ᐅ Central ventilation system – is humidity recovery necessary?
Created on: 30 Dec 2019 16:17
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ludwig88sta
Hello everyone,
for our planned single-family house with 2 floors and a basement, we have decided on a central ventilation system for all rooms with heat recovery within the thermal envelope. The brand and model are not yet determined.
I have read here on the forum a few times about an additional moisture recovery feature (MRF?), since otherwise the indoor air tends to become too dry (of course, the ventilation system also removes the moisture that naturally occurs in the room, which usually makes the air feel comfortable). I wanted to ask if anyone can confirm these issues and whether this is more common in timber frame houses or also occurs in brick houses?
Did you install additional moisture recovery in your ventilation system? How much extra cost does it involve? I assume there are also additional maintenance costs because it involves another motor and related components.
Best regards
ludwig88sta
for our planned single-family house with 2 floors and a basement, we have decided on a central ventilation system for all rooms with heat recovery within the thermal envelope. The brand and model are not yet determined.
I have read here on the forum a few times about an additional moisture recovery feature (MRF?), since otherwise the indoor air tends to become too dry (of course, the ventilation system also removes the moisture that naturally occurs in the room, which usually makes the air feel comfortable). I wanted to ask if anyone can confirm these issues and whether this is more common in timber frame houses or also occurs in brick houses?
Did you install additional moisture recovery in your ventilation system? How much extra cost does it involve? I assume there are also additional maintenance costs because it involves another motor and related components.
Best regards
ludwig88sta
M
Mottenhausen9 Jan 2020 09:31What surprised me the most are the air exchange rates I see recommended here.
People suggest 0.5 air changes per hour. For our house (approximately 425m³ (15,000 ft³) of air volume), that would mean an airflow of more than 200m³/h (118 cubic feet per minute). With such unrealistic values (who in the world still recommends such absurd rates these days?), it’s no wonder that practically all indoor humidity is being vented out immediately.
Do you also consider the system’s power consumption and heat loss? With airflow rates over 200m³/h (118 cfm), heat recovery efficiency might still be around 75 to 80%, but the rest is lost to the outside.
If you allow 30m³/h (18 cfm) per person in the household, then with a four-person household you end up at 120m³/h (71 cfm), and then the heat exchanger can actually reach the 90 to 95% heat recovery rate advertised by the manufacturer. The fan’s energy consumption decreases exponentially (keyword: flow resistance / Bernoulli’s principle / exponential increase with flow velocity…), and indoor humidity won’t drop drastically even without an enthalpy exchanger.
The best option is to program daily schedules with increased ventilation during mealtimes, shower times, and so on. Also, week-day dependent settings are useful. Our Vitovent 300-W offers many configuration options. The airflow rate for each ventilation step can be easily programmed through the service menu—just check the manual.
Currently, my daily average (!) is only 0.2 air changes per hour or 85m³/h (50 cfm), and CO2 levels are no problem throughout the day. Something that might also help some people: Adjust the supply and exhaust vents so that airflow matches the room size and duct length. Our installers were not very precise here and opened everything roughly the same distance based on feel, resulting in most circulation happening between the living room, utility room, and kitchen, while the bedrooms saw hardly any air exchange…
People suggest 0.5 air changes per hour. For our house (approximately 425m³ (15,000 ft³) of air volume), that would mean an airflow of more than 200m³/h (118 cubic feet per minute). With such unrealistic values (who in the world still recommends such absurd rates these days?), it’s no wonder that practically all indoor humidity is being vented out immediately.
Do you also consider the system’s power consumption and heat loss? With airflow rates over 200m³/h (118 cfm), heat recovery efficiency might still be around 75 to 80%, but the rest is lost to the outside.
If you allow 30m³/h (18 cfm) per person in the household, then with a four-person household you end up at 120m³/h (71 cfm), and then the heat exchanger can actually reach the 90 to 95% heat recovery rate advertised by the manufacturer. The fan’s energy consumption decreases exponentially (keyword: flow resistance / Bernoulli’s principle / exponential increase with flow velocity…), and indoor humidity won’t drop drastically even without an enthalpy exchanger.
The best option is to program daily schedules with increased ventilation during mealtimes, shower times, and so on. Also, week-day dependent settings are useful. Our Vitovent 300-W offers many configuration options. The airflow rate for each ventilation step can be easily programmed through the service menu—just check the manual.
Currently, my daily average (!) is only 0.2 air changes per hour or 85m³/h (50 cfm), and CO2 levels are no problem throughout the day. Something that might also help some people: Adjust the supply and exhaust vents so that airflow matches the room size and duct length. Our installers were not very precise here and opened everything roughly the same distance based on feel, resulting in most circulation happening between the living room, utility room, and kitchen, while the bedrooms saw hardly any air exchange…
Mottenhausen schrieb:
Our installers were not very precise and more or less adjusted everything to the same opening based on feel, resulting in most of the circulation occurring between the living room, utility room, and kitchen, while there was hardly any air exchange in the bedrooms... Our Zehnder system was calibrated by Zehnder themselves for about 2 hours. A good approach.
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ludwig88sta9 Jan 2020 10:07lesmue79 schrieb:
To prevent mixing of airflows, usually a KVS is used, but it often exceeds the (financial) limits. Could it be that KVS is not offered for single-family homes for this reason? I haven’t seen any information about the prices of a KVS. Do you have specific figures or experience?
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Mottenhausen9 Jan 2020 23:24Lumpi_LE schrieb:
More important than the air exchange rate (which probably very few can actually calculate) is the air quality, and the best way to measure that is by the CO2 level.But even here, a lot of unnecessary alarm is being raised. If the user struggles to convert percentages to ppm and therefore has no proper sense of the measured CO2 concentration, it doesn’t help much.
- Lethal: 8% = 80,000 ppm
- Safe: anything below 2.5% = 25,000 ppm
- "Like fresh air": anything below 0.15% = 1,500 ppm
- Fresh air 0.04% = 400 ppm
Even in a completely sealed bedroom with two people and the door closed, CO2 levels rarely exceed 2,500 ppm by the end of the night. So, even then, you’re only at about 10% of a concerning CO2 concentration. If the door is even slightly open, levels stay below roughly 1,500 ppm.
But of course, the construction industry comes out with a completely unrelated study claiming that in a 4 m² (43 ft²) bedroom, headaches from CO2 exposure occur after just 2 hours, and you absolutely need to consider active ventilation.
I’m generally in favor of mechanical ventilation systems, which is why we have one. However, people shouldn’t panic and run it constantly at maximum power, causing dry air, high electricity consumption, and significant heat loss just to force some hypothetical air exchange rates, keeping the CO2 meter as close to 400 ppm as possible.
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