Danfoss FP715 not switching pump off at end of programme

[entee]

Hi,


A bit of a strange request…

I have a FP715 which is controlling a pair of HPA2 valves. In the past, despite both valves working fine, the ‘pump’ microswitches have failed closed and kept the pump running. Determining the culprit by manually operating the valves showed which one was duff. Over the years several HPA2s have been replaced, the most recent 9 months ago.

9 times out of 10, I have to manually drop the mains supply to switch the pump off at the end of a programme period. Reinstating the mains after a minute frequently works with a short burst from the pump whilst it switches off. To my ageing electronic engineer mind, this sounds like the relay in the controller is not dropping out as it should.

Testing the valve microswitches shows they are opening fine and both fully open when the system is off – as I would expect.

I’m wondering if a kind expert out there could please advise? Does this sound like a duff controller?

Many thanks for reading this!

----------------------

Danfoss FP715
Danfoss HPA2 x 2
Grundfoss UPS3
Ideal Classic

 

[stem]

The sequence of control is:

Programmer.....> room thermostat.....> motorised valve.......> microwsitch......> operates boiler / pump

So, an easy test when you feel that the programmer is misbehaving and has not switched off, simply turn the room thermostat right down so that it clicks off. If the pump stops then, it's the programmer, if it still keeps on going it's not. (you may have to wait a few minutes, see below re pump overrun)

You don't say if it's the heating or hot water channel you suspect to be duff, so if it's the hot water the same test can be done by turning down the hot water cylinder thermostat.

There are lots of variants of Ideal Classic, so without a model, I can't check how your pump is connected, but some boilers have a pump overrun facility whereby the pump is wired directly to the boiler, so the boiler controls it, such that it will run the pump for a few minutes after the boiler has switched off to remove residual heat from it. In such cases the boiler PCB is what is controlling the pump and may be the cause of the problem.

Testing the valve microswitches shows they are opening fine and both fully open when the system is off – as I would expect.

That indicates it's not the programmer or thermostat. They are solely responsible for opening and closing the motorised valves. That's it, they have no direct link to the boiler or pump. So if the valves are opening and closing as they should, then the programmer and thermostat are working as they should.

 

[entee]

Thanks for getting back so quickly!

There is no electrical room thermostat for heating and this system relies upon individual thermostatic valves for room temperature control. Turning down the hot water cylinder thermostat when running in HW only mode made no difference, neither does manually switching both CH & HW off at the controller. I understand the over-run timing thing, but when working normally, the pump will switch off within a second or so.

Coincidentally I have just watched the 11am shut-off sequence for both CH & HW and despite the LEDs going out, the pump continued until I switched the mains off manually using the isolating switch adjacent to the hot water cylinder and control gear in the airing cupboard. Switching the mains back on after few minutes sometimes works mostly. Occasionally it needs to be off for a while before it can be switched back on and the pump remains off.

I know the FP715si has a full master reset facility - does the same wok on the old PFP715 I wonder? Just pressing reset places the time settings back to factory defaults.

 

[stem]

OK thanks. In that case, when the FP715 switches 'off' and the pump continues to run, are the motorised valves open or closed? If they are open it's the FP715 that is faulty, if they are closed it's not.

As you suspect a reset will just put it back to factory settings. If a programmer is staying 'on' when it shouldn't be, it's usually a component failure such as a relay.

EDIT
If you are not sure how to tell if the valves are open or closed; the manual operating lever will feel loose and move freely if it's open, if there's a resistance when you slide it to the manual position it's closed.

 

[entee]

Both valves always return to the self-settling power-off closed position so I guess it is looking like it might be the boiler PCB - argh! I was hoping it was the controller because it is a 2 minute swap out!

Incidentally the Classic is an SE24 FF

Thank you so much for your wise guidance.

 

[stem]

Maybe not the boiler PCB. Below is a diagram from an online SE24 FF manual and as you can see the pump is connected to the wiring centre, the only connections going to the boiler are its L, E & N. The pump should be connected directly to the microswitches from the motorised valves via the wires I have highlighted

Which is odd (if your boiler is wired the same as shown) when you say...

Testing the valve microswitches shows they are opening fine and both fully open when the system is off

If the microswitches are 'open' the pump (and boiler) won't be getting any power.

 

[ianmcd]

I guess it is looking like it might be the boiler PCB

Wont be the boiler PCB as it only has a switched live no permanent live and no pump over run, your stray current is coming from somewhere else, double check the zone valves, unless of coarse you have a frost stat fitted

 

[entee]

Yes, looking at the circuit diagram the live can only go through the 2 microswitches - operating singly or together. One of them is definitely 'on' when it shouldn't be, albeit intermittently.

As a test I have swapped out the oldest HPA2 for a new one [CH]. Of course live testing won't make the fault appear! If the issue remains later I shall knock up a simple twin switch box so I can isolate either of the orange cables when the unit is still running.

A theory: Microswitches don't last forever and there are various grades from 'Chinese Jim' to aviation. I suspect these are somewhere in the middle and generally do a pretty good job.

This particular environment is a little harsh because the pump motor sits at the full system operating temperature. I have been measuring the microswitch functions, not only when cold, but with a low test-meter voltage. Stuffing AC mains through a tired overheated microswitch could well be a different story. Furthermore, I have tested some old HPA2s that had failed with a test-meter, the switches are miraculously working again!

I will report back, even if good news, for the benefit of others.

Thanks to all for taking the time to offer advice - greatly appreciated.

 

[ianmcd]

The micro switches stick on them for fun, the valve will close but the micro-switch will still be in the made position

 

[entee]

Reporting back as promised....

On the basis that two swallows don't make a summer, we have so far enjoy two programmed "switch-off" events, so it appears the microswitch was sticking in the on position, albeit intermittently. Checking the microswitch on the removed HPA2 shows it working but it is cold. Possibly age and heat are the issue. At least we are working again and thanks again to all.

 

[ianmcd]

An easy DIY test to see if it is indeed a sticking micro switch, turn the boiler thermostat down to as low as your home is still comfortable, if when the system shuts down , and the pump is still running, turn the boiler thermostat to full , if the boiler fires for a min or so then it is indeed one of the micro switches sticking

 

[DP]

Easiest thing to do here would have been to put hand on boiler when pump running but programmer not calling ( and motorised valves self close when power removed from valve). If pump running, boiler would be firing too unless it had overheated so would be hot to feel.

Entee, do you not have a multimeter? Did you not think of checking the grey / orange wires for mains voltage that was running the pump? Me thinks your boiler would have been responding to pump run voltage also

just read Ian’s message. Another way to skin the cat