central heating problem

We have a problem with our central heating.

We have a Main 32he combi. When running the central heating, the boiler gets up to heat and then cuts out, cools down and the heats up again. If we turn off the upstairs radiators, the downstairs seem to work fine and visa versa.

Do you think there is a blockage? Anyone have any suggestions to clear it? i have noticed that one pipe running under one rad doesn’t seem to get very warm. If I isolate this, the next rad gets hot, open it up and the flow seems to stop.

The boiler has been checked and we have been told that its fine and must be the pipework for the heating. Also, there are 2 drain offs under this rad, and I cannot get either of these to release any water

Any help or suggestions would be great! Very short of money at the moment so anything that I could try myself would be very helpful

Thanks

Rob

The system needs balancing you can do it your self

I did a hunt for boiler installation manual and got Main 30 HE which says output 11.9 ~ 30.9 kW so it is designed to modulate (turn down output) this means the return temperature is important, so every radiator needs to have the same temperature drop across it, normally set to 20°C but big thing with all TRV wide open it needs to be the same with each radiator.

This is set with the lock shield valve, and you would not believe how many times I have found one wide open, what this means is the water returns hot from that radiator and turns down the output from the boiler.

With no thermostat what I would do starting closest to boiler turn off lock shield valve wait for pipes to cool, the turn on ¼ turn at a time until one pipe get warm then move to next, not how it should be set, but it seems to work.

thank you, i will try this out.
one thing to note that this is a new problem, its actually my sisters house and she had an engineer out as the boiler stopped working when it was freezing a couple of weeks ago. turns out that it was a frozen condense pipe, but she still had the engineer out to make sure everything was ok. when he arrived the heating was working perfectly. the engineer drained the boiler for whatever reason, and since then she has had this problem.
before that, the heating was working perfectly fine!
just a coincidence?

Ok so Ive tried to balance the radiators and I have them all hot! Which is better than where we were last week. however I still have the problem with the boiler starting and stopping.
the boiler fires up, gets up to 80c and runs for 20-30 seconds, then the flame cuts out. It cools to 30c and then fires up again. And this cycle just going round and round. many more suggestions please?

Guys
Can anyone help? Getting a bit desperate now

It might be an air lock close all the rads except one downstairs, when that gets hot open a different one and close the one that's got hot......do this throughout the house on all the radiators......check if the boiler needs manualiy bleeding in the manual

Oh and bleed each rad one by one when your heating them up and don't forget to keep the pressure topped up at the boiler.

It will be an airlock balancing doesn't cause this kind of z problem.