howto drain and refill central heating

[hartnellg]

I'm planning to add some Sentinel X400 sludge remover to my central heating system and at the same time fit a towel radiator - I think I can get away with a partial drain

After I've run the sludge remover for a couple of weeks, I was then planning to fully drain down, remove and flush each radiator in turn, fit TRVs in place of the standard valves and also replace the lockshield valves that are leaking

I'm concerned about getting the filling correct so that I don't
a) damage the boiler by running it dry
b) damage the pump by running it dry

Where the pump is located there appears to be some sort of vent - the only marking I can see is 'Flamvent'. This was fitted over 20 years ago

I thought there was 'How to drain and refill a central heating system' but can't find it - anyone know where it went to

Thanks for any advice

 

[hartnellg]

I realised that I had given very little information about the system
So here are some more details:

Boiler is a Potterton Netaheat 10/15
Fully pumped two pipe system fed by feed/expansion tank in loft
3 motorised valves that isolate the hot water, ground floor heating and first floor heating
Where the motorised valve heating pipework combines at the pump is the venting device ("Flamvent")

The system was installed in 1984 by my (late) father, who had been a plumber since the year dot

Any contribution welcome

Thanks

 

[Expertboy]

Be careful with the partial drain, seen people do this before with bad consequences.

If you:

Knock off the boiler
Turn mains water off
All zone valves set to manual
Open up a downstairs rad valve
and then drain away until it stops.

Remember when filling you may need to check your motorised valves are still in the manual position to re-fill the system.

 

[Paul Barker]

To drain down, tie up float valve or whatever. Turn boiler and pump off. Manually open zone valves, attach hose open drain cock, starting upstairs open all rad vents and if fitted a vent on the hot water coil, move downstairs and open those vents.

When refilling make sure zone valves are still manually open, leave upstairs vents open, open all downstairs vents a tinsy winsy bit, then open one a full turn (I never open any vent more than a full turn unless it's blocked, usually half a turn is adequate) wait with it to fill move on, at least get some water in all downstairs rads before closing vents and going back through them one by one. Same for upstairs.

Usually this involves a little gambling and a lot of ruinning around and a few educated guesses but avoids air locks.

If you do get an air lock you have to start employing cunning.

I usually find starting pump on different speeds, stopping it and listening for air bubbling up the vent and/or feed. When it stops start pump again, ad nausium.

If when you come to reliught boiler a certain zone doesn't get hot, shut all valves to rest of system and see if pump will pump out the air lock, usually does.

Last resort, hardly ever necessary backfill. Involves a little gambling or plenty of confidence in overflow pipe.