Originally Posted by Ko_deZ
What about smell food smell? Gas leaks? Should the ventilation run at maximum when there is a fire? Temperature? Certainly, a non-humidity recovering system like the window unit mentioned earlier should not be running if the outside temperature is below freezing since there would be a huge buildup of ice in the outgoing path. That would actually have to measure outside humidity as well, as that is an important factor in the efficiency of the HX and would determine the lowest temperature possible.
There are too many factors for this to be a good system. Sure, efficiency will be higher, but how much higher than >90% will you get? If you manage to cut the energy loss in half again, that sounds like a lot, but if your energy loss was just a few watt-hour anyway? Is it worth significantly reducing the experienced indoor climate (smell) because there is only one person in the house?
Also worth considering is that you without a ducted system will have to have one in each room, making the one in the bathroom much more troubled with icing up. Having a normal fan vent in this room would be throwing away a whole lot of energy since the hot and humid air during and after showering contains a whole lot of energy.
If your ceiling height is high, then it is very efficient to pull out energy from the high area and regenerate the heat to other parts of the house.
Not having a close to average temperature in your house will possibly cause moisture problems in your wall. The humidity-block (directly translated from norwegian. We use plastic for this) is inside the wall, thus at a cooler place than the inside air. Humidity has a tendency to even out troughout the house, but relative humidity will be higer in cooler rooms for this reason. If the humidity then also spreads into the walls, as it does, it might hit dew point before meeting the humidity-block in your wall. So a evenly heated house is important in cold climates.
A ducted system also keeps a positive pressure on the incoming air, and a negative on the outgoing as long as you have set it up correctly. Total should be a small negative pressure in the house. This will avoid pushing moist air out trough holes in the humidity-block, avoiding dew and humidity damage. A per-room based system must also have pressure sensors on both indoor and outdoor to make sure it does the same.
This is becoming a whole lot of work, with a whole lot of possible errors and bugs, and all it does is save a tiny bit of energy. Yes, I agree, make it as good as possible, but also consider the amount of time and effort, and investment, compared to the gain. I considered making my own system too, but I decided that it was not worth it.
Example of my house:
93% efficiency, 300m³ per hour, 20C inside 0C outside.
The outgoing air will have a temperature of 0+(20*0.07) = 1.4C. Difference is 1.4K
The specific heat capacity of air is about 0.001297J/cm⁻³k⁻¹.
My energy loss per hour is 1.4K*300000000cm³*0.001297J/cm⁻³k⁻¹ = 544J. That is not very much. 1W for one hour is 1J/s*3600s = 3600J.
If I run my ventilation unit at minimum, I have 150m³, and at max, 600m³. It would still not be a whole lot at max. Actually the fans use more power than that.
I suggest spending time and money insulating and swapping windows instead.
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