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What you need is a "2 stage thermostat" and a 24 volt relay. The thermostat would first "energize" your space heater via the 24 volt relay, and then when that can't maintain temp it would kick in the central heating. Normally these 2 stagers wouldn't be used with a space heater which is why you would need the relay to take the 24 volts from the t-stat and switch on the heater's regular electricty.
Honeywell, White-Rogers, tekmar et al make and sell them.
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