Central Heating programmer or motorised valve ???

Have ideal classic boiler and having problems with 3A fuse blowing in fused spur. After heating goes off can see that motor on valve ( diverter valve ? next to system pump in tank cupboard ) is not returning - so valve held open. If I turn power off at fused spur can see that valve returns, If I don't do this after the end of each time period then when next time period comes on fuse blows and mcb trips out.

Replaced motor last year. I guess power to motor not getting "signal" to turn off when heating goes off.

Is it likely to be programmer ? Small PCB on valve ? Boiler ?

curious said:

... Is it likely to be programmer ? Small PCB on valve ? Boiler ?

Click to expand...

It is more likely to be a fault with the circulator pump that is causing the excessive current to be drawn.

When the diverter valve is in its resting position, and the programmer calls for heat (CH and DHW), the pump receives its power, initially, from the cylinder stat alone. However, when the diverter valve is in the open position, the pump powered from the room stat (through the diverter valve's Orange wire) in parallel with the cylinder stat. It could be that cylinder stat wiring is limiting the initial current to the pump, just enough to prevent the fuse from blowing.

Nevertheless, it would be worth while checking the switched live voltages at the diverter valve. With the room stat and cylinder stat both in the calling for heat position, the voltages on the diverter valve's White, Orange and Grey wires should as follows:

NB It is normal for the diverter valve not return to its resting position when the cylinder stat (or programmer) stops calling for DHW before the demand for CH ends. The diverter valve is held in place by the 240V that is applied to its Grey wire, from the programmer's 'DHW Off' connection or the cylinder stat's third (heat satisfied) terminal

CH4 thanks very much for that, think I'm getting out of my depth but it will help if I now call in engineer.

I checked this morning and noticed something a little strange.

1. If I advance the CH off, leaving HW on the valve return and shuts off flow to radiators.

2. If I reduce room stat setting whilst CH and HW are on again valve returns and shuts off flow to rads.

3. If I reduce cylinder stat whilst CH and HW are on pump goes off momentarily.

However at end of time cycle when the motorised valve doesn't return, it actually moves even further to the open position. Does this make any sense to your original thoughts ?

It does make sense, and further suggests that your programmer and 3-port motorised valve are working correctly. A brief description of the operation of your CH system, with diagrams, may make things clearer.

If the programmer's power supply was interrupted, the initial state of your system will be like this:

The motorised valve's return spring will have moved the valve to its resting position (DHW only).

If the programmer first calls for DHW the pump/boiler will be energised via the cylinder stat (the path of the electric current is indicated in Red):

The motorised valve will remain in its resting position because no power has been supplied to its motor. In this position the valve only supplies hot water to the cylinder.

If the programmer now calls for CH, as well as DHW, 240 volts is applied to the valve's motor via the room thermostat:

The motor advances the valve towards its mid position.

When the valve reaches mid position, its first micro-switch (the upper switch in the diagram) toggles over:

Now the valve's motor is no longer powered directly from a 240 volt supply. Instead, it receives its power via a 13k0 resister and a diode. The resultant reduced power is insufficient for the motor to advance the valve any further, but is just sufficient to prevent the return spring pulling the valve back to its resting position. So the valve remains in its mid position, supplying hot water to both the cylinder and radiators.

When the cylinder stat is satisfied, it toggles over:

Besides applying 240 volts directly to the valves motor, enabling it to advance the valve beyond the mid position, this switching action also removes the power supply to the pump/boiler. This is the reason why you noticed the pump go off momentarily (the third item in your posting).

As the valve moves further across, it toggles the second micro-switch. Thereby, restoring power to the pump/boiler:

The valve continues to move across until it hits the end stop - stalling the motor. In this position the valve only supplies hot water to the radiators.

When the programmer stops calling for CH and DHW, the valve remains in the fully across position because its motor is still directly powered - this time from the programmer's DHW Off terminal:


If the supply is not interrupted then, when the programmer next calls for CH and DHW, the pump/boiler will be initially energised via both the room and cylinder stats. This is because the motorised valve will still be in the fully across position (it takes a few seconds for the return spring to move the valve to back to the mid position):

This is the point at which your fuse blows.

Hope this helps,

John

CH4, thanks very much for the last post - now feel like I have a much better understanding.

Honest this is final query.

Whole system seems to work as you have indicated, other than voltage at Grey wire when DHW is on only - this reads 150V approx ?? ( but I am not sure if I checked this when Cylinder stat was satisfied or not - would this explain voltage reading ? but why only 150V as coming direct from DHW on thro' stat ? )

From you final comments on last post, is it therefore normal for motor to remain with 240V 'till next CH or DHW call ? ( for example if I turned both stats down so they didn't call should it leave 240V supply on indefinitely unless supply is removed ? )

And finally,

If system is working exactly as described in your last post (which I believe it is) and this is the normal working sequence, then does this confirm you first thoughts to be the pump faulty ?

Many thanks for your time.

When in DHW only mode, with the cylinder stat calling for heat, the Grey wire should be floating (not connected to any power source) - please see the second diagram from the top, in my last post. Hence, there should be no voltage present on the Grey wire, other than a small amount of noise (electrical interference) picked up from adjacent wires.

However, in combined DHW and CH mode the Grey wire is connected to the junction between the motorised valve's resister and motor (please see the fourth diagram from the top). The resister and motor act as a potential divider, so to measure 150 volts there would be reasonable (it does vary from one make of 3-port valve to another, that is why I only noted it as "Reduced Voltage" in the table of voltages above). Is it possible that the valve had just moved back to the mid position when you took the reading?

If the system works as described then this would tend suggest that the programmer, 3-port valve, cylinder and room thermostats are not causing the problem. It does not, however, confirm that the pump is the cause. Only further tests or substitution would determine that.

Just a slight correction to your explaination above C4.

When power to the motor is supplied to motor via the diode it is not so that a reduced voltage is applied to the motor.

The two microswitches toggle at a definite position of the valve, the mid position, and at this point power is supplied via the diode so that it is fed with DC current.

When DC is applied in this way a sycron motor, as fitted to these valves, acts like a brake and holds the valve in this set position.

Alan

Thanks Alan for correcting my mistake.

Several years ago I dismantled a 3-port valve to try to understand how it worked. I assumed that the motor was asynchronous because I did not think that the inherently low starting torque of synchronous motors would be able to overcome the stiffness of the return spring. The gearing must be much higher than I imagined.

John

John I can assure you I also gave much thought to how these mid position valves worked and came up with many theories including yours.

All was revealed however at a Honeywell training day. The had working cut away versions of the valve and as the valve operated you could see the DC braking effect.

The gearing is sufficient to overcome the return spring but not so strong to cause damage as when the valve is fully opened the motor is stalled. Power being applied all the time.

Alan

Well done CH4 for the best explanation I've seen of how these valves work. I once took a Honeywell one apart to draw out the internal circuit; it took me quite some time to work out what was going on.

They may vary. There was a Satchwell Sunvic 3 port 6ft from where I'm sitting which used to go through heads every 3 years or so. Some of the replacements were made by others. One of them drove me nuts. In CH + hw position, it would slowly motor and spring back between the two microswitch positions a few degrees apart. Annoying to listen to, to say the least. If I remember correctly it did it from new. The only other possibility I can think of is that the diode had failed short-circuit.

(Horrid valve - it only seemed happy with its screws slightly loose. Finally I noticed that our HW was getting too hot. Seemed the valve was letting by so I thought I'd take the head off and check. One screw was a bit tight so I pushed, as you do. Then water dribbled everywhere. I was about to go on hol to NZ for 3 weeks so I had to drain the darned thing and leave it dead to come back to!
The valve had leaked enough to rust the screw in tight. Now replaced with a Honey).