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Twiddling my knob — 17 Comments

  1. We too have a closed system and over a long period the air slowly leaked out of the expansion vessel.  I didn't know this at he time and kept topping up with water until one day there was nowhere to expand to and the safety valve on the boiler started blowing out hot water.  I guess I have a slow puncture in the diaphragm because I need to re pressurise the expansion vessel about once a year.  This is a lot easier than fitting a new expansion vessel, it only takes a couple of minutes to do.  There should be a valve like you have on a car tyre somewhere and you simply give it a few strokes with a tyre pump and all should be well.  You don't want a dodgy heating system when this winter's global warming strikes.

    • I checked the big red blob thing [expansion tank?] and there is a screw or bolt or something on top.  I'd be wary about mucking with it though as it's probably holding the whole lot together.

      I have the system running at full tilt at the moment [it's gone a little nippy here in the mountains] and it seems to be holding up well.

      So far….

  2. Sounds like you might be spending too much time twiddling your knob and not enough time trying to fix the heating. I knew a plumber like once ………..

  3. Hi GD. Not wishing to alarm you, but I had a similar problem. The result was that the expansion tank burst in the middle of the night and mains water ran from the upstairs boiler for about  4 hours. Got up in the morning and splashed my way to the kettle for my morning brew…. Bastard thing!

    Best get a plumber out.

    • Bugger!  As I didn't have enough on my plate.

      I'm not that worried about things going bang as water pressure here is almost nonexistent, and if any of the bits above do blow there is nothing underneath them.  I'm not kidding.  I never bothered with flooring under them so it's a clear drop to the foundations and below.

  4. The red expansion vessel is just a larger version of the "Spheres" used on Citroen Hydropneumatic suspension, and similar to British Leyland's Hydragas suspension units, which I did a lot of experimentation on. The seam in the middle holds a rubber diaphragm which should have air one side (the top, where the small filling valve is), and the bottom which is connected to your heating system. The idea is that it accommodates the variation in volume as the water heats up and cools down. Without this "give" the blow-off valve will constantly be working when the heating is warming up, and then you'll have airlocks forming as it cools down.

    The flexible braided hose should be the filling connection – it appears to be fed from some sort of pressure reducing valve immediately below the black knob. I take it the 15mm pipe is your cold water supply, and the 22mm stuff is the heating? Turn off the heating and let it cool down, then shut off the black fill valve. Now open a vent somewhere until no water flows out, but leave the vent open with a container under it. Remove the cap at the top of the expansion vessel, and there should be a normal car type Schrader valve underneath. See what (if any) pressure it holds. If none, connect a pump and apply a few strokes (to the pump, NOT your knob) and see if any water comes out of the open vent. If the diaphragm is good there may be a small leakage, but this should stop when it is fully extended to the bottom of the vessel. If it's split you would expect water to carry on flowing, as the air you're pumping in would escape into the heating circuit.

    What you do now depends on whether it's OK – if it holds pressure go for about 15psi. Then close the vent and open the fill valve gently. You really need to be able see the system pressure as you do this, I can't be sure from your picture which of the two gauges will give this – probably the one on the left, which looks as if it's on the feed side of the blow-off valve. If your mains pressure really IS low you might not get much more than the expansion vessel pre-charge value, but 20-30 psi would be ample. Go round and bleed all the radiators and any other vents, whilst someone else keeps an eye on the pressure (topping up if needed), then close the fill valve. Finally run the heating and see if it's made any improvement. You would expect to see the pressure increase with temperature, but this should fall back to the charge value when it's cooled down.

    Right, that'll be £30 for my time, thank you very much…

      • Very remiss of me to omit my thanks! 

        Six cheers for you, Microdave.

        I'm letting the system run as-is at the moment and it's holding out well.  I'll give the above a full run trough when and if I have sobered up a bit.  :;

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