3 litres from humidity overnight

[quanglewangle]

We have a small (2m x 3m) greenhouse, which we like to work hard (read crowded). Worried about botrytis I have started monitoring humidity (RH). It is generally a satisfactory 50% - 70% during the the day. Interestingly, regardless of whether the vent is open or closed. It rises to 90% - 100% overnight.

Not wishing to vent at night, well not until it gets warmer, and not as if it seemed to bring the RH down much in the day, I tried a dehumidifier.

Very interesting: dehumidifier yielded 3 liters of water in about four hours overnight before the tank filled and it only lowered RH from 90% to 75%. Able to tell because timed to start at 02:00 and could see the step in the graph. Will try it with a hose to drain tonight so it runs longer.

But so much water? Very surprised.

[burnie]

It's the principle that causes fog and mist during the night/early morning, heat in the air and ground releases moisture when the temperature drops, causing condensation.

[ameno]

Is this truly necessary? It's natural for relative humidity to increase when air temperature drops, as cold air cannot hold as much water. Relative humidity is a measure of vapour saturation, not of total water in the air. The amount of water in the air has not increased; it is the same as it was in the day.
What's more, I'm fairly sure botrytis need warm, damp conditions to thrive, but your night time conditions would be cold and damp.
Also, by removing water from the air and reducing the humidity you also increase the rate at which water is lost from your plants, meaning you will need to water them more to make up for it.

[quanglewangle]

Agree with ameno with all about the physics of RH but...

[...]I'm fairly sure botrytis need warm, damp conditions to thrive, but your night time conditions would be cold and damp.[...]

While the moisture is gaseous I suspect it does little harm: the problem occurs when it condenses on the plants ie overnight. When the sun comes up on the now wet plants is when the worst conditions are created.

Also, by removing water from the air and reducing the humidity you also increase the rate at which water is lost from your plants, meaning you will need to water them more to make up for it.

Exactly - but this is a good thing. This is the mechanism that brings food up into the plant. Google "Vapour Pressure Deficit"

[ameno]

Exactly - but this is a good thing. This is the mechanism that brings food up into the plant. Google "Vapour Pressure Deficit"

It's a good thing to an extent, but if evaporation ends up exceeding the rate at which you can provide the plant water, or the plant's roots can absorb water, then you have a problem.

[quanglewangle]

It's a good thing to an extent, but if evaporation ends up exceeding the rate at which you can provide the plant water, or the plant's roots can absorb water, then you have a problem.

Optimum vapor pressure deficit is well documented.

Next winter's project may be to control VPD automagically ... Oh dear...

[Jungle Jane]

You’re probably getting so much water/condensation because you’re using a heat mat in there I think I remember another thread,for the early tomatoes. It’s not a natural environment.

[quanglewangle]

You’re probably getting so much water/condensation because you’re using a heat mat in there I think I remember another thread,for the early tomatoes. It’s not a natural environment.

Probably right.

Not an actual heat mat but cables in damp sand so similar enough.
.
Of course, greenhouses in general are not natural environments

Dehumidifier set to 50% run for 4-5 hours overnight seems to fix the worst of morning condensation.