Central Heating Problem

[Sandeee]

Thought I'd try here to see if anyone had any ideas about a central heating problem we're having. We have a megaflo hot water tank and an Ideal Classic FF330 boiler. We have noticed that the central heating does not appear to be coming on and have tried turning the thermostat up to 28 dec and above with no results. I can hear the thermostat click when it passes about 20 deg so it seems to be able to sense the temp in the room but dosen't seem to be switching the central heating on.

At the moment I have the heating and hot water both set to on all the time and when I run any hot water and the hot water tank starts heating up the radiators then become hot.

Grateful for any advice

Sandy

[The Natural Philosopher]

Sounds like a motorised valve problem.

In general the thermostat drives a valve which opens up the rads and also switches the boiler on.

Need to know more about the installation setup to pinpoint where to start poking.

Since rads do get hot when HW is on, it may be a loose wire in the electricals..sounds like the valve is opening anyway..can you confirm that the thermostat switches off the heat when you have ut set to HW and CH both?

[Sandeee]

Hi

Thanks for such a quick reply...

It sems like the thermostat (located in the hall) isn't having any influence over the operation of the radiators. i.e. what I think is happening is when the hot water tank needs to be heated the radiators also get hot and when the hot water reaches required temp then it stops heating and then the radiators go cold!! We are then unable to make the radiators heat up using the thermostat control. I suspect turning the thermostat down when the radiators are heating wouldn't work as it seems like the need for the water tank to heat up is controlling the radiators rather than the need to warm the house!!

Sandy

[Owain]

Yes, but you need to understand that the roomstat doesn't usually talk to the boiler directly. The stat tells the motorised valve to open the heating circuit, and the valve mechanically operates a switch to turn on the pump/boiler.

It sounds as though the mechanical link between the valve and the microswitch has come undone. You are therefore relying on the HW stat to call for heat and turn the pump/boiler on.

The valve itself is operating because when the boiler is running (with a call for heat from the HW) the rads do heat up.

You can probably take the cover off the motorised valve and see the microswitch actuators - CAUTION they operate on 240V mains.

Owain

[Roger Mills]

It's definitely a motorised valve problem. How many valves have you got - and what type? You will almost certainly have either one 3-port valve or two

2-port valves. The diagnosis will be different depending on what you've got - so please let us know about your setup.

[Sandeee]

Many thanks for all the help so far...

I've had a dig around under the hot water tank today and there is a honeywell two port valve and a drayton zone valve actuator.

When I switch on hot water and heating at the timer the two port valve moves and I can hear the HW tank working. The zone valve actuator sits at A when this happens. At this point the radiators heat up as well. When I select heating only and move the room thermostat up to 30 the ZVA moves to B but the two port valve dosen't move. I've tried manually forcing it over and latching it but that has no effect. From watching everything switching etc it looks like the two port valve and ZVA are doing the right things. Do you think the problem might be the internal parts of the two port valve?

Cheers

Sandy

[Roger Mills]

I'm a bit confused by your terminology!

All 2 or 3 port zone valves are in two parts - the wet part which directs the water, and the actuator (the electrical part which rotates the spindle of the wet part to make it do its business). I presume that you've got two complete valve/actuator assemblies - one Honeywell and one Drayton - rather than the unlikely combination of a single Honeywell wet bit controlled by a Drayton actuator?

If so, one valve will switch the HW flow, and the other one will switch the CH flow. This is called an S-Plan system. The valves are opened by the programmer and respective (room or tank) thermostats. Each valve has a set of secondary "volt-free" contacts which close when the valve opens and switch on the boiler and pump. It sounds as if the CH valve is opening - allowing hot water to flow to the radiators when the HW is being heated - but its secondary contacts are not working. Thus when *only* CH is required, the boiler and pump are not being switched on.

[Sandeee]

Roger

Thanks for that- The S Plan system you described sounds like what I have with one honewell valve and a separate drayton actuator. What you have suggested sounds right as when the drayton actuator moves to the central heating position nothing else happens. I'll have a look into replacing the actuator

Many thanks for your help

Sandy

[Roger Mills]

If you look at the S-Plan wiring diagram in

you'll get a clue as to how it's *supposed* to work.

When the CH zone valve is open, the volt-free(1) contacts should be closed, connecting the orange wire to the permanently live grey wire - and thus turning on the boiler and pump via pin 10 of the wiring centre.

The most likely cause of your problem is a duff microswitch - although a wire *could* have dropped off. You *can* replace just the microswitch - but it's a bit fiddly, and easier to replace the whole actuator - which you should be able to remove from the wet part of the valve without any need to drain the system.

(1) Volt-free in this context simply means that the switch is completely isolated from the motor circuit - and can be used to switch anything which needs switching on when the valve is closed.