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.
Musketier schrieb:
@GrymIgnoring furnishings, a 150m² (1,615 sq ft) house with a ceiling height of 2.70m (8.9 ft) contains about 400m³ (14,125 cu ft) of air. With a circulation rate of 225m³ (7,943 cu ft) and factoring in furnishings, you exchange the air roughly every 1.5 hours, increasing the temperature by about 0.5 to 1°C (0.9 to 1.8°F) each time. Sorry, but that is simply incorrect. That’s exactly why the calculation is necessary.
Heat or cold is stored in the building components, not in the air. You need to calculate the actual energy input. In those well-insulated houses heated by air-to-air heat pumps, the air is sometimes blown in at 50°C (122°F) or higher, with much larger volumes per hour. Yet people still have trouble warming the house—even in KfW40 or KfW55 energy-standard homes. It’s not the case that after half an hour the entire air volume has been exchanged and suddenly the room temperature is 50°C (122°F).
To compare, consider the energy input from a south-facing window exposed to direct and diffuse solar radiation of approximately 1 kW/m² (650–700 watts direct and the rest diffuse). A window measuring 2.00m by 2.30m (6.6 ft by 7.5 ft) has an area of 4.6m² (50 sq ft), which equals about 4.6 kW of solar power. A triple-glazed window transmits about 55% of this, so roughly 2.53 kW enters the room.
Mechanical ventilation with heat recovery in the living room: 0.0136 kW
South-facing living room window: 2.53 kW
Running mechanical ventilation with heat recovery for one hour: 0.0136 kWh of energy input
An unshaded south-facing window in 19.35 seconds transmits the same 0.0136 kWh of energy input
Conclusion: Mechanical ventilation with heat recovery contributes no noticeable heat input. The waste heat from just three LED bulbs of 5.5 watts each is higher than the energy input from the mechanical ventilation.
Kaspatoo schrieb:
Are you trying to prove or demonstrate that a mechanical ventilation system with heat recovery barely increases the house temperature, and that turning off the system would only have a minimal effect?
I still think that those without a separate air conditioning or cooling system can turn off the ventilation unit. How much warmer it would get is debatable. But the fact that it would get at least slightly warmer and you can notice minor temperature differences shows that you could still switch off the system to achieve some "cooling." Even if it’s just 1-2°C (34-36°F). With a night bypass, the supply air temperature is even higher, which feels much more noticeable to the body.I did some research on the specific heat capacity of solid (massive) houses and found values between 30 and 70 kWh/K. Let’s calculate using 30 kWh/K: to increase the house temperature by 1 Kelvin, you must supply 30,000 Wh. If we take the 61.2 Watts for the whole building from above and assume it lasts from 12 pm to 8 pm on average, that equals 489.6 Wh, or just under 0.5 kWh. The house would warm up less than 0.02 K due to ventilation. That’s really negligible.
By comparison, an unshaded south-facing window like the one above, with the same 8 hours of sunlight and no shading, introduces 20.24 kWh of energy. The temperature through a single unshaded window would rise by 0.67 K. That’s considerably more than barely 0.02 K—and you usually have mechanical ventilation plus many windows.
The downside is that with the system off, there is no air exchange. Opening windows probably brings more warm air into the house than mechanical ventilation.If opening windows lets in heat with an 8 K temperature difference instead of 1 K, that’s eight times the energy input. You would need to ventilate only one-eighth as much, which would be below the minimum ventilation rate necessary to protect against moisture.
On the other hand, we currently live in a top-floor apartment. In summer, with lots of sun, we open everything for cross-ventilation and hope for a breeze. You don’t get that with a mechanical ventilation system with heat recovery.Hope. Right…
Personally, this calculation doesn’t help me much; in practice, it will probably be different anyway because of other factors and no lab conditions.In practice, you won’t notice a temperature difference between the outgoing and incoming air with mechanical ventilation with heat recovery in summer. It will feel as if fresh air is replacing stale air at exactly the same temperature.
Oh dear, even if the heat recovery efficiency were 85 percent, it doesn’t change the scale of the issue.
Mechanical ventilation with heat recovery (MVHR) in the living room: 0.0136 kW
1 south-facing window in the living room: 2.53 kW
Now let’s assume 80 percent (the minimum heat recovery efficiency required for KfW certification) and say there are 3 south-facing windows:
Mechanical ventilation with heat recovery (MVHR) in the living room: 0.0272 kW
3 south-facing windows in the living room: 7.59 kW
One hour of MVHR with 80% heat recovery delivers as much heat into the room as comes in through the windows in 13 seconds.
Don’t get fixated on insignificant decimal places. Air is not an effective heat transfer medium, and heat recovery at 80% or 90% still “eliminates” most of the temperature difference. In practice, the MVHR contributes almost nothing to heating.
On the other hand, you could connect a ground source heat exchanger to the MVHR and ideally blow air into the room that is 5–7 degrees Celsius (9–13 degrees Fahrenheit) colder. This is not about a 1–2 K (1.8–3.6°F) temperature increase, but a 17 K (31°F) drop. Even then, many say it barely makes a difference. Despite the tenfold temperature difference!
And if you see it that critically, then reduce the MVHR airflow to 100 m³/h (roughly 60 cfm). Then the heat input in the worst case equals just under 6 seconds of unshaded window exposure. Assuming the heat recovery just meets the KfW55 minimum standard and is not over 90 percent as often advertised.
I also don’t believe MVHR manufacturers have developed software that detects when the system is being tested and then offers a special 90 percent heat recovery boost, while normally only providing about 80 percent heat recovery. So if the 90 percent figure is correct and you reduce the system to 100 m³/h, then the heat input from one hour of ventilation corresponds to less than 3 seconds of unshaded south-facing windows...
Mechanical ventilation with heat recovery (MVHR) in the living room: 0.0136 kW
1 south-facing window in the living room: 2.53 kW
Now let’s assume 80 percent (the minimum heat recovery efficiency required for KfW certification) and say there are 3 south-facing windows:
Mechanical ventilation with heat recovery (MVHR) in the living room: 0.0272 kW
3 south-facing windows in the living room: 7.59 kW
One hour of MVHR with 80% heat recovery delivers as much heat into the room as comes in through the windows in 13 seconds.
Don’t get fixated on insignificant decimal places. Air is not an effective heat transfer medium, and heat recovery at 80% or 90% still “eliminates” most of the temperature difference. In practice, the MVHR contributes almost nothing to heating.
On the other hand, you could connect a ground source heat exchanger to the MVHR and ideally blow air into the room that is 5–7 degrees Celsius (9–13 degrees Fahrenheit) colder. This is not about a 1–2 K (1.8–3.6°F) temperature increase, but a 17 K (31°F) drop. Even then, many say it barely makes a difference. Despite the tenfold temperature difference!
And if you see it that critically, then reduce the MVHR airflow to 100 m³/h (roughly 60 cfm). Then the heat input in the worst case equals just under 6 seconds of unshaded window exposure. Assuming the heat recovery just meets the KfW55 minimum standard and is not over 90 percent as often advertised.
I also don’t believe MVHR manufacturers have developed software that detects when the system is being tested and then offers a special 90 percent heat recovery boost, while normally only providing about 80 percent heat recovery. So if the 90 percent figure is correct and you reduce the system to 100 m³/h, then the heat input from one hour of ventilation corresponds to less than 3 seconds of unshaded south-facing windows...
Oh Grym, or should I say BauGrym
How many times have you changed your opinion over the years since you started dealing with house construction?
Quotes from July 2015
How many times have you changed your opinion over the years since you started dealing with house construction?
Quotes from July 2015
For a long time, I was very much in favor of controlled mechanical ventilation, but I have now revised my opinion.
Since we currently live in an older building and our ventilation system is basically an always-tilted window, summer and winter: There is no freezing cold air. It mixes so quickly with the rest of the air that we don’t notice anything. That’s theoretical nonsense from the controlled ventilation salespeople.
So... in spring, summer, and fall the ventilation is unnecessary because usually multiple windows in the house/apartment are kept open permanently anyway (when it’s 5-10°C (41-50°F) outside and a window is tilted open, ventilation losses are minimal compared to transmission losses), and in winter the air is too dry. Great system.
This is not an opinion but a calculation. The mechanical ventilation system with heat recovery practically does not contribute to heating.
And my earlier observation that there is no freezing cold air when a tilt-and-turn window is open is also correct. Why? Because air is not a good heat transfer medium...
But I don’t really see what the three quotes have to do with the fact that a mechanical ventilation system with heat recovery does not contribute to heating in summer? The three quotes are as relevant to the topic as the ventilation system is to heating in summer.
So, whether you keep the system running or not is thermally irrelevant. Moisture protection is still ensured, as well as fresh air, and some people are critical about turning it off. The constant, gentle airflow in the supply ducts ensures that no dirt can enter there. Completely switching it off means giving up this protection.
I would always keep the system running, both in summer and winter, and open windows as you wish (generally allowed, since the air is fresh and clean anyway). Use the exhaust hood and, at the same time, open windows in the traditional way. And that’s it...
And my earlier observation that there is no freezing cold air when a tilt-and-turn window is open is also correct. Why? Because air is not a good heat transfer medium...
But I don’t really see what the three quotes have to do with the fact that a mechanical ventilation system with heat recovery does not contribute to heating in summer? The three quotes are as relevant to the topic as the ventilation system is to heating in summer.
So, whether you keep the system running or not is thermally irrelevant. Moisture protection is still ensured, as well as fresh air, and some people are critical about turning it off. The constant, gentle airflow in the supply ducts ensures that no dirt can enter there. Completely switching it off means giving up this protection.
I would always keep the system running, both in summer and winter, and open windows as you wish (generally allowed, since the air is fresh and clean anyway). Use the exhaust hood and, at the same time, open windows in the traditional way. And that’s it...
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