Central Heating Confusion

I have a confusing problem with my central heating system, and any assistance would be gratefully received.

System is a fully pumped system fired by a Potterton Netheat back-boiler. When the system is activated, all seems to be correct: 3-way valve turns, pump starts and boiler fires. (Very) Hot water starts to flow into the 3-way valve and flows into the correct exits from the valve, depending upon CH/HW selection. The radiators normally get hot and the hot-tap water starts to heat.

However, after a period of time things normally start to go awry. Although all components seem to be running OK still (pump running, boiler firing), the water into the 3-way valve is not as hot as it should be. The radiators are warm, but not hot, and the hot water is the same. As the rads/water is not getting as hot as it should, the thermostats never turn the system off and the boiler/pump run all day to no effect (boiler turns itself off when thermal cut-out activates, then starts up again when cooler). System can be forced to start working again by de-selecting both HW/CH at the controller, leaving for a few minutes and re-starting everything.

I’ve tried bleeding radiators, but nothing comes out so I’m assuming that there’s no airlocks, but I’m not sure what else to try.

The last thing that I did to the system was flush with Fernox in March (but didn’t put in a treatment upon re-filling system), and system seems to have been running OK just on HW over the summer. Problem just seems to have started now we’re using the CH in earnest.

Other posts talk of noises coming from their system – this problem is causing a noisy whining sound to come from the missus, so any help would be gratefully received.

Could try lubricating the missus?

Thought of several reasons why this could be happening but have discounted them all - need more finkin time.
Pump, sludge, air... Is the boiler stat turned up ?

TJ said:

I've tried bleeding radiators, but nothing comes out . . .

Click to expand...

Not even water?

Potty Netaheat boiler? Never seen one.

If 'nothing' comes out of the air bleed on a radiator (with the system OFF), then you either have a major airlock or the cold feed to the boiler is empty or blocked. Water should come out of the bleed screw if there's nbo air in the rad. Check the following:
- sufficient water in the F&E tank
- water comes out of bleed screws on rads and (especially) from 'finger vent' bleed screw near 3-port valve (there SHOULD be one).
- no ticking or whooshing noises from the pump

If all are OK and circulation still not happening, call an engineer. It'll be quicker and cheaper in the end.

It sounds like the boiler is at fault, possibly locking out if its electronic ignition.

Cheers for the quick responses - first time I've tried this 'on-line problem solving' - it's great.

Apologies for the confusion - when I said there's nothing coming out of the bleed valve, I meant no air. Water is coming out of all radiators, as normal.

There is water in the header tank (what do you mean by 'sufficient water'), but cold water feed did get blocked on one occasion a couple of years ago. I had to cut pipes then, and I'm hoping that I don't have to this time. Could cold water feed blocked cause these symptoms?

Problem seems to be that (I think) circulation happens all of the time that the pump is running, it just seems to be that the temperature of the circulated water drops after a period of time.

Intrigued by the idea that problem could be caused by faulty boiler. Boiler is fading and firing again during running, but circulated water temperature seems unaffected by this. Could I be ignoring this boiler glitch and it's the main cause of the problem?

Level of whining will increase with onset of winter. May have to resort to croydoncorgi's suggestion of calling in someone who knows how it works.....

TJ,

The problem that you describe is symptomatic of a slow running pump. This can be caused by a build-up of sludge on the rotor or badly worn bearings.

If after refilling the system, the air was not fully vented from the pump's motor housing, then the pump may have been run dry. This can quickly ruin the pump's bearings because they are designed to be to water lubricated.