We need to replace our propane furnace. One contractor said he needs to replace our existing 2 inch external PVC pipe with a new 3 inch one (requiring drywall work). Another says they can use a non-direct vent setup using inside air. I've read other posts about pros and cons but one thing that wasn't mentioned is whether the non-direct will suck all the warm air from upstairs down into the basement. My house is a mix of 100+ year old and modern and the heating is very uneven from room to room. Will non-direct make that better or worse?

  • No, and it'll bring basement smell into the hsuoe as well. Mar 15, 2017 at 18:23

2 Answers 2


First of all, warmer air always tends to rise, and colder air tens to descend. So inside a room you will find warmer air near the ceiling and colder air near the floor. (anyway, the difference is not huge - maybe 5 °C ). My opinion is that your furnace will never suck the air from the upper half of your living room, yet it may suck the air from the lower half - or may not - depending on how well your windows and doors are insulated. I shall explain that briefly.

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As shown in the diagram, your furnace is by no means a vacuum pump; every bit of air that rises is immediately replace by an equal amount of air. This mass of replacement air can come either from above (from the flue) or from front (from the ventilation holes in the furnace's door)

Situation 1: your house is hermetically insulated - all your windows and doors are perfectly air proof (this rarely happens in practice) This means that the replacement air from the furnace's door will have to be sucked from the basement, which in turn will suck it from the rest of the house, meaning that some amount of air will leave the house without being replaced, thus your furnace would work like a vacuum pump. BUT PHYSICS DON'T WORK THIS WAY!!! Because the air pressure inside the house will tend to be equal to the air pressure outside the house, all the replacement air will be sucked from above, via the flue. So, in this situation, NO AIR CIRCULATION inside the house.

Situation 2: your windows and door are not perfectly air proof Still, the largest amount of replacement air will be taken from the above, via the flue, but some air will be also taken from the furnace's door vents. This air will be sucked from the basement, which will suck it from the rest of the house, which will suck it from the outside of the house, via your non-hermetic windows and doors. But, the air flow WILL ALWAYS FOLLOW THE SHORTEST PATH POSSIBLE. As shown in the picture, you will get some air circulation, but only in the lower half of your room. This means that the air in the lowest half of the room will get replaced by outside air, which is colder; the upper half of the room will not get disturbed, but since air masses will exchange heat until thermal equilibrium, this means that OVERALL, YOU WILL END UP LOOSING SOME HEAT.

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  • one more aspect: let's not forget that the furnace is, by itself, a source of heat; so, for the cold air circulation going down to the basement, there is also a hot air circulation , on the same path but opposite sense. This air is basement air heated by the furnace, and will go up to your living room. This phenomena cancels the cooling effect generated by the entry of the outside air, into your house, underneath the door Mar 15, 2017 at 17:30
  • 2
    If you have situation 1, I hope the batteries in your CO alarms are good.
    – Tester101
    Mar 15, 2017 at 17:36

While obviously some amount of air will be sucked up for combustion, it is not significant in terms of effecting the conditioned space temperature. Although, I must point out that it seems irrelevant because you only have two choices for obtaining combustion supply air, direct or non-direct.

  • I'm not sure I understand why it would be irrelevant. My understanding -- perhaps wrong -- is that direct comes from outside the house and non-direct refers to using the air immediately surrounding unit
    – StepByStep
    Mar 15, 2017 at 16:11
  • Irrelevant with respect to improving comfort throughout the home, maybe.
    – isherwood
    Mar 15, 2017 at 17:23

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