I'm about to fit a thermostat to my Worcester Bosch 24i combi boiler.
The installation manual shows the thermostat connected with 3 wires a
neutral and the 2 others for a switching a live connection. My
understanding is that the neutral is for supply to a resistor to
generate heat in a thermostat that uses a bi-metalic strip to make it
The stat I am looking at is a Maplin digital thermostat which has
connections for COMMON, NO (Normally Open) and
NC (Normally Closed). Is the neutral wire not required with this stat
and I just wire it between common and normally open?
Do digital stats not need the internal resistor?
If it hasn't got a terminal for neutral, then DON'T connect the neutral wire
to any of the other terminals - 2 wires are sufficient. The 3 contacts are
only provided for the benefit of installations which need a Demand Satisfied
as well as a Demand Unsatisfied signal.
Incidentally, you will probably need to use the COM and NC (as opposed to
NO) contacts, so that it switches OFF when the demand is satisfied.
You need to use the Common and NC (normally closed) connections, you want
your pump to run until the target temperature is reached.
Digital stats do not need a resistor as the internal electronics provides
the required hysteresis (spread between actual on and off temps, situated
around your target temp). This is needed to prevent rapid on and off duty
cycle due to very small changes in the ambient temperature. Some of the
better stats gradually train themselves to your environment so can pre-empt
the switching points to maintain a closer control of temperature.
The old mechanical stats use the resistor to warm up the bimetallic trip a
little to reduce the amount of hysteresis as a straight bimetallic strip has
a large difference of temperature between on and off.
Does it not tell you in the thermostat instructions?
BTW Maplin not long ago were offering a digital thermostat at a remarkably
low price but when I phoned them about it they told me immediately it was
not suitable for use with a combi boiler .... I hope yours is not this one.
You will get a big bang if you connect neutral to one of the outputs of the
stat, as it will short the live supply to earth. You do not need to connect
neutral - the stat acts as an in-line switch (just like any switch). For CH
use, the NO connection is not required as it will only signal "CH off" which
is not of great use. Conversely, in fully-pumped systems, you do need a "HW
off" signal to the 3-way valve and this comes from the time-switch or HW
stat or both.
For people who like peace and quiet: a phoneless cord.
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