Our dehumidifier is a simple manual one so there is no humidistat or thermostat.
I know that condensation only occurs when dew point occurs which is a function of temperature and humidity, is there a dew-point controller I can buy rather than a humidistat as the humidistat will only switch on humdity levels and does not take into account temperature?
The dehumidifier we have is actually very good at what it does, it easily pulls over a litre of water out of the room every 24 hours in the deepest depths of winter.
In previous years I used to run it non stop once conld weather started and then turned off when cold weather ended
Given teh cost of electricity, I thought a dew point controller was a sensible thing to do.
You are right, he should have added 'above the dew point'. For example, air at -10°C will still hold ~4% water vapour (g.m^3). The absolute water vapour content (g.m^3) of air containing ~4% water vapour at 30°C (i.e. pretty dry air by any standards), won't change as the air is cooled until it reaches -10°C. However, the _relative_humidity_ (RH) will rise as the temperature falls to -10°C, at which point the RH hits 100%.
The issue is often where it occurs, often you need to take a lot more moisture from the air in cold weather if the problem is condensation on walls or window frames,since your device is not located in those areas to measure them locally. Bathrooms even those with fans etc, seem almost impossible to stop getting condensation. Brian
So turn it on when the temperature reaches dew point, to stop condensation?
That is equivalent to saying you will turn it on when RH reaches 100%. The dew-point is the temperature at which condensation occurs because the air is saturated with water.
Saying "the air is saturated with water" is another way of saying "the relative humidity is 100%"
Exactly ! Walls and windows especially in poorly insulated rooms will be significantly colder than the air the dehumidifier or sensor is monitoring. One is trying to reduce condensation on these cooler surfaces. RH% is not useful for this, setting a Dewpoint temperature of say 12C may be.
Or it may not. If the ambient is 22 and the dew point is 12 you are unlikely to get condensation. Why because the RH is around 53%
On the other hand if the dew point is 20 and the ambient is 22 you will get condensation on any cool surface, because there is a lot of water in the air, the RH is 88%.
The dew point tells you zilch about the probability of condensation. You need to know the likely hood the air will be cooled to that temperature.
what is important is the difference between ambient and dew point which tells you the relative humidity.
Agree, but if the aim is to economically reduce the risk or amount of condensation on cold surfaces then monitoring RH isnt an efficient way of doing it My poorly insulated Laundry is running at 17C 77% RH so a dew point around 13C .
No, you would have to monitor the temperature of the coldest surface and turn the dehumidifier on when it is getting close to the dew point temperature.
It would certainly be possible with your own system but not really feasible for a dehumidifier to do.
So we are back to RH% again because that is what a humidity meter reports. Most of the meters I have seen have a probe on a wire that can be placed near the coldest part of the wall or window, but they are expensive...
The air near the coldest part of the wall must be closer to dew point? Will it be good enough to place the sensor close to the window.
PI's are getting expensive so how about an Arduino? In fact some one has a project to do that sort of thing :-
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and the sensor is available on E-Bay for less that a fiver
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and mains relay boards are cheap
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as the sensor report ambient temp and rh% you can calculate the dew-point...
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so you could stick the sensor on the window, or even have multiple sensors on multiple windows....
Or would it be enough to simply detect condensation is occurring with something like too bits of alarm tape stuck to the window..
.. and now we have it sorted when to switch on, what about switching off...
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