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.

[Chi Ma]

Plant the black lead of your multimeter on a neutral (white wire or ground) Then your red probe will tell you where the voltage is.

[Bill Moore]

Okay... let me make sure I understand what you're saying.   The wire coming in the outlet has three wires:  black, white, greenish blue.    I connected the white wire to one of the terminals on the thermostat. I wired one of the leads from the heater element to the black wire. I taped off the ground wire.  Photo to follow.  

So, I will connect the black lead from the multimeter to the green ground?  Where do I place the red lead/probe?

[Bill Moore]

Here is internal wiring of the thermostat.

[TCLynx]

Voltage is the potential or difference.  So when the element is off, there is no connection and therefor the difference is at it's greatest.  When you turn the element on and measure across it, you won't get a difference because the current is flowing through the element, or what you will measure is the voltage drop due to the resistance of the unit if there is much.

At this point I think you may be better off getting some help or doing much more reading and research before wiring anything up.

[Bill Moore]

No kidding!   Why do you think I posted this?  ;-)

[TCLynx]

I'm just a little worried for you getting info on a forum for something like this.  As in I'm say please be very careful!!!!!

I'm a little worried that the heating element seems to be wired to the Hot and the switch box is on the neutral side.  I'm used to seeing the switch hooked up on the hot side, that way there is a minimal amount of added safety in case the longer wires over to the heating element get cut, at least they are on the neutral side of the switch so if it is off when the accident happens then tragedy perhaps averted.

I think we have already had some one melt down plastic fittings trying to make a home made fish tank heater using hot water heating elements.  Some one else tripped the breaker and killed some fish for lack of power to the pumps and air pumps.

Heaters draw a lot of current and need heavy duty wiring and ample circuits to power them.

Now does your meter have an ohm meter?  With the unit unplugged from power.  You might measure across the "thermostat" and see how the resistance changes while you turn the knob.  I'm guessing that the "thermostat" is actually just a big adjustable resistor or potentiometer (if there is a knob on the other side of the box.)  Probably, the higher the resistance is set, the lower the temp you will get from the heating element.  Now if it is simply an on/off switch in that box well then not much control to be had and you would need to get an additional temp controller to plug it into.

[Patti Taylor]

I have a 2k element in mine working since nov with a pex programmable thermostat.You might want to search the forum, Rob nash has good plans for a diy heater.@ 2kw hooked up 120v your pulling close to 17 amps, so use wire rated for 20 amps(12 ga),make sure your circuit breaker is sized accordingly.Think safety there is water involved... might be a good idea to hook it up to a gfci circuit.Go with a stainless steel if you can as other elements are coated and contain copper, which I hear can be detrimental to fish.Looking at picture above: it would be a good idea to get some romex connectors so your wires don't get yanked out.I'm with TC on this.... Please be careful

[Bill Moore]

Alex...  thanks for the recommendations.  I am using 10 ga. wire.  The circuit is dedicated GFCI at 30 amps (twin 15A). 

Bill

[Bill Moore]

TC...  I rewired the thermostat.  Black wire now connected to thermostat. Neutral is connected to element.  

TCLynx said:

I'm just a little worried for you getting info on a forum for something like this.  As in I'm say please be very careful!!!!!

I'm a little worried that the heating element seems to be wired to the Hot and the switch box is on the neutral side.  I'm used to seeing the switch hooked up on the hot side, that way there is a minimal amount of added safety in case the longer wires over to the heating element get cut, at least they are on the neutral side of the switch so if it is off when the accident happens then tragedy perhaps averted.

I think we have already had some one melt down plastic fittings trying to make a home made fish tank heater using hot water heating elements.  Some one else tripped the breaker and killed some fish for lack of power to the pumps and air pumps.

Heaters draw a lot of current and need heavy duty wiring and ample circuits to power them.

Now does your meter have an ohm meter?  With the unit unplugged from power.  You might measure across the "thermostat" and see how the resistance changes while you turn the knob.  I'm guessing that the "thermostat" is actually just a big adjustable resistor or potentiometer (if there is a knob on the other side of the box.)  Probably, the higher the resistance is set, the lower the temp you will get from the heating element.  Now if it is simply an on/off switch in that box well then not much control to be had and you would need to get an additional temp controller to plug it into.

[Eoin Ross]

Whew - Glad to see the hot is not switched now!  I see from your profile that you're in Nebraska. So I'll asume that you're in the USA :)

Is this thermostat and heater being plugged into a 120V circuit - a plug like what a toaster or computer would go into? I'm guessing that will be the case.

You DEFINITELY must make this a GFCI receptacle - yours, and the fishes safety, and the national electrical code will require it. 

The greenish blue wire (Should be more green than anything) will be the ground wire and would connect to the ground on the plug. - it will be the partly rounded pin. This will need to be continuous to the body of the heater element and a grounding connection into the water - maybe a stainless steel bolt. This connection doesn't do anything normally, but will trip the ground fault or breaker in the event of a short to ground. Heater elements can corrode and the live parts can contact the water.

The hot wire is black, and in a polarised plug is conneccted to the narrow flat pin. this is the one that should have any switching (The thermostat) in it. Once the switch opens, any wires between the switch and the white wire will be at ground potential, connected through the heater element.

The white wire is called the grounded conductor in the code, and will be connected to grround at the service entry to your house - and there should be no other connections to ground in the 

[Eoin Ross]

Carrying on... #(%U(#$ windows ....

The white wire is called the grounded conductor in the code, and will be connected to ground at the service entry to your house - and there should be no other connections between the neutral and the ground rod in the building.

Please read (Pretty pictures too) the following links a few times and feel free to ask questions!

Bonding and grounding requirements of pools Grounding vs Bonding

A ground-fault circuit interrupter (GFCI) is the only protection device designed to protect people against electric shock from an electrical system. GFCI Basics

The NEC specifically bars switching the grounded (neutral) conductor or the grounding conductor Article about switches

[Eoin Ross]

Here is an image from another article showing a method of grounding the water... FAR better that the suggestion of a bolt above

Article: swimming pool bonding requirements

This article gives a required minimum of 9 inches square on the conductive area of that fitting.