Using ventilation for overnight cooling

[gravelld]

Plenty of chat on Twitter over the past few weeks about using overnight mechanical ventilation to, if not cool the house, at least provide some contribution to cooling.

 

For anyone that has tried this - how has it worked out? What sort of ACHs are you aiming at to make a difference? Are you running in some sort of boost mode?

[Stones]

I've just updated my blog which details our cross ventilation cooling strategy.  It does work, but only because the ambient temperature is lower and there is usually always some kind of breeze to assist with air flow.

[gravelld]

Good timing!

 

Although you seem to be using natural ventilation, so it's a bit difficult to work out how this can work for others, because it's so variable (a bit like why it's a bad idea to use natural ventilation for air quality).

 

Thoughts? Can you provide a quantity for wind speed, and how much your windows are open as a ratio of the volume?

Edited July 28, 2018 by gravelld

[Stones]

My fault for missing the 'mechanical' bit! 

[gravelld]

@StonesNo - sorry I just added another bit to my post - just wondering how windy you are and how much your windows are open.

Edited July 28, 2018 by gravelld

[Jeremy Harris]

If you get a significant drop in night time temperature, and you have a house that has both a high heat capacity internally, and adequate thermal conductivity between the internal air and the areas with highest heat capacity, then "night purging" as a cooling technique can work very well.

 

Adding a mechanical fan to assist airflow sounds like a good idea, and would also allow the air temperature to be reduced further if an evaporative cooler was added after the fan (what the Americans often refer to as a "swamp cooler").

 

However, if the house itself has a relatively short decrement delay, then the effects of any night time cooling will be short-lived, as the short decrement delay will lead to the house heating up relatively rapidly again the next day, if it's hot.

 

Relatively cheap and easy to set up though.  In an existing house I'd be inclined to use a positive pressure ventilation approach, making up a board to fit a window on the North side of the house and fitting a fan into it, perhaps with an evaporative cooler for greater effect, and see how effective it was.  Something like I did when making up this air test fan maybe, as a temporary test installation:

 

[Ferdinand]

I have been running my bathroom extractor and kitchen extractor hood with some success to draw in air from the other end of the house through open windows or doors.

 

But not overnight.

 

The kitchen one has proved very useful when using the oven.

 

F

Edited July 29, 2018 by Ferdinand

[ProDave]

For overnight ventilation cooling, you need to understand you need a cross flow of air from one side to another.  I am just back from 5 very hot nights in a hotel room, with just one opening window, and nowhere for the air to flow to, so overnight cooling was pretty poor. 

 

We have our bedroom windows all open a little at night and it helps keep the place cool, then close them in the day if it is going to be hotter outside (doesn't happen often up here)