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Flushing heating systems

How to flush a heating system with mains water


The steel elements (radiators) in heating systems tend to corrode over time, the speed of this depending on how much oxygen (air) is available and whether a chemical inhibitor has been added.

The corrosion products are mainly hydrogen (which will collect at the top of radiators) and iron oxide sludge, which will spread around the system and settle on virtually all surfaces. Any surplus will collect mainly where water flow is slow, like the bottom tube of radiators.

The symptoms are radiators that are cold along the bottom, especially in the middle, and/or cold tops of radiators where hydrogen needs to be vented from the air vents. You can test for hydrogen by holding a lighted match to the escaping gas. Hydrogen will burn (careful!).

The sludge that blocks the bottom tube of the radiator (and can ultimately clog up the entire system!) is difficult to remove. Obviously one option is to close the valves at each end of a rad, disconnect and remove to outside, where a hose can be inserted to blast the sludge out.

However this is a tedious procedure if, as is likely, many radiators are affected. There is also a likelihood that sludge will spill onto carpets, etc. and ruin them. Most professionals will recommend a “power-flush”, but that is an expensive option (£300 upwards?) which requires specialist kit and a complicated procedure which isn’t really a DIY option.

This post therefore describes a simpler alternative (mains flushing) that can have satisfactory results, particularly if combined with some chemical treatment. It has to be said that others may not agree, and a search of this forum will turn up some interesting debates on this.

Sealed system (combi and system boilers)

The process is easier to set up with combi and system boilers so this is described first. It is necessary to identify two connections on the CH system pipework where mains water can be added and sludgy water can be flushed out to a drain. On sealed systems the filling loop (see this thread) is readily available to add mains pressure water to the return pipework. But in most cases a suitable connection on the flow pipework will not exist.

I find it most convenient to fit a flushing point to the flow pipe under the boiler near the filling loop, so that mains water can be added via return or flow pipework, and sludgy water can also be flushed out from either, allowing for flow directions to be reversed (this helps dislodge sludge).

This flow connection can be made using a 22mm brass compression T fitting. A soldered fitting is of course neater and can be used instead, although plastic fittings should not be used so close to the boiler. The branch of the T will have an isolating valve fitted. 15mm size is sufficient but 22mm will allow for an even more rapid exit of sludgy water.

It is then necessary to identify a suitable drain point for the sludge and to set up a large bore hose secured at both ends. Ordinary garden hose can be used, but the bore is a bit restrictive and it is prone to kinking. I prefer polythene tube of 16mm bore which fits over 15mm copper quite easily. Being transparent it also allows the colour of discharge water to be easily seen.

If the copper insert has an olive compressed onto it and a hose (Jubilee) clip used, a very secure connection from poly to copper can be made. (The end of the poly tube can be pushed over the olive and then clamped beyond it.) The other end of the copper can then attach to your flow and return connections using compression nuts and olives.

The end of the hose sitting in the sink or WC or whatever will need to be weighted down to stop it jumping about (and spraying everything with sludge). An outside drain is obviously safer if you can access one. There is something to be said for running the discharge water into some kind of settlement tank so you can have the satisfaction of seeing what crud has been flushed out (at least the heavier stuff).

Closing boiler circuit

Before beginning flushing, we need to close the circuit through the boiler so that water can’t short circuit through the boiler and has to flow through the radiators. In most cases the boiler isolating valves will be available and shutting one (the flow) is sufficient, leaving the return connection open to give a pressure reading from the boiler pressure gauge (you don’t want to risk excessive pressures).

Be sure to remember to check that the isolating valves are open before using the boiler again. A word of warning - old isolating valves will often develop a leak once disturbed. It is difficult to repair such leaks and you may have to obtain new iso valves as (expensive) spare parts. If you’re worried about this it may be better to fit a second pair of generic 22mm iso valves just below the boiler ones.

Drain down rads

The final element of the preparation is to ensure that all the radiators are drained down. This is to force the mains flushing water through the lower channel of the rad where most of the sludge lies. If the rad is full of water the flow will also pass through the upper channel, reducing the cleansing effect.

Flushing procedure

We are now ready to flush. First the discharge valve is opened followed by the filling valve. If the filling connection is to the return pipe, the boiler pressure gauge can be observed and the pressure limited to say 1 Bar to start with and then 2 Bar when you have more experience with the process. Initially the water will flow through all the radiators (subject to rad valve settings).

The discharge water will initially be sludgy, but will clear. You then need to go around the system closing all the radiators but one. All the force of the mains pressure flush will then apply to that rad alone, resulting in very sludgy discharge water. Note that the boiler gauge pressure should ideally be monitored at all times - if this is not possible at certain stages it’s better to temporarily close the filling connection.

When the first rad discharge water runs clear, you can open the next rad and close the first one (doing it in that order avoids the circuit being completely closed at any point). Again flush until water goes clear, then move on to the next rad, etc. etc. The order in which the rads are flushed doesn’t really matter.

If flow through a particular rad is slow or non-existent, check that both rad valves are open. You may also find Thermostatic Rad Valves (TRVs) juddering, particularly when flow is the opposite to the normal direction through the valve. In this case open the valve fully or remove the head completely.

At the end of the process, you can reverse the fill and discharge connections and repeat the whole process in reverse. This will dislodge even more sludge. When reversing the flow, you may find that you cannot use the boiler pressure gauge to monitor the filling pressure. In this case it’s best to fit an additional pressure gauge to the filling connection, or at least to try to limit filling rate.

Finally, when the process is complete, you should briefly flush through the boiler itself (return to flow direction) and then refill the system. Ideally you should add a chemical cleaner to the system water at this stage, so that the residual sludge (there will be some!) can be broken down for later flushing.

The system can be vented and run as normal for a week or whatever period is recommended by the chemical cleaner manufacturer. At the end of this period the flushing process should be repeated. Final filling should include an chemical inhibitor to limit future corrosion.

Open systems

It is possible to mains flush an open system (those filled from a Feed & Expansion {F&E} tank), but certain modifications have to be carried out. Such modifications should only be attempted if you thoroughly understand the set up of the open system, since an error could result in a dangerous heating system.

The main modification required is to isolate the F&E tank and open vent, otherwise the mains water would simply rush up to these points and run out the overflow pipe. This isolation is usually done by disconnecting the feed pipe from the F&E tank and temporarily joining this to the Vent pipe, with the addition of an isolating valve. The valve is closed when flushing through other circuits, but opened to allow flushing through the feed and vent pipes. But do not attempt to run the boiler with this cross connection in place.

An alternative approach on some systems is to isolate the heating flow and return pipes near the boiler. The pump isolating valves will allow isolation of one pipe, requiring an isolating valve to be fitted to the other. For convenience it is best to fit new isolating valves (full bore gate valves or ball valves) to both flow and return since the pump valves may not be serviceable and are awkward to operate.

With the heating circuits isolated from the open side of the system (boiler, gravity cylinder circuit, F&E pipe, F&E tank and Vent pipe) the heating circuits can be flushed in the same way as described above for sealed systems. Of course suitable mains and waste connections have to be identified, but this can be anywhere on the heating circuit, for example by removing a kitchen radiator and using the flow and return valves.

More care needs to be taken when introducing high mains pressure since the system will previously only have been subject to very low pressures (determined by the height of the feed tank above the system). It is more likely that the high mains pressure will “find” potential leaks, or even cause the catastrophic failure of a rad if the rad was so badly corroded internally that it simply couldn’t withstand the pressure. The result could be a lot of nasty sludgy water spraying everywhere!

A further complication with open systems may be the presence of an anti-gravity or check valve in the heating flow pipe. This must be by-passed or removed to allow flushing in a reverse direction. It may be preferable to disassemble the anti-gravity valve and remove internal components for hand cleaning.

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