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[neo]

Glow-worm boiler, Honeywell mechanical thermostat and separate honeywell timer.

Problem - central heating no longer works independently of hot water. Hot water function is fine but boiler does not fire up if only central heating is selected. Any ideas ?

[htg engineer]

is it a combi ?
Back Boiler ?
what's the boiler model ?

I take it with the question, you have been able to have heating on by itself before ? with gravity hot water systems you cannot have heating only on.

Check room thermostat is calling for heat, and check actuator head and 3 port valve are free to move (if your system has one).

can't really say much more, you haven't given much information about the system to go off.

[neo]

It's not a combi. It's a Glowworm Fuelsaver.

I'm not too familiar with technical terms. The boiler heats both domestic hot water and central heating. There are two electonic valves near the cylinder. These I believe are to direct the heated hot water either to the central heating or to heat the water in the cylinder, or both. So (as I understand and we have in the past) we should be able to have the central heating on only. This makes sense (in my mind) as you shouldn't lose central heating just because the dhw has reached its optimum temperature and 'requested' boiler shutoff. Surley if you still wanted ch the electronic valve to the cylinder would just close in this case so that all the heated water went to the ch ?

[htg engineer]

You've solved the problem yourself really, like you say the valve should turn directing the water to either the heating, water or both.

The actuator probably isn't working corectly and requires replacing. If you remove the actuator head and move the spindle is should move freely.

Replacing the actuator/motorised valve should solve your problems.

Only attempt this if your competent and confident enough

[neo]

There are 2 motorised valves - one I understand to allow/stop heated water going to dhw ie. it stops it when the water in tank is hot enough and the other to allow/stop water going to the radiators. If the radiator actuator was faulty why doesn't the boiler fire up anyway from cold when heating only is selected ? Surely a faulty actuator would just mean that heated water was going in wrong direction ! The pump doesn't run either when heating only is selected. Could this be a problem with the boiler circuit board?

[The Heating Doctor]

I suspect the system is wired as a 'S' plan and the heating circuit is only triggerd once the valve actuator has gone to the full open position. In the full open position there will be an end of traval (auxillary) switch and only when this switch is closed will the circuit be made to start boiler & pump. So 1) Either the switch no longer functions. 2) The motor which drives the valve is faulty. 3) The thermostat which drives the valve is faulty. If you are a competant electrician then turn the heating clock to on, bridge out the room thermostat and see if the valve moves / boiler fires, if not then it's a fairly safe bet it's the valve actuator. 4) The valve spindel is jammed preventing the actuator from moving, follow htg engineers (earlier post) to check this.

[neo]

Fantastic! Checked the actuator which controls water flow to heating system. It is a Danfoss 2 port motor (type HPA2). There is a lever to the side which sets it to auto or manual. I removed the actuator and found that when the lever moved the motor moved but a plastic slider which should move with the lever was broken. I can only assume that this had something to do with the end of travel switch. Anyway, I found a new actuator on the internet for £23, replaced it and now everything works as it should.

Many thanks to the 'the Heating Doctor' and 'htg engineer'.

[slr1981]

I have a very similar problem too (CH works only if HW is on as well), although unfortunately the solution here does not work for me.

I've taken apart each actuator, and everything looks fine (toggle switch is hit when system demands either HW or CH, and in both cases, the pump starts working.

1) If just the CH demand is on, the pump goes but the boiler doesn't
2) If both the CH + HW is on the pump goes and the so does boiler, then hot water flows through whole system.
3) However, if CH demand is on, and i manually set the HW actuator on (so that the vavle on both are open, but only the electric switch is on in the CH actuator) the pump works AND so does the boiler.

This is confusing the hell out of me, cause i cant work out what signal is being sent to the boiler to make it fire

Any help is very appreciated.

[neo]

I have since had the offending actuator to pieces and found that there was a loose wire in side. Not sure if this was anything to do with the end of travle switch but it obviously wasn't helping!

[slr1981]

Something else which confuses the hell out of me is the switch in the actuator for the HW has three cables coming out of it, and the switch in the actuator on the CH only has two!

Could it be that the wrong type of actuator has been fitted in the system.