Should my central heating boiler be active all the time?

[Glyn Lloyd-Jones]

First of all let me thank you for taking the time to read my post.

I don't really have a problem as such, but I am confused about my central heating setup.

I recently moved into a house with an Ariston Clas HE boiler, external programmer (Honeywell DT90E) and room thermostat. The timer built into the boiler is bypassed (set to '1') so the external programmer has full control. In this condition, the boiler is 'active' all the time, 24 hours a day. When the programmer sets the heating off, hot water continues to recirculate and the boiler fires up occasionally to keep it at temperature. When the heating is on, everything works as expected. I tried setting the built-in timer and noted that the boiler is completely inactive when this is set to turn the heating off, which is what I would expect.

So my question is: is this normal behaviour with an external programmer, and won't it be wasting gas (albeit a small amount, I hope) recirculating hot water all day?

I would be very grateful to anyone who can shed some light on it.

Cheers,
Glyn

 

[flameport]

Not normal - with controls off, the boiler should also be off.

A DT90E isn't a programmer, it's a room thermostat.

 

[Glyn Lloyd-Jones]

Thanks for replying. Of course you're right about the DT90E, it's the thermostat, the programmer is ST9400C, my bad.

So is it wired up wrong? The Ariston manuale says to connect the room thermostat in series with the programmer, so how can it know when heating is off rather than just the thermostat?

 

[stem]

As you have a dual channel programmer, it would suggest that your Clas HE is a 'heat only' version (i.e. not a Combi boiler) Two channel programmers are used with systems that have central heating and stored hot water (aka a hot water cylinder) In which case, is it possible that when the heating is off, the hot water part is still working the boiler?

Alternatively, there are some installations (not many) that are fitted with a 'thermal store'. This is a well insulated large volume of water that the boiler keeps hot permanently and when required, water is pumped from it around the radiators. An internal coil supplies hot water to the taps on demand. With this type of installation, the boiler timer is normally left 'on' permanently so will still fire up occasionally to maintain the temperature of the thermal store even if the heating isn't running.

But in either case it would be odd that the boiler has an integral single channel timer fitted, and an external two channel timer. If it's a traditional system with radiators and a hot water cylinder, then the two channel timer would have been fitted from the start, so why fit a boiler with an integral timer as well? On the other hand, if it's a system with a thermal store, the hot water is instantaneous and so there's no need for a dual channel programmer to be fitted. All just seems a bit odd.

Can you confirm which type of boiler you have? [Clas HE comes as a heat only and a combi] What does the hot water channel of the ST9400C actually control? Do you have a thermal store?

You may also get a clue as to what's happening from the display, when it's running and you don't think it should be.

 

[Glyn Lloyd-Jones]

Thanks for chipping in. Although it doesn't explicitly say whether it's a combi or not anywhere, neither in the manual nor on the boiler itself, here is a cylinder (Ariston Primo ITI 210) with a built-in immersion heater (which I have never turned on), which I assumed was just for DHW. When the heating is off the display shows a lower case 'c' and the water temperature (e.g. c039), which I understand means it is in bypass mode. It does behave normally if switch off the programmer and use the internal time clock (although this doesn't keep time correctly so I can't leave it configured that way).

The thing that I don't get is the boiler only has terminals for connecting a room thermostat, apparently no provision for a programmer, and the installation manual says connect the contacts of the room thermostat and programmer in series to the room thermostat terminals if you want to use a programmer, but I really don't see how that can work properly. Maybe this model is just not suitable for use with a programmer? I've no idea what the hot water channel does, I suspect it does nothing....

 

[stem]

A combi boiler will fire up and heat the hot water on demand as soon as a hot tap is opened, so it's quite east to test. (make sure the central heating is switched off at the time of the test). However, now you have told us you have a hot water cylinder, it is extremely unlikely to be a combi. (the electric immersion heater in the cylinder is usually just there as an emergency back up).

As you have the installation manual (provided that it is the one that was supplied with the boiler, and not one you have found online) it tells you on the cover which version you have.

The thing that I don't get is the boiler only has terminals for connecting a room thermostat, apparently no provision for a programmer, and the installation manual says connect the contacts of the room thermostat and programmer in series to the room thermostat terminals if you want to use a programmer,

That's perfectly normal for any make of boiler. The terminals are there for external controls to be wired to, and tell the boiler when it should 'run'. The boiler doesn't know what the external controls are, and if there is a separate programmer and thermostat, when they are wired in series to the same terminals, it means that both the programmer must be 'on' and also the room thermostat must be 'calling for heat' at the same time for the boiler to operate. For heating systems fitted with motorised valve/s they will be wired to the boiler terminal instead of a thermostat.

With systems that have a hot water cylinder to provide stored hot water, the heating controls (programmer and thermostat) are normally connected to a motorised valve and not directly to the boiler, the motorised valve is then wired to the boiler terminals.

I've no idea what the hot water channel does, I suspect it does nothing....

I would expect it to be controlling the times that the hot water cylinder is heated. If it does that may explain why the boiler is running when the heating is off. (ie because it is heating the hot water) Turning off the boiler timeswitch should stop heating and hot water. You could always try switching the hot water channel of the programmer to 'off' and see if you run out of hot water the next day.

When the heating is off the display shows a lower case 'c' and the water temperature (e.g. c039), which I understand means it is in bypass mode. It does behave normally if switch off the programmer and use the internal time clock (although this doesn't keep time correctly so I can't leave it configured that way).

If you turn the boiler's integral timeswitch 'off', can you use the external controls to work the central heating, or does nothing happen?

 

[Glyn Lloyd-Jones]

I really appreciate you taking the time to help me out.

Ok, the manual says system boiler, so I can confirm that it's not a combi.

...it means that both the programmer must be 'on' and also the room thermostat must be 'calling for heat' at the same time for the boiler to operate.

Ok, I get that, but I was comparing it to how it works when I run the boiler with the built-in timeswitch. If I program the built-in timeswitch and force the programmer to heating 'on', it's as if I had just the room thermostat connected. In that case the boiler comes on when the timeswitch is 'on' and heats when the thermostat calls for it. When the timeswitch is on and the thermostat is off, the boiler goes into 'c' mode (heating post circulation) and when the timer is off the boiler is off (standby).

If you turn the boiler's integral timeswitch 'off', can you use the external controls to work the central heating, or does nothing happen?

When the built-in timeswitch is off, nothing will get the boiler out of standby, so to use the external programmer I have to set the timeswitch to '1', i.e. on all the time. Then, as you say, I have heating when both the programmer and thermostat are on, but when either (or both) turn off, it always returns to heating post circulation and stays there until the next request. My original question was really because this behaviour is completely different from that of the built-in timeswitch. Turning the hot water programming off has no effect on this behaviour and if I leave it off I still have hot water the next day.

With systems that have a hot water cylinder to provide stored hot water, the heating controls (programmer and thermostat) are normally connected to a motorised valve and not directly to the boiler, the motorised valve is then wired to the boiler terminals.

There is a motorised valve on the cylinder inlet (flow from boiler) so I presume it is connected as you say. All wires go to/from a junction box, which I haven't opened yet so I don't what's connected where.

Update:

I've followed the pipes to and from the boiler and cylinder, and the system I have looks like this:

Solved!

I spent Sunday morning poking about in the wiring.

It turned out to be a faulty motorised valve actuator (Drayton ZA5) on the hot water tank valve.

The contact was always closed, so i pulled it apart (don't worry, I may know nothing about heating, but I am an electrical engineer so I'm ok with electromechanical things) and found the plastic turrets holding the return springs had broken off, leaving the valve in the open position. I put a couple of M4 bolts in place of the plastic turrets and now everything works as expected with programmable hot water and heating – and the boiler goes into standby when neither are requesting heat.

Many thanks for the info that eventually got me on the right track.