DIY Doctor UK DIY Forums

Our central heating was power flushed a couple of years ago but, despite the use of inhibitor, the sludge is back.

We recently had a new boiler installed and the plumber said it's an open system rather than a closed one with an expansion tank. As such he said it will always let in air and sludge up over and over again.

My schoolboy chemistry says this makes sense. Radiators are steel, oxygen dissolves into the water, particularly in summer when the water is cold and rusts the rads (and possibly the copper too) from the inside.

If this is true why aren't all systems closed?

Hi chasman

You have a valid point. But, a correctly designed open system will not sludge up in five minutes. In fact, many years ago that would be the only way, and without any inhibitor. It really all comes down to proper design of the system.

These days sealed systems are common, and some installers will use them all the time, but there are a few pro's and con's. A system should always be designed to suit the property, and the users requirements.

In your case, I would suspect that there must be some other problem with the system causing it to sludge in such a short time. This really needs looking at before the new boiler becomes damaged.

• What are the pros of open?
• What are the cons of closed?

It's a two story barn conversion with rads upstairs and down. We have a hot water tank upstairs with no complementary immersion heater All upstairs rads now have thermostats which were fitted at the same time as the boiler. Boiler is downstairs. Pipework is a mixture of copper and plastic... the plastic pipe appears to be of poor quality as our insurer refused to cover it.

What other details are needed to get some idea of the problem? What are the likely scenarios for the sludging and how do I determine which one is correct?

There are other supposed problems with the design which I'll mention in case it's relevant.

1) The system apparently doesn't have a "bypass circuit" and the new bloke seemed to find that incomprehensible... we were told it did and to never shut the valves on one rad near the hot tank. New crew have fitted a thermostatic valve to it. I don't know who to believe.

2) We had a MagnaClean fitted and he reckons it's in totally the wrong place (up by the hot tank instead of downstairs by the boiler) filtering water which has already been through dirtying up the system. They didn't find anything in it after two years of use so he seems to have a point. It was fitted by a reputable supplier. Again I don't know who to believe.

I just want a definitive answer to end all the uncertainty and give me system which runs efficiently for the next decade without any expensive fiddling about.

Put it this way, my own open system is now about 30 years old. When I installed it, although the components for a pressurised system were available, I chose to go for open in common with most of the systems we were installing at the time for the sake of simplicity. I did not want a pressure vessel, filling loop, and gauge in the airing cupboard.
OK, I'm lucky to have a loft for the tank, and it would be unlikely to freeze up, but I know that the pressure is always constant, and as the design is correct there will not be any problems.
In all these years I have not had sludge problems, replacement valves etc, and a new boiler, yes, but not sludge.
I'm not trying to say anything against sealed systems, just being a bit old fashioned !

Back to your problems - I can't understand how someone can change a boiler knowing full well that there are problems with a system, and do nothing about it. Even the plastic pipes can cause sludge if of the wrong type.
The whole system needs to be inspected in detail, and all remedial work carried out by a competent heating engineer. He may well suggest converting to a sealed system anyway as they are generally not so critical of system design.

Thanks, I understand now. I suspect the plastic pipes are the root cause. As there's no way we can empty the house and tear up all the carpets and floors I'll have to compromise by flushing the sludge with a system cleaner, fitting a combined magnetite and particulate filter and regularly adding a reputable inhibitor.

I know better than to ask for opinions on which filter/inhibitor to use :-) I'll more or less get an even number of replies favouring each of the three major players... if there's an unbiased test anywhere please post a link!

One more question though. Much ado is made about the position of the filter. On the return close to the boiler appears to be a (rare) point of consensus. What is the thinking there? Once the particulates are in the filter they are no longer in circulation causing a problem... why does the location of the filter affect its efficacy?

I think that unless you tackle the problem, no amount of inhibitor and filtration is going to solve the problem.
The best answer I can give on filter positioning is to follow the makers instructions.