Compact Pump Group Installation Guide - electrical connections

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Note: This document must be read in conjunction with the Compact Circulation Pump Kit Installation manual, and the User/Installation instructions for the Compact boiler.


Note: The following instructions have been superceded by the introduction of a pre-wired plug and socket on the pump group assembly, and the wiring harness of the boiler respectively (see the Pump Kit Installation manual, Page 53). However this article may aid the installation and wiring of a replacement pump on an appliance supplied without the pre-wired plug.


The Compact range of boilers (except 15 Slim and 45 models) are supplied with the pump group assembly as a separate loose item, to be installed on site. This is because there are four different pump group variants that the boiler can be ordered with:


  • Standard pump without DHW kit
  • Standard pump with DHW kit
  • High efficiency pump without DHW kit
  • High efficiency pump with DHW kit


The boilers are now ordered from MCZ in the high efficiency variant only, and the DHW kit is not offered for sale in the UK due to its poor performance and the risk of increased tarring of the combustion chamber & heat exchangers.


There are two main sets of electrical connections to be made between the pump group and the boiler:


  • The power supply lead for the pump itself
  • The air and water pressure switch connections.


The pump power lead is found as a loose item in the carton with the pump. It has a plug on one end that connects to the dedicated socket on the pump The other end are bare conductors - see picture below:


Compact Pump power lead


Connect the plug to the socket on the pump and place the pump group assembly loose onto the platform at the rear of the boiler - see below:


Compact Pump in situ


Now refer to the pump group installation manual - page 54 (extract shown below):


Compact Pump Group Install Wiring


The pump live and neutrals need to be wired into the motherboard at the terminals shown. The motherboard, in the rear of the boiler and above the pump position, is orientated at 90 degrees to that shown in the drawing above. See below for the correct terminal positions as you will actually see them:


Compact pump wiring to motherboard - Locations


Note that the neutral position will already have a connection in it (it is the neutral for the 3 way valve terminals) so care will be needed to introduce the pump neutral. There is a loose crimp terminal for the earth to connect to. Once the conductors have all been connected, the wiring should appear thus:


Compact Pump Wiring to motherboard - Done


The next step is to connect up the terminals on the water pressure switch on the pump . Again, refer to the pump group installation manual - page 54 (also see extract of drawing above).

In the void where the pump group will sit, you will observe two sets of unconnected leads - one with green insulated sockets, and one set with black - as follows:


Compact Pump Pressure Switch leads - Location


It is the leads with green insulation that connect to the water pressure switch. The leads with black insulation are Not used in this installation and should be left unconnected (they would connect to the flow switch on the DHW version of the pump group).

Referring to the green leads: One lead can be traced back to the motherboard whilst the second lead connects to the air pressure switch. The lead that connects to the motherboard needs to be connected to the normally open terminal on the pressure switch, whilst the other lead that goes to the air pressure switch connects to the common terminal. Once done, it should appear thus:


Compact Pump Pressure Switch leads - Done


The plumbing connections (boiler flow, return, pressure gauge) can now be made. The pre-formed flow & return pipes (supplied loose) connect to the pump as shown below, using the clips and seals supplied (pump removed from boiler for clarity):


Compact pump with boiler pipework attached


For the pressure gauge fitting please also see article 'Compact 18-24 circulation pump installation - pressure gauge connection'

The pump is then screwed down to the boiler structure to complete installation.