ᐅ Mechanical ventilation with heat recovery and still keeping windows open at night
Created on: 30 Aug 2016 14:23
K
Kaspatoo
Hi,
I would like to have a mechanical ventilation with heat recovery system in our newly built house, but we always sleep with the window open at night (mainly because of the cool, fresh-feeling air; warm air doesn’t feel fresh to me).
Here in the forum, I’ve read several times that many people just do this “without any issues.”
I’ve also often read that this could interfere with the mechanical ventilation system (it might "malfunction"). It was mentioned that this leads to increased wear and tear, but I couldn’t clearly identify exactly how and on which components this higher wear would occur. Apparently, this only happens if the system uses some kind of dynamic pressure control and doesn’t operate with a constant static pressure.
I have also read that this not only cools down the bedroom with the open window but, in the worst case, could cool the entire house because the ventilation system causes a temperature equalization. So either the heating has to compensate or the other rooms get colder.
For me as a layperson and reader, this means:
- If you have a mechanical ventilation system, make sure it does not have dynamic pressure control to avoid the “malfunction” problem.
- When planning the ventilation, ensure that at least the attic and the ground floor have separate circuits for the mechanical ventilation and are not connected “in series.”
Regarding the latter: As far as I understood correctly from a planner, the pipe layout would look like this: assuming you have four rooms in the attic (bathroom, 3 bedrooms), two rooms would get supply air ducts, and two rooms would get exhaust air ducts (one of those definitely the bathroom). The airflow then passes under the door.
1) If I open the window in an exhaust room, I would expect the following:
- At most, only my room cools significantly due to colder outside air coming in through the open window.
- It might be that little happens (almost no fresh air in the room), except that the outside air flows quite directly into the exhaust.
- Other rooms lose their air exhaust; the air might stagnate there, causing the air pressure to rise and the pressure increase to reach the supply air fan. This results in more resistance and could lead to higher wear (it’s like a freight train with locomotives at front and rear: if there’s no locomotive pulling at the front, the one at the back has it harder, although it won’t supply more power than set). In the extreme case, this would be like holding the supply air fan in place, which I believe is not good for the component in the long run.
- The question is: how serious is this or am I overthinking?
2) If I open the window in a supply air room, I would expect:
- In the worst case, the supply air flows directly outside, and I get nothing from the open window.
- The “pushing” locomotive has more load because the “pulling” locomotive is absent.
If the answer is: yes, opening windows is a bad idea with mechanical ventilation with heat recovery, then my follow-up question is: how do I prevent mold if I can’t regularly manage to open windows?
In summary, it seems to me there are only four possible options:
- Spend a lot of money on individual controls.
- Forget mechanical ventilation with heat recovery, ventilate manually and, if you ventilate too rarely, just skip the insulation and build a house like in the 1970s.
- Install mechanical ventilation with heat recovery and live without opening windows.
- Install mechanical ventilation with heat recovery, ventilate anyway, and accept the consequences (energy loss, system wear, disturbed indoor climate).
What do you think?
Which of my statements are correct, which are not?
Thanks a lot for your answers.
I would like to have a mechanical ventilation with heat recovery system in our newly built house, but we always sleep with the window open at night (mainly because of the cool, fresh-feeling air; warm air doesn’t feel fresh to me).
Here in the forum, I’ve read several times that many people just do this “without any issues.”
I’ve also often read that this could interfere with the mechanical ventilation system (it might "malfunction"). It was mentioned that this leads to increased wear and tear, but I couldn’t clearly identify exactly how and on which components this higher wear would occur. Apparently, this only happens if the system uses some kind of dynamic pressure control and doesn’t operate with a constant static pressure.
I have also read that this not only cools down the bedroom with the open window but, in the worst case, could cool the entire house because the ventilation system causes a temperature equalization. So either the heating has to compensate or the other rooms get colder.
For me as a layperson and reader, this means:
- If you have a mechanical ventilation system, make sure it does not have dynamic pressure control to avoid the “malfunction” problem.
- When planning the ventilation, ensure that at least the attic and the ground floor have separate circuits for the mechanical ventilation and are not connected “in series.”
Regarding the latter: As far as I understood correctly from a planner, the pipe layout would look like this: assuming you have four rooms in the attic (bathroom, 3 bedrooms), two rooms would get supply air ducts, and two rooms would get exhaust air ducts (one of those definitely the bathroom). The airflow then passes under the door.
1) If I open the window in an exhaust room, I would expect the following:
- At most, only my room cools significantly due to colder outside air coming in through the open window.
- It might be that little happens (almost no fresh air in the room), except that the outside air flows quite directly into the exhaust.
- Other rooms lose their air exhaust; the air might stagnate there, causing the air pressure to rise and the pressure increase to reach the supply air fan. This results in more resistance and could lead to higher wear (it’s like a freight train with locomotives at front and rear: if there’s no locomotive pulling at the front, the one at the back has it harder, although it won’t supply more power than set). In the extreme case, this would be like holding the supply air fan in place, which I believe is not good for the component in the long run.
- The question is: how serious is this or am I overthinking?
2) If I open the window in a supply air room, I would expect:
- In the worst case, the supply air flows directly outside, and I get nothing from the open window.
- The “pushing” locomotive has more load because the “pulling” locomotive is absent.
If the answer is: yes, opening windows is a bad idea with mechanical ventilation with heat recovery, then my follow-up question is: how do I prevent mold if I can’t regularly manage to open windows?
In summary, it seems to me there are only four possible options:
- Spend a lot of money on individual controls.
- Forget mechanical ventilation with heat recovery, ventilate manually and, if you ventilate too rarely, just skip the insulation and build a house like in the 1970s.
- Install mechanical ventilation with heat recovery and live without opening windows.
- Install mechanical ventilation with heat recovery, ventilate anyway, and accept the consequences (energy loss, system wear, disturbed indoor climate).
What do you think?
Which of my statements are correct, which are not?
Thanks a lot for your answers.
Grym schrieb:
Why? A delta of 23 degrees to 24 degrees equals 1K.
The 31-degree (87.8°F) warm outside air also passes through the heat exchanger to the supply air outlets. If 23-degree (73.4°F) exhaust air is expelled and 31-degree (87.8°F) outside air is taken in, then with 90% heat recovery, the resulting exhaust air temperature is 30.2 degrees (86.4°F) and the supply air temperature is 23.8 degrees (74.8°F). I once rounded the difference up by about 25% to 24.0 degrees (+1.0 instead of +0.8).
But night cooling could be done just at night, not throughout the day.
Wh = watt times hour. And if I calculate that per hour: Wh/h = watt.
Or in other terms, 225 m3 air exchange per hour equals:
(calculated with an exact difference of 0.8K)
0.34 Wh/(m3*K) x 225 m3/h x 0.8K = 61.2 watts
This applies to the entire house. If the volume flow 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 Thanks for the explanations.
- I now understand where the delta T comes from, thank you.
- As far as I know, even the highest setting (party mode) is preset at about three-quarters of the maximum capacity from the factory. If you want to run 400 m³/h at night, an oversized system for many single-family homes would be needed, or the highest setting’s capacity is increased to 100% by a professional, which could negatively affect the system’s lifespan. The power consumption during that time would be roughly equivalent to 2 to 3 lamps of 60 watts each running overnight.
- The unit watt times hour is correct, but it seems this calculation assumes the night lasts exactly one hour. If the night operation lasts, for example, 10 hours, then the total energy must be divided by the number of hours to get the power [watts].
@TE sorry for the off-topic.
B
Bieber08151 Sep 2016 22:37Kaspatoo schrieb:
where my questions, as I hope, have now been answered.May I ask how? So, what is your conclusion?
also:
theoretically, it is possible that the mechanical ventilation with heat recovery (MVHR) brings in more heat during the day than it removes at night. it all depends on the temperature difference. if it is 35°C (95°F) outside and 20°C (68°F) inside, the heat is transferred inside with a 15K difference (high heat transfer). if at night the temperature inside is 25°C (77°F) and outside is 15°C (59°F), only a 10K difference would remove heat again.
practically, however, this is unlikely because, on the one hand, the heat capacity of the air is very low and, on the other hand, the air intake side (very often on the north/west side of the house) is usually in cooler shade.
in fact, you can do without the ventilation system in summer, but it does no harm to run it on a low setting.
humidity in summer is another issue. thanks to the high dew point, mold hardly ever occurs except in basements and similar areas...
theoretically, it is possible that the mechanical ventilation with heat recovery (MVHR) brings in more heat during the day than it removes at night. it all depends on the temperature difference. if it is 35°C (95°F) outside and 20°C (68°F) inside, the heat is transferred inside with a 15K difference (high heat transfer). if at night the temperature inside is 25°C (77°F) and outside is 15°C (59°F), only a 10K difference would remove heat again.
practically, however, this is unlikely because, on the one hand, the heat capacity of the air is very low and, on the other hand, the air intake side (very often on the north/west side of the house) is usually in cooler shade.
in fact, you can do without the ventilation system in summer, but it does no harm to run it on a low setting.
humidity in summer is another issue. thanks to the high dew point, mold hardly ever occurs except in basements and similar areas...
Payday schrieb:
...in fact, you can skip the ventilation system in summer... Then you could also skip the ventilation system in winter, because the fresh air being blown into the house is colder than the warmed air leaving the house.
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