Central Heating - how to find the blockage?

[darkwood]

My CH system is an old single-pipe system about 30 years old. The boiler is about 15 (Potterton Kingfisher II 50, the pump new and most rads have been replaced.
The downstairs rads take a long time to warm up and even then they are only warm at one end and cold at the other. Upstairs is not bad. The header tank fills with hot water hence theres a lot of noise and air in the system. This only started this winter. The furthest radiator is cold, though its never been very warm.. As a test I reversed the pump then the furthest rad was very hot but the rest didnt heat much at all.
I guess there is a restriction but where is it likely to be? I've tried flushing the system and I've cleaned all the rads. They werent too bad.

I thought about converting the downstairs to two pipes while upstairs still one pipe. But it should work better as is.

Any help / ideas much appreciated.

 

[gas chap]

have you tryed balancing the system, ie turning the closest one to the boiler down etc

 

[darkwood]

Yes I've tried shutting the early ones off completely. It only helps a little. The water in the pipe is reasonably hot but it doesnt rise up through the rads and they are clear. Its like there isnt enough flow. The pump is in the return and the temp there is 45deg - same as flow into the lounge radiator which is 3rd from last. There are all bled regularly because of the air problem.

 

[hellomum]

what speed is the pump on, try shutting the bypass gate valve if it has one

 

[chrishutt]

There are all bled regularly because of the air problem.

If you've had a long-standing air problem you probably have a serious corrosion problem as well. The result will be sludge collecting in rads, especially at low points in system, and lining pipes. You need an expert eye to identify the root cause of the air problem and to carry out remedial treatment of the system.

 

[darkwood]

The pump is on the highest speed and is a 60. Ive tried lower speeds, just gets worse.

The rads are all clean. The pipes may be sludged though ive tried to flush them out. Should I replace the pipes or can they be cleaned out? I've tried a chemical flush agent.

 

[gas chap]

If you have drained the sytem,

does it take a long time to re-fill ?

 

[darkwood]

No, I wouldnt say was slow to fill up.

 

[gas chap]

How many pipes have you coming off your boiler ?

 

[darkwood]

There are four pipes to the boiler. Near the top theres a 25?mm going up to the tank and a 22mm from the pump. Near the top theres a 25?mm going up to the tank and a 15mm going down through the floor.

thanks for replying.

 

[gas chap]

Ok fist of all if your pump is pumping back to the top pipe of the boiler turn it back round.

are you including the gas pipe in the 4 pipes?

 

[darkwood]

AH! the 15mm would be the gas... doh!

The pump returns to the bottom of the tank. I put 'top' twice sorry it was late!

I was thinking of cutting the pipe from upstairs to downstairs and attaching a mains hosepipe and trying to flush the downstairs. The drain is on the return by the pump. Would this be enough pressure to do any good?

 

[ChrisR]

First I'd check the pump. If you take the 4 allen screws out you should be able to split it and look at the impeller - two discs, which can fill with grot so the pump does nothing much. Then, see how hard it is to stop the pump spinning. If you can do it by pressing against the flat of the impeller with your little finger it's probably had it. You should need strongish thumb pressure.
I've read here that really weak ones can be stopped with a screwdriver through the hole where the big central screw is, but never tried it. I imagine they'd have to be very weak to be able to do that (esp a 60).

Flushing with the mains can do wonders, though the flow through a drain cock isn't great. You can remove a radiator and put a garden hose on the rad valve union.

 

[gas chap]

Are you shure darkwood that the 15mm pipe is the gas?

 

[darkwood]

I think its the gas, I dont see any other.

Chris, I have a new pump, it was a 50, I got a 60, it makes no difference if its on middle or top speed. I've cleaned the rads out.