Heating system overhaul - comments please

[Martin Pentreath]

Hi to all,

The story so far (please don't judge me).

The heating system in question has a combi boiler and is in a small two-bedroom flat. For some time there was a drip from the pressure relief valve. To solve the problem of the drop in pressure which this drip caused, the filling loop was left permanently open. Magic. The system remained at (fairly low) mains pressure and all was well. (Except the annoying drip). Eventually I got round to diagnosing the cause of this. It was fairly simple - the expansion vessel had lost its air, so that whenever the system came on the pressure shot up to three bar and the pressure relief valve opened. At some point some crud had found its way into the valve seat so that it stuck permanently open. Blah blah, I think this is a common tale of woe. In my slight defence I don't live in the flat, so once the filling loop bodge got things working I kind of forgot about it.

Anyway, I've made my confession and that's all behind me. I've reprimed the expansion vessel, closed the filling loop and stabilised the pressure, and that's all OK now.

On to the point: because for some time there was a constant flow of water in and out of the primary circuit, with any corrosion inhibitor long gone, the system is now full of a serious amount of crap. What's more the rads are 30 years old, and were looking a bit dodgy before all this happened. Judging from what I just rinsed out of the completely clogged secondary heat exchanger, and the pretty orangey colour of the water throughout the system, there is a serious iron II oxide problem.

So, I'm going to replace all the rads, they need to go from a cosmetic point of view anyway. My real question is if the rads are all going how much major flushing do I need to do? I guess quite a lot of crap will have accumulated in the pipe work, but I've a feeling that that stuff is going to be pretty hard to shift. And on a secondary point, which if any flushing chemical is good at dissolving iron II oxide (ie rust). I'm not aware that any are, but I stand to be corrected.

Cheers!

Martin

[newshound]

Agreed, I think you could have a considerable problem with stuff in the pipes. I don't know how effective the professional flushing kit is. One thing you might consider is adding a magnetic "filter" which will at least help to separate out magnetite (the black oxide). I'm not surprised you have a blocked secondary HX, this is likely to re-block unless you can get the system adequately clean.

[Martin Pentreath]

As a salutary warning to anyone thinking of falling into lazy ways when it comes to maintaining their heating system, today was the second time in four weeks that I've taken out the secondary heat exchanger for remedial flushing (because on both occasions the hot water was unusable), and I've never seen such a mess! I normally reckon on giving them a rinse once a year, and even then I would always find far less crud than this.

[Andrew Gabriel]

I would first try to clean it whilst the old rads are still in, to get as much out as possible before fitting the new ones. You can probably use the filling loop and drain valve to force water through the system. Might be worth temporarily removing the non-return valve so you can get a better blast of water through. (I have a separate inlet for this on my system, which is normally capped off, but can have the filling loop connected to it.) Hopefully there are enough valves to allow you to force the water through each radiator, one at a time, to try and clear them of loose material and flush all pipes. (You'll need to fully open the balancing valves, and reset them afterwards.)

Then run the system with a cleaner in it, instead of inhibitor, according to the instructions (usually for something like 1-7 days).

Drain down, and replace the radiators. Refill, and with the system cold, pressurise up to the normal hot running pressure. Go around checking for leaks, and repeat the check after an hour or two. (Doing this with the system cold means you're more likely to see the leaks, as they won't dry up due to hot plumbing.)

Finally, flush this out (it may have any debris which was inside the new radiators), and refill with inhibitor to the right cold pressure.

[Martin Pentreath]

Thanks Andrew, as a variation on your plan what about if I took off and scrapped all the old rads first (wouldn't leaving them on while flushing just encourage the crud which they contain to enter the general pipe work?), and closed all the rad valves while I ran the system with some sort of cleaning agent in it (I was thinking maybe Fernox DS40). The circuit would be pretty short with no rads, just the boiler and the pipework, but really that's all I'd want to clean out. Assuming the weather is about to get warm this would have the added advantage of not turning the flat into a sauna in early summer. I could then flush the system (I was thinking of connecting a hosepipe to one of the rad valves via a washing machine hose, with another washing machine hose on the opposite rad valve going to the drain). I'd then put the new rads on, open up all the valves, and fill with inhibitor.

[TheScullster]

"Martin Pentreath" wrote

Hi Martin

Having inherited a system in a similar state to yours (though mine was vented non-combi), I would be inclined to do as follows (this involves rather more work).

Remove all old rads and flush out completely connecting mains pressure water via an old rad valve or whatever so you can blast them at full pressure/velocity until you are convinced they are all clean.

Systematically work through the house connecting a hose from each rad valve to drain (one at a time), then apply mains pressure water to blast water out of each and every orifice in turn. Make sure you don't over pressure the existing boiler doing this. If you have some fine gauge netting, tape this over the drain hose end to catch the crud for identification/quantity. I am convinced that water velocity is important in shifting crud, hence the "connection to each point" method rather than simple recirculation. I used the recirc method a few times with appropriate chemicals but that was not totally effective.

Once all points have been blasted and all radiators flushed, re-assemble the old system and fit a Magna Clean type filter in an accessible spot. Run the system with Fernox chemical cleaner (their tech support is good and worth a call on this) and see what comes out in the filter. The filter test will be a good indication of the cleanliness of the system and give you confidence to spend the money/time on new rads, boiler whatever. Then use inhibitor and clean the filter crud out every six months.

Phil

[BartC]

In one house, over a period of 9 years, I went through 3 boilers, and replaced a DHW heat exchanger twice (or was it three times?) and had a 3-way valve overhauled once, and replaced once. In the end I asked for a Magnaclean to be fitted into the pipework.

(I also encountered quite a few incompetent engineers: for example, one time the boiler kept cutting out (because the clogged DHW exchanger wasn't extracting the heat fast enough and it was getting too hot). His suggestion was to remove the thermistor that detected overheating of the primary circuit and stopped the thing blowing up! £70 for that 'advice')