As discussed previously I suspect I have a fault in the 3 port valve. Drayton Mid Position Actuator MA1.
For diagnostics, do I have to do more that check that when CH is called for by the programmer power goes to the thermostat, and when the thermostat clicks on power comes back to the valve?
I am assuming that the sequence at the valve is:
(1) CH request from the thermostat
(2) Mid port valve moves to open CH port
(3) Controller then calls for pump and boiler
This has the second assumption that if (2) fails to happen then (3) will not happen (which seems to be more or less what is happening).
Timeclock feeds to stat, stat feeds through to valve actuator, when valve actuator has set itself to the correct position, its micro switches then feeds through to the boiler and pump. On more modern systems, it just feeds through to the boiler alone and the boiler then turns the pump on, that allows the boiler to run the pump beyond the time the boiler is firing, to remove heat from the boilers heat exchanger.
So if the valve actuator fails to achieve it required position, the boiler will not fire. Sticking actuators, especially the spring return type are a common fault.
Next a view of the wiring instructions inside the lid.
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Finally the wiring diagram from Drayton.
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When the power is on, the programmer has CH turned on and the thermostat is calling for heat then there should be current flowing to/from the white wire in the PDF.
Looking at the PDF it seems as though the common terminal is live and the calling terminal is neutral. I assume with 240V AC this is largely irrelevant for basic diagnostic purposes.
So for testing purposes, working from the table inside the lid of the wiring loom, there should be power between COM and CALL (2 and 3) and 2 should be linked to 4 which should then pass power to the white wire from the Mid Port Switch.
Or is it?
This is where I get horribly confused because in my first round of testing it seems that if I have power and the programmer calling for CH but the thermostat turned right down, I get 240V between 2 & 3 and 2 & 4. If I have the same with the thermostat turned right up then I get 0V between 3 and both 2 & 4.
This is the opposite of what I was expecting.
Regardless, with 240V or no 240V between 3&4 the pump doesn't run and the boiler doesn't fire.
However to proceed with the diagnostics I need to know that my first step is correct.
I think my next step will be to take the thermostat off the wall and check the wiring there. There should (according to the PDF) be live and neutral into the thermostat (but why?) and a third wire to carry the call for heat.
This does seem to be reasonable, because if the programmer is calling for heat then you can hear the thermostat click on and off, but if the programmer isn't calling for CH the thermostat doesn't click.
Any advice most welcome.
I keep adding to this. Would a valid test be to disconnect the white wire from 4 so that I'm only testing the thermostat output assuming 3/COM is the live feed?
Can you remind us whether this has *ever* worked properly. If it hasn't, it's most likely that it's wired incorrectly. If it *has*, it has presumably developed a fault - most likely associated with the 3-port valve.
The only wiring diagram you should need is this one:
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Clearly your connection box doesn't use the same numbered terminals for the same things, so you will need to work out the equivalence. In order to design any diagnostics, it's important to understand how a Y-Plan system works and, in particular, the part played by the 3-port valve.
There are two electrical inputs to the valve actuator - white (heating demand) and grey (hot water off or satisfied) - and one output (orange - switched on by the valve in the heating-only state).
When there is a HW demand but no CH demand, the valve is unpowered and its spring return holds it in the HW position. The boiler (and pump if the two are connected together as shown)[1] is powered from (1) on the cylinder stat. The white and grey wires will be at 0v but the orange wire will be at 240v even though it's not being switched by the valve, because it's connected to the boiler.
If you now add a CH demand, the white wire will be at 240v, and will cause the valve to motor to the mid position. The boiler and pump will still be powered from (1) on the cylinder stat.
If you now remove the HW demand, so that there is only a CH demand, the grey wire will go to 240v. [This assumes that there is a HW-off connection on the programmer and that the cylinder stat has change-over contacts, as shown - without which the system won't work]. This causes the valve to motor to the CH position, at which point a micro-switch is operated which switches live from white or grey (can't remember which) to orange. The boiler is no longer powered from (1) on the cylinder stat but is powered by the orange wire instead. Clearly, if that micro-switch doesn't do its job, there will be no power to the boiler and pump.
Y-Plan systems can be difficult to diagnose because connections which you don't expect to be live often are because they're connected to something else which is live (e.g. the orange wire) or through induction or the internal workings of the valve actuator.
I've known actuators simply getting 'confused' - requiring the system to be completely powered down, after which they work ok again - but I guess you've tried that.
I had lots of problems with my system when it was configured as a Y-Plan. In the end, I bit the bullet and converted it to S-Plan - replacing the 3-port valve with two 2-port valves - and have never looked back.
[1] If your system requires pump over-run, the pump may be connected to dedicated terminals on the boiler, and not simply wired in parallel with it.
I can't honestly say if it has ever worked as designed - it is not my house.
I have a suspicion that it may have been faulty for a long time because when the hot water is on via the programmer, the CH is on via the programmer, and there is a call for heat from the HW thermostat then the valve is (usually) in the mid position and the radiators heat up.
It is only because the house started from very cold and took an unusually long time to heat up that I started to investigate, first by feeling all the radiators (which need balancing but not a major problem) and noting that they weren't heating up consistently when the programmer and thermostat were calling for heating.
I suspect that the owner thought that the heating wasn't very good but couldn't be arsed to get someone to investigate in detail.
If you are in constant occupation and use a significant amount of hot water every day then the house stays warm.
Anyway, I'm going to replace the 3 port valve head (wish me luck) and see if that does the trick.
I am not enthused to undertake a full diagnosis of the wiring - I would if it was my own system but this is a house which is a long way away from my home and needs to be left empty but frost free over the winter.
I would just leave it with the heating and hot water programmed, but every now and then the 3 way valve moves to the HW only position and doesn't seem to move back. I am assuming that either the valve head is confused/ faulty or it is seeing a call for HW but no call for CH. Which is again a puzzle because AFAIK this can happen when the CH is calling for heat.
Oh, and thank you for your comprehensive explanation; just one thing, though; I thought when there was no call for HW or CH the valve, unpowered, returned to the mid position. To go HW only or CH only required power to move the valve to close off the other feed. But I may well be wrong.
"The valve has 3 pipe connections, in a T formation. The centre leg of the T is the inlet and the two short arms are the outlets, usually referred to as ports A and B. Actuation controls whether the flow from the inlet passes to the A or B port. In the resting state flow is to the B port (usually the hot water circuit)."
Correct. Although, when both demands are removed (but HW-off is still live) the valve tends to stay in the position it was last in until a new demand appears. That's why you have to remove ALL power in order to re-boot it - so that it doesn't even have a HW-off signal. When you remove the power, you can often hear it whirring back to the HW position.
You mean you just swapped out the motorised valve head and not the valve itself?
They motorised parts certainly do fail - I've had to change both the HW and CH valve actuators last year as they were getting stuck and buzzing. Not worth refurbing the gears or changing motors. One is a Honeywell the other a Danfoss, installed about 15 years ago when we moved in and redid the system.
And yes, our wiring loom is under the stairs and hard to get at...
Two views of the original valve head. It appears to have been sprung open at some point, though how and why remains a mystery.
The clip on the back which is supposed to secure the valve head to the valve had jumped out of its guides so the valve head was essentially free floating.
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Picture of the boiler front.
It has obviously had the original controls disabled and an external controller fitted.
The boiler temperature thermostat still works but one built in timer is missing (HW) and the other (CH) is thankfully not doing anything obvious.
Does have a sticker on the top saying that the CH and HW are now controlled by a separate controller.
History, probably. They are a development of the earlier diverter valve
- which was either HW or CH but not both at the same time. These always defaulted to the HW position, and you didn't get any CH until the HW demand was satisfied.
I suppose it makes sense in that you generally want HW all year round but only want CH when it's cold. If the valve failed in the summer, you wouldn't want CH but no HW.
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