Connecting to an S-Plan or Y-Plan system
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 consumption, reduce efficiency and cleaning/servicing intervals.
It is possible to connect a pellet boiler to a standard S-Plan or Y-Plan arrangement, with respectively either a motorised valve for hot water and a motorised valve for central heating, or single 3 way mid-position valve, as is fairly standard for domestic gas or oil fired boiler installations.
For S-Plan systems, it is important 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 dissipate 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 between 5 and 20 minutes, after which the boiler will go into a full shut down.
Should a demand be placed on the boiler while at minimum power 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 engaged, 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 from 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.
The call for heating or hot water from the S-Plan or Y-Plan controls 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/stove cannot perform this function. The boiler should therefore either be left on with no on/off times set, or at least timed to be available to operate at least during the times that the external S / Y-Plan 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, etc.) the appliance should be set to System Configuration 4 (within Settings menu). This means that the Auto Eco function, and external thermostat options will now have been automatically enabled.
For MCZ Hydro stoves (Ego, Musa, etc.) the 'Thermostat' and 'Auto Eco' functions need to be enabled.
The wiring diagram for the systems can be found below:
NOTE: The above S-Plan wiring solutions replace previously published versions where the double pole relay may not have been included, and the output from the micro switches on the valve heads (wired Grey & Orange) would have been wired directly to the External Thermostat terminals on the appliance.
There had been several reports of installations using the original S-Plan wiring solution for MCZ Hydro Stoves, with no relay, 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 have been inducing voltage into the volt-free micro switch circuits that then feed calls for heat to the stove thermostat terminals, which in turn produce the unusual characteristics described above. It is therefore recommended that the above diagrams are followed in order to isolate the motorised valves from the appliance.