> When the system is cold, pressure shows zero, despite refilling only
> today. Does this indicate a faulty expansion vessel?
It could indicate a problem with the pressure vessel - but it may be one which you can fix yourself.
The pressure vessel has a diaphragm inside with (compressible) air on one side and water on the other. If there is insufficient air in it, the water has nowhere to expand to when it gets hot - resulting in the pressure rising too much and the safety valve opening, causing the pressure to drop.
The vessel should have a Schrader valve (like a car type valve) at which you can measure and top up the air pressure. With the water system unpressurised, the air pressure should be about 0.7 bar (~10 PSI). Check it with a car tyre pressure gauge, and top it up with a car tyre pump if necessary. If water comes out of the Schrader valve, the diaphragm is shot and the vessel will need to be replaced. Otherwise, just top it up as necessary.
Then use the filling loop to pressurise the water system to 1 bar when cold. Then, when the system heats up, the pressure should rise to no more than about 2 bar - and certainly not enough to trigger the safety valve, which operates at about 3 bar. When it cools down, it should return to 1 bar.
The fact that there is a switch for water heating indicates that it is unlikely to be a combi. A combi boiler produces hot water on demand, so is "always on" and requires no switch.
(cont...) So back to my question, why does nothing happen when I flick the HEATING swtich, but the radiators warm up when I flick the WATER switch (both on the wall timer)? :-S
My guess is you have a three port valve that is stuck. It may be a mid position valve, and the valve has been set manually to the mid position (many valves of this type have a leaver on the side that will do this.
(you neglected to mention if the hot water also got heated)
It appears that you may be right. If I move the valve lever to off, then turn the HEAT switch on, the valve lever doesn't return to on. However, it does work the other way automatically. You can hear a click when it doesn't move, indicating the wall timer is sending the correct current.
Is the simple solution a new valve or could this possibly be anything else?
The other strange thing as mentioned in my above post, is that only the WATER switch turns the system on. E.g.: Water off = gas boiler off, valve off, pump off Heat on = nothing Water on = gas boiler on, valve on, pump on, radiators on
Thanks for the excellent info. Have now done all the tests, topped up the pressure from 5 PSI to 10 PSI, topped up the water to 1 bar when cold. When warm it rises to just below 2 bar, *but* when cold again it drops to zero, doesn't remain at 1 bar.
Faulty pressure vessel [re-check to make sure it's *still* at 10psi]
Faulty pressure relief valve [find where it discharges to the outside world, and hang a plastic bag on it to see whether it collects any water when the system is running - it shouldn't!]
Water leak elsewhere [inspect all pipework, junctions, radiator valves, etc.]
It sounds as if you have a Y-Plan system, with a 3-port mid-position motorised valve. Your system is probably wired incorrectly. You can find a wiring diagram in the Y-Plan section of
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Pay particular attention to the connections to the grey wire on the 3-port valve. This needs to be live when HW is either switched off at the programmer or satisfied at the cylinder stat. So there must be a HW-off connection to the programmer, and the cylinder stat must have change-over contacts - with the one connected to valve-grey still live when the water is hot. In this condition, it's the valve which switches the boiler on via the orange wire, but it needs grey to be live to be able to do this.
If the wiring *is* ok, you could have a faulty microswitch inside the valve actuator. It sounds from an earlier post as if the actuator may be suspect, anyway. With most makes of valve you can replace the actuator without disturbing the wet part of the valve.
OK, I just had a look at the install manual for you boiler. Looks like we may be talking at crossed purposes!
You boiler appears to be designed for gravity circulation for the hot water heating (i.e. there is a separate feed and return connection on the boiler for connection to the hot water cylinder). The central heating is handled separately.
With these types of system you don't really get fully independant control of heating and hot water - however it does depend on how it was all plumbed in the first place. Some boilers of this type can be configured to run fully pumped while simply ignoring the gravity circulation. However usually any time the boiler is on there will be heating to the water via non pumped circulation.
This would suggest that your motorised valve will be a two port valve (i.e. only two pipes in and out and not three). Often positioned close to the hot water cylinder. This valve is normally driven from the cylinder stat such that it closes when the cylinder is up to temperature.
You would expect water on to give boiler on, pump off and valve under control of the cylinder stat.
Heating on would give boiler on, pump on (or possibly under control of the room stat), valve under control of cylinder stat.
We need to understand a bit more about how your system is connected up - what pipes go where, and what control wiring there is.
However in the simplest configuration you would only have simple "on" or "off" control of your system (and the fact that the *programmer* can cope with independant water and CH could just be a red herring!). The pump would be controlled by the room stat, the valve by the cylinder stat, and the call for heat signal to the boiler would be generated by the combined outputs of the stats in serise with the timer.
Yes it is rather confusing... the boiler the OP provided a link to provides for gravity circulated hot water - although the talk of pressure guages etc does suggest a sealed system. It is possible that it was plumbed as a sealed system and the gravity circulation option simply ignored (or it might be the link posted was to the wrong boiler! ;-) Three port valve fully pumped is what I initially assumed... perhaps if the OP could look at the valve and count the pipes connected to it?
Whatever the case, it sounds like this is a wiring / valve related problem.
Hi, definitely a sealed system, at least that's what it says on the tin (on the boiler).
The valve has one inlet and 2 outlets, the pipe going up is for Heat, down is for H/W.
Questions remaining:
1) Why does the HEAT switch on the wall timer have no effect, and only the H/W switch turns the boiler and radiators on?
2) I note that the pressure does slowly drop, and that there are constant slow drips coming from the hooked pipe outside the property, connected to the boiler. What does this indicate, and how is it fixed?
Did you see my earlier post about the necessary wiring for a Y-Plan system? That explains it.
Is this water - leaking from the safety relief valve? If so, the valve has probably got some crud in it and needs cleaning out or replacing. Or, if you have a condensing boiler, it could be condensate - which is normal, but wouldn't explain the pressure loss.
Thanks, so you're definate I have the Y-plan from what I've said now? If so will take a look at the wiring diagram and my installation.
All I can see is that the water comes out of the safety pipe outside my property, so I assume that is the relief valve? How do I get inside to clean out the crud or replace?
You said "The valve has one inlet and 2 outlets, the pipe going up is for Heat, down is for H/W." In my book, that's a pretty good description of a 3-port mid-position valve as used by Y-Plan. [OK, it just *could* be a diverter valve as used by W-Plan systems, but that is less likely].
I can't speak from first-hand experience on this one - so hopefully someone will be along who can. But I've heard it said that once a relief valve opens, it's difficult to get it to seal again. ISTR that you left your filling loop connected, causing the system to over-pressure and blow off.
There are two types of three port valve. A diversion valve and a mid position one. The diversion valve can be in one position or the other - not between both. Typically the hot water is given priority such that if the cylinder is calling for heat, then it will service that (and only that) output. Works well with rapid recovery cylinders since they can be reheated in short order and the valve then returns the flow to the heating.
The second type is a mid position one. This has the capability to split the flow between both outputs when it gets concurrent requests for CH and HW. Better on slower recovery cylinders since it prevents the cylinder disabling the CH for long periods.
Two possibilities: 1, it has not been wired / setup correctly and hence would never have worked, or 2 something is broken! ;-)
The first task is to eliminate 1, which means some detective work to see how it is wired up. Looking at the outputs from the timer will be a good start - they ought to be labelled with their function (failing that, a manual may be downloadable from the web that will tell you there function.
Once you are happy that the system *could* work, it is a case of working through each stage with your multimeter[1] and seeing what is wrong.
e.g. verify that when the programmer calls for CH that a signal does in fact come out of it.
Verify that the room stat does call for heat when required.
Verify these signals reach the three port valve.
Verify that it moves correctly in response to the signals.
Verify that the micro switch outputs (if present) on the valve that indicate its position are working, and hence if any call for heat that should be passed to the boiler via this route does in fact arrive.
[1] remember that many of these signals are likely to be at mains voltage - so take care. If you don't feel confident that you can do this without electrocuting yourself then this would be the time to consult a competent heating engineer.
It sounds like your pressure relief valve is failing to seat correctly. This can happen when a valve is forced to operate (under normal circumstances it will never open). You overfilling exercise may have caused this. Clean or replace the valve I guess.
If it's a diverter valve, the system will be W-Plan, and if it's a mid-position valve (more likely in my view) it will be Y-Plan. Wiring diagrams for both can be found at
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