Connecting to an S-Plan or Y-Plan system

NOTE:

Prior to considering the following article, it should be noted that Specflue recommend that All MCZ boilers and Hydro stoves are installed together with a buffer or thermal store sized at 25 Litres per net KW output of the appliance. Installing an appliance without this provision may increase fuel consumpion, reduce efficiency and cleaning/servicing intervals.

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It is possible to connect a pellet boiler to a standard S-Plan arrangement, with a motorised valve for hot water and a motorised valve for central heating, as is fairly standard for domestic gas boilers.

The following schematics detail the various ways this can be accomplished.

Schematic of S-Plan system where room thermostat is used to control temperatures.

Schematic of S-Plan system where TRVs (thermostatic radiator valves) are used to control temperatures.

It is important when connecting to an S-plan to fit a by-pass circuit that allows a flow of water through the boiler at times when both motorised valves are closed.  This allows the boiler to dissapate heat that builds up in the boiler and helps prevent overheating, while also protecting the circulating pump from a dead-head.

Pellet boilers are usually setup to run using the 'Eco-Stop' mode, whereby the boiler will go into a tick-over state once a demand is satisfied.  The Eco-Stop lasts for a set period of time, typically 20 minutes, after which the boiler will go into a full shut down.

Should a demand be placed on the boiler while in Eco-Stop, the boiler will simply switch to full output, rather than have to go through a full start-up procedure.

Without the Eco-Stop used, there is a danger that the boiler will go into a full shut-down and full start-up each time a demand signal cycles, making operation very inefficient.

During Eco-Stop the boiler will continue to burn pellets, although at a minimum rate.  It is advisable that the by-pass circuit is taken through a towel rail or radiator to allow dissipation of the generated heat.  If the by-pass simply feeds water fom the boiler flow to the boiler return there is increased risk that the system will lock out on overheat as there is nowhere for the heat to go, and it may be necessary to reduce the Eco-Stop time should this occur.

Wiring:

The call for heating or hot water is made via the external thermostat terminals on the terminal block at the rear of the boiler. As these terminals are 'volt free', it is imperative that no attempt is made to connect any live 230 volt input to these terminals, or irreparable damage will be done to the motherboard.

It should also be noted that an external timer/programmer is required to control the central heating and hot water thermostats - the onboard timer on the boiler cannot perform this function. The boiler should therefore either be left on with no on/off times set, or at least timed to be in operation during the times that the programmer will be set to operate. A few minutes allowance should be given at the start time of a programmed call for the boiler to go through its startup and ignition procedure.

For Red 365 boilers & stoves (Compact, Primula, Gardenia) the appliance should be set to System Configuration 4 (within Settings menu) and External Thermostat set to 'on' (again withing Settings menu). The Auto Eco function will now have been automatically selected.

For MCZ Hydro stoves (Ego, Musa) the 'Thermostat' and 'Auto Eco' functions need to be enabled.

The wiring diagram for the systems can be found below:

S-Plan Wiring for Compact

Y-Plan Wiring for Compact

S-Plan Wiring for MCZ Hydro stoves

Y-Plan wiring for MCZ Hydro stoves

UPDATE:

With regard to the MCZ Hydro stoves on S-Plan system - there have been several reports of installations using the S-Plan wiring solution described above where the stove has exhibited unusual operating characteristics, incorrect information displayed on the control panel, possibly not responding to calls for heat, etc.

It is believed (although not proved conclusively) that certain types of motorised valves may be inducing voltage into the volt-free micro switch circuits that then feed calls for heat to the boiler thermostat terminals, which in turn produce the unusual characteristics described above. If this does seem to occur, the solution is to instead use a relay (greys becomes perm. live and oranges provides power to relay coil) rather like the Y-Plan wiring to then give the volt-free input to the stove terminals - as follows:

S-Plan Wiring for MCZ Hydro Stoves with Relay