Controlled residential ventilation and still open windows at night

  • Erstellt am 2016-08-30 14:23:28

Grym

2016-09-01 00:25:29
  • #1


If it is 23 degrees inside (exhaust air) and 31 degrees outside (outside air), then the supply air comes in at about 24 degrees. With 225 m3 air exchange per hour and a specific heat capacity of air of 0.34 Wh/(m3*K), this results in a heat input (at midday, assuming 31 degrees outside temperature) of: 0.34 Wh/(m3*K) x 225 m3 x 1K per hour = 76.5 W = 0.077 kW So as much as a light bulb... This is of course distributed over all rooms, so more like 1/5 of a light bulb for the living room, for example.

This applies to the warmest hours of the day.

If it is then 17 degrees outside at night and 23 degrees inside and bypass as well as output are increased to 400m3, this results in a cooling capacity of: 0.34 x 400 x 6K per hour = 816 W = 0.816 kW

By the way, a floor cooling system achieves several kW of capacity.
 

AOLNCM

2016-09-01 09:40:06
  • #2
I cannot follow your calculation. - In the first formula, Delta T is incorrect. - In the second: 400 m³ in a single-family house? Even if you have a relatively large ventilation unit (e.g. with basement ventilation), you normally run it at such a volume flow only briefly, if at all. With a typical power consumption between about 5 - 170 watts, you usually do not run the system at max. - And the unit Wh does not mean W per hour.
 

Grym

2016-09-01 11:39:04
  • #3

Why? Delta from 23 degrees to 24 degrees = 1K.
The 31-degree warm outside air also comes through the heat exchanger to the supply air openings. If 23 degrees exhaust air is blown out and 31 degrees outside air is introduced, then with 90% heat recovery afterwards it results in: 30.2 degrees exhaust air and 23.8 degrees supply air. I once rounded up by 25 percent (the difference) to 24.0 (+1.0 instead of +0.8).


But you could, for night cooling. Also only then at night and not all day.


Wh = Watt times hour. And if I then calculate that per hour: Wh/h = Watt.

Or differently, 225 m3 air exchange per hour equals the following:
(calculated with exact difference of 0.8K)

0.34 Wh/(m3*K) x 225 m3/h x 0.8K = 61.2 watts

That applies to the entire house. If now the airflow in the living room is 50 m3/h, then the heat input in the living room within one hour is:

0.34 Wh/(m3*K) x 50 m3/h x 0.8K = 13.6 watts = 0.0136 kW
 

Musketier

2016-09-01 12:04:56
  • #4
Can you sleep when you blow through the 400m³ at night? Moreover, your calculation actually proves exactly what I said. Turning off the system during the day is better than running it continuously. Ignoring the furniture, in a 150m² house with a ceiling height of 2.70m, you have about 400m³ of air. With a circulation of 225m³ and taking the furniture into account, you exchange the air roughly every 1.5 hours and increase the temperature by 0.5-1° each time, in addition to the heat input that comes through the windows and walls anyway. At the end of the day, you have a nice warm building, then you try desperately to fall asleep with the warm temperatures and the fully running controlled residential ventilation system, and while you sleep, the house gradually cools down. You also get your daily quota of air exchange if the system runs a bit more in the evening.
 

Kaspatoo

2016-09-01 12:04:58
  • #5
Do you now want to prove/derive with this that the house only heats up slightly more due to a controlled residential ventilation system and that turning off the system would then only have minimal effect?

I still think that those who do not have an extra air conditioner or cooling can still turn off the system. How much warmer it would get is debatable. However, the fact that it would at least be marginally warmer and that you can also notice slight temperature differences shows that you could still turn off the systems to create "cooling." Even if it ends up being only 1-2°. With a night bypass, you might achieve about the same again, and that already makes a big difference in terms of how it feels to the body.

The disadvantage is, of course, that no air exchange takes place when it is turned off. And opening the window probably brings more warm air into the house than the controlled residential ventilation system. On the other hand, we currently live in an attic apartment. In summer, with a lot of sun, we open everything for cross-ventilation and hope for a breeze. You wouldn’t have that with a controlled residential ventilation system.

And if the energy-related differences are only marginal anyway, presumably everyone will try everything out for themselves regarding turning off, bypass, windows open, and doing nothing and will soon notice what is effective. Personally, the calculation does not help me; in practice, it is probably different anyway because of other circumstances and not a laboratory experiment.

Overall, this also does not lead to my original intention of this thread, although my questions, as I hope, have now been answered. I suggest you start a separate thread if the exact calculation is really important to you.
 

Kaspatoo

2016-09-01 12:07:00
  • #6
Oh, I see:

Many thanks to everyone for the helpful answers =)
Now I only have my other threads about Kfw70 vs. 55 and costs for a shell construction *Werbung* left
 

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