Thermostat: Electrical Engineering Question

[Bill Moore]

Since the 500W aquarium heating couldn't get my 100 gal fish tank (+50 gal sump,  50 flood & drain and 16 ft NFT) higher than about 71F, I built DIY heater using 2000W electric water heater element and a Eaton thermostat for a hot tub, which came with no setting indications for even OFF/ON. 

I figured that I could use my multimeter to determine OFF/ON, as well as amount of voltage being used to regulate the heat level of the element, but what I discovered is completely counter-intuitive.  When I check the Voltage in what I think is the OFF position (the element is cold) I see 115-120V.  When I go to what I suppose is the full ON position, I see zero volts. It remains at zero volts nearly all the way back to the OFF position, only showing 115 volts in the last 15-20 degrees of arch. This is not at all what I expected.

Anyone here in the Community an electrician and familiar with how this type of thermostat works?  BTW... I have the 48" temperature sensor submerged in the tank.   

Thanks.

[Eoin Ross]

I have not been able to find a manual online for that temp controller. I have seen that its good for 25A (2kW @ 120V will be 17A so you're good there). It does appear to be an On/Off control type.

 

From your photo and posts afterwards...

• Cable from the plug has Black, White and Green
• There are two red wires from the heater element
• The black wire goes to one terminal of the thermostat
• A red wire goes to the other terminal of the thermostat
• The white wire is connected to the other red wire.

 

IF this is correct... the measurements...

• Put your multimeter on 200 or more AC Volt range
• Place the red probe in the white wire's wire nut
• Place the black probe on the thermostat's terminal that has the red wire on it.

 

You're now measuring the voltage across the heater - when the heater will is on, your multimeter  will read 120Vac

 

[Bill Moore]

Years ago I invented "Moore's Second Law."  The first law, of course has to do with computer processing power. Gordon Moore (no relation) created that one.  The second, my law, goes like this:  If there is a 50/50 chance to do something either right or wrong -- like which wire to connect to the thermostat -- there's a 90% probably of picking the wrong one -- in this case the black.  

Just to reassure everyone.  The circuit is a GFCI and installed by a professional electrician who helped me set up the wiring for the system, including upgrading the necessary breakers for my 400W  230V metal halide lights.  The only question is how am I going to ground the element (green wire).  I think I'll have him come back over and help with this.  Want to get it right and in code. That's what this whole adventure has been about: learning, experiencing.

[TCLynx]

Good to hear you have some one who can come in and help and/or double check your work for you.

[Eoin Ross]

Cool, it looked a bit iffy from the photo and jumped right on it - should've read a bit more. Don't want to see someone hurt/killed. :-)