Boiler running fine and HW production fine also, but nearly all the radiatiors are not really heating up and then only at the top and are cold at the bottom.
Another thing I’ve noticed is that the radiator in my bedroom used to make lttle pinging noises as soon as the heating came on which I thought was due to the increased pressure from the pump – this doesn’t happen now.
My feeling is that the pump is pooped – any other suggestions other than bad CH Aids? Pump is a Gundfos and is about 10 years old.
Any checks I can carry out?

You can hear a pump running and feel it vibrate if you touch it. If you’re comfortable using a multi-meter you could just see if it is drawing current (although if you’re not happy with live mains I wouldn’t try this at home kids).

Another symptom of a dead pump is the boiler short cycling, firing up then shutting off every few minutes as the heat isn’t taken away quick enough and the heat exchanger hits cut off temp too quickly.

Another symptom of a dead pump is the boiler short cycling, firing up then shutting off every few minutes as the heat isn’t taken away quick enough and the heat exchanger hits cut off temp too quickly

I should add that it’s a very old boiler that is way too dumb to do something like this!
There is ‘some’ vibration when the heating is on.

The only other thing I can think of is that the pipework is blocked somehow. A flush seems fairly pricey though.

http://www.ebuild.co.uk/topic/4808-radiator-hot-at-top-cold-at-bottom-whats-wrong/

your pump circulates water through both the radiator circuit and the hot water circuit, so if you’ve got hot water then you’re pump cannot be goosed. It is most liktly to be your motorised valve as this is the valve that diverts hot water from your boiler to your hot water circuit or your radiator circuit or both.

just checked the MV and it’s moving when the heating comes on – if it wasn’t working surely no hot water would get to the radiators?

Sludgefest, possibly, but if it’s a sudden change, an obstruction or a part-malfunctioning valve is more likely.

Do you have TRVs on the rads? Try shutting off all the rads but one, and opening that valve right up to give it the full force for a bit. If your pump has a higher setting you could give that a shot at the same time.

does your system have inhibitor in – and has it always had inhibitor in ?

id be tempted to drop the water – take an affected radiator into the garden and give it a flush with the hose if the TRV trick doesnt work. easy check for broken TRVS – are the inlet pipes hot and rad cold or the whole lot – if the whole lots cold id be thinking blockage/valve or airlock but if the inlet pipes hot and the rads cold id think sticking TRV

Unlikely to be the MV as they normally seize shut off, which means no heat / hot water.

Pump could be very weak, or your HW / one radiator is shorting out the system when the CH is on and re-circulating all the warm water back to the boiler….

Sludgefest, possibly

That’s a lot of sludge to half fill all the radiators….

They are in parallel, so for them all to heat up only at the top, you’d need them all to be half full of rust……

Realistically they probably aren’t as well treated with inhibitor as they should be. The system has been drained within the last 6 or 7 years if that makes a difference.
Feeds to the rad aren’t particularly hot and nor is the outlet from the pump.

Last time this happened to us it was the motorised valve, old two way ones so one for CH and one for HW, CH one looked to be moving but wasn’t.

We had this and I got my mate round who’s a plumber. Everything checked out OK (pump, boiler, TRVs, etc.) so he did a power flush and after that everything was perfect. Equally hot radiators throughout the house.

You should be able to manually open the motorised valve by taking the ‘cover’ with the motor inside off it. Would help you exclude that as a cause.

You should be able to manually open the motorised valve by taking the ‘cover’ with the motor inside off it. Would help you exclude that as a cause.

They normally have an over-ride lever poking through the top of the lid…..

Nobody has mentioned balancing the radiator system yet.
May be that the flow through them is too fast.
Back the outlet valve right off to closed then open to about 1 to 2 turns, the water will then flow through slower and give up? its heat to the radiator/room.
Worth a try, but thinking back it does sound like sludging in the radiators.
If the boiler is an old cast iron heat exchanger then more likely to be iron deposits/sludge in the rads if a decent/any inhibitor has not been used.