I have this hydronic parking heater. When I run it I can heat the coolant circuit with the engine OFF.

I found an upgrade kit from Webasto for my van which let's you install a 3-way solenoid valve in the engine bay controlled by 2 relays. This modification allows you to bypass the engine block, heating a smaller circuit, so the cab matrix gets up to temp quicker and more efficient.

The conditions for the valve to be energised are:

  1. heater ON
  2. engine (ignition) OFF

Now I wanted to be able to manually switch between small and big coolant circuit myself. For example:

  • I want to heat up the engine block before going to work with my digital timer (big circuit)
  • When waiting in the van I only want the cab to heat up quickly (small circuit)

So I was thinking to put a switch in the (+) blue wire {7}

What I don't seem to like about this Webasto circuit is the K2 relay. If the ignition comes ON, the relay breaks the negative to the valve, but what if the relay fails??
I absolutely want to make sure I run the coolant circuit through the engine block with the ignition ON!!
* I know that the heater comes on automatically when driving in <5°C outside temp, so this is a situation where I don't trust the K2 relay..

So I was thinking some more.. what if I put a double pole switch (DPST) in the (+) blue wire {7} and the (-) brown wire, somewhere?
Maybe in wire {6} or even {3} that last one would break all relays right?

Good thinking?
My manual switch would only come on when waiting in the van, getting toasty, right?
All other situations I would be confident I will not ruin my engine?
Any other suggestions?

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  • \$\begingroup\$ The thought that occurred to me is can you get a valve that if unpowered will circulate coolant through the engine and that requires power to close and stay closed? This makes that valve fail safe in that if something goes bad the valve opens and lets the coolant through. \$\endgroup\$
    – zeta-band
    Nov 20, 2019 at 19:47
  • \$\begingroup\$ Yes the valve does exactly that, when unpowered coolant will flow through the engine block. Whenever the heater starts (also automatic) the (+) to the valve is made by K1. The (-) is broken when the ignition comes on with K2, but what if K2 fails, with the heater on? \$\endgroup\$
    – Anton PTM
    Nov 20, 2019 at 20:49

1 Answer 1


Relay coils are very reliable, the most usual failure mode of a relay that's switching an inductive load is for the contacts to "burn" and fail open-circuit.

In the case when the relay nevertheless fails closed you still have the protection of the engine's over-temperature alarm system.


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