Bear with me, this could be long winded. Just went though the NACHI HVAC course and also the video course. Needed a refresher. They talk about the combustion air coming in high & low within 12 inches top and bottom. I never see two sources, if it’s outside air though tube it comes in at floor level. There is also return air coming back from house. Is this only for natural draft (which I don’t see much any more) or also mid-level Induced draft? Should I be looking for something else and I’m not seeing it?
One other thing, in the gas furnace section it talks about if furnace upgraded to a mid or high-level efficiency that the masonry chimney flue might not be able to handle it because of gases. Does that mean that all masonry chimneys should have a metal liner? Saw something else that said if chimney is on exterior wall it should have metal liner because of cold.
Well, there ya go, you might just read this and go WTF is this guys problem! I’ll check back later, thanks :shock:
Return air has nothing to do with draft or combustion air.
There is still natural draft with residential boilers and the burners will usually be “open” and near the bottom. I do not see forced air furnaces with natural draft at all, anymore. I think that the video is talking about what area is important to keep open and clear for the the proper intake of combustion air, but I am not sure.
Some mid efficiency and high efficiency units use outside air for combustion and either exhaust the combustion gases through the wall or up a chimney, depending on temperature of gases (or whether it goes through a secondary heat exchanger). The reason some of these are required to go directly through the wall, up a lined chimney or chimneys on the outside need a liner is because of temperature. In any of these situations, the combustions gases can be cooled too quickly to escape and will end up condensing in the chimney and creating corrosive and acidic liquid.
Generally, it is always required to have a liner, if there is a chimney, in new installations and probably a safe practice, even if not required.
I hope this helps and didn’t make things worse.
Most of what I see now is the single opening line coming in and going to floor. I keep seeing and reading about the openings 12" from ceiling and 12" from floor and wonder if I’m missing something. I know it’s not all installations that’s what I’m trying to determine which ones should have it? thankyou for your feed back
Mark feel free to call me and I will explain everything, as that might help you more than a forum answer.
I explain this stuff to clients all the time .
Thanks for the talk and the info Bob! It’s a lot easier talking about it then doing 20 posts for info. Had to take that call the daughter just had our first grandson and needed to catch up. thanks again 
Anytime.
I am much more educational and informative on the phone than typing.
Unless in the report of course.
A pic is worth a lot more than words here is typically what I see in my area high and low vent natural air flow/movement as a result of temp differential between the attic and the enclosed closet
Rule of thumb is 1 square inch per 1,000 btu.
What if one has no Thumb:D:D 3 inch pipe is used if water heater only in the closet when furnace and water heater both in the closet size is increased to 6 inches
Really?
No matter what size?
Well no silly basically talking 40 and 50 gallon water heaters jump to 80 and going to increase the pipe size to 8 inch and so forth. Your quote was correct 1 square inch per K BTU is correct I was just messing with you gona get Linas after you
Not true leaky returns can depressurize a mechanical room interferring with combustion air and venting.
So we are gonna play diff scenarios on top of all this?
Oh man …you really think he needs prodding ?:twisted: