What thermostat not to use.

[Sam Takoy]

Hi,

My boiler uses just two wires for relay. I've been told that means that I can can't use certain common thermostats. How do I tell which ones I cant use. (I realize that I can't use "inline" thermostats.)

Thanks in advance,

Sam

[Twayne]

Well: How many wires does the new thermostat have to use, and does the boiler require a closure of opening of contacts in order to be run? You want a temp control to work with two wires and provide either the open or closed output your boiler requires. In general you'll want contacts to close when heat is called for, but it's possible to be the other way round.

HTH,

Twayne

[ransley]

use a 2 wire unit.

[Lp1331 1p1331]

You can use any mercury bulb or mechanical thermostat. Some digitals require a "common" wire at the stat. If this is the case you will have to have three wires at the stat. Some thermostats refer to this as being "hardwired." A digital that uses only batteries, or is "power stealing" would work. Digital are much more acurate and have less of a temperature swing. than mechanical types. If you are looking at HD/Lowes,the box should tell f it needs a common wire or not. Good Luck Larry

[RBM]

The only thermostats you couldn't use would be uncommon ones. You have 2 wire low voltage, (series 80) which is about as common as it gets

[Sam Takoy]

Yes - that sounds right. Is having a common a property of the boiler or just the wire. For instance, if I replace the wire to have a common would I have nothing to attach it to at the heater end?

Thanks!

Sam

[RBM]

You will always have a common at the boiler. It is part of the power supply that feeds the thermostat. Typically the thermostat only requires a switch leg (2 wires), and the common is connected to the relay that the thermostat is controlling.

[Lp1331 1p1331]

Sam, the common wire would go to the other low voltage side of the transformer than the red lv wire going to the thermostat should be hooked to. IOW- the transformer should have on wire going to the relay-that is common. the other lead from the transformer goes to the thermostat, and the comes back form the stat to the relay. It would be a lot easier just to get a stat that doesn't need a common. The stat is nothing more than a switch that completes the circuit from the red side of the transformer to the relay. A common wire to the thermostat is there only to power up the electronics of a digital stat. Actually, on some, you can have the display backlit all the time if you want. Obviously, on one that is run solely off batteries, this is not possible without changing the batteries every day.

[trader4]

Yes, exactly. If you go to HD or Lowes you'll find almost all the thermostats are either just mechanical or battery powered electronic ones. I wouldn't be surprised if they all fell in those categories and work with 2 wires, which is the simplest system you can have.

[terry]

Sounds like low voltage thermosta operating a relay at the furnace?????????????? Possibly just a pair of contacts that close when heat required. If more complicated than that get help from someone who understands the circuitry involved!