Pump runs continuously (no CH/HW call; boiler switched off)

Hi all,

Here goes my first post on DIYnot – what a brilliant website – thanks to all its contributors!

Our CH/HW system has been in and running reliably for about 8 years – Baxi Solo 3 PF, Siebe Lifestyle LP241 programmer, Drayton MA1 mid-position valve, Grundfoss Selectric pump, Drayton CTS3 cylinder stat, Drayton room stat. Looks like it’s set up with the Honeywell Y Plan.

A couple of weeks ago, in the still of night, I noticed that the system’s pump was running. In fact it runs continuously, day and night, even without CH or HW demand, and even with the boiler thermostat control knob turned fully OFF (then the boiler doesn’t run – just the pump).

Because of the super cold weather at that time, I thought it may have been due to the boiler’s “integral frost-stat” (from the boiler's manual) keeping the pump going to avoid frozen pipes. But now with the milder weather, this theory cannot be correct – the pump still runs at midday, with no HW or CH demand and the boiler dial switched OFF.

I have seen hints elsewhere (did I mention that DIYnot is a brilliant website!?), that one cause might be a stuck/faulty micro-switch on the valve actuator. The actuator unit looks like costing £40-50 if I have to replace the whole thing. Are the micro-switches replaceable? How would I go about testing the micro-switches or the whole thing?

Many thanks!
RIPE

So the pump runs 24hours a day now? does it stop at all when you select Hw to CH and all combinations? Usually as the valve moves from HW only position to CH the pump will stop briefly as the microswitches are broken then remade.

You could also be looking at a knackered PCB on the boiler or even frost stat which is keeping a simulated demand on the system. The system will have a pump over run and sometimes the board shorts causing the pump to continually run.

Hi Dave,

Thanks for your thoughts! Yes, it runs 24/7, no matter what the controller or thermostats are telling it.

It doesn't seem to stop when switching from HW to both to CH or back, however, just now as I write this, the HW On is calling but the MV remains in the CH position, so I think the HW is up to its set temp on the cylinder stat.

The only way to get the MV to move back to HW without manual help, but using its own return spring) is by turning off the whole system at the isolator switch. That's also the only way to turn the pump off.

When the isolator is switrched off, I can manually move the valve and latch it into the mid-position, where it stays when I switched the isolator on again (but unlatched and returned to HW by the spring when turned off again).

I switched it all on again, and with the MV in the HW position, I tried again, first calling for HW on the controller, and the MV stayed put. Then I also called for CH on the controller, but instead of moving to the mid-position (CH and HW), the MV went straight across to CH. No change when turning HW off at the controller, but also no change when turning off the CH again at the controller (MV stays at CH). This supports the thought that HW is On but Satisfied.

I will try again later after drawing off some HW, but I am pretty sure that the pump does not even stop momentarily when the MV is switching across in either direction.

Doesn't the pump need the boiler to tell it to come on and off? If so, then would the MV microswitches not be turning the boiler momentarily on and off the way you suggest?

The Baxi manual shows the overheat stat and a thermostat sensor, both of which are mounted on the water flow pipe. It also mentions but doesn't show the integral frost-stat. Is it possible that this is one and the same as the thermostat sensor on the flow pipe? If not, where is it, and how can I test it (I have a multimeter)?

Here is the wiring diagram for a Y plan

Are you sure there is not a fault with the room stat/cylinder stat?
Try having the programmer calling for both but then manually turn down the room and cylinder stat and see if the MV moves to the corresponding port.

I.e..... Turn the system off and make sure the valve is sitting at its last position.
Turn both room stat and cylinder stat down to min.....
Turn power back on and have programmer calling for CH and HW
Is the pump running now??

Basically, the programmer sends voltage to the room/cylinder stat which if calling sends power to the relevent MV wire and thus to pump and boiler.
If either stat is not calling (switch internally not made) even though programmer is on, then there should be no power to MV to get it to move to power pump.

Check the stats with the multimeter to see they are making/breaking contact - do this with the power off tho!

If they are ok, then the pump is getting a live supply from somewhere, and if I remember right the boiler has a permenant live in order to power the pump overun, it could be the PCB

sorry forgot the link


http://i20.tinypic.com/2efhkl4.gif

Try also disconnecting the orange and grey wires and testing for continuity to see if the microswitches are stuck

you can rule out the actuator micro switch sticking kepping system live that only happens on two ports becuse they have a permant live going to them, 3 ports get thier power from romm stat or cyl stat, so if the micro switch was sticking on a 3 port when the stats aren't calling for heat it would have no power going to it so the boiler and pump wouldnt run.
most likely cause is integral frost stat is stuck on closed postion (continuos circuit) so is on all the time.

D'oh, went blank there, considering i put a y plan diagram up made the school boy error of forgetting it was on a 3port not 2 port, tah for the correction

its really common for the pump overrun on the solo's to fail keeping the pump running, so you will prob be looking at a new pcb (the pump overrun is controlled by the pcb)

I drew down some hot water from the cylinder, and can confirm that the MV moves to all three positions correectly and as expected by the controller and the two stats.

The pump runs continuously no matter which position the MV moves to, and does not even pause momentarily when the actuator moves across.

I can hear one "click" when the MV passes the mid position in either direction, and another when it arrives at the CH position (or departs from it). That seems to indicate that there are two microswitches and that both are working (clicking) at least mechanically, if not electrically. There are five wires to the MV (as per your diagram).

From the terminal block, there are five wires to the boiler: (1) Permanent Live, (2) Neutral, (3) Earth, (4) Switched Live, and (5) Pump Live. I think you are correct that (1) enables the pump to overrun (and obviously keeps continuous power to the boiler's PCB when (4) is not on).

When anything calls on the boiler to run via (4) going on, the boiler in turn causes the pump via (5) to run, and (5) should stay on for a while after (4) goes off again. BUT (5) should go off again sometime after (4) does.

Using a neon tester screwdriver, I found that with both HW and CH controls off, the (4) Switched Live shows dim, but (5) Pump Live is bright (on), so it looks live the boiler is telling the pump to run (5) even though the controls are not telling the boiler to run (4). Does that seem to eliminate controls and microswitches as the cause of the problem?

That would indeed mean it's either the boiler's PCB, or its integral frost thermostat.

The Baxi manual does not show the frost stat at all (just mentions it in text). Could it be the same thing as its thermostat sensor (attached to the flow pipe)?

gas_man_gaz said:

its really common for the pump overrun on the solo's to fail keeping the pump running, so you will prob be looking at a new pcb (the pump overrun is controlled by the pcb)

Click to expand...

Thanks Gaz, I have had other boilers (esp Potterton) needing PCBs replaced, but before we put the total blame on the PCB on this one, how can I first check and eliminate the frost stat? Where is it anyway?

Where is your boiler? Look for a stat clamped onto the pipe.

Its going to be the PCB as I suspected firstly.

Dave T
IEng MIGEM

Yes, Dave, you might well be right.

The boiler is downstairs in the kitchen (the controls and cylinder are upstairs in a cupboard off my office).

The thermostat sensor fitted to the boiler flow pipe shows a resistance of about 1.3 M-Ohm at room temp, falling to below 1.0 M-Ohm as it warmed up in the exhaust fan from my laptop. After I put the sensor in our freezer for a few minutes, the resistance was initially greater than 2.0 M-Ohm (the limit of my multimeter), and then fell towards 1.3 M-Ohm again. So it *seems* to be working, BUT I am still not sure if this also has the function of frost stat, nor what the "good" readings should be.

I removed the PCB and it shows no external signs of component damage (such as blackened parts, melted tracks, popped capacitors etc). If I stare at it hard enough and long enough, I can almost see some faint discolouration of the white label on the mini-relay for the pump (on the PCB). Interestingly, that relay is the *only* component on the whole board that has a separate label stuck on top.

The white label makes me think that (1) it is such a frequent replacement item that Baxi have their own part number for it (it is identical to two other relays on the PCB, which don't have such extra labels), and (2) because they don't want us to know that it's a common item to replace (lucrative sales)! Cynical, aren't I? Comes from bitter experience with a certain major UK maker of large kitchen appliances.

If that relay *were* faulty (e.g. stuck in the "pump on" normally-closed position), then that would explain the particular problem with continuous pump running. I would need to either (1) try replacing the relay, or else (2) run the system with the PCB hanging out to test the relay.

The MV works fine with the boiler's PCB removed (and of course the pump remains off when the PCB is out).

Q1: Is that thermostat sensor the same thing as the frost-stat?
Q2: If it IS the PCB at fault, will it most likely be that relay?
Q3: Did I mention yet that this website is (thanks to its contributors) brilliant?

Its a failure on the PCB. I had the exact same fault on the exact same boiler. Replacing the PCB fixed the problem.

Even Baxi admitted its a common fault when i spoke to their helpline.

As mentioned, just get a new PCB, don't bother messing about repairing it.
Hope this sorts it for you

Dave