but the simple answer is constant air flow

The draft will change in the chimney as exhaust vents towards it – especially when going from cold air to hot. A draft hood is placed above the upper most part of the gas furnace to draw air into the chimney and makes it possible to draw more or less air through the chimney as necessary to create a constant flow.

This makes it possible for the burner to enjoy consistent air flow without any wind gusts or sudden temperature spikes or drops. Hot air, if not put through a draft hood would create a strong air flow through the burners.

A draft hood cools the air as it is released by the burners from 500 degrees F (260 °C) to between 300 degrees F and 350 degrees F (150 °C - 180 °C). The cooling needs to be carefully calibrated to avoid condensation build up in the chimney however – a problem that occurs when the temperature gets too low.

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    This really doesn't make sense from a physics point of view (and your link doesn't back it up). The only reason hot air wouldn't rise above cold is if something else were pushing it (e.g. wind, or some other source of air pressure). – Daniel Griscom Oct 30 at 10:26
  • @DanielGriscom Yeah, I have to eat crow there. This is what I had been colloquially told and I thought I had a link to back it up but I couldn't find it. I dug deeper and found a better answer. Thanks for the correction – Machavity Oct 30 at 12:36
  • Props for adapting, but with such a complete change in content, shouldn't this be a new answer? (I'm not sure you did the wrong thing, but...) – Daniel Griscom Oct 30 at 13:16
  • Good reference, but it could use some clarification/elaboration. The two questions it leaves are: (1) It says the cooling provided by the draft hood "needs to be carefully calibrated." Picture in the OP shows one of the hoods loosely – one might say haphazardly – attached. Neither the hoods nor the gap are sized to the chimney's airflow characteristics in any installation I have seen. So how are they "calibrated?" (2) Is the draft hood there also to control the airflow to the burner, not just to address the exhaust problems? That makes as much sense as pushing on a rope. – feetwet Oct 30 at 15:05
  • #1 sounds like an excellent question to ask as a different question. Heck, I'd like an answer to that too – Machavity Oct 30 at 15:08

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