Humidistat working backwards?

[Captain Scarlet]

Hi, we have a village hall type building, brick built with a corrugated metal roof and suspended ceiling.

We suffer from condensation dripping down from the metal roof and damaging the ceiling tiles. A few years ago we fitted extractor fans in the roof space set on a timer to run overnight during winter months. This addressed the problem but is very expensive to run as they draw 2kW for 10 hours every night.

We recently had humidistats fitted, the idea being that the fans would only run when humidity and the risk of condensation is high. From what I now understand it is all a bit more complicated than that, and the Relative Humidity setting must be varied according to the outside temperature.

However, even accounting for that, they are not working as expected. In particular, during the recent spell of cold weather, the fans were not running enough, and my colleague found that to get them to run more we had to raise the RH setting on the humidistats, from 80% to 90%. This doesn't seem to make sense, I would have thought lowering the setting would turn the humidistat "on" more.

Have I totally misunderstood or does it sound like they have been fitted incorrectly? I haven't yet asked the electrician who fitted them as I would like to understand the issue better first before doing so.

Many thanks

 

[AndyPRK]

They usually get adjusted backwards to what you would expect

 

[EFLImpudence]

and my colleague found that to get them to run more we had to raise the RH setting on the humidistats, from 80% to 90%. This doesn't seem to make sense, I would have thought lowering the setting would turn the humidistat "on" more.

You are correct.

That IS the wrong way round - so can only assume something is not working or contacting as it should.

 

[rsgaz]

They probably have a "normally open" and a "normally closed" contact. The wrong one has been used.

 

[JohnW2]

Hi, we have a village hall type building .... We suffer from condensation dripping down from the metal roof and damaging the ceiling tiles. .... We recently had humidistats fitted, the idea being that the fans would only run when humidity and the risk of condensation is high. From what I now understand it is all a bit more complicated than that, and the Relative Humidity setting must be varied according to the outside temperature. .... However, even accounting for that, they are not working as expected. In particular, during the recent spell of cold weather, the fans were not running enough ...

I think you may have highlighted the conceptual problem, namely that 'consumer' humidistats are probably not all that good at measuring relative humidity (RH).

Condensation will occur if the temperature falls below the 'dew point' for the amount of water vapour/moisture in the air (absolute humidity). When the temperature is equal to the dew point, the air is totally saturated with water, and that is the situation of "100% RH" (temperature equal to dew point).

In the normal bathroom situation, the temperature is fairly constant ('room temp') and the risk of condensation arises when, at that temperature, the amount of water vapour in the air rises - so a humidistat which senses the rise in (absolute) humidity level will do roughly what one wants, even if the 'correction for temperature' (to get RH).

In your village hall, the situation is probably roughly the opposite of this. I would imagine that the (absolute) humidity level is probably fairly constant, and that the (condensation) problem arises when the temperature falls below the dew point for that prevailing level of (absolute) humidity. If that is the case, it could well be that you would do better by having the extractor controlled simply by a thermostat, set such that the fan comeson when the prevailing temp falls below the level at which (as you have observed by experience) condensation becomes a problem.

... and my colleague found that to get them to run more we had to raise the RH setting on the humidistats, from 80% to 90%. This doesn't seem to make sense, I would have thought lowering the setting would turn the humidistat "on" more. Have I totally misunderstood or does it sound like they have been fitted incorrectly? I haven't yet asked the electrician who fitted them as I would like to understand the issue better first before doing so.

As you say, that sounds back-to-front. However, it's not the first time I've heard people complaining that the marking of settings on a humidistat are confused/confusing (or just plain wrong!) - but, in practice, one can easily (as you have done) determine which direction one has to 'turn the knob' to get it to switch on!

However, as has been suggested, it's very probable that it has simply been 'wired incorrectly', which could be very easily rectified.

Kind Regards, John

 

[Captain Scarlet]

Thanks all,

It does sound like it's been fitted backwards but John your suggestion we would be better off with a thermostat, as what we are actually battling is lower temperatures in the roof space, rather than varying levels of humidity in the body of the building, makes sense. I might not go back to the same electrician though!

 

[JohnW2]

Thanks all, It does sound like it's been fitted backwards but John your suggestion we would be better off with a thermostat, as what we are actually battling is lower temperatures in the roof space, rather than varying levels of humidity in the body of the building, makes sense.

Indeed. As I said, it's the opposite situation from the familiar one in a bathroom. With a bathroom, there is an essentially constant temperature, and condensation problems arise when humidity increases - so control of the extractor by means of a humidity sensor is logical. With your hall, there is an essentially constant humidity, and condensation problems arise when temperature falls - so, in this case, control of the extractor by thermostat would seem logical, and probably more likely to 'work'

I might not go back to the same electrician though!

I wouldn't be too hard on him/her. Since it's hard to think of why/when such functionality would be desired, I suspect that many of us would not even consider the possibility that there was a way of wiring it (if that's what he/she has done) that would cause it to 'switch on' when humidity fell. I suppose that what's a bit less forgivable is that he/she presumably didn't 'twiddle the knob' to see when the fan came on and off (aka 'testing'!) - since doing that would have revealed 'the problem'!

Kind Regards, John