CH control, or lack thereof...

[Ed]

Where to begin? The symptoms: CH doesn't always start in the morning, it seems that the colder the house the less likely it is to start, but that may just be our perception..! Secondarily it seems to be ignoring the DHW tank stat. The system is a very dull indirect setup with a Baxi back boiler at the front, a Tower / Grasslin Mid pos'n valve (replaced two years ago) controlled by a Tower roomstat (with acc), an old Tower tankstat and a Honeywell digital 2 channel programmer, the square type. All has been fine until a few weeks ago. The timer is a 2nd hand one fitted 3years ago (give by a mate whose system I updated) and seems fine, it replaced an old Towerchron mechanical (DHW only or CH+DHW, no CH only) timer with pins (for which previous student tenants had lost almost all the pins,how?). The 3 way replaced what you woud have expected to be a 3 port diverter due to the old type timer it had been installed with when new, but it was a mid pos'n one and after much learning I got it all wired and it worked fine for about a year (until a small crack in the baseplate allowed it to distort enough under the spring pressure that the cam sometimes missed the second microswitch, depending on whether heat from the pipework had made it expand a little more. I only mention this in case someone finds it useful in the future.) I have been all through the wiring and am sure that both stats and the timer are working fine. Having fiddled with the stats to study every possible permutation of inputs to the 3 way (not just in isolation but also trying every input that can follow from each possible previous set of inputs.) The only consistent weirdness I can find is: With BOTH stats enabled by the timer and Both calling for heat, the 3 way moves to the centre AB pos'n as it should. If you then reduce the setting on the room stat (to simulate the room coming up to temp) so that the room stat is now satisfied I would expect the 3way to spring return to the DHW ony pos'n, but instead it stays in the AB permiting heat to flow to both rads and DHW. This can't be right, can it? The only way to get it to spring return from there is to turn DHW stat to Satisfied, or to remove the mains power from the system. You can disconnect ALL the wires in the 3way and it still stays in the AB pos'n. Aha! thought I, that'll be the flow from the pump holding it open, disconnected the pump, no change, gave-up. However it doesn't explain either of my symptoms, tank too hot and CH not starting. I have a feeling but have been unable to confirm yet that the Ch starts on mornings when the DHW stat AND the CH stat are BOTH calling for heat when the timer powers the system up at 6.30. If the DHW isn't calling for heat, then the CH takes a lie-in too regardless of what it's own stat has to say. As I say this is a hunch so far but I'll be provoking it at the weekend when I have time, otherwise it's over to you lot, help!

[Ed Sirett]

From the sypmtoms you give I would expect the 3-port valve is sticky and the valver head is not returning to the HW only position under spring power alone. When the HW cylinder is up to temp then the boiler will only fire if the valve winds all the way to the radiators only position.

The boiler is fired when the orange wire on the 3-port valve is on.

[Set Square]

I can't pinpoint the exact problem - but I'm pretty sure that it's something to do with your mid-position valve and/or its actuator. This device - and its horrible little microswitches - plays a key role in the control logic for a Y-plan system - and your symptoms are typical of what happens when all is not well in that department.

The first thing to check is that the spring return takes the valve to the HW only position when you remove *all* power from the system. This means turning it off at the FCU which powers the whole caboodle - not just at the programmer and stats.

If it doesn't return, there is either something wrong with the spring return mechanism in the actuator, or the valve itself is seized. With many of these things (but not all) the actuator can be removed from the valve with no danger of water spillage. If yours is like this, remove the actuator and check that the spindle of the valve turns fairly freely. You should be able to turn it with finger and thumb - or at any rate with a pair of pliers, exerting only a small force. If it is seized, you'll need to replace the valve - which requires draining the system. If the valve is free, the actuator is duff. With many of these valves, it is possible to replace just the actuator - or even to rebuild it with new motor or microswitches if you are good at soldering and like small fiddly things!

If the spring return does work when you remove all the power, but the CH still doesn't work, the most likely cause is still a duff actuator.

[I know you replaced the 3-port valve a couple of years ago - but some of them don't last any longer than that!].

[Ed]

The spring return is smooth and fast, no problems there. As I write the system is behaving fine so it's a little hard to do diagnosis. When I did the wiring checks I did find one wire in the connection unit that could have been tighter but really didn't seem loose enough to be a troublemaker. I've given it a little tweak anyway so now I just have to wait for the next cold morning. Still baffelled by the way it sits on AB with ALL wires disconnected, what is holding it there? If you pull the power at the FCU it spring returns sharply... Watch this space...

[Set Square]

It is possible for mid-position valve actuators to get "confused" sometimes - maybe a microswitch doesn't quite make when it's supposed to, or something like that. Sometimes when this happens, the thing can be reset by removing all the power and starting again. This may have happened in your case.

If you disconnect ALL 5 wires to the actuator, this should have the same effect as pulling the power. Can't explain it if it didn't! Are you sure that *something* wasn't still connected?

[Harry Bloomfield]

If it is a Sunvic valve actuator, it is a well known problem...

The two positions away from the 'home' position are held by constantly cycling the motor on and off against the spring..... The motor runs until the microswitch controlling the motor is opened by the cam, the spring causes it to back off a little, microswitch closes, motor runs again until the switch opens.

Sometimes the mechanisms sticks, with the spring unable to return the valve to the home position. Undo the two screws when the valve is stuck, and it will likely then go to its correct position. Sometimes just knocking the actuator is enough to make it return.

The long term solution is to replace the valve with new, about £45-ish.

[Ed]

Sure as can be, Synchron has only two wires, both disconnected, + orange white and grey for good measure, stays put on AB until you shut off at the FCU, then instant smooth spring return. Anyone for an exorcism?

[Dave Plowman]

On the ones I've seen, the micro switches are of a generic size used on many other applications. However, as with much else, they vary considerably in quality (and also price) between makers. A quality make will give in its spec the expected life in number of operations - which is vastly in excess of the likely use in this context. Assuming, of course, they are kept within their rating.

[Set Square]

Well I'm sorry, but something *is* still connected! If the actuator is

*totally* disconnected from everything, turning off the FCU can have no possible effect on it.

Tell us again where you are making the disconnections. The actuator should have a 5 core cable going to it, with the opposite ends connected into a junction box as per Y-plan in

synchron motor is an internal component inside the actuator. Have you been disonnecting the internal wiring inside the actuator or the external wiring in the junction box?

[The Right Rev. Peter Parsnip]

The Lord alerted my mind to the presence of this EVIL article by Harry Bloomfield, and I thusly replied:

That's your solution to everything.

[Pirx Teh Caravan Pilot]

It won't cure my piles.

[The Left Rev. P. Parsnipe]

The Lord alerted my mind to the presence of this EVIL article by Pirx Teh Caravan Pilot, and I thusly replied:

How many piles do you have?

[ah]

Try math!