CH Room & Frost Thermostats

[Richard Sterry]

Daft question coming up

As far as I can see, browsing at the DIY stores, room thermostats, and indeed many frost thermostats, do not simply have a pair of contacts which make and break at the set temperature, but they also appear to require a permanent live feed. At least, that's the impression I have.

Why? I could understand it in the case of electronic programmable stats, but simple passive ones...

I want to add a frost stat to our system some time, (bit late for this winter I know). When I did this at our last house some years ago, I used a Honeywell frost stat with a pair of contacts and nothing else - no live feed required. I would quite like to do the same here, and it just so happens that a redundant twin-and-earth cable is already in the right place.

[Rick Dipper]

I made the same mistake, Twin and Earth has THREE conductors, there is no way that I may have used the thirs one for a purpose other than the one intended.

Needless to say this is one bodge I am not proud of.

Rick

[Richard Sterry]

*SHUDDER*

(other) Rick

[Andrew Gabriel]

It powers an internal heater (resistor) which reduces the hysteresis of the switching temperatures (i.e. the difference between the ON and OFF temperature at any setting). Without it connected, you might find there is a difference of a few degrees C between the ON and OFF temperature. The internal heater is designed to heat the sensor by nearly this amount in order to cancel out most of the hysteresis.

If you are only going to use the switch contacts and not the internal heater, select a unit with the lowest current rating you can get away with for the load, as this is likely to have the lowest hysteresis without using the internal heater.

You need to think carefully about what you want the frost stat to do. This depends where you are positioning it in terms of inside or outside the normlly heated area.

[Dave Plowman]

They use an accelerator coil - a small heating element - to improve the response time, but can be used without.

Most electronic types use a far more accurate temperature sensor than the traditional bi-metallic type - and are, in fact battery operated, so only require two wires.

[Richard Sterry]

Andrew Gabriel wrote: [sniip]

Thanks for the nice, clear explanation. All understood.

[snip]

A degree(!) of hystersis in a frost stat would be no bad thing. I intend to place it such that it will react before any part of the system can freeze up, but such that it will (eventually) receive heat from the system and switch off once the crisis (so to speak) is averted. In my last house, it was in the integral garage close to, but below, the wall mounted boiler. This worked very well. In this house, I will probably position it in some roof storage space on the 1st floor. This is insulated from the outside world, but would get cold a little before the rest of the house. As I mentioned earlier, there is a redundant cable routed through to this area already.

Rick

[Andrew Gabriel]

You should not connect the froststat so it directly demands heat. If the area it protects is one you want space heated, then connect it to bypass the normal central heating timer, so the heating will come on but is still controlled by the room stat. Otherwise, if it's somewhere like a loft, the rest of the house might get up to some stupidly high temperature before the loft gets up to the switch-off temp. If it's in an area which you don't want to space heat like a garage, then connect it together with a pipe stat on the boiler return set for perhaps 30-40C such that it just fires up the heating for long enough to take the chill off the plumbing.