Adding "on" light to 240V thermostat

[DoubleDogDan]

I'm renting a place that has 240V radiant floor heat, basically a series circuit of resistive heating element cast into the concrete floor. The control is an adjustable bi-metalic single pole single throw on-off switch. When fully powered, the circuit pulls about 25 amps, as read by my somewhat accurate clamp on ammeter. I would like to add a small light (LED? Neon?) to the thermostat to indicate when the element is drawing current/the heat is "on". Since the power is 240, it's not just a simple matter of connecting one lead of a neon bulb to ground and the other to the switched side of the thermo, since both sides are "hot" and the light will be "on" regardless of the thermo's switch. Since the thing pulls a fair bit of current, would it be possible to build a simple circuit which would sense the current flow, perhaps by a few coils of wire wrapped around one of the high current leads to detect when flow is occurring & light an LED? I don't doubt thermostats exists with this obvious feature, but again I'm renting, plus my $$$ is low at the moment (which is why I'm obsessed with knowing when this cash-hogging heater is running), and I'd like to accomplish this as cheaply as possible. Any suggestions appreciated!

TIA

Dan

 

[Martaine2005]

Hello,
The simplest is a neon. It will light when the thermostat comes on. No extra circuitry needed.

Martin

 

[Minder]

Are you sure there is high voltage AC on the thermostat?
Usually low voltage, and Neon's that are made for appliance indicator have a minimum strike voltage.
M.

 

[Martaine2005]

As Minder poster above, an ammeter is not important with this.
The voltage at the thermostat is!
Your ammeter will only be useful if the LED, neon is in series with the circuit..

Martin

 

[DoubleDogDan]

This is not your typical forced air thermostat setup. This is a line voltage thermostat, not the usual 24 volts you see with forced air. Very basic series circuit, think source (240v), resistor (heating element), single pole single throw thermostatically controlled switch. Since each side of the switch has 120v in reference to ground, I don't see how to connect a neon lamp directly.

Thanks for the replies.

 

[Martaine2005]

If there is no neutral in the box, you can run one!
Or run a live from the box to another with the neon and neutral!
Again, amperage has nothing to do with what you what to do!!.

Martin

 

[duke37]

Just connect the neon across the heater circuit. Make sure it has a series resistance to limit the neon current. This is often built into a neon indicator.
There is no need for any ground reference.

Are you really 100?

 

[Tha fios agaibh]

If your renting, you should ask the owner if he will hire a professional to install one.

 

[DoubleDogDan]

Duke - That would work, but I don't know if I have access to both sides of the load from within the thermostat's electrical box, I'll have to check that. If not, I'd have to run wiring, which I'm trying to avoid in a rental.

 

[Martaine2005]

As if!!!!!!
The owner probably gets a cut of the electricity used!!

Martin

 

[duke37]

My mains thermostats have a neutral to supply a diddy resistor to reduce the dead band. The mains situation is different in the US.

 

[Alec_t]

Simply wiring the neon (with series resistor) across the thermostat switch would provide an indication; albeit non-ideal in that the neon would light when the switch opens rather than closes.

 

[Tha fios agaibh]

Imo, if you don't even own the place, you have no right to alter premises wiring. Even if the owner says it's ok.
You are altering a listed device and possibly opening yourself to liability.

Another reason is, if you add a pilot light from the switchleg (load side of tstat) to neutral, you will trip the gfci breaker that's required for a heating element encased in cement. Another words current draw must match on L1 and L2.

 

[Bluejets]

GFCI (Earth leakage) operate on earth leakage current,i.e. an imbalance between line and neutral current, not on balanced line to neutral current as it would be if connected line to neutral.

 

[Tha fios agaibh]

That's true, but it depends on how its wired. I'm referring to the commonly wired split phase 240v circuit where 240v is derived from 2, 120v legs.
If that's the case, the 2 wire 240v circuit requires a 2 pole gfci, and will trip if you try and add line to neutral load that draws >4ma.

 

[GPG]

https://www.nema.org/Products/Documents/NEMA-GFCI-2012-Field-Representative-Presentation.pdf
Only 240 here.

 

[Tha fios agaibh]

Op has a avatar and a profile page showing his home as Seattle, where 2 pole 240 is the norm.