Inaccurate thermostat

[Bratters]

During the winter when our central heating is on, the temperature of our house seems to vary widely. I have confirmed this by temporarily putting a digital thermometer next to the room thermostat on the wall which controls our heating. The room stat is set to 20C - the digital thermometer shows the temperature going as low as 18.1C before the thermostat activates the heating - it then goes as high as 22 before it switches off again. In other words, its "range" seems to be very wide. 4C is a lot - 18C feels pretty cold, and 22C is uncomfortably warm. As such, we're constantly altering the thermostat to make the heating come on/off, which only makes the problem worse.

I've looked at newer, digital thermostats (e.g. Drayton), but even some of these seem to have "ranges" of over a degree.

My digital thermostat is accurate to 1/10th of a degree C, and I wish it was possible to control the activation of the heating system with such accuracy. Anyone got any ideas?

I realise there would still be some "bounce" effect (for want of a better term!) - i.e. when the house reaches 20.0C, and the heating is switched off, the rads are still going to be hot, and the temperate will continue to rise a bit. I can live with that - but having a thermostat where the difference between "on" and "off" temps is nearly 4C seems crazy.

 

[Stoday]

The thermostat may be wired incorrectly.

Most mechanical thermostats use an accelerator heater to reduce the hysteresis you describe. When the heating switches on, the accelerator heater switches on too, providing extra heat to switch the thermostat off quicker.

Does have a drawback because it creates an offset, but you can easily overcome that by turning it up during cold weather, when the offset tends to reduce the room temperature.

 

[ricarbo]

Most, if not all, thermostats of the mechanical type, intended for use with central heating, have an 'accelerator', which is a resistor that warms up the bimetallic strip. This effectively cures the problem you describe. you can tell if you have the right type by taking the cover off. there should be three wires (live in, live out and neutral), plus any earth wire. If the thermostat doesn't have the neutral terminal, it's not meant for central heating control. If there isn't a neutral wire, this type of thermostat won't ever work correctly. Most of the electronic programmable 'stats are battery powered, do not need 'accelerators' and so can be used with just two wires (live in, live out).
The 'dead band' you refer to is needed to stop the system from starting and stopping too frequently and you will not find you notice the small change in temperature between on and off.

 

[Bratters]

Thanks for the information. I've been out and bought a Drayton Digistat 3, which apparently has a "dead band" of 0.6C - I defy even my wife to be able to say it's too warm/cold within that range...

Thanks for the help.