Two issues with Worcester MT10 boiler

[JoT]

We have two problems associated with a Worcester MT10 boiler.

1 - Its pressure was reset only 3 weeks ago but it has dropped back to nearly zero despite still working well. I read that this suggests a leak somewhere. However, there is no visible sign of any leak. I have bled all the radiators but only water came out.

2 - An additional problem is that one of the radiators remains cold. If the central heating is put on high, it gets mildly warm (barely perceptible) near the top at one end only as if the thinnest trickle of warm water is getting in. Only water comes out when it is bled.

Any suggestions would be much appreciated. Thank you

[aeromech3]

1) Do you mean the pressure gauge reads Zero but the boiler continues to work?
As you had trouble 3 weeks back, I would re-check all the parts you touched and that the valves are back to the original position.

[JoT]

Yes. The boiler was actually working alright 3 weeks ago but because the pressure gauge showed almost zero we thought we should repressurise it. It has dropped back down again but still continues to work well. We have ensured the valves are back to their original position.

[aeromech3]

Just possible the gauge is faulty, but your boiler is a Greenstar what model, as MT10 refers to the timer?

[JoT]

D'oh!. I'm not sure. It has an installation sticker for a Worcester Greenstar 25CDi/30CDi/35CDi/40CDi

[aeromech3]

Well there is a leak somewhere, have you checked the pressure relief (think the copper pipe going across your other 5 vertical pipes leads outside the house and comes from the relief valve) as this might be leaking ?
A combi boiler should not run without water pressure and can be dangerous if it does.
Report back and maybe Razor has time and some ideas.

[ericmark]

I seem to remember my mothers greenstar would run with no pressure, there is a flow switch rather than pressure switch, so as long as there is flow it would run.

However you clearly do need to find where the water is going, and also get all radiators working. In the main radiators not working is down to balancing, each radiator has two valves one each end in the main, there are some where all one end, but the one which controls room temperature the TRV (thermostatic radiator valve) is slow moving, even my electronic type take 3.5 minutes to exercise midday every Saturday, so it is important the radiator does not have too much flow so there is time for the TRV to work before the radiator gets stinking hot and causes an over shoot, and a large hysteresis in room temperature.

This is done using the other valve (lock shield valve) we are told there should be around 15 degs C difference between water temperature in and out, however most people don't have a differential thermometer to measure this.

So in the main you close the lock shield, then allow pipes to cool, then open 1/4 turn at a time until one pipe starts to get warm, starting closest to boiler, and working out. As well as allowing time for the TRV to work, it also means there is a pressure differential between flow and return, so all radiators get a share.

My old house you would see no leaks in winter, but in summer you would see a slight leak from some lock shield valves, adjusting the balance you may find the leak, there are other valves, I think the by-pass valve is inside the boiler, but for some reason mother also had an external by-pass valve, which had been boxed in.

Faulty expansion vessel can cause loss of water, there is a small tank with a diaphragm in it, gas one side which can be compressed as the water expands and water other side, normally a tyre type valve to re-gas it, if the diaphragm fails, and it gets water both sides, as the boiler heats up the pressure release valve will let excess pressure out, and once cool there will be no pressure.