MVHR units work on a standby trickle plus boost as required, either manual or via sensor(s). I note that some forum members may cut the power completely to prevent trickle ventilation.
I recommend to use the trickle, particularly as that is when heat recovery is at its most efficient. Trickle ventilation alone would eventually clear bursts of humidity but you run a structural/aesthetic risk in the mean time if the humidity reaches near to the dew point. Mould doesn’t actually need condensation to grow as it can extract moisture from the air at high humidities. Therefore I also recommend using the boost when humidity exceeds 65% until in falls below 60%. Those are my own figures and not scientifically proven, although 60% is the recommended maximum domestic humidity. I know that many people trigger ventilation at 70% or above but I am a cautious individual.
My normal average extract humidity (whole house, trickle) is 49-51%
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This is the boost rate required for the extraction regardless of whether it is MVHR. I think that theoretically it could be less for MVHR as you are replacing extracted air with dry(er) warmed air but the regulations don’t recognise that.
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We’ve had our Kair 150 a couple of years now. We had a 100mm hole blocked up and a new hole drilled in a different wall. The drilling took 2 hours with our Accrington bricks!
My first observation is, we didn’t have a clue what was going on until we bought a cheap digital humidity meter. Now the unit’s seemingly erratic behaviour is easier to understand.
The main thing is that the humidity in the bathroom is not at all uniform. When I have a quick shower the other meter says 80% but the fan doesn’t come on. If I flick my towel around the room a few times I can usually encourage it to come on. I have noticed sometimes it cycles on, off, on, off. suggesting that it is reducing the humidity in one part of the room, but not elsewhere. After a while the humidity averages out and the unit comes back on. It is at one end of a room approx 3x2.5metres. Or perhaps the stat is detecting the lower humidity in the incoming air.
It is quiet enough in trickle mode not to worry about closing the door at night.
Even though we may be replacing it with full MVHR within 3 years of installing it, I’m not sorry we had it installed. It means I can push ahead with Airtightness measures in the bathroom now without worrying about the humidity.
I too wondered about how the seasons would impact on operation. In winter the outside air is cold and humid. The unit warms up the air and reduces the relative humidity so that the incoming air is less humid than the internal air it is replacing.
But when it is the same temperature outside and in, the incoming air may not be any less humid than the air you are expelling. Sometimes the unit comes on spontaneously throughout the day, unrelated to human activity in the bathroom. I have increased the humistat level for the warmer seasons in previous years. This winter I may have just left it at a higher level through the winter too.
The fan became noisy after just less than a year, Kair sent a replacement under warranty.
This is a vital piece of information for others thinking of getting a single room MVHR. Hassles over warranty are best avoided.