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Thread: Do I control intake or extraction fan with variac?

  1. #1

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    Default Do I control intake or extraction fan with variac?

    Hi, been a while but I'm just putting the finishing touches to my loft set up and I have a techie question regarding extraction control.
    I have a 6 inch extraction and 4 inch intake (plus two passives I can block up if required). Before I can afford a proper fan controller set up I've bought a variac and a few thermostats.
    Should I...

    Use the variac on the intake fan so it slows down when temps are lower.

    Use the variac on the extraction fan so it slows down when temps are lower (possible negative pressure problem?).

    Use the variac on both fans which slows overall air flow when temps are lower.

    The variac is rated well above what both fans are rated at so the last option isn't a problem.

    I'm trying to avoid having the fans kicking in and out all the time so I've designed a circuit which will potentially give me three stage control on intake (full, half, off) and two stage on extraction (full, half).

    Just after opinions really if anyone has experience of controlling temps in this fashion, what set up worked best for you.
    Let's get it on!

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    I have always had a fully sealed air tight room so i can use an over sized extractor fan run real slow until needed and a passive inlet (no fan). Creates a strong negative pressure in the room and makes sure nothing escapes. Couldn't open my door till i turned it down at one point!

    If your in a tent or something that can get sucked inwards or isnt as well sealed then thats another story.
    Last edited by channel; 22-09-18 at 01:38 PM.

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    Both unless you want varying negative pressure

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    Quote Originally Posted by gardro View Post
    Both unless you want varying negative pressure
    What would be the issue with having a varying negative pressure. So long as it was always negative?

    I guess the main concern is that if the neg pressure drops significantly then smell could get out? If that's your concerned you could add an inline variable inlet like a back draft or damper flap to keep the negative pressure a bit more constant.

    Just imo. The benefits of exhaust only in a sealed room are numerous. Fewer parts to go wrong. less initial cost and overall running cost (not that it ever was a lot to begin with). Quieter (when running normally at 25-50% power). Pulls everything into the carbon filter.

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    Quote Originally Posted by channel View Post
    What would be the issue with having a varying negative pressure. So long as it was always negative?

    I guess the main concern is that if the neg pressure drops significantly then smell could get out? If that's your concerned you could add an inline variable inlet like a back draft or damper flap to keep the negative pressure a bit more constant.

    Just imo. The benefits of exhaust only in a sealed room are numerous. Fewer parts to go wrong. less initial cost and overall running cost (not that it ever was a lot to begin with). Quieter (when running normally at 25-50% power). Pulls everything into the carbon filter.
    I was going to type sommet very similar. Literally.

    Negative is negative innit? If you're negative, then it's still being drawn through the filter?

    Is it beacuse the available through-flow of air is lowered?

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    Quote Originally Posted by channel View Post
    What would be the issue with having a varying negative pressure. So long as it was always negative?

    I guess the main concern is that if the neg pressure drops significantly then smell could get out? If that's your concerned you could add an inline variable inlet like a back draft or damper flap to keep the negative pressure a bit more constant.

    Just imo. The benefits of exhaust only in a sealed room are numerous. Fewer parts to go wrong. less initial cost and overall running cost (not that it ever was a lot to begin with). Quieter (when running normally at 25-50% power). Pulls everything into the carbon filter.
    This only applies if you have and intake and exhaust ( as per OP )

    Given the generality of the question, i replied in general terms.

    In a sealed room ( not sealed since you have at least 2 holes when the door is closed but i know what you mean ) the same would still apply.

    Imagine if your room had no leaks whatsoever with intake + extract fans, as you slow down the extract but kept the intake constant, at full speed, what would happen to all that air.....? The extract would be forced to spin faster. No need for the variac in the first place....

    This is without the smell being mentioned

    Hope that explains

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    Ok i get what you were saying now, I thought it was a responce to my suggestion of using the exhaust fan only and a passive inlet. I've not been on forums for a while it's obvious now your comment was a reply to OP not me.

    You were saying OP should set both inlet and exhaust on a variac or OP's pressure will vary and the fans will be inefficiently working against each other and could even create positive pressure in the grow space and possibly push smells out.
    Last edited by channel; 22-09-18 at 08:19 PM.

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    Thanks for the input fellas. My room is a wooden framed square shape (about 1.7M x 1.7M) with a pitched roof which follows the slope of the house roof if that makes sense. It's about as air tight as I can make it but not 100%, it's fairly well insulated though as is the loft space.
    My thinking with controlling both intake and extraction is I'll always have negative pressure if I reduce the speed of both fans by the same amount. The passive holes can be blocked up as well obviously, they are only there as a just in case thing. With lights off I'm guessing it's going to be cold up there so again I'm thinking reducing air flow will stop more cold air coming in. I have a heater on hand btw for warming the room up.
    Another option would be to just have passive intakes and regulate the extraction only. It's obviously going to be a bit of a learning curve until I get the right balance, I was just wondering if anyone else had come across a similar situation and what was the best (most efficient) set up.

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    Quote Originally Posted by Beezerk View Post
    Thanks for the input fellas. My room is a wooden framed square shape (about 1.7M x 1.7M) with a pitched roof which follows the slope of the house roof if that makes sense. It's about as air tight as I can make it but not 100%, it's fairly well insulated though as is the loft space.
    My thinking with controlling both intake and extraction is I'll always have negative pressure if I reduce the speed of both fans by the same amount. The passive holes can be blocked up as well obviously, they are only there as a just in case thing. With lights off I'm guessing it's going to be cold up there so again I'm thinking reducing air flow will stop more cold air coming in. I have a heater on hand btw for warming the room up.
    Another option would be to just have passive intakes and regulate the extraction only. It's obviously going to be a bit of a learning curve until I get the right balance, I was just wondering if anyone else had come across a similar situation and what was the best (most efficient) set up.
    If you get a dual fan controller it will take care of this for you.

    By undersizing the intake fan you will almost always maintain a good vacuum.

    Passive works fine but you need to ensure the intake hole is big enough for flow when extract is at full speed, but small enough to maintain smell control when speed is turned down

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    Which controller is recommended? I did have it written down but I can't find the notebook lol.

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