Noisy Boiler after flushing

We recently had to change the Grundfos pump on our central heating system. I bought it and a local plumber/heating engineer fitted. he said that the previous one had failed due to scale in the system. (The system was probably fitted about 20 years ago when the bungalow was built), so he put some flushing compound in the system and advised us to leave it in for a month or so, then he would drain down and refill, adding an inhibitor at the same time. This drain down was done about a week ago but the inhibitor was not added because he was \\\"....unable to get out all of the flushing compound because there is no drain cock on the boiler....\\\" and, being in a bungalow, the feed from the boiler to the tank goes up into the roof and then back down again. Ever since the system was drained and refilled the boiler has sounded like an old, rather large, kettle. Lots of bubbling and banging. Prior to this latest work the system had always been perfectly quiet.
My first reaction is that there must be air in the system in the region of the boiler. All radiators (which have individual feeds from the tank), the towel rail and the water system are getting hot O.K. and there is no sign of air in any of the radiators. Is there an air bleed valve on the boiler? It is a \\\"Baxi FS501RS\\\", and is it true that there is no drain tap on it. The \\\"heating engineer\\\" supposedly serviced the boiler at the same time as draining and flushing the system. Any advice would be most welcome. Thanks in advance. Tony Norton.

It is possible to put inhibitor in the system while the flushing chemical is still there, but not desirable. Once it is flushed though, the dilution should be adequate for the inhibitor to be added with out a problem.

There should be an air bleed point at all of the highest points in the pipework. If there isn't, it's worth getting some put in.

The noise sounds like there is air in the boiler.

Thanks for your prompt response Oilman. There is on the 'out' side from the boiler a standpipe that has a 'gismo' on top of it with a Schrader (car tyre) type valve on the top of it. I depressed the valve core and it bled water. As I was reluctant to flood the loft I only let a little go so I don't know if there was any air behind the water. Is this supposed to be an automatic bleed valve? It is not at the highest point in the system, the top up tank is the highest, in the apex of the roof. Is it possible for these valves to malfunction? If so, is refurbishment a matter of shutting of the stop valve below it, removing and cleaning, or is it a replacement job? If it is not automatic is there a special tube one can attach to it to bleed into a bucket or into the top up tank.

There is what appears to be an overflow return. An open pipe leading back to the top up tank which is gently spurting hot water into the tank when the system is on.

Any further advice would be much appreciated.

Tony Norton

.....All radiators (which have individual feeds from the tank).....

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This is unusual, is this waht you mean?

What is the "gismo" what colour and shape is it? It sounds like an expansion vessel, but that doesn't sound right with a header tank in the loft.

There shouldn't be water spurting into the tank. That indicates the heat is not getting away from the boiler. It could be debris has accumulated in the boiler, so you may have to disconnect the boiler pipes and flush it through if there isn't a drain.

Hi Oilman,

["This is unusual, is this what you mean?"]

Yes. There are a mass of small bore pipes running from the main pipework by the tank. The towel rail even has it's own supply from the main feed, just after the pump, with a valve, between the 2 take off points, that has always had a label hanging on it marked 'under no circumstance should this valve be turned off'. I guess this is probably because to do so would divert all the output from the boiler thru the towel rail and its supply pipes.

The ' gismo' mentioned appears to be of solid brass (or bronze) construction, is about 3" to 4" high and about 1.5" to 2" dia at it's largest point and made up of 2 or 3 pieces (all the foregoing from memory) screwed together. The Schrader valve is at the very top of it. The whole assembly is at the top of a vertical straight length of pipe that tees into the pipe from the boiler. I thought this was possibly an automatic bleed valve, as mentioned by the 'heating engineer'.

I think your comment on debris accumulating in the boiler is probably valid. The descaler will obviously have loosened up a lot of crap and the boiler, being the lowest point in the system, is a likely place for it to land up.

If ["the heat is not getting away from the boiler"] could this mean that there may, in addition to debris within the boiler, be a partial blockage in the output pipe? I suggest 'partial' because the hot water system and radiators seem to be functioning O.K. Have you ever come across a boiler without a drain point?

Thanks again

Tony Norton