COB heat

[NanoGadget]

A/C is exactly what im trying to avoid....lol

I have a vague understanding of VPD... so when you say too hugh VPD youre meaning not enough humidity for light/heat, which would mean too low VPD would be the opposite?

Also... wouldnt 70ish RH be a little high in bloom, risking mold?

70% is way too high in flower. I keep my RH at 45% or so during flower with no issues.

 

[Randomblame]

VPD means vapor pressure difference. The lower the humidity is -at a certain temperature- the more water will be absorbed and transpired thru the stomata. The 70% RH are recommended for plants in the early and mid vegetative state; in bloom 60% are okay and in the 2nd half around 55%. But there is a way to deal with it..
Imagine the following:
When the VPD is too high the plants take up more water and therefor also more nutes; especially calcium. But too much calcium uptake blocks magnesium and within a few days you'll see a magnesium deficit in the upper half of the plants(yellowing between the veins, chlorosis).
it is often confused with bleaching from too much light.
Let's say the VPD is 20% too high you can do two things. The best way is to simply increase the humidity but sometimes it is impossible for some reason(risk for bud rott, hot summer, ..). In this case you can also lower the nutrient strength by 20%. Plants can now use more water but would still uptake the same amount of nutes.
Usually its easier to deal with a high VPD when you use a hydro system. In soil you have less control over the EC unless you use light mix.
I'm using the VPD charts below and use only a dehumidifier in the 2nd half of the flowering stage and only at night.
60% with lights on and <55% at night and it works great.

 

[Gquebed]

VPD means vapor pressure difference. The lower the humidity is -at a certain temperature- the more water will be absorbed and transpired thru the stomata. The 70% RH are recommended for plants in the early and mid vegetative state; in bloom 60% are okay and in the 2nd half around 55%. But there is a way to deal with it..
Imagine the following:
When the VPD is too high the plants take up more water and therefor also more nutes; especially calcium. But too much calcium uptake blocks magnesium and within a few days you'll see a magnesium deficit in the upper half of the plants(yellowing between the veins, chlorosis).
it is often confused with bleaching from too much light.
Let's say the VPD is 20% too high you can do two things. The best way is to simply increase the humidity but sometimes it is impossible for some reason(risk for bud rott, hot summer, ..). In this case you can also lower the nutrient strength by 20%. Plants can now use more water but would still uptake the same amount of nutes.
Usually its easier to deal with a high VPD when you use a hydro system. In soil you have less control over the EC unless you use light mix.
I'm using the VPD charts below and use only a dehumidifier in the 2nd half of the flowering stage and only at night.
60% with lights on and <55% at night and it works great.

View attachment 4255758

Thanks for the info

 

[burnpile]

I can never stay within the chart, with lights on at 84 deg it's about 40%, then during the day with lights out and a 200w heater it's about 67 deg and 65%. It does vary with the outside conditions. I always have used mild nute strength so sounds like that helps.

 

[Rahz]

So Im going to hydro. I plan to put 1300w of COB light (cree 3590s) over two 4x4 flood tables, in a room that is 11X11, 7.5' high. So i did a test... just fired up the lights to see how fast the entire room (completely empty right now) would heat up. In 2.5 hours it hit 85f.

I was hoping to cool the room with a 6" 440 CFM fan, filter and a temp switch (perfectly straight shot out a window, total run about 5').

Think thatll be enough CFM?
11x11x7.5 = 900 cu ft. / (440× 80%) = full room air change in 2.5 mins???

Have you bought yet? And any specific reason you're using 3590s?

 

[Gquebed]

Have you bought yet? And any specific reason you're using 3590s?

Ya....
I bought em off you a 1.5 years ago. 4x T4 2100s. Lol
I used them for a couple rounds. Went back to HPS for a crop then shut down for a basement reno. Now im back to em. I recently messaged you about a fan on one.