Last rad cold at the bottom - bit of an odd one as I don't think its blocked...

[tolly]

When you next take the rad off get a bucket and an old washing machine hose. Screw one end of the wash mac hose onto the rad valve and put the other end into the bucket and open up the rad valve. Check to see that the water flows freely into the bucket and this will tell you whether there is any blockages in the pipework feeding the rad.

 

[tolly]

Do so on each of the rad valves

 

[M7R]

Ok will do next weekend when we whip it off again, it seems to be flowing really well now though all the others are turned off,

 

[michael c]

Tolly gives good advice there mate, i would expect the rad to have flowed as only route now it does sound though that your bay window rads need restricting

 

[CM25]

Sounds like a circulation problem to me not the rads as it's hot hot all over when uve shut everything off
Check if a bypass is fitted and ensure its not fully open, does the problem occur all the time or just when heat and hw on? If this is the case a balancing valid on hw rtn maybe req, do u have a combined air/dirt separator for the cold feed and vent cos I've know them to get blocked, check the pump, if the systems old it can block/restrict the unions on the pump but the pump appears fine.
Hope this helps but definitely sounds like a circulation/balancing issue not the rad

 

[M7R]

The pump is as old as the boiler, I have stripped and cleaned it as it got blocked on the last clean due to the crap x800 which had been frozen... Cleaned and cleaned the system, all is good. The pipe work Round the boiler and pump is new too as it was all changed when the boiler was added.

hw balance is only 1 and a bit turns open, but the problem is the same even when he is not demanding heat and the valve closed.

i have balance the system again and the problem is back, I'm sure the bays are t helping, as they have old valves where it's only one turn from closed to open, both are set about 1/2 open or just under, this is where the inlet is very hot and the outlet is just warm to hot.. So a out right, the rest of the rads are about the same with the valves being new and about 1 to 2 turns open apart from the hall which is wide open.

 

[CM25]

How bout the bypass it's btwn the pump and the 2/3ports and goes straight to the rtn, and if this is too open it'll short circuit back to the boiler

 

[CM25]

Also check the cold feed/vent arrangement, if uve got a combined one its possible it's blocked. Where's the hall rad that's fully open in relation to the one that's not working, and waters lazy an it'll take the easiest path.
Just to confirm if the other rads are off the problem rad is hot all over?
Still sounds like a circulation fault tho

 

[M7R]

I have a separate vent and feed, when we drained and refilled the system the vent was gurgling and spitting water every so often so I know that's clear.

the problem rad is the hall one, is the last on the loop and a 1.4m single, which comes after the 1.4 old rad in the dining room bay, a big old rad with out fins, -this is being replaced in a couple of months, along with the old bay rad in our bedroom.

if its still a prob after the other 2 old rads have been replaced then ill be annoyed..

 

[StillLearnin]

Have you considered pressurising the system, that bit of extra pressure might help the flow a bit.

 

[michael c]

A Friend of mine recently had a problem with his boiler over pressurising due to a failed part, he was convinced he could fix without replacing the part. He drained down and flushed his system numerous times and was convinced the system is clean. He has disturbed debri in the return leg from a rad and now has similar to yourself.
From everything you have written i still believe you need to powerflush the system and balance correctly as you have indicated all rads are currently set the same.

 

[M7R]

I thought of that but was advised against it due to some bits of the system being 40 years old, there was a higher risk of leaks, and little to gain really.

 

[StillLearnin]

There is obviously a risk but more often than not it only ever helps and is normally pretty straight forward to achieve. Make sure your boiler has an overheat stat on it though.

 

[CM25]

Did u find and check the bypass?

 

[M7R]

I haven't seen any of the pipe work as it is all under the floor boards, I will ask next door next time I see him in the week, see if he remembers how he imbed t all in, but I doubt there is a by pass