#luften

Update:

I managed to measure the boundary layer temperature of my heavy load-bearing wall somewhat directly. I basically just used an additional air temp sensor, put it on the wall, and covered it with a blanket, leaving a small hole that was faced away from airstreams to account for moisture.

The data is not yet fully clear but as you can see via the graph, the green air temp stops raising when it approaches the yellow wall temp.

I am somewhat sure now that the temp of this wall is the most significant indicator of room temperature after I closed my windows and rolled down the blinds.

Until around 2:30 AM I pointed both of my fans to the windows, after that I let one of them cycle between the window and the wall.

I am still not sure if the fan pointed to the wall to get rid of the boundary layer is significant or if it would be better to just point the fan to the window.

There is almost no air movement I can measure or perceive at the wall if I point both fans to the window.

I think thermal inertia of the wall is quite impressive! It got blown on the entire night with 2C lower air and only lost temp very slowly. So it is either very bad at conducting heat or it is a good thermal battery.

After calculating the wall is about 3m³ of concrete which is round about 1.75 kWh/K. Which is lower than expected.

The convection exchange of this wall should be pretty low as it is obviously vertical... after looking into convection vs forced exchange (blowing onto the wall) calculation examples I guess that the temp exchange is about 3 to 5 times as high as with normal unsupported convection.

Thinks I want to test and see if they are relevant:

• Messure different setups and distances from the window to the fans

• Messure outside temps to optimize the "closing window" time. I think this should be the time when outside temps are maybe 0.5C under the wall temp as air kWh/K is super low and will be heated almost immediately to wall temp if closed. So even if indoor temp is cooler then outside temp it does not make sense to close the windows as long as outside temp is lower then wall temp.

• Calculate how long and how often a given location on the wall must be blown upon to hit diminishing returns regarding the removal of the boundary layer to determine if it would be better if one fan just randomly blows at all walls, floor and sealing.

• Set up better temp sensors for measuring wall temps.

• Search for heat bridges

• Blowing air at the upper portion of walls since I expected that the delta is bigger there.

Until now it is quite amazing that I can basically safe each day about 3 to 5 EURO on energy because I do not use a AC... which is also not very practical in my apartment.

There are times when this will not work like Sunday when we will have 25C at night and 40C at day.... but yah it is also questionable if a mobile renter friendly AC can even pull down the room temp at these outside temps especially if it is only a unit with 1 or 3 hoses instead of a split AC unit... which are a lot more expensive like 1k EUR and gigantic space wise... for about 3 days a year I cannot cool with Lüften.

Let's see how I think about this at Sunday

#climatechange #ac #cooling #lüften #heat #extremeheat

So I am maybe 2 mm away from deciding to drill a hole in the wall and put a temperature sensor into it to get reliable core temps.

The last few days have been brutal with 2 days 36C and then 2 days 40C in a row.

So my apparent walls have heated themselves up to about 28C. It was not possible to cool them overnight because the temps only reached 24C and there was not a lot of wind what lead me to measure my fans how much their cone of air is and what the perfect distance to my windows are.

But this still has no effect because fans where just to bad at creating static pressure. I then sealed a window completely and made 2 holes for the 2 fans which resulted in great static pressure but obviously not much of transit air.

Tonight it was 17C with good wind so I was able to allocate both fans to the walls… but the thermal mass is just so high that my thermal camera still reads 26C at these wall but my wall sensor reads 24C.

So there are 2 issues.

1. the wall has a very high thermal mass that I can not cool down at night with the current equipment.

2. wall must have a bad internal conductivity

This creates issues in my planing when I have to get up to close the windows.

So there are 4 things I will do.

1. I will build a window sized fan out of PC fans that have great static pressure and are silent. That way I can set up my blinds and windows in a way that I don’t need to be worried that it will rain into my apartment while at the same time having great airflow.

2. I will drill a hole I my wall (3mm or something) maybe 10cm deep and then I will put a temp sensor there to get the core temps of the wall that is clean on not just a sensor on the wall with a blanket over it.

3. I will put one of the fans statically without moving on a wallpoint and measure how the core temps change and how the surface temps will change.

4. I will measure airspeeds and input and output temps of my apartment to measure the rough thermal mass and thermal lag. (And also I want to know how much heat in watt I can shed with just Lüften)

5. with all this data I will be able to create a good thermal model of my Appartement and that will give me a good idea how I can behave in the days before a heat wave and while it is going.

There is maybe a way to put my diy fan into my home assistant and let the fan only run between 2AM and 6AM when it would be beneficial to Lüften.

And I want a system that tells me „in 7 days we expect a heatwave, you have to begin to cool your apartment tomorrow at night.“ while considering dew points and mold prevention if my apartment could become too cold - although I don’t think this is an issue.

And I will be able to get a estimation how hot the apartment will be while a heatwave is going on and maybe just take vacation or get a hotel with AC or something if it gets too extreme.

Like yah yah I could get myself a monoblock and diy it to a 2 hose system but these things are quite big, expensive and expensive to run while I only need them for 3 days a year like this one. And additionally I think small monoblocks are just too weak to cool a apartment of this size idk I would have to calculate it. In any case I want to prevent getting a AC because of space constraints and energy aspects.

I mean the energy price are going up a_lot if a heat wave is coming in and i totally expect that Germany, if a even more extreme heat wave is rolling it would need to shed private homes because the grid would collapse otherwise. So a Balkonkraftwerk with batteries would be needed if this should be somewhat usable and financially acceptable. Last Sunday it would have been about 0.9€ per hour of running AC at its peak, while still having to pay for the AC and having the hassle of DIYing and putting it up.

I just hope that I can optimize Lüften in my Appartement to a point where it is just not necessarily anymore to have one, even in a heat wave.

I keep you updated if I have new data or ideas.

#heat #hitze #climatechange #klimawandel #diy #luften #cooling