Active + Hydro stoves pump running constantly

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The Active + Hydro stoves (Duo, Flux, Swing, Toba) are currently available with either a standard type circulator, or ‘A’ Rated high efficiency type. This article relates to reported instances of the High Efficiency type pump running constantly, even with the stove switched off & cold.


The high efficiency pump used in these applications is of a modulating type that uses a PWM (Pulse Width Modulation) signal to control the speed at which the pump runs, and to switch it on or off. This signal is of a low voltage DC type, and is separate to the mains voltage supply that turns the pump. It is therefore observed that two separate cables run to these pumps.

The PWM signal delivered by the motherboard controls the pump speed/operation by sending pulses of signal of varying duration. The longer the pulse duration, the slower the pump runs. The mains power supply to the pump is permanent; Therefore if no PWM signal is delivered (i.e. pulse of zero duration, the pump runs constantly at full speed. Conversely, if the PWM signal is constant (i.e. pulse of 100% duration, the pump stops completely. It can therefore be seen that in order to stop the pump from running, e.g. when the stove is off or the water jacket temperature is below the set point threshold, the pump must receive a constant signal to stop it from running. The only other means of preventing the pump from running is to isolate the power supply to the stove via the rear On/Off switch where the power lead connects.

Identifying Cause of Issue

If this problem is observed, the conclusion is that the pump is not receiving the PWM signal in order to prevent it from running. The following steps should be followed to locate the cause.

Step 1

Check that the PWM signal cable is correctly plugged onto the motherboard: You should find it along the top edge of the board as viewed from the rear of the stove, to the right of the black control panel cable. The letters ‘PWM’ are printed on the board below the terminals. The plug should be pushed square onto the board, and the terminals on the plug and board should be properly aligned.

Duo motherboard with PWM cable connection position marked

The plug has three spaces but only 2 wires are connected, in the middle & right hand positions (black & red respectively).

Step 2

If this seems all ok, the next thing to check is signal output from the motherboard. When the stove is fully off (State 0 as per the Stove State menu), these terminals should be delivering a steady 9V d.c. approximately. This can be checked by removing the plug from the board and testing with a meter. If this is not observed there is a fault with the motherboard.

Step 3

If still no problem found the next thing to check is the plug on the pump: This has the two sets of wires for the power supply, and the PWM signal, connected to it. See the attached assembly drawing:

[1] Active_+_PWM_Pump_wiring_plug_connections

If the plug is carefully opened up, you should find that the live & neutrals are wired to a 2 terminal plug (Item 2) whilst the PWM cables are wired to a 3 terminal plug (Item 3). When viewed as seen in this drawing (power supply to left, PWM signal to right), the PWM plug should be wired, from left to right as:

  • Red
  • Black
  • Empty

Power and PWM cables in pump wiring plug: The PWM cable is the lower one in the 3 pin plug

If this is not the case, the conductors should be moved accordingly, if this is possible.

Step 4

If the connections are in the correct positions, the final thing to check is the continuity along each conductor between the pump and the motherboard. Assuming that Step 2 gives a positive result, this can be done either by:

  • Ensuring the cable plug is connected at the motherboard end, then checking across the pins of the plug at the pump end for the a steady 9V d.c.
  • Disconnecting the cable at both motherboard and pump ends, then checking for continuity along each conductor with a meter.