Worcester Combi 20/25 Oil Boiler - Central Heating Pump

[RJA67]

Hello everyone,

I'm new on here, so please bear with me


I have a Worcester Heatslave Combi Oil Boiler 20/25, installed in 1998. I've had very little trouble with it over the years, well nothing I couldn't sort myself....eventually! However, recently I have developed an issue which I can't get to the bottom of.....


My boiler is starting and heating up fine, but the CH pump doesn't seem to be running to pump the water out to the radiators. The diverter valve is fully swung (powered) over making the micro-switch (checked by multimeter) which, to me, should initiate power to the pump motor? The pump doesn’t seem to be running though, so the boiler internals are understandably really hot. However, when running a hot water tap, water then seems to be pumped to the radiators and they become red hot! So, the pump is working, but not as it should in my opinion. The hot water is hot too, by the way.


Once the radiators have been supplied with heat, the CH stat on the facia becomes responsive again i.e. it will on/off the burner. This stat is very course to move though, whereas the HW stat is free (recently replaced). I'm thinking to replace the CH stat due to it's sticky nature.


I have checked the diverter valve and it is free to turn by fingers. I applied a bit of lubricant to the spindle and reassembled the top parts.


I replaced the Flow Switch two days ago, as it was weeping and I couldn't get it tight (I eventually cracked the casing of it). I suspected that it might be part of the problem. Alas, I think it has helped with another issue, but not this one.


Do you have any other suggestions as to what might be causing this problem? Could the internals if the diverter valve be an issue?


Regards,
Robert

 

[Terrywookfit]

There should be no flow to the rads when running a hot tap.

 

[Gasguru]

I'm not familiar with these models but the manuals got a good wiring diagram so you should be able to see where the problem is but for starters I'd take a look at the pcb and have a good look for broken solder joints and whether the relays contacts are cooked.
The heating stat sounds iffy too.

 

[RJA67]

Thank you for the replies so far.

Update: I had a shower this afternoon and the water was ok. I felt it could have been warmer though. When I got out the radiators were piping hot and the Central Heating was off. Later on I had another play around with the CH dial on the facia panel. This time it went past it's stop point i.e. the dial is going around 360 degrees, so the stat is now officially goosed. I think it was on it's way out anyway. Luckily I have one on order.

I have also noticed that the bottom area of a couple of lower floor radiators are cold, whilst the top of the radiators are roasting, So it would appear I have a serious sludge issue too. Tomorrow will be a flushing out day!

Once the system is flushed and the new stat is fitted I'll see where were at then. I'll have look at the PCB too. I have the drawings.

 

[Gasguru]

I have also noticed that the bottom area of a couple of lower floor radiators are cold, whilst the top of the radiators are roasting

If the difference between the pipes is no more than say 15 degrees you almost certainly do not have sludge.
99% of diyers that post on here about radiator sludge actually have a simple balancing issue due to a large temperature difference between the radiator flow & return pipes.

 

[Terrywookfit]

The diverter valve needs to be checked for correct operation before any flushing is done!!

Sounds like it is sticking or fooked.
Is it the VC series head (plastic) or V404F type.

 

[RJA67]

The diverter valve needs to be checked for correct operation before any flushing is done!!

Sounds like it is sticking or fooked.
Is it the VC series head (plastic) or V404F type.

Terry, it is the V4044 type.



At the moment, the actuator motor and the diverter valve are free to turn. I've replaced the actuator motor on numerous occasions, it's the most common fault I've had over the years. I'm suspicious of what's going on inside the valve though, as I can't understand why hot water is getting pumped to the radiators when we turn on a hot tap? I don't know what the valve internals consist of i.e. metal/plastic/rubber etc?

 

[Terrywookfit]

It's a rubber ball that sometimes detaches itself.

 

[RJA67]

I don't know what the valve internals consist of i.e. metal/plastic/rubber etc?

I've now googled it and think I've found out.

 

[RJA67]

It's a rubber ball that sometimes detaches itself.

I see. This is actually starting to sound more like it could be the problem.

I have also noticed that that water pump is scorching hot when the central heating is on and no heat is coming to radiators. Could it be that the pump is trying to pump against a closed valve, so it is just overheating as it can't pump? Then when a tap is turned on, the valve is open and the heat goes out to the radiators. It's like the ball is 90 degrees out?

 

[Gasguru]

Extremely rare for the ball to fall off...seen a few times in 20 years.
If the seats are ok a replacement ball & plate kit is cheap.
Cheap motors can magnetise in the open position and the springs can fail to return the mechanism.

Untitled

• Gasguru
• 30 Sep 2008
• 1

 

[RJA67]

Mmmm, that's not what I thought it looked like. Obviously I've found the wrong type . Looking it again, it may be the VC type of internals I found. It was a cartridge assembly.

 

[Gasguru]

Are you sure you don't have a convention Honeywell V40... valve?
The picture shows an old style actuator with the ball & plate...missing the valve body.
The VC type actuators can't be pulled apart and the motor is not accessible.

 

[RJA67]

The motor is accessible on mine, as I've changed it many times. By "looking at it again" I meant looking at what I'd found on the internet for the internals. I think I found VC internals.

Mine looks like this.....

 

[oilhead]

On heating only, it sounds like you are pumping against a closed valve. If you are getting a cool shower and hot radiators, then something is opening the valve when you want hot water only. From 1998, the original actuators were changed, and if you have replaced the actuator, then the wiring may not be compatible. From memory, a small change was required, so check your wiring at the PCB.