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.
We also appreciate the controlled residential ventilation system in the summer... simply because it keeps the air smelling fresh in the morning—even without opening the windows—and it still does when you come home from work. The effect on humidity can sometimes even be negative, depending on the weather.
S
Sebastian792 Sep 2016 18:12What exactly becomes dangerous there?
Payday schrieb:
that's why you can also use one with heat recovery – it doesn’t blow cold air inside and also transfers part of the warm exhaust air to the fresh air.
a ventilation system is mainly a comfort feature. it saves you from manually airing out and helps prevent mold growth inside the living area from the start. in winter, it saves some heating costs while still providing fresh air. when 3-4 people take warm showers in the morning, the bathroom gets very humid. in such cases, a 10-minute window ventilation won’t be enough to make everything feel fresh again. the ventilation system simply ventilates over several hours while no one is home and removes the moisture. the only important thing is that there are no thermal bridges in the exhaust air ducts, otherwise condensation can form and that becomes dangerous.A ventilation system is neither a heating system nor an air conditioning system.
It is not intended to heat or cool.
Its purpose is to supply fresh, unused, oxygen-rich air into the house.
Since the outdoor air rarely matches the indoor air temperature, manufacturers include a heat exchanger to avoid excessive heating or cooling of the indoor air.
In recent years, ventilation systems have become popular in detached single-family homes with the argument: “The house is so airtight that no air can get in or out otherwise.”
The downside of a ventilation system is that even with the best heat recovery efficiency of 95%, it works against the heating system in winter by bringing cooler air inside and raises indoor air temperature further in summer.
Not every homeowner is aware of this, and sales often do not emphasize this point.
Two extremes as examples:
Heat recovery efficiency of 90% (more realistic)
Summer:
Outdoor air 31°C (88°F)
Supply air 25°C (77°F)
Exhaust air 24°C (75°F)
Discharge air 30°C (86°F)
In this case, the fresh supply air is 1°C (2°F) warmer than the exhaust air and continuously heats the indoor air during the day.
Winter:
Outdoor air -20°C (-4°F)
Supply air 18°C (64°F)
Exhaust air 22°C (72°F)
Discharge air -16°C (3°F)
Here, the heating system must compensate not only for the house’s heat losses but also for the ventilation system.
Another disadvantage is that the system must be heated with an electric preheater (usually 1000 watts) at very low temperatures to prevent the heat exchanger from freezing.
To prevent the house from cooling down too much during winter months, manufacturers offer electric or hydronic post-heaters as options. The electric versions consume even more power, while the hydronic one can be connected to the heating system.
In all cases, energy that was saved through insulation is lost again.
Anyone wanting a ventilation system should consider complementing it with an earth heat exchanger. This reduces the extreme fluctuations in outdoor air temperature. That way, the heat exchanger has smaller temperature differences to handle, reducing the temperature difference between supply and exhaust air.
In our climate zone, soil temperature at 1 to 2 meters (3 to 6 feet) depth is about 2°C (36°F) in March and about 15°C (59°F) in September.
Positive temperatures here make preheating unnecessary.
The temperature difference of outdoor air without an earth heat exchanger was 51 Kelvin (51°C or 92°F).
With an earth heat exchanger, it was 13 Kelvin (13°C or 23°F).
Even though the air in the earth heat exchanger does not exactly reach the soil temperature, this example aims to illustrate the pros and cons of a ventilation system.
AOLNCM schrieb:
Two extremes as examples:
Heat recovery efficiency of 90% (which is more realistic)
Summer:
Outdoor air 31°C (88°F)
Supply air 25°C (77°F)
Extract air 24°C (75°F)
Exhaust air 30°C (86°F)
In this case, the fresh supply air is 1°C (2°F) warmer than the used extract airThis is not an extreme, but practically a zero-sum game. The answer here is not 42 but "the waste heat from 3 LEDs." I calculated this above. Air is not a good heat transfer medium, so it doesn’t matter. That's why ventilating 5 times a day for 10 minutes each time isn’t a problem. Nothing cools down because air is a poor heat carrier. It’s just... inconvenient, because you lose 50 minutes of time per day.Winter:
Outdoor air -20°C (-4°F)
Supply air 18°C (64°F)
Extract air 22°C (72°F)
Exhaust air -16°C (3°F)
In this case, the heating system has to compensate not only for the building’s heat losses but also for the ventilation system.
Another disadvantage is that at lower subzero temperatures, the system requires electric preheating (usually 1000 watts) to prevent the heat exchanger from freezing.You have to ventilate anyway. The difference is that the temperature comes in at 18°C (64°F) instead of minus 22°C (-8°F).Anyone who wants to have a ventilation system should try to complement it with a ground heat exchanger.Nope.S
Sebastian793 Sep 2016 08:09Apart from that, we only experience prolonged, significant subzero temperatures in a few regions, where additional measures might be considered.
Thanks to the enthalpy exchanger, I can draw air in down to -10 degrees Celsius (14°F), then the unit switches off to prevent freezing.
And then there’s the fact that I’m unlikely to suffocate. A ground heat exchanger is a nice concept, but depending on the design, it can pose hygiene issues and is not always feasible. I have the controlled mechanical ventilation located in the attic — in the north, it draws air in shielded from the wind.
Thanks to the enthalpy exchanger, I can draw air in down to -10 degrees Celsius (14°F), then the unit switches off to prevent freezing.
And then there’s the fact that I’m unlikely to suffocate. A ground heat exchanger is a nice concept, but depending on the design, it can pose hygiene issues and is not always feasible. I have the controlled mechanical ventilation located in the attic — in the north, it draws air in shielded from the wind.
Now I have a somewhat different question.
The scenario mainly relates to the winter season.
If I understand correctly, a mechanical ventilation system with heat recovery is set to exchange the entire indoor air about twice within 24 hours (at least in theory/calculation).
If I were to open the windows fully for ventilation in the morning and evening, would that theoretically also result in exchanging the entire indoor air twice within 24 hours?
If yes: why do many then say that ventilating twice a day is not enough?
And: why does mechanical ventilation cause such dry air compared to window ventilation?
In both cases, in my opinion, moist indoor air is expelled and replaced by drier outdoor air.
I am asking this mainly because we consider the resulting dry air a significant disadvantage.
Partly because we already experience major eye issues due to air conditioning in the office, and based on stories from my brother-in-law about his mechanical ventilation system (it is very difficult to recover healthily at night during illness because the mucous membranes dry out completely; if you run your hand over the door veneer, you can feel it warping due to dryness in the house, built in 2007, where winter humidity is often around 30%).
Yes, there is heat and moisture recovery, but I do not believe it adds an extra 30%.
I also don’t want to create a tropical rainforest indoors with plants to compensate.
My thoughts are:
- From the perspective of dryness, mechanical ventilation seems overwhelmingly oversized.
- Would reducing the ventilation rate not improve the situation?
- But exchanging the air twice a day isn’t much. Two times window ventilation also means two air exchanges, but retains more humidity?!
- I am afraid that manual ventilation may be insufficient and, due to the time required, might be skipped sometimes. However, mechanical ventilation seems to swing too far in the opposite direction. Something sensible in between would be needed (especially for KfW5 standard).
The scenario mainly relates to the winter season.
If I understand correctly, a mechanical ventilation system with heat recovery is set to exchange the entire indoor air about twice within 24 hours (at least in theory/calculation).
If I were to open the windows fully for ventilation in the morning and evening, would that theoretically also result in exchanging the entire indoor air twice within 24 hours?
If yes: why do many then say that ventilating twice a day is not enough?
And: why does mechanical ventilation cause such dry air compared to window ventilation?
In both cases, in my opinion, moist indoor air is expelled and replaced by drier outdoor air.
I am asking this mainly because we consider the resulting dry air a significant disadvantage.
Partly because we already experience major eye issues due to air conditioning in the office, and based on stories from my brother-in-law about his mechanical ventilation system (it is very difficult to recover healthily at night during illness because the mucous membranes dry out completely; if you run your hand over the door veneer, you can feel it warping due to dryness in the house, built in 2007, where winter humidity is often around 30%).
Yes, there is heat and moisture recovery, but I do not believe it adds an extra 30%.
I also don’t want to create a tropical rainforest indoors with plants to compensate.
My thoughts are:
- From the perspective of dryness, mechanical ventilation seems overwhelmingly oversized.
- Would reducing the ventilation rate not improve the situation?
- But exchanging the air twice a day isn’t much. Two times window ventilation also means two air exchanges, but retains more humidity?!
- I am afraid that manual ventilation may be insufficient and, due to the time required, might be skipped sometimes. However, mechanical ventilation seems to swing too far in the opposite direction. Something sensible in between would be needed (especially for KfW5 standard).
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