Having got my exhaust fan replaced, I'm now measuring the humidity within the exhaust port (close to the fan) of the Helios unit.

I'm seeing the RH never dropping below 80%. I do see it rise (to about 95%) when the shower is in use. General humidity is around 50% inside the house. Outside, about the same.

Does anyone else measure the humidity at the MVHR exhaust for comparison?

I'm going to move the sensor to the extract manifold and see what the RH is there.

Sounds like a heat exchanger doing its job to me!

if you have the temperatures inside and outside, and the temperature of the exhaust and the of the supply to the rooms, I am sure that someone can do the maths.

RH is relative humidity - the Relative being the important part.

Assuming my maths are correct, if you take 50% RH air at 21 degrees and cool through the heat exchanger to 15 degrees, the RH climbs to about 73% (but of course the absolute amount of water in the air does not change). What does change is the capability of the air to hold the water - hence lower temp can hold less water, hence RH is higher for the same amount of "wetness" if you like as the temperature drops.

Posted By: snyggapaif you take 50% RH air at 21 degrees and cool through the heat exchanger to 15 degrees, the RH climbs to about 73%

Yes that makes sense. I do see a correlation in the difference between incoming temp and exhaust temp to the RH.

I'll turn off the HE and see what happens to the RH.

Posted By: tonyYes, totally expected , indeed condensation forms on the outgoing heat exchange matrix in winter and there should always be a condensate drain to lead it away

Yes there is. A warning to others though is that the trap tends to dry up in the summer so needs a good clean out right now else you might get a flood!

So the bottom line is these fans are working in a fairly damp environment!

Ideally you want as much water to condense on the heat exchanger as possible.

When you ventilate the house to dry it, moisture evaporates from the fabric, which has an evaporative cooling effect. Like sweating, evaporative cooling is very powerful, due to the high latent heat of water vapour. You want some of that vapour to condense in the exchanger, to recover some of that latent heat.

If the outgoing air has condensation happening, it has more overall heat capacity than the incoming air, so ideally the mhrv would run unbalanced (more incoming than outgoing air) to match the heat capacites and maximize heat recovery. I don't know if any mhrvs are clever enough to do this....!

Posted By: borpinA warning to others though is that the trap tends to dry up in the summer so needs a good clean out right now else you might get a flood!

That's why I fitted a dry trap - a HepVO - plus it needs less vertical space.

Posted By: snyggapaI would have expected (but not checked) that the designers would put the fans on the "warm" side of the heat exchanger, so both on the "room" side of the unit to minimise the issue.

Me too, but my unit (Brink) has the fans on the downstream side of the heat exchanger, just like Brian's. It may be something to do with not overpressurising the heat exchanger I suppose.

The fans on my unit are plastic, so I expect the motors and bearings are not sharing the damp atmosphere, although I've never dismantled it.

It's possible to get enthalpy exchangers as plug-in parts instead of heat exchangers for some models of MVHR and they recover some of the outgoing moisture and deliver it to the incoming air (by being made of paper AIUI).

Some dehumidiers work by cooling the air until condensation takes some moisture out and then reheating it before returning it to the room. I think they use a heat pump to exchange the heat.