central heating draindown problems

[IanJ]

Hi

I'm installing a couple of replacement downstairs rads - different sizes to originals so need to drain down system. Not the first time it's been done, but first time I've done it myself. I'm planning to leave it drained down for a couple of weekends while I do the work with decorating etc (and while the weather is so warm )
System is standard gravity-fed - a 20-year old mixture of 15mm copper/plastic and 10mm microbore, powerflushed a couple of years ago.

Followed steps of plumber who last did it a few years ago when fitting TRVs - turned off CH, shut off header tank supply, and drained via hose to draincock on lowest rad. Opened upstairs rad bleed valves and they drained quickly - but 3 downstairs rads still have water coming from bleed valves, and no more water from hose. Took off one of the old rads to be replaced and opened valve to let out remaining water - but 3 rads still don't drain. Header tank empty.

I now have both old rads removed - no water from valves, so I can get on with the job - but what have I done wrong?

Also, I assumed HW would be fine to use while CH drained. But when HW on, boiler and pipes make banging noises. Again I can manage with immersion for a couple of weeks until system is refilled, but would prefer not to - what have I done wrong?

Any help appreciated.
Cheers

 

[45yearsagasman]

With regard to using the boiler.........you cannot while the system is drained as the HW primaries would also be empty.
The rads you couldn't drain, are they connected to drop supplies? If so then they will not drain via a hose and would have to be manually drained.

 

[IanJ]

Thanks for the quick reply.
Immersion it is for the hot water for a little while then!
Not sure about the rad connections to be honest - but I'm assuming they are connected as you suggest. The pipes I need to work on are dry now, so I'll carry on and then refill the system in a week or two when the work is done.

Any tips on avoiding an airlock when I refill? The boiler was banging away for a couple of mins this morning when the HW came on, before I got to it to switch off ... does that make for a bigger airlock risk when I refill?

Cheers