Reversing thermostat logic

Thermostatic control of exhaust fans is necessary in almost all grow ops. Thermostats, however, are not cheap- unless you know where to look!

Broken aquarium heaters have thermostats as do old waterbed heater controllers. Mad, free thermostats!

Problem is, they do exactly the opposite of what most growers need to do with a thermostat!

Thermostats from heating devices either increase resistance or open a set of contacts, breaking the circuit and switching something off, when temperature rises to the setpoint.

If you need to switch something on, such as an exhaust fan, when temperature rises to a certain setpoint, you need swap the logic. Here's how to do that, with the aid of a cheap relay and a 12V DC power pak transformer.

A 12V powerpak is used as relays with mains AC voltage coils are less available and more expensive than those with 12VDC coils. If you happen to have a SPDT relay with an AC mains voltage coil, just eliminate the powerpak from the drawing and connect the wires as though it were not there.



Add a 12V power pak and a SPDT (Single Pole, Double Throw) relay as shown. The relay contacts should be rated for your local line voltage at about 10 amps.

The thermostat contact closes when cooler than setpoint, turning on the 12V power pak. The Normally Closed (NC, closed when relay coil is de-energised) contacts then open, turning off the exhaust fan.

When no current is applied to the relay coil, as when the air temp is higher than the thermostat setpoint, the NC relay contacts are in the 'closed' or 'connected' resting position, completing the circuit to the exhaust fan.

A suitable relay should be $7-10. Any 12V DC powerpak can be used, hopefully one in your junkbox from some device you no longer have.

This information would really be useful for me if I understood it. You are using the 12v power pack to open the normally closed switch, which shuts off the fan. Guess I'll have to go out to the old work shop and experiment a little. These are the types of controls I need to start setting up. Keep posting, I'll catch up eventually. VV

VictorVIcious said:

This information would really be useful for me if I understood it.

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But you do!

You are using the 12v power pack to open the normally closed switch, which shuts off the fan.

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Yep, exactly! You've got it!

I'm sorry it looks complex, but a schematic is the best way I knew of to communicate a wiring diagram. I'll think about how to simplify the appearance of the drawing, but it seems to have gotten the message across- you figured it out straight away and a couple of things are happening in this dwg.

I bought one of those fans you can mount in line and a thermostat for it. The idea is to mount the thermostat on the outside of the ductwork, and drill a hole for air to hit the thermostat and start the fan when the air is warm enough or cold enough if you have it on air conditioning. The problem I had was that it never shut off once it started. 70 degrees in the room and the fan still runnning, which of course starts my heater going. I had check into thermostats before but none came with a transformer. It looks like the 12v power pack and coil are used instead with this system. I'll continue working on it tilI get it right. I looked at your bud dryer as well.

VictorVIcious said:

I bought one of those fans you can mount in line and a thermostat for it. The idea is to mount the thermostat on the outside of the ductwork, and drill a hole for air to hit the thermostat and start the fan when the air is warm enough or cold enough if you have it on air conditioning.

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Oh yeah, there's commercially made fan thermostatic controllers sold at my local hydro shops... but they cost a packet! Remember, I'm dirt cheap.

However, there's one they have which is specialised enough that I'd consider buying it instead of building a copy. This one I'm thinking of both controls exhaust fans and a dehumidifier. Shuts off the dehumidifier while the exhaust is running- no need to dehumidify air if it's going straight out the exhaust. It also has a time delay feature so the dehumidifier is not switched on and off too quickly. It takes several minutes for the little aircon unit inside most dehumidifiers to build up pressure and begin condensing moisture on their coils. It is hard on the compressor to switch it on and off unless it has been off for about 5 mins. The time delay solves that problem. The cost of the unit is less than I would put in to designing an equivalent of their time delay logic.

VictorVIcious said:

The problem I had was that it never shut off once it started. 70 degrees in the room and the fan still runnning, which of course starts my heater going.

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hmm... I would have thought the thermostat sensor itself would be placed at plant level instead of in the duct. If it's in the duct, it is constantly being warmed by the flow of air coming off the lights, ballast, etc. If it is at plant level, the warm air will tend to rise to the top of the room, losing a little heat energy by radiation. The fan should then turn on and off as the temp rises to the setpoint. I wonder if just moving the tstat won't solve this prob.

VictorVIcious said:

I had check into thermostats before but none came with a transformer. It looks like the 12v power pack and coil are used instead with this system. I'll continue working on it tilI get it right.

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If the relay used has a line voltage operated coil, you don't need the transformer. I only used a powerpak because my local (insert giant electronics chain name) shop only had relays with 12V coils.

If it's convenient to move that tstat, have a go and please let me know if that changes anything.

VictorVIcious said:

I looked at your bud dryer as well.

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Thanks. It does the job for me!

I'll let you know.VV

Al B. Fuct said:

If the relay used has a line voltage operated coil, you don't need the transformer.

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heh, I discounted the possibility that a commercially made thermostat unit would most likely use a thermostat with the correct temp coefficient- that is, closes contacts when the hi temp setpoint is reached. Wouldn't need a relay at all in those circumstances. No wonder you wouldn't have seen transformers on properly made commercial units.

My circuit is junxploitation- bodging up some otherwise useless piece of crap to give it a second life.

Ok, looks like I'm the only one having any problems so here we go. First, I live in Michigan. Too much heat is a problem for about a month in the summer here. I have a room that is about 10x20, concrete floor, insulated walls etc.
And I bought a gas space heater. Venting requirement is fresh air, and 1' away from walls. It looks like a fireplace. The sensor I believe it uses for firing up is in the back near the floor. This makes it less than ideal for control. I fight to keep a 10 degree fluctuation, constantly moving the knob up or down. So i want to control it with a Thermostat that I can place where I want it, up a little higher on the wall. I don't mind it being a little cold at floor level, its going to be anyway with a concrete floor. This unit has an electric circuit and an internal fan that starts when the unit fires up. If I unplug it the fan won't work (duh) but it will still fire up. This suprised me. Maybe you know how this works. I don't.
We are starting to get a few days warm enough that I will be able to go without the heat for a day so I can tear it apart and look at it. My idea is to in effect bypass the sensor, by putting it at a higher temperature setting than necessary, which will make it a closed circuit, always on. Then controling past that with a thermostat.
The exhuast fan is a separete issue. The control has a 4 position switch, off, air, heat and on. It has a dial for setting the temperature for turning on, and a minimum or maximum variation for turning off. I placed the fan in the attic and don't always hear when its coming on, so I'm going to plug a light into the circuit with it so I'll see when it comes on and goes off. I did put it on the wall level with the lights. I just have to take the time to mess with it. VV

Can you remove the thermostat itself from the heating unit and remotely mount it in a surface mount electrical wall box, extending the wires with some common lamp cord?

If you can't remove it and remote it, we need to find out how the existing thermostat works so we can find an easily remote-able unit.

There ought to be only 2 wires going to the tstat responsible for the on/off function. You'll need a little volt/ohm meter for this job, $10-20 at the hardware or Radio Shaft. Set it to OHMs function and touch the probes to the 2 tstat wires. Turn the temp knob until you hear it switch. Make note of the resistance value when the air temp is both above and below the setpoint. Most likely, it will open when the air temp is above the setpoint. If that's the case, you will probably be able to get a new tstat at the hardware or a local heating and aircon supply house.

Let me know when you decide if you can remote the existing tstat or if you need to replace it.