Boiler/hot water pump never switches off. Sorted !

[Aunt Maud]

I have a Bosch condensing system boiler with a hot water tank on the side.

The thing has timer controls and an external thermostat. If the heating and hot water are switched off and the system allowed to cool, the circulation pump continues to run.

The pump has two sides to it, one circulates the central heating water and the other circulates the domestic hot water from the tank to the heat exchanger. I have fitted a DHW circulator yesterday, but the pump was constantly running before that.

Any ideas before I splash £200 on a new pump, only to find that the old one was ok ?

[mrcharly-YHT]

If I get this right, your current pump is actually a double pump - two entirely separate systems, is that correct?

Usually the heating circuit is on a 'call' wire from the thermostat. I'd check the wiring diagram for your pump, find out which wire/terminal that is to your pump and disconnect the wire. If the pump stops, you know there is a wiring fault to the pump (rather than a fault in the pump). Ditto for the other circuit - that will be on a thermostat on the tank I would have thought.

The only system at all similar to yours I had, the heat exchanger pump was in the boiler itself and the thermostat was on the tank. When the tank cooled, it signalled the boiler to fire up, that also started the heat exchanger pump (which was part of the boiler).

[Aunt Maud]

It's this one here.....https://www.idealo.de/preisvergleich/OffersOfProduct/2376529_-ups-15-50-ttao-junkers.html

There are the normal flanges for the central heating then a separate connection to the hot water tank. I'm assuming there is some kind of diverter valve or second impeller that just services the flow from hot water tank through the heat exchanger.

I've now got it on its lowest flow rate to quieten it down, but I'll have to wait to see if it is enough to do the heating. The hot water is doing fine and the noise is bearable. It does use a metric shed load of electricity though and is responsible for using nearly 50% of our total electricity use.

The circulator pump I fitted uses nothing in comparison and is a pump of equal capacity.

[Quisling]

Modern boilers normally have a pump overrun command so that when the thermostat stops calling the boiler for heat the pump continues to run after the boiler has stopped firing.  This ensures that any residual warmth in the boiler heat exchanger is dissipated into the central heating circuit and protects the boiler.

Is it possible that the boiler is erroneously continuing to overrun the pump?

[Aunt Maud]

I think it might be something like that or a faulty thermostat as mrcharly-YHT said.

Low speed setting on the pump heats all the rads; but it's still running all the time, which I want to put a stop to, as I can't afford it.

[mrcharly-YHT]

Don't want to teach granny etc, but you have bled it, haven't you?

[Aunt Maud]

Yes, we've only got 4 rads on a very short circuit at the moment as I'm busy installing a manifold system at the moment. I was singing the praises of the DHW pump when the boss said "but it still makes a racket".

I did the rod against the ear thing and listened to the pump, but it wasn't clattering about like something was worn, just humming loudly to itself all the time.

[Canardly]

We recently had a boiler problem where a faulty pump was the chief suspect. New one was duly purchased with a slightly higher head rating. Problem still existed with heat rise stat and boiler cutting out. Turned out to be sludge/partical blockage in the boiler which was subsequently shifted via a power wash, applied to the boiler only, as the circs are clean. Pump does overun for some time in normal operation. The latest room stats have led indicators which light when calling for heat.

[phil d]

Not sure if it's relevant here, but on my heating system I control the heating side with motorised valves, which have an auxiliary switch so when a valve opens (because the relevant thermostat calls for heat) power is supplied to the boiler and pump.  Sometimes the auxiliary switch sticks (always, it seems, in the switch closed position) which causes boiler and pump to stay on.

The switches are non-serviceable, so a new motor / solenoid has to be put in.

[Aunt Maud]

I think what's happening is that it's dumping heat into the heating system, because the DHW flows through it and the DHW circulation pump on the return is on a constant speed setting and running all the time.

I'll fit a timer for the pump and put the hot water on twice daily.

[Aunt Maud]

Maud, you're a dopey prick......

That switch with the moon-clock-sun on the boiler control should be switched to CLOCK! you cock, otherwise the chuffing boiler runs continuously for years, whilst you go around and scratch your noggin about it.

[Aunt Maud]

Oh!

[Canardly]