Boiler cutting out perhaps due to restricted flow or return

[zebu]

I'd be very grateful for help with the following.

Problem
The boiler cuts out 5 minutes after starting because it reaches 80 degrees. It then takes 10 minutes to cool down enough to restart, though it can sometimes be restarted by flicking the HW on and off or clicking the room thermostat on and off.
The cutting out happens even if all the radiators are off. If the radiators are on they can all get warm but not hot enough.
The problem could have been building up for several years.

System
Pumped non-combination open vent system with 16 radiators and one HW cylinder. All radiators have new TRVs except for the two in the room with the room thermostat. The boiler is 25-30 years old but seems to be working properly. The visible pipework is copper and has no isolating valves. The water is hard.

Actions taken so far
Tested boiler and room thermostats by clicking on and off.
Replaced pump (unnecessarily, in retrospect).
Tried all three pump settings.
Cleaned out F&E tank, removing lots of brown sludge.
Removed and re-attached two-port motorised valve for CH.
Power flushed twice, once through a radiator inlet and outlet and once through the flow and return near the boiler, finally getting clean water coming out.
Baffled heating engineer and power flushers.

Suggested diagnosis and cure
A restriction in the pipework (not the radiators) impedes flow. Removing it is likely to involve replacing pipework.

Questions
Are there any other diagnoses and cures?
Where is the restriction likely to be besides at the junction of the cold feed and the return?
What length is the restriction likely to have?
Which rooms should have their carpets and floorboards lifted first? The house has three storeys, with the boiler on the ground storey, and one room has a tiled floor.
How can the restriction be detected most easily? Putting a magnet next to a pipe might work for iron, but would not work for lime. Different pipes might have differences in temperature or in rates of warming up that a thermometer (or just a hand) could find.

Thanks

 

[gas4you]

Is this happening on HW as well as ch?

 

[zebu]

I don't think it happens on HW as well as CH. However, that's a little difficult to test, as when I've tried it the HW has already been quite hot and and the circuit to/through it is short, so I wouldn't have expected the heating to stay on for long. (I'm not at the house for a ccoulpe of days now, so I can't test immediately.)
If it happens on HW as well as CH it might point to the restriction being in the shared part of the return (or, I suppose, the shared part of the flow); alternatively it might point to a problem in the boiler, but the boiler thermostat clicks off all right.
If it doesn't happen on HW then the shared part of the return is free from blame.
Are there any other implications of its happening or not happening on HW?
Incidentally the return is arranged with the cold feed teed off before the cylinder and the radiators.[/quote]

 

[bellairaphon]

filter???? check below the pump or above the boiler on the flow. airlock too. what boiler is it?

 

[zebu]

Sorry, bellairaphon, I don't understand what you're suggesting.

The boiler is an Ideal Standard E Type RS.

 

[heatingman]

sounds like the spreader in the base of boiler has corroded and blocked up, and strangly enough they display similar symptons when they come away/rot. Reckon its time for a new boiler.

 

[Agile]

Although the OP has written a lot of words there is nothing about any temperatures around the system.

I would expect a thermal survey of the system would deliver some useful clues!

Tony

 

[zebu]

How would I tell whether the spreader has given way or there is a restriction in the pipework, heatingman?
Temperatures have been measured with a thermometer on the outside of the boiler only. The temperature there is about 80 degrees when the boiler cuts out (as expected) but continues to rise to about 87 before falling slowly to about 67. The heat from the each radiator was erratic and unpredictable, perhaps because of air locks or sludge moving around. It may now have settled down to gentle warmness in them all. What measurements do you recommend, agile?

 

[Agile]

A good idea of whats going on can be obtained by measuring the flow and return temperatures at the boiler and at each radiator.

In a properly functioning system the flow temperatures should all be around 80° C and the return at about 69° C.

Of course if the boiler thermostat is set lower then the differential should still be about 11° C.

It should operate with the pump setting at "2".

Tony


Edited to correct the misprint where it said the return should be 79° in stead of 69°

 

[Paul Barker]

It could easily be the boiler heat exchanger.

You don't have to lift boards, just put the powerflush on the furthest rad valves, shut all rads but one and isolate boiler. Step through the rads one by one and finally the boiler.

You will definately find the area of the blockage. Then you know either to lift that particular floor or throw that particular boiler in a skip.

 

[Deleted 98600]

on older types on systems, sludge can cause the boiler stat to operate simply because the water isnt going n e where.

 

[zebu]

Thanks. I'll try surveying the temperatures. If that fails to show where the restriction is, power flushing at each radiator inlet and outlet could do so, but I'd probably make a mess. Any other ideas? These are all helpful.