Cold Central Heating after draining and refilling

9 Jan 2012
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United Kingdom
Any suggestions please ?

Moved into a house a couple of years ago that had air continually circulating no matter how much radiators were bled

Recently had a leaking radiator valve (bedroom upstairs and furthest from the boiler) so drained down system and replaced. Refilled system from make up tank in loft

All radiators now NOT getting any heat

Towel rail in the bathroom almost above boiler gets plenty of heat.

Heard a couple of 'clunks' in pipework but nothing like the air circulating in pipes as before.

All radiators are full of water with no air arriving when bled

From cold pump appears to run ok and boiler ignites. After a couple of minutes the flame goes out on boiler thermostat as if local part of circuit is satisfied heat wise.

The pump (in airing cupboard upstairs) continues to run

Any suggestions to try please. Could it be an air lock ? or perhaps the pump is not pumping even though it feels like it when touched, An internal slight vibration can be felt

Any suggestions will be most welcome

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I put my own system in years ago and have had the same problem once, it turned out to be the CH zone valve that wasn't opening even though it was being called to open

You might have a diverter valve but same problem might be happening

Check this first in my limited experience
Hello craftor

There are no zone valves or anything of the sort. It is a very basic system and badly needs updating.

System consists of a boiler, basic timer/programmer, circulating pump in airing cupboard and radiators plus heated towel rail in bathroom

Thank you for taking the time to reply

in that case hit the pump with a hammer gently could be sludge build up had that before as well, it sounds like its turning, you will still get a vibration but might of stalled

Hope this works
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Me again

I gave the pump a tap with a large adjustable spanner and it spluttered and whined (loudly), halted briefly then started turning again.

Unfortunately it hasn't cured the problem so I think I will bite the bullet and fit a new pump. God knows how old the existing one is !.

Details on the existing pump are:-

Two Speed Grundfos Super 4

No - F804 Type - UPS 18.45 Class F r/min - 2500 44W 220V 4uF

I am looking on the Screwfix (very local to me) website for a replacement pump and they have the following one in stock which physically fits in between the pipe connections:-

Grundfos UPS2 15-50/60 Central Heating Pump

Product Code: 42635

Suitable for working temperatures up to 95°C. 1½" BSP connection. IP42 rated.
•3 Speed Settings
•ERP Compliant
•2 Year Manufacturer's Guarantee
•6m Max. Delivery Head
•1½" BSP Connection
•2°C to 95°C Max. Temperature Range
•3m³/h Q Max. Flow Volume
•IP42 Rated

240V. 50/60Hz. 130mm port to port. Max. temp. range: +2°C to +95°C. Weight 2.65kg.



Would you be able to tell me if this is a suitable replacement when comparing specifications please ?


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yes thats the standard pump for most houses. Unless you live in a mansion or skyscaper it will be ok

As i'm probably tight on spending money I dont have to I would completely flush the system from the lowest point keeping the water topping up the expansion tank

Turn the boiler temp to zero but keep the CH programmer on and keep tapping the pump until it frees up. If its heavily sludged you might get the same prob with your new pump

When its running free for about 1/4 hr turn off CH,shut water to expansion tank to completely drain the system

Add boiler treatment to expansion after closing drain valve, or inject into any radiator, top up system whilst bleeding the radiators, downstairs first
and voila all should be well

Fingers crossed
Have you balanced the radiators?

I have had the same problem as you in the past and it was the rads needed balancing.

How I do it is turn off all the radiator valves and turn the heating to constant and then one by one i open each valve a quarter of a turn.

You just need to remember that the water will take the path of least resistance so if you have all the valves open roughly the same then the water will flow evenly.

Also turn the pump up to number 3 (the most powerful setting) if you have this option as that will get the water flowing faster.

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