C/H boiler and room thermostat not communicating- possible causes?

usignuolo

We have a Potterton Performa boiler which is around 4 years old and postdates our gas central heating system which was installed 24 years ago. As a result not all the radiators have individual thermostats (although they all work, and we had a system flush with the new boiler).

We also have a room thermostat which is in the inner hall. It is a Drayton digistat. The boiler is set to run twice a day and the thermostat is set to maintain a constant temperature of 21degrees. The problem is that the thermostat and the boiler do not talk to each other although both appear to be working.

The boiler comes on at the correct time in the morning and fires up amd reaches 21 degrees but then switches the ch circuit off and does not fire back up when the temperature drops below 21 so that it does not maintain a consistent 21 degrees in the house.

Once the ch is off, the boiler can be coaxed into turning the ch back on by resetting the room stat manually to a higher temperature, but even then it will only work for so long before it switches off again. And when the temperature falls below 21 it does not turn back on. The problem remains the same if we leave the heating on continuously.

Once on the ch heats all the rads OK it is just it will not stay on.

Does anyone have any idea what might be the cause? I should like to have some grasp of the possible reasons before I call out the local c/h engineers.

DVardysShadow

Cause? Thermostat is in the inner hall, which is likely to be a warm location with a stable temperature. You should place your thermostat in a location with the least stable temperature in the house, on the theory that if the thermostat stabilises the temperature there, then the rest of the house will be more stable. I have had most success with a wired thermostat mounted on an external wall, with a small hole allowing a draft from the cavity.

Your description of the boiler restarting when the thermostat is reset to a higher temperature indicates that contrary to your title, the thermostat and the boiler are communicating.

Also, how are you measuring the temperature to conclude that the thermostat has not operated? Most thermostats have a good 1 deg C differential between lowest off and highest on. The differential is important to stop the system switching on and off over very short periods.

usignuolo

I agree stat could be better sited but do not think that is the main problem. There are no rads in the inner hall where it is and which leads to the back of the house. It is usually colder than the rest of the house.

Tonight I went into the hall and thought it was cold. I checked the stat which was showing 17 degrees (although program setting called for it to be 21 degrees, I checked that too.) Went into kitchen and checked boiler which was not firing and the temperature for central heating on front of boiler was on lowest operational temperature. Waited ten minutes to see if it fired up, Nothing happened.

Went back to stat and manually changed actual temperature to be 21 degrees. Went back into kitchen and boiler was firing . One hour later the room stat was showing 17 degrees again. Back in kitchen and boiler had stopped firing, on ch circuit.

Shouldn't it be maintaining constant temp of 21 degrees during operational hours if that is what room stat is set to and shouldn't it do this automatically?

penrhyn

Is it an RF room stat?, does it have batteries and are these OK.

When the thermostat calls fro heat their should be a little flame symbol, is there?

Does the boiler have a separate timer?

DVardysShadow

Then the thermostat itself is not working. But quite plainly, the communication link is working between the 2.

usignuolo

When the boiler is firing then there is a little flame symbol alongside the temperature reading on the room stat. The stat works off batteries and appears to be working fine, all its functions locally on the stat appear to work.

But after the initial start up which I assume is set off by the room stat, the temperature reaches 21 degrees and then the boiler basically just idles on the lowest water temperature on the CH gauge, even when the house temperature drops well below 21 degrees (as recorded on the stat). After that first start up the boiler never fires back up to 21 degrees automatically, I always have to force it manually to do so.

So there is some initial communication between room stat and boiler but not continuous. This is what is puzzling me, if it fires up first thing, why doesn't it fire up automatically during the rest of the day when the temperature drops noticeably below 21 degrees. The stat shows the target temperature to be 21 and the actual temperature to be 17 say, but the boiler does not fire. I can make it fire up by increasing the target temperature which can take a while of course, but once it is achieved, it just starts idling again.

I am trying to get a feel for whether the fault is in the stat or the boiler.

DVardysShadow

usignuolo wrote: »

When the boiler is firing then there is a little flame symbol alongside the temperature reading on the room stat. The stat works off batteries and appears to be working fine, all its functions locally on the stat appear to work.

But after the initial start up which I assume is set off by the room stat, the temperature reaches 21 degrees and then the boiler basically just idles on the lowest water temperature on the CH gauge, even when the house temperature drops well below 21 degrees (as recorded on the stat). After that first start up the boiler never fires back up to 21 degrees automatically, I always have to force it manually to do so.

So there is some initial communication between room stat and boiler but not continuous. This is what is puzzling me, if it fires up first thing, why doesn't it fire up automatically during the rest of the day when the temperature drops noticeably below 21 degrees. The stat shows the target temperature to be 21 and the actual temperature to be 17 say, but the boiler does not fire. I can make it fire up by increasing the target temperature which can take a while of course, but once it is achieved, it just starts idling again.

I am trying to get a feel for whether the fault is in the stat or the boiler.

To me quite plainly the communications are working fine.

Also, the communications between stat and boiler do not need to be continuous. Indeed, I would not expect this. I would only expect communications when the stat detects an event which demands a change to the boiler state from off to on or vice versa.

You have tried whacking the thermostat up and you see the boiler fire. Once this happens, whack the stat back down and see the boiler switch off. If this happens the fault is plainly in the stat.

Now I know nothing about modern digital wireless stats, so you should take a careful look at the manual. Is there some kind of setting on the stat for 'temperature differential' or similar? If there is, then you may have it set too wide. Otherwise, I think the stat is broken.

Canucklehead

Good morning: the OH stopped fitting the Digistat as he had several in a row that failed during the warranty period (and our own out of warranty...Drayton coughed up a refund) and all were replaced by Honeywell products. See what I mean here. Needless to say, he doesn't fit the Digistat anymore but opts for the Honeywell range.

Let us know how you get on.

Canucklehead

usignuolo

Thanks everyone for advice so far.

The Digistat room stat in the hall is not wifi. It has a battery but it also has cabling going into it which I assume connects it to the boiler in some way. In fact I am beginning to think this is where the problem is.

The Digistat was installed with the previous boiler and always worked fine. The new(er) boiler was installed in a bit of a rush as it was winter and it broke down the day before we were going on holiday. (Lucky really we could get anyone to replace it at 24 hours notice.)

Anyway as I said it is a Potterton Performa 28 and it in fact has an analogue timer (by Grasslin) installed in it - circular timer on right hand side of bottom fascia under the control panel. This controls when it comes on and off (on, off or set timings using the tripper slides round the outside).

There is no temperature option on the Grasslin timer and I get the impression that in its basic mode you just set the timer on the boiler for when you want ch to come on and then adjust the boiler dial which controls CH temperature to heat up the rads. Is that right?

The Digistat is set to come on twice a day and heat up to 21 degrees. Is anyone able to give me a simple explanation of how the timer control on the boiler should interact with the room stat?

macman

The mechanical timer on the boiler should be set to be permanently 'on' (the options will be on off, timed). The Digistat then controls both the timing and the temp.
If the boiler clock is set to timed, then it will effectively override the Digistat, so that it will only respond to a call for heat when the clock is at 'on'.

DVardysShadow

usignuolo wrote: »

Thanks everyone for advice so far.

The Digistat room stat in the hall is not wifi. It has a battery but it also has cabling going into it which I assume connects it to the boiler in some way. In fact I am beginning to think this is where the problem is.

The Digistat was installed with the previous boiler and always worked fine. The new(er) boiler was installed in a bit of a rush as it was winter and it broke down the day before we were going on holiday. (Lucky really we could get anyone to replace it at 24 hours notice.)

Anyway as I said it is a Potterton Performa 28 and it in fact has an analogue timer (by Grasslin) installed in it - circular timer on right hand side of bottom fascia under the control panel. This controls when it comes on and off (on, off or set timings using the tripper slides round the outside).

There is no temperature option on the Grasslin timer and I get the impression that in its basic mode you just set the timer on the boiler for when you want ch to come on and then adjust the boiler dial which controls CH temperature to heat up the rads. Is that right?

The Digistat is set to come on twice a day and heat up to 21 degrees. Is anyone able to give me a simple explanation of how the timer control on the boiler should interact with the room stat?

OK, Your thermostat is probably not correctly wired in that you may only have an unswitched live and a switched live. The thermostat probably needs a neutral connection to make it work with close temperature differentials.