Central heating flushing ideas, advice?

[Matthew]

Hi All,

All our radiators were replaced last year (15 of them). We had an underfloor leak on the CH which I had previoulsy asked for advice about. Once we get further in towards the summer and the central heating isnt used I need to drain the system down and fix the problem.

In the meantime we are doing some work in the concervatory and I removed the associated rads. When I drained them I noticed that although the system was fernoxed there was some black water at the bottom (presumably because of the leak continuously bringing in fresh water). Once the job is finished I was going to flush the entire CH system out again and refernox. I dont want to take off the rads and manually flush with a hose due to time and effort so I have a number of options;

1) I can hire a power flush from TP for this, is it worth it?

2) Alternatively how about flushing the entire system with mains pressure water somehow. I dont quite know how I would do this though. Maybe attach a hose onto the header tank somehwere and cap off the expansion pipe whilst draining to the outside.

3) Or how about this, is it possible to use the CH pump itself with the boiler switched off to continuously flush the system to a drain. I dont know whether this would be powerful enough but providing the header tank was continuously filling OK would it work (we have a grunfoss superselectric so its reasonably powerful)?

BTW how do the other (cheaper) CH agents compare to Fernox?

Thanks for any thoughts, Matthew

[Andy Hall]

The whole exercise can be done in less than half a day, but OK.

That's another option.

Possibility, but if the hose comes off you are going to have water coming through the ceiling perhaps.

You would also need to have a way to direct the flow individually through the radiators to make sure that the crud was carried through and also avoid it being carried through the boiler. This needs careful thought and possibly the addition of an isolating valve.

Not powerful enough.

No idea. I've used Fernox for about 30 years and know that it works properly and does what it says.

One might be able to save £10 or so by buying some unknown product. In the context of a heating system that would cost several £k to fix, I think that that's a false economy.

[mcbrien410]

I try and flush my system once a year but I think it is more like two years.

I connect a hose pipe to the drain plug at the back of the Boiler, feed it out of the door and down a drain. I then turn all radiators off except one. This is the highest one in the system. I then turn the CH pump to the high speed and then open the drain valve. The ball c*ck in the header tank opens and starts to keep up with the drain valve. I flush that radiator for say ten minutes. Turn the second radiator on and the first one off. Work around the system one radiator at a time.

Turn the drain c*ck off and let the header tank refill.

I dod need to bleed the air out for a couple of weeks but that about that.

Chris.

[John]

Black water may be better than sludge - it could be the Fernox doing its job and keeping the muck in suspension.

I believe that flushing the pipes (as they are small) is more significant than flushing the radiators.

For my old radiators I made an adaptor to screw into the top plug of the radiator - this can be connected to a hose and mains pressure. I open another radiator's drain valve and force a bit of mains pressure around selected parts of the system. If the header tank starts to overflow I know I am overdoing it.

My ideal system would have a transparent settling chamber and a dosing chamber to help with maintenance.

[Matthew]

Sounds like a good plan for starters.

This may be a stupid question but how do you switch the pump on without the boiler firing?

Matthew

[Andy Hall]

Let me expand slightly on the earlier post.

If the system is badly enough sludged up that a powerflush is actually

*required*, then the options are really to do that or flush through at the radiators using mains pressure in some way. The reason is that the sludge material is relatively heavy and a CH pump simply won't move much of it round to leave via the drain. Filling and emptying the system won't do much either.

However, if you do an exercise of fill system, run pump, stop pump and drain and the water remains brown to black after a couple of times, then you should suspect that there is heavier sludging.

If the system is only lightly silted, which, reading again, yours probably is having had inhibitor, then you could use the approach of adding and running some desludging chemical in the system hot for a few days. This is recommended on new installations anyway. It isn't a panacea for really bad systems but will deal with those which have a little. This approach, followed by several flushes and inhibitor may well be adequate for you.

If you want to run the pump without the boiler firing, it will often work if you turn the heating on but turn off the boiler gas supply. This is because even if the boiler controls the pump directly, and most do in some way, the pump needs to be running before the boiler fires. Of course in the case of external pumps, the boiler doesn't really "know" that the pump actually *is* running but just powers it up. It's possible that some boilers have electronic control and if there's a pilot know that it's not lit. If this proves to be the case, you might have to temporarily wire the pump to a permanent live inside the boiler and use the spur switch to it to turn the pump on and off.

A word of warning though..... Be careful if you run the pump while flushing. There is a risk of water being completely drained from the system and the pump running dry. If you do that for any length of time, which can be quite short, you will knacker the bearings and require a new pump. It's safer to fill the system run the pump, stop the pump and empty.

Finally, if you think that air is being introduced through pumping over or sucking down at the header tank, then it ought to be fixed. Even inhibitor won't help if it's too bad.

[Dave Plowman (News)]

Don't think this is a good idea - it's the oxygen in fresh water that causes the corrosion.

[Matthew]

I understand about the oxygenated water but I would add inhibitor to the system again following flushing and bleed the rads etc. The problem is flushing the black sludge out of the system. As you say I need to ensure that the rate of header tank refilling is more than the power of the pump. I can check that with an experiment and I could wire a switch in series with the pump just in case the pump is too powerful (there are 3 speeds so room for flexibility). Matthew

[Andy Hall]

That's fine, but beyond a certain point, if the water is continually being oxygenated as a result of pump over, then the inhibitor will fail to work. In effect, the useful lifetime of the inhibitor becomes shorter than the manufacturer normally specifies.

The real question is how much.

What can happen if you are doing this is for air to be sucked down the vent pipe or even the feed pipe as a result of the pump forcing water out of the drain - I've seen it happen.

I am not saying that it *would* happen, but I am sure you don't want to have to buy a new pump if you don't have to do so.

You could. Another thing is that you can close all radiator valves and open them one at a time in turn, thus forcing all the flow through one radiator. You can also improve flow through a given radiator by opening the lockshield valve fully. However, you will then need to rebalance the system. Realistically, if you are going to do a proper cleaning job, opening valves like this would be needed regardless of the method used.

[mcbrien410]

Matthew, my CH system is very simple with a solid fuel Bont ESSE boiler, no zone valves, no controllers. The pump is either on or off and is selected by hand. In Aberdeenshire the CH system does not get too hot. The boiler is permanently alight from September to July and quite often in between too. In the 1983/84 winter we were without lekky for nine days which prevented the pump from rotating. I had ooodles of hot water but nothing in the radiators. I was giving hot water to the neighbours as they were ALL electric. The one gravity fed radiator was glowing too so after three days I moved my bed into that room.

I must get a UPS in case that happens again.

Chris.

[Matthew]

Thanks for all the pointers I think I will try a couple of 'careful' experiments in the late spring using the existing CH pump on different speeds. I have gone off the idea of teeing in with a couple of valves and using the mains pressure as a flushing mechanism. I think using mains pressure (seems to be especially high in our area) might be dangerous I wouldnt want to upset anything. If it works OK I will post to the group to let you know how it went in case your interested.

Matthew

[Dave Plowman (News)]

If it's had the correct inhibitor there won't be any black sludge.