Central heating control problem

[BDLB79]

Hi all

This is my first post so please forgive me if I am in the wrong forum.

My father in law asked me to change the room stat in his house recently from an old dial type to a more modern digital style. having performed this task in my own home and for my father, I was confident that I could do the job in pretty short order. I swapped the stat no problem (red into A, yellow into B, earth cut short and neutral terminated and tucked away) and all indications appear normal when switching the system back on. There is a small flame logo in the corner of the stat telling the boiler that a central heating demand is required and the boiler has fired up. This is where the problem begins, the boiler will not shut off once the required temperature has been reached. In order to get the boiler to turn off I have to set the program controller to off at which point the boiler shuts off. I am not sure if there are a number of issues with his system as the hot water light also comes on when the CH is set to 'ON' or 'ONCE' even though the hot water control is set to off which would suggest that the DHW is calling for heat.

When I asked him if this is 'normal' he said that he turns the CH off at the controller when the house is warm enough rather than letting the thermostat control what is happening and has done so for years and has not noticed anything unusual with the lights on the controller.

My first thought is that the program controller could be faulty however I cannot find anything on the web to suggest this. The new room stat is a Honeywell DT90E and the controller is a British Gas UP2, all in conjunction with an Ideal classic boiler. The system is a pressurised system.

Any help with this would be much appreciated although I have suggested that he call a gas safe engineer to check out the entire system as it is roughly 20 years old and could probably do with some TLC.

 

[Steelmasons]

Did you actually check for voltage @ room stat before installing replacement???..is room stat cable actually connected to programmer/MV?

 

[BDLB79]

Steelmasons thanks for your reply. Unfortunately i did not have my multimeter with me at the time so in answer to your question I do not know. I can only assume at this time that there is voltage as the system has been working, albeit perhaps not correctly, however I will check when I next get an opportunity this week.

All cables are connected and after scouring the internet for installation instructions for the programmer the connections all appear correct.

 

[Steelmasons]

I can only assume at this time that there is voltage as the system has been working.

Never assume...going by what you posted regarding the programmer controlling the boiler/MV then it is likely the room stat has been disconnected at some point , this being the case then obviously the controls system will still "work" albeit without the influence of room stat.

 

[BDLB79]

Went and had a quick look after work to satisfy my curiosity, and after further inspection of the hot water cylinder and airing cupboard i cannot find any M/V. I was expecting to find a 3 port valve or at least 2 single zone valves so that i could check the position indicator, however there were none that i could see. There are 2 Grundfoss pumps, a cold water isolation valve (which i is the cold water feed for the DHW system, looks like a water stop cock) a few more pipes with pressure relief valves etc. but no M/V. I have not come across a setup like this in any of my previous homes so am unfamiliar with it. Unless i am mistaken this should have been at the very least a Y or S plan system which would require the use of zone control valves. Any info you can provide on this setup would be very much appreciated.

 

[Echo the husky]

earth cut short

 

[Johnmdc]

From your description of the cylinder if sounds like you have a thermal store.
When the clock is calling for hot water the boiler and one of the pumps will run these are in turn controlled by the thermostat on the cylinder.
When you turn the heating on the other pump will run, this pump is also controlled by the room thermostat. So the room thermostat only controls the second pump not the boiler.
Post some pictures of the cylinder and pumps for confirmation.

 

[BDLB79]

Echo, when I say cut short I meant cut shorter, the earth was roughly 4" long and wouldn't fit within the back plate as it is very cramped so I cut it back to around 1.5" and terminated it in a block behind the stat. This should leave enough wire should the stat require changing again in the future.

Johndmc, have you got any good schematics that show this particular setup? Also if the room stat doesn't control the boiler, how does it turn it off when the target temp is reached? And If it doesn't what is the purpose of the room stat in this type of system other than to start the pump? Will try to get some photos later in the week. Thanks for your help

 

[Echo the husky]

That's OK then!

 

[Johnmdc]

If you Google "thermal store" there are loads of explanations of how they work, but basically the boiler and first pump heat up the cylinder to the temperature set by the cylinder thermostat usually set to about 75°.
The second pump then circulates the heated water through your radiators. The room thermostat controls this pump to maintain your house at the set temperature.
The reason the hot water light comes on on your clock is because it has been set up that way. There would be no point in running the heating pump if the cylinder was cold, so when you turn the heating on it also fires the boiler and other pump up.
This is all assuming that you do have a thermal store. Until we see pictures we can't be sure.

 

[ollski]

Yes pretty sure it will be a thermal store where the up2 has been set to gravity to prevent someone turning the water off and still expecting to get heating

 

[BDLB79]

It has been a while since I last posted butii was unable to get down to my in-laws to take another look, however after making the trip, my father-in-law has told me that he is getting an engineer out to take a look at the system to make sure it is all OK and give it a service. The system all seems to be working fine at the moment so no issues with the new stat. Thanks for all your help on this issue.