6 ms·

You don't need AC to remove humidity. MVHR removes it and dumps it into the drains when the warm outgoing air goes through the air to air heat exchanger to warm

by willyt 9y ago

You don't need AC to remove humidity. MVHR removes it and dumps it into the drains when the warm outgoing air goes through the air to air heat exchanger to warm up the incoming air.

maxsilver 9y ago
Then where does the heat from your dehumidifier go? MVHR seems to assume outside air is always cooler and fresher than inside air, and uses that for free cooling. Which is great for like 70% of the time. But what about the ~30% of time when it isn't there.
- dangravell 9y ago
  It doesn't assume that. MVHR has a bypass to disable the heat exchanger if that's important. You don't have a dehumidifier. One of the biggest benefits of fabric first is the reduction in complexity.
  - falcolas 9y ago
    If you don't have a dehumidifier, all of those big bags of mostly water that inhabit the building will expel some of that water with every breath, and going to increase the humidity fairly noticeably. I'm also going to guess that the MVHR is also not 100% efficient. It's either going to create heat to completely cool down the incoming air (hey, look, an AC, complete with dehumidification), or the incoming air is going to average out with the out going air, creating an increase in temperature. Plus, all of those bags of mostly water are going to be expelling heat, too.
    - willyt 9y ago
      MVHR dehumidifies the air inside, I explained in another comment above. We have used units which are 96% efficient.
    - dangravell 9y ago
      MVHR is a ventilation system - it removes the humidity. A decent MVHR runs at double figures in terms of watts, so like an old fashioned light bulb.
- wheresmyusern 9y ago
  what are you talking about? who mentioned a dehumidifier? mvhr simply exchanges air without changing its temperature. its not great for cooling or heating by its very definition. its good for humidity and freshness. using insulation and hr ventilation simply allows you to have a heating and air conditioning solution that is massively reduced in size. in some cases none at all.
brudgers 9y ago
Warming incoming air is the opposite of what people in the US mean by "air conditioning". It seems to me that what you are describing is the opposite of how thermodynamics works. Warm air holds more moisture and condensation occurs when air is cooled below the dew point.
- willyt 9y ago
  I was responding to the point about using AC all year to remove humidity. The thermodynamics works like this: The air inside your house is more humid than the air outside; warm air at the same pressure holds more moisture than cold air. When you extract the warm air from the house the heat exchanger cools it down, therefore reducing its moisture carrying capacity and causing condensate to form which you can remove. The heat is transferred to the incoming cold air via the heat exchanger, the warmed up incoming air now has a much greater water carrying capacity even if it was already humid outside so it absorbs humidity from your house, when this air is extracted again the cycle repeats. Passive houses have very low humidity inside because of this.
  - maxsilver 9y ago
    > I was responding to the point about using AC all year to remove humidity. That's not something I claimed. Most people in the US are only using AC in the summer (or warm season), to remove humidity from their home when the humidity outside is much higher than indoors.
    - willyt 9y ago
      Sorry I was confused when you said “if not for the heat then to at least remove the moisture”. When your insulation is being used to keep heat out, your heat exchanger would operate in reverse to use extract air to cool incoming air, some MVHR units have optional precooling of incoming air. I expect a modern standard AC unit will be doing some cold recovery as well to improve efficiency. But ideally a fabric first approach will keep a lot of heat out when combined with external blinds and brise soliel to keep high angle summer sun out.
maxerickson 9y ago
The "even in Michigan" was carrying implicit information. The relative humidity outside right now it 51%. It's -10°C, the dew point is -18°C. So during the winter houses that aren't incredibly well sealed need humidification more than they need dehumidification. During the summer it gets quite humid even when the temperature isn't all that high.

by willyt 9y ago

You don't need AC to remove humidity. MVHR removes it and dumps it into the drains when the warm outgoing air goes through the air to air heat exchanger to warm up the incoming air.

maxsilver 9y ago

Then where does the heat from your dehumidifier go? MVHR seems to assume outside air is always cooler and fresher than inside air, and uses that for free cooling. Which is great for like 70% of the time. But what about the ~30% of time when it isn't there.

dangravell 9y ago
It doesn't assume that. MVHR has a bypass to disable the heat exchanger if that's important. You don't have a dehumidifier. One of the biggest benefits of fabric first is the reduction in complexity.
- falcolas 9y ago
  If you don't have a dehumidifier, all of those big bags of mostly water that inhabit the building will expel some of that water with every breath, and going to increase the humidity fairly noticeably. I'm also going to guess that the MVHR is also not 100% efficient. It's either going to create heat to completely cool down the incoming air (hey, look, an AC, complete with dehumidification), or the incoming air is going to average out with the out going air, creating an increase in temperature. Plus, all of those bags of mostly water are going to be expelling heat, too.
  - willyt 9y ago
    MVHR dehumidifies the air inside, I explained in another comment above. We have used units which are 96% efficient.
  - dangravell 9y ago
    MVHR is a ventilation system - it removes the humidity. A decent MVHR runs at double figures in terms of watts, so like an old fashioned light bulb.
wheresmyusern 9y ago
what are you talking about? who mentioned a dehumidifier? mvhr simply exchanges air without changing its temperature. its not great for cooling or heating by its very definition. its good for humidity and freshness. using insulation and hr ventilation simply allows you to have a heating and air conditioning solution that is massively reduced in size. in some cases none at all.

brudgers 9y ago

Warming incoming air is the opposite of what people in the US mean by "air conditioning". It seems to me that what you are describing is the opposite of how thermodynamics works. Warm air holds more moisture and condensation occurs when air is cooled below the dew point.

willyt 9y ago
I was responding to the point about using AC all year to remove humidity. The thermodynamics works like this: The air inside your house is more humid than the air outside; warm air at the same pressure holds more moisture than cold air. When you extract the warm air from the house the heat exchanger cools it down, therefore reducing its moisture carrying capacity and causing condensate to form which you can remove. The heat is transferred to the incoming cold air via the heat exchanger, the warmed up incoming air now has a much greater water carrying capacity even if it was already humid outside so it absorbs humidity from your house, when this air is extracted again the cycle repeats. Passive houses have very low humidity inside because of this.
- maxsilver 9y ago
  > I was responding to the point about using AC all year to remove humidity. That's not something I claimed. Most people in the US are only using AC in the summer (or warm season), to remove humidity from their home when the humidity outside is much higher than indoors.
  - willyt 9y ago
    Sorry I was confused when you said “if not for the heat then to at least remove the moisture”. When your insulation is being used to keep heat out, your heat exchanger would operate in reverse to use extract air to cool incoming air, some MVHR units have optional precooling of incoming air. I expect a modern standard AC unit will be doing some cold recovery as well to improve efficiency. But ideally a fabric first approach will keep a lot of heat out when combined with external blinds and brise soliel to keep high angle summer sun out.

maxerickson 9y ago

The "even in Michigan" was carrying implicit information. The relative humidity outside right now it 51%. It's -10°C, the dew point is -18°C. So during the winter houses that aren't incredibly well sealed need humidification more than they need dehumidification. During the summer it gets quite humid even when the temperature isn't all that high.