Gathering ideas for humidity based fan / ventilation control

[nctnico]

I'm contemplating fitting my home with a heat-recovering ventilation system as a replacement for the simple ventilation system which just pushes warm air out of my home and pulls cold air in. One of the goals is to lower humidity coming from the bathroom and drying laundry in the attic. Right now I need to keep windows open throughout the year which causes quite a bit of extra heating costs.

Now it is possible to buy heat recovering ventilation units which have a built-in function to work ventilate more the indoor humidity rises based on measuring the humidity in the air that is sucked into the unit. The problem however is that these are far from ideal where it comes to the form factor. I looked at nearly 20 different units but only found one unit which would be an possible physical fit. But this model doesn't have the ability to control the fan based on incoming humidity. It has 3 manual fan speeds and thats it.

Now I'm wondering how difficult it would be to make something as a one-off project to make the ventilation unit work smarter by itself. I'd envision a setup with 2 humidity sensors. 1 sensor measure the air blown into the house, 1 sensor measures the air coming from the house. When the humidity coming from the house is much higher than going in, switch the fan to a higher speed until the humidity drops again. Or is this too complicated and would comparing a long term average to the current value also work?

Does anyone have hands on experience with something like this?

[brucehoult]

Right now I need to keep windows open throughout the year which causes quite a bit of extra heating costs.

If your need is for heating, not cooling, at the same time as lowering humidity, then why not use a dehumidifier?

I've been using small portable units for several decades. For a long time I was using De'Longhi, but both units I had finally died (after maybe 20 years of 24/7 use from mid April to mid November!) and I now have a Panasonic F-YCL27N which is actually quite a bit more efficient. Both the De'Longhi and the Panasonic put out about 900W of heat, the old De'Longhi used 330W of electricity to do that, the new Panasonic 280W (measured). It's the start of winter still, I was away all of April and May and there was summer before that so the house walls and furniture has quite a high water content. So it's currently (after exactly 1 month of use) outputting about 12-15 liters of water a day. That will decrease by maybe half as the house and contents equalise to the ~50%-55% I keep the air at (it's usually 70% to 90% outside). That takes about 3 months of 24/7 operation.

The input power -> output heat ratio is not as good as a heat pump with indoors and outdoors heat exchangers (they push 4:1) but it's still pretty good. That unit currently costs NZ$732 (US$445).

[Messtechniker]

Just a hint: you need to compare the absoluty humidity (g/m3) or the dew point and not the relative humidity, for your ventilation to work properly.

Just search for "arduino" "dew point"

[brucehoult]

Just a hint: you need to compare the absoluty humidity (g/m3) or the dew point and not the relative humidity, for your ventilation to work properly.

Yup ... all we pilots are familiar with this :-) Every ATIS and METAR lists dew point, not other measures of humidity.

I don't know what electronic sensors natively measure. I'd have thought it's *not* RH, but they have a temperature sensor included and do some calculation, but I don't know the physics of an electronic humidity sensor. Need barometric pressure too, if a conversion (in either direction) is needed. Of course MEMS sensors for that are almost as cheap as temperature these days.

[nctnico]

Right now I need to keep windows open throughout the year which causes quite a bit of extra heating costs.

If your need is for heating, not cooling, at the same time as lowering humidity, then why not use a dehumidifier?

That is an option and I already have airco units which can de-humidify but I also want to have fresh air going into my home without losing too much energy.

[Messtechniker]

Just a hint: you need to compare the absoluty humidity (g/m3) or the dew point and not the relative humidity, for your ventilation to work properly.

Yup ... all we pilots are familiar with this :-) Every ATIS and METAR lists dew point, not other measures of humidity.

I don't know what electronic sensors natively measure. I'd have thought it's *not* RH, but they have a temperature sensor included and do some calculation, but I don't know the physics of an electronic humidity sensor. Need barometric pressure too, if a conversion (in either direction) is needed. Of course MEMS sensors for that are almost as cheap as temperature these days.

So the BME 280 sensor would be for you. Does relative humidity (rh), temperature (temp) and barometric pressure.

Must calculate dew point fron rh and temp.

As to operating principles, see here:

https://www.electronicshub.org/humidity-sensor-types-working-principle/