Worcester 240 - Hot Water OK, Central Heating Trips Overheat

I have a Worcester 240 boiler. The hot water works fine, but if I try to use the central heating, the heat does not leave the boiler (ie the output pipe stays cold), and the boiler overheats, tripping the overheat switch. When I run the hot water, I notice that there is no run on through the radiators, but when I stand next to the boiler, I can hear the pump running. It is as if the pump cannot circulate the water. I had the gas board out to this, and they have serviced the ignition system, and replaced the pump and the diverter diaphragm. They say that my pipes must be blocked, but I have a radiator right next to the boiler, and the water could flow into that one. The pipes literally just come out of the bottom of the boiler, into a tee, which on one side travels about four feet to the radiator before a return to the boiler. I opened the drain valve, and opened the water inlet to a high pressure, and water spurts out at full pressure from the drain. I doubt there is a blockage. Also, I am in a soft water area, and the pipes do not fur up here. I still suspect a fault in the boiler. Is there some sort of valve or something that may not be opening, preventing the water from circulating?
I have read lots of horror stories of people being sold power flushes but this not fixing the problem. The gas board want 500 quid from me to do a powerflush.

Not an expert on WB, but I would be looking at the diverter valve. Sounds as though it could be stuck in the DHW position.

I thought that, but the diverter diaphragm has been replaced, and on the Worcester, when the diverter is in the hot water position, a little button which I think is on the diaphram spindle, pops out, and activates the electrical switches that demand hot water. There is no hot water demand during heating, so I presume that the diverter is not stuck. I thought maybe there could be a blockage inside the boiler at that position. I wonder if there is some other valves inside the boiler that could be stuck. If I open the drain, and open the water supply tap, water sprays out of the output drain under pressure. If I then create a hot water demand whilst this is happening, I still get cold water coming from the drain, but the boiler heats up, and the overheat switch trips. It is as if the boiler is not releasing its hot water into the radiator output.

You have not made it clear if the pump is running on the CH mode.

You also mention BG as if they have only just been to it.

If the pump is running on CH then check the isolating valves underneath the boiler at the back as its sounds as if the BG man may have closed them when working on the boiler and forgotten to open them both !

It will not be the diverter valve as even if it were jammed in DHW the boiler would still control itself. In any case the pin would be out and you seem to imply thats not the case.

Boiler pipes block with metal oxides from the rads but this is unlikely as it happens progressively and not suddenly.

Very rarely the isolation valve heads snap or bend so they look open but are still closed. They usually leak on your boiler after they have been moved !

Tony

when the parts were changed, was the boiler drained by isolating it or by draining the whole system,with the age of your boiler most pros would know better not to bother with the iso valves. the reasons agile has stated.

He didn't drain the whole system, so must have used the isolation valves. This is a sealed system anyway, so it is easy to de-pressurize. Hi did inject a tube of sludge remover into the inlet tube, but this never circulated because I get no output to the radiators. I later drained the whole system. If the output valve is blocked, I would not get a jet of water in the output drain, when I open both the drain and the filling valve, would I? Is there some sort of "shunt" that allows water from the inlet, to go to the outlet, somehow bypassing the heating tank altogether?

The pump is working in Central Heating mode, but the output pipe does not get warm. The output valve is in the open position, but I cannot easily unhook it to check if it is open internally, because the pipework is tight around it. I really need to change the output valve, because when I close it, water spurts from the control screw. I wonder if I can get a spare.

Just to be clear British Gas had just attended the boiler, but left it in a non-working state, saying that I needed a system flush, and that I would have to pay £500 for this. I don't believe that there is a blockage in the radiators. If there is a blockage, I believe that it is in the boiler, or in the output valves.

Both the flow and the return iso valves have to be open to allow a flow round the heating circuit from the boiler. You did not mention the return valve !

I can test the opening of the iso valves by pressurising the system and opening the PRV in the boiler and seeing if water passes from the heating circuit into the boiler as each iso valve is opened on its own.

Tony

Wow! Opening the valves one at a time was a good idea. I closed the input valve, and pressurized the system. The pressure in the boiler increased, so it must have been back filling through the output. I then opened both valves, switched on the boiler, and the output pipe was hot. I left this running, and wahey ... my radiators are starting to get warm. I guess that there must be have been some crap in the boiler output valve, and the reverse fill dislodged it.

Cheers,

Mark.