I am trying to use a Honeywell 'Round" electronic thermostat to control a 1500W 120V heater.

The first one had a mercury switch which turned on a 12VDC wall wart that turned on a power relay and works just fine.

Can't find another mercury type so I bought an electronic one and have spent hours proving that it doesn't work but am coming to the conclusion that it may be a cockpit problem.

When I connect it to an AC transformer, the relay in the thermo clicks on and off as it should but I can't figure out how to make that control the outboard relay. There is no measurable current flow through the thermostat either in the on or off position.

I can not find a schematic of this thermostat so I have no idea what is going on inside.

Aside from that, does anyone know where I can get a mercury type?

Thanks,

Jack

Mercury stats are no longer available. A 24 volt stat can handle 12 volts, as 24 volts is the maximum the stat is rated, and your new unit can handle 0 to 24 volts.

What is the make and model number of the stat you have?

Honeywell CT87K.

I am an engineer and thought I understood how this works but I am in the dark on this.

When off line the relay does not respond to the temp setting. When connected into my 12V DC system, still no response.

If I connect a 12 or 24 VAC transformer, the relay responds but the circuit does not make or break. It is simply open and stays that way.

When I disconnect it from the AC, the relay responds for a while and then ceases as though a stored charge has faded away.

I see nothing in the instructions about DC operation but don't know why it should care if it is just a relay and the electronics are powered by the battery.

Are you absolutely certain this will operate on 12V and how about DC?

Js

There is an old saying "DC welds, AC breaks. If not rated for dc, maybe contacts welded.
Why not use a 24 Volt AC transformer?
Is 12 Volts DC all that is available, or is that what your contactor coils are ready for.

Sounds like you need that thermostat, a 24 Volt Transformer, and a 24 volt Contactor rated for the load of the heater, as the thermostats relay can't handle it.

Do you want a mercury thermostat or do you just want a low voltage, mechanical thermostat?

http://www.homedepot.com/webapp/wcs/stores/servlet/ProductDisplay?productId=100476730&storeId=10051&l angId=-1&catalogId=10053&ci_sku=100476730&ci_kw={keyword} &cm_mmc=shopping-_-googleads-_-pla-_-100476730&ci_gpa=pla

I wanted another mercury thermostat but can't fine another one. The link posted is exactly the one that I purchased but it is not mercury and is not a mechanical thermostat. It has a computer chip and all sorts of electronic components and a 3 volt battery.

I was going to buy another Ranco but thought I could save some money from the junk box.

The relay is 12VDC and will switch 220 @ 20 amps but I can just put a bridge rectifier in front to convert the AC to DC so that's no problem.

I have both 12v and 24v AC transformers and in addition DC but the relay in the thermostat just clicks but does no make contact through the R and W terminals.

As far as welding the contacts goes, they usually get welded shut, not open.

Substituting an equivalent resistor for the coil, I get about 1.5 micro amp when the thermo is in the on position and just noise when off. This is with 12VAC driving it. In other words, the relay in the thermo is not really a relay but just a high resistance.

I have two identical thermos and they both do the same. It is unlikely that I fried them both.

Js

Sorry about that. I guess I assume the title reflected the workings of the device.
My old round mercury Honeywell model number is T87F2873.

Try this site.

Honeywell T87f Thermostat - Compare Prices, Reviews and Buy at Nextag - Price - Review

You are right on contacts welding shut, I was thinking in an off chance it was a spdt, or dpdt, where it is welded in the other condition, it really should never see a load on the contacts to weld shut, just an idea.
Do you show continuity on relay contacts when set to heat, and should be red and white connecting, and no connection to voltage or heater? If not the thermostat may be bad.