Does a double throw thermostat exist?

[1974stephen]

I have a customer who's tried to install a Simens industrial heating actuator unit to replace a faulty one, but the new unit seems to need switch voltage to open and close, rather than just to open, closing with a spring action like most domestic motorized valves. They're using it with a very old single pole stat.

Ideally they want to replace the stat as well, so are there stats available which offer double pole which will work, or will I need to install a relay in the circuit to offer the dual voltage switching needed? If the former, can anyone suggest a make and model?

 

[ianmcd]

the valve you are describing is called a MOMO valve (Motor open/Motor off) as opposed to motor open spring return, you need the correct valve

 

[oldbutnotdead]

Do you mean double pole or double throw? Think most room stats are double throw nowadays, problem you'll have is getting an extra core from the stat to the valve. If you need double pole double throw (cos closing involves reversing polarity or similar) then relay will be the way forward- that or fitting the proper valve

 

[1974stephen]

I'll need to check about the number of cores at the stat, I know there's three core and earth at the valve, but it's wired in singles to the connection box, so not 100% where they all head. If I install the relay at the valve, then I'd only need the neutral and switched live to give me two opposing switched lives off the relay.

Regarding the double pole thermostat, assuming I have enough conductors, I was trying to describe something that would give me two switched lives, one when calling for heat and another when not calling for heat. Checking my pole and throw definitions has meant I now realize I did actually mean double pole! The few domestic type stats I've worked with just seem to have N L L1 connections, rather than N L L1 L2.

 

[stem]

A good percentage of room thermostats, in fact probably the majority have a NO (Normally Open) and a NC (Normally Closed) connection. When the thermostat is 'off' the NC connection is live and when it's 'on' the NO connection is live.

The terminals are not always labelled NO and NC, sometimes they have other names such as 'Heating call for heat / Heating satisfied' or 'Heating on / Heating off. Or the terminals will just have numbers (which will change from product to product) and a diagram will show the switch as something like this.

In this example (Nest)
NC is 1
Common is 2 (live supply)
NO is 3

If you find a thermostat you like the look of, you will be able to find the instructions on line and check the connection details, or post back details of its make / model on here and someone will be able to advise you.

 

[oldbutnotdead]

I'll need to check about the number of cores at the stat, I know there's three core and earth at the valve, but it's wired in singles to the connection box, so not 100% where they all head. If I install the relay at the valve, then I'd only need the neutral and switched live to give me two opposing switched lives off the relay.

Regarding the double pole thermostat, assuming I have enough conductors, I was trying to describe something that would give me two switched lives, one when calling for heat and another when not calling for heat. Checking my pole and throw definitions has meant I now realize I did actually mean double pole! The few domestic type stats I've worked with just seem to have N L L1 connections, rather than N L L1 L2.

Nah, you mean double throw (electrically the thermostat you want would be called SPDT for Single Pole Double Throw). See stem's post above