I am wanting to add a Horstmann HRT2 thermostat to an existing central heating system. With it not wired I can hear it clicking when I turn it to above or below room temperature. This would suggest I could connect it using 2 core cable (live and switched live). The instructions say terminal 4 must be connected to neutral. This means I should ideally use 3 core and earth (I don't like using the earth wire for anything other than earth). Is that neutral connection really necessary?
HRT2 Thermostat Wiring
- Thread starter fidom
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The neutral is often needed to prevent large temperature swings (hysteresis), it can be wired using 2 core/twin and earth, but like I said large temperature swings. If you can’t get a neutral to it consider a battery operated thermostat which negates the need for a neutral wire. The alternative is a wireless thermostat, which still needs a neutral but only for the receiver wired via the boiler or wiring center.
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As @CBW says, but this can be an advantage, it depends what the wall thermostat does, this may seem odd, but today the TRV controls room temperatures, but it can't turn the whole boiler on/off, so we want a method to turn the boiler off on a warm day, and back on with a cool day, typical is to have a wired thermostat in the hall, set so central heating only turned off on a warm day, for that job the large droop would not be a problem, but when used to control room temperature we are looking for a droop of 0.5ºC or less.
I don't have TRVs because it is a solid fuel system and the radiators are acting as a heat sink to prevent the boiler from overheating. I'm adding a pellet boiler in addition to the log burner and the room stat is for use with that.
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In that case, what does the stat control, a pump?
On the subject of roomstats, they often incorporate a booster heater, energised when the stat calls, which needs a neutral. This anticipates the heat from the rads, giving better temperature control (less hysteresis).
The stat would operate the motorised valve to switch to heating rather than hot water (the default). On the existing setup a pipe stat on the feed pipe out of the boiler switches to CH at a given boiler temperature. The hot water is on gravity so the pump is switched by the microswitch in the motorised valve.
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