Temperature Differentiation Affect on Stack Effect
1869farmhouse
| Posted in General Questions on
I’ve been playing with some modeling software and reading more about thermodynamics/pressure and how air moves in a home.
I’m curious about how zoned temperatures for first and second levels where the upper level was cooled several degrees lower than the first level in the summer, how that would affect stack effect?
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I have commented before, temperatures tend to be quite even in open floor plans providing the house is well insulated and airtight. The double envelope concept of years ago relied on a slow convective loop to balance temperatures and in some cases distribute heat from lower level thermal storage. My experience is from building very airtight and highly insulated structures starting in the early 1980's. Foundations and basement slabs need adequate insulation to negate earth coupled cooling in cold climates. All of the expensive and sophisticated heating and cooling equipment will not better superinsulation principles.
Makes sense. The home in question has R-28 worth of rockwool between the first and second levels, but it really was just theoretical as much as anything. All in the name of understanding building science more throughly.
Imagine the case where the upper level is 75F and the lower floor and outside is 95F. You would have a single floor of stack effect (1/2 the pressure). But there is little effect if it's 75F up, 78F down and 95F outside.
The single floor stack effect makes sense, but even at the proposed temperatures, let’s assume the thermostat is mid wall and we have 73 at the floor level in the second story and 81 at the ceiling level of the first story. The air masses meet at the stairwell, would it just be the pressure from the “single level stack effect” that would pull the cooler airmass up the stairs? It seems like the heat would move towards the cooler air mass and there would be reverse stack effect if anything at the stairwell.
The cooler air will tend to sink towards the bottom level, and the warmer air at the lower level ceiling will try to replace it. A smoke pencil would show that the cool air is sinking in part of the stairwell and the warm air would be rising in a different part.
Overall, the stack effect reverses in a house that is cooled in summer - the cool air in the house sinks towards the lower level and eventually exfiltrates out through holes in the envelope. Hot outdoor air infiltrates at holes in the upper level.
If you are cooling the upper levels more, it would just mean that the main floor AC will not run.
For example with a two story house you can get away with a 1.5 ton AC for the 2nd floor and install a 3/4 ton unit on the main floor (I've done this and works great for cooling).
This is also the reason that if you want good cooling performance with a single air handler, the most important item is a large return near the ceiling of the 2nd floor. Without that, the 2nd floor will never be comfortable in the summer.
For heating the situation reverses. With doors open upstairs, you can pretty much heat the whole house with a single heat pump on the main floor.