Blockage in Central heating

chewycov

Right this is somewhat a follow up to my last thread.

We thought the issue with my central heating was my pump, I replaced it, I also had a gas safe pal replace the thermostat on the boiler for good measure, no real improvement. Decided to drain the system to see how dirty it was. It drained so much but the header tank in the loft didn't drain one bit, so clearly we have a blockage somewhere. I've read other forums across the Web and the general concensus for others with similar issues is that the blockage is most likely at the "tee where the cold feed meets the system". I don't seem to have such a thing, I've followed the pipes as best as I can and I assume they must meet actually IN the boiler. I'll go over the pipes in the coming days with a magnet to see if I can find the blockage, though I have a feeling that it'll be at the elbow that comes off the tank - don't have anything long enough to poke down there to see. If it's not there, does anyone else have ideas?

It's not a massive concern as the whole system will be ripped out to make way for a combi system in a few months, but it sure would be nice to save electricity in the meantime!

Alex1983

What boiler do you?

chewycov

It's a Myson Orion something or other. A plumber pal of mine reckons it's about 30 years old by now

Alex1983

Is it fully pumped? How many pipes come off the top of the boiler?

chewycov

There are 4 pipes off the top of the boiler, I believe one is cold feed in, one is return, one goes to the pump in the airing cupboard which then goes through a 3 port diverter valve and from there to where it needs to go. Unsure of what the 4th is. I'm not sure what fully pumped means but the pump in the airing cupboard serves the radiators and also to heat the cylinder

chewycov

Oh actually 4th pipe will be vent won't it?

Alex1983

Yes from memory the 4 pipe fully pump version was flow, return, cold feed and open vent.

w0z

First make sure the outlet of the header tank (inside the tank) is clear, they sometimes block preventing water flowing into the system.
It's probable that where the header tank feed tees into the flow (or return), crud from the header tank falls down the pipe and builds up at the junction. You'll need to get to that junction which will almost certainly be below the floor, follow the header tank feed pipe until it meets another pipe. You may need to cut out a section of pipe near the tee.
Alternatively if you can work out what to disconnect it may be possible for your plumber to use pressurised water down the feed pipe to clear it.

chewycov

The cold feed from the header tank seems to just go straight into the boiler with no tees anywhere on the way

J_B

We had something similar a couple of years ago.
Turned out to be one of the solenoid valves that had seized up!