How to control a dehumidifier?

[Chris Hogg]

But can you control dew point directly? I think not.

Dew point is a function of absolute humidity (the amount of water vapour per unit mass or unit volume of air, sometimes quoted as the partial pressure of water vapour in the air). Only one parameter influences the dew point - the humidity. At any given humidity, when the temperature falls low enough for the air to become saturated, condensation will occur. That is the dew point (it's a temperature) for that degree of absolute humidity. The only way to control the dew point is to control the humidity. To control the humidity you need a dehumidifier. Whether that dehumidifier uses relative humidity or absolute humidity for its control, is neither here nor there. AIUI most, if not all, dehumidifiers control on relative humidity and do a perfectly acceptable job.

Of course, you can vary whether or nor condensation occurs by varying the temperature, keeping it above the dew point for that degree of humidity, but that is not controlling the dew point.

[zall]

Corse you can by dehumidifying the room.

But you can obviously change the humidity with a dehumidifier.

And that is what he is using.

It is when you want it to do what it needs to do automatically rather than having to be turned on and off manually as required.

His does too, but he wants to avoid having to turn it on and off as required.

Not possible with his outside walls.

But turning the dehumidifier on does.

[Animal]

Then you'll get mould. Keep it down to 65% to avoid mould.

[Animal]

Funny how people like to use computers where 2 contacts would work. Put contacts on the window glass to detect condensation directly.

[Animal]

under £2 last time I got them. Ebay.

[zall]

Not in this case

MUCH better to calculate if condensation will form there before it happens and turn the dehumidifier on earlier.

As you corretly point out, you need to the RH below 65% to avoid mould and so a computer to consider both would give a much better result whenj they are so cheap with an Arduino.

[David Wade]

I suggested that further down

Dave

[David Wade]

Sensors yes, pre-built meters with dials knobs and readouts about £30.

[Animal]

To do that you'd need to measure RH, room temp & glass temp & use a computer to calculate. Way easier to just sense window condensation.

It's pointless overkill. If you have near zero window condensation you'll be ok.

[zall]

Yes.

But you get a much better result when condensation never forms.

And it isn't just glass that gets condensation in that situation.

No.

It isn't just windows that get condensation, particularly with double glazed windows.

MUCH better to never have any condensation anywhere.

[Animal]

Obviously if your windows get nearly zero condensation you don't get condensation anywhere else. Only Rod could fail to work that out.

[zall]

Depends on whether the windows are double or triple glazed and construction of the outside wall and whether it is insulated at all.

[Animal]

Yeah, all those people with 4" walls & triple glazing beware.

[zall]

Or just double glazed. Its a lot easier to replace the windows than the external walls.