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:
The statement referred to your previous claim that a mechanical ventilation system with heat recovery cannot cool or heat. This is logical because such a system is neither an air conditioner nor a heater. The heat exchanger only tries to transfer as much energy as possible from the exhaust air to the incoming fresh air (or vice versa in summer). However, it will never reach 100%. Therefore, with each air exchange, the indoor air temperature tends to rise.
Additionally, every electrical device generates heat. From this perspective, the mechanical ventilation system can be counterproductive in summer, unless it is located outside the thermal envelope.
So I wonder why energy (electricity) is used to heat a house when I want it to be cooler.If it is 23°C (73°F) inside (exhaust air) and 31°C (88°F) outside (outdoor air), the supply air will enter at about 24°C (75°F). With an air exchange rate of 225 m3 (7950 ft3) per hour and a specific heat capacity of air of 0.34 Wh/(m3*K), the resulting heat gain (around midday, assuming 31°C (88°F) outside) is:
0.34 Wh/(m3*K) × 225 m3 × 1 K per hour = 76.5 W = 0.077 kW
This is about the heat output of a light bulb... Of course, this is distributed across all rooms, so for example, the living room would receive about a fifth of a light bulb’s heat output.
This applies to the hottest hours of the day.
At night, if it is 17°C (63°F) outside and 23°C (73°F) inside, and the bypass and airflow are increased to 400 m3 (14,130 ft3), the cooling effect is:
0.34 × 400 × 6 K per hour = 816 W = 0.816 kW
By comparison, a chilled floor system can provide several kW of cooling capacity.
I cannot follow your calculation.
- In the first formula, the Delta T is incorrect.
- In the second: 400 m³ (14,130 ft³) in a single-family house? Even with a relatively large ventilation unit (for example, including basement ventilation), it is usually only run at such a volume flow rate briefly, if at all. With a typical power consumption between approximately 5 and 170 watts, the system is generally not operated at maximum capacity.
- And the unit Wh does not mean W per hour.
- In the first formula, the Delta T is incorrect.
- In the second: 400 m³ (14,130 ft³) in a single-family house? Even with a relatively large ventilation unit (for example, including basement ventilation), it is usually only run at such a volume flow rate briefly, if at all. With a typical power consumption between approximately 5 and 170 watts, the system is generally not operated at maximum capacity.
- And the unit Wh does not mean W per hour.
AOLNCM schrieb:
- In the first formula, Delta T is not correct.Why? The difference between 23 degrees and 24 degrees is 1 K. The 31-degree (Celsius) outside air also passes through the heat exchanger to the supply air outlets. If 23-degree (Celsius) exhaust air is blown out and 31-degree (Celsius) outside air is taken in, then with 90% heat recovery, the resulting exhaust air temperature is 30.2 degrees (Celsius) and the supply air temperature is 23.8 degrees (Celsius). I rounded the difference up by about 25% to 24.0 (+1.0 instead of +0.8).
- For the second: 400 m³ (cubic meters) in a single-family house? Even if you have a relatively large ventilation unit (for example, with basement ventilation), you usually run it at such a high volume flow only briefly, if at all. With a typical power consumption of about 5 to 170 watts, you don’t normally operate the system at maximum output.But you could do so for night cooling. Again, only at night and not continuously during the day.- And the unit Wh does not mean W per hour.Wh means watt multiplied by hour. And if I then calculate it per hour, Wh/h = watt.Or put differently, 225 m³ (cubic meters) air exchange per hour equals the following:
(calculated with the exact difference of 0.8 K)
0.34 Wh/(m³*K) × 225 m³/h × 0.8 K = 61.2 watts
This is for 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) × 50 m³/h × 0.8 K = 13.6 watts = 0.0136 kW
@Grym
Can you really sleep at night when you push through 400m³ (14,130 ft³) of air?
Also, your calculation actually proves exactly what I said. Turning off the system during the day is better than running it continuously. Without considering furniture, a 150m² (1,615 ft²) house with a 2.70m (8 ft 10 in) ceiling height has about 400m³ (14,130 ft³) of air. With an air exchange rate of 225m³ (7,945 ft³) and accounting for the furniture, you replace the air roughly every 1.5 hours and increase the temperature by about 0.5 to 1°C (0.9 to 1.8°F) each time, in addition to the heat gain already coming through windows and walls. By the end of the day, you end up with a nicely warmed building, then you struggle to fall asleep in warm temperatures while the ventilation system is running at full capacity, and while you sleep, the house gradually cools down.
You still get your daily air exchange if you run the system at a higher rate in the evening.
Can you really sleep at night when you push through 400m³ (14,130 ft³) of air?
Also, your calculation actually proves exactly what I said. Turning off the system during the day is better than running it continuously. Without considering furniture, a 150m² (1,615 ft²) house with a 2.70m (8 ft 10 in) ceiling height has about 400m³ (14,130 ft³) of air. With an air exchange rate of 225m³ (7,945 ft³) and accounting for the furniture, you replace the air roughly every 1.5 hours and increase the temperature by about 0.5 to 1°C (0.9 to 1.8°F) each time, in addition to the heat gain already coming through windows and walls. By the end of the day, you end up with a nicely warmed building, then you struggle to fall asleep in warm temperatures while the ventilation system is running at full capacity, and while you sleep, the house gradually cools down.
You still get your daily air exchange if you run the system at a higher rate in the evening.
Are you trying to prove or demonstrate that a mechanical ventilation system with heat recovery only slightly increases the indoor temperature, and that turning off the system would therefore have minimal effect?
I still believe that those without additional air conditioning or cooling can switch the system off. How much warmer it would get is debatable. However, it is clear that it would at least be marginally warmer, and small temperature differences can be noticed, so it is possible to turn off the system to achieve some "cooling," even if it’s only by 1-2°C (2-4°F). Using a nighttime bypass might double that effect, which can feel much more comfortable.
The drawback, of course, is that when the system is off, there is no air exchange. Opening windows probably brings more warm air into the house than the mechanical ventilation system with heat recovery. On the other hand, we currently live in an attic flat. In summer, when there is a lot of sun, we open everything for cross-ventilation and hope for a breeze. You wouldn’t have that option with a mechanical ventilation system.
And if the energy difference is actually only marginal, everyone will likely try different approaches for themselves—turning the system off, using the bypass, opening windows, or doing nothing—and then notice what really works. For me personally, the calculation isn’t very helpful; in practice, it’s probably different anyway because of varying circumstances and no controlled lab conditions.
Overall, this doesn’t really relate to the original purpose of this thread, although I hope my questions have been answered by now. I suggest opening a separate thread if the exact calculations are really important to you.
I still believe that those without additional air conditioning or cooling can switch the system off. How much warmer it would get is debatable. However, it is clear that it would at least be marginally warmer, and small temperature differences can be noticed, so it is possible to turn off the system to achieve some "cooling," even if it’s only by 1-2°C (2-4°F). Using a nighttime bypass might double that effect, which can feel much more comfortable.
The drawback, of course, is that when the system is off, there is no air exchange. Opening windows probably brings more warm air into the house than the mechanical ventilation system with heat recovery. On the other hand, we currently live in an attic flat. In summer, when there is a lot of sun, we open everything for cross-ventilation and hope for a breeze. You wouldn’t have that option with a mechanical ventilation system.
And if the energy difference is actually only marginal, everyone will likely try different approaches for themselves—turning the system off, using the bypass, opening windows, or doing nothing—and then notice what really works. For me personally, the calculation isn’t very helpful; in practice, it’s probably different anyway because of varying circumstances and no controlled lab conditions.
Overall, this doesn’t really relate to the original purpose of this thread, although I hope my questions have been answered by now. I suggest opening a separate thread if the exact calculations are really important to you.
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