Central heating sweltering upstairs, normal downstairs

[AVR2]

We've just turned the CH back on for the winter. It's controlled by two Drayton Digistat +3s, one in the hall and one on the landing.

Every night since turning it back on, downstairs has been fine (21C) while upstairs has been sweltering (typically a displayed 24C). I've been through the programming on the upstairs Digistat and it's programmed to heat to 21C.

Last night I put the upstairs one into Manual mode (which is supposed to override the programming until you switch back to Auto) and adjusted the temp down from 24C to 21C. When I got up at 5.30am, it was still in Manual, but it was reading 26C, and the bathroom was like a sauna.

The only fix I've been able to find is to switch the CH back off overnight.

Can anyone suggest what's going on?

 

[stem]

So for some reason the upstairs heating isn't being switched off when the set temperature of 21 degrees is reached. This could be as a result of a fault with the thermostat, or the heating system elsewhere. If you have the two wire version. The thermostat is battery operated. If your is, have you tried a fresh set of batteries?

To test if it is the thermostat that is faulty or not, you could swap them over. Then if the downstairs gets too hot and the upstairs is OK, you will know that the fault is with the thermostat. I seem to remember that the wiring is fixed to the backplate with this model, and the programmer simply clips into place so you won't have to touch the wiring to try this.

 

[AVR2]

Cool, I'll give that a try tonight. There is a battery compartment on the front of each stat, so I'll try new batteries as well.

 

[stem]

Cool.

Let's hope so.

 

[AVR2]

At the risk of sounding stupid, I presume that our CH is multi-zone, hence the ability for the upstairs temp to be controlled independently of the downstairs? This being the case, presumably the stats are hard-wired to their respective zones rather than being assigned through a programming setting?

I ask this because there's nothing in the Digistat installation guide or user manual about assigning a stat to a zone, in fact the user manual doesn't say anything about using more than one stat and actually says "If your heating system is a boiler with radiators [which ours obviously is], there will usually be only one programmable room thermostat to control the whole house".

 

[Hot&Cold]

Two thermostats = two separate zones so each thermostat will operate a zone valve.

The manual mode will have a set temperature value,sounds like yours could be set rather high.

Check all the temperature/time settings and if needed/confused you can restore the user settings to its original settings (similar to a hard reset).
Then enter your personal settings.

There again the zone valve on that circuit could be defective,try the settings first.

 

[AVR2]

The manual mode will have a set temperature value,sounds like yours could be set rather high

It's not, that's the issue. I've been through all the temp/time settings and none of them go higher than 21C. Despite this, if we don't switch the CH off, upstairs will get up to 28C (as happened this morning).

 

[ericmark]

I had same with father-in-laws house, found three faults, one thermostat out of calibration needs setting to 10°C to control room at 20°C, this had been found and room was set to 10°C, also the programmer was set for thermo siphon hot water 10 programs instead of 16, this also really did not cause the problem, the last was the motorise valve micro switch was stuck on so boiler never switched off, however again since valves were working this also should have not caused over heating. However once corrected the temperature went down. It would seem valve not fully off and with pressure with pump running 24/7 enough water got through to over heat house.

Also with my own house we have a wired thermostat battery operated, it has a little sign come up when batteries are low, however I know from bitter experience well before the sign comes up, the thermostat stops working, batteries last about 2 years, I change every year to be on safe side.

Before having instant hot water installed my house has thermo siphon domestic hot water and pumped central heating, there was no motorised valve and to get hot water without central heating all it did was switch off the central heating pump, all plumbed in micro bore so theory no pump = no central heating, however found if pump was started even for a very short time, then up stairs would continue to heat with thermo siphon.

So how are the zones controlled, by which pump runs or by a motorised valve, is the valve stuck if a motorised valve. There are normally leavers or other things where you can see if valve is working or not, in the main a thermostat controls the valve, then valve controls the boiler, so if valve stuck it will over heat.

Mothers central heating the thermostat was wireless and it lost contact with base, likely some thing local interfering it however was a cheap one and did not repeat command, so if the shut down heating signal was lost then heating carried on, I had to move set temperature to over actual temperature then back down again for heating to turn off. My fault for using a cheap thermostat.

 

[Hot&Cold]

if we don't switch the CH off, upstairs will get up to 28C (as happened this morning).

If you have 100% checked the digistat settings then the fault is idea:assuming the system is correctly installed) either the digistat is faulty or one or more of the zone valves are at fault.

As a test,have you swapped the positions of the digistats (they are hard wired) to see if the fault is moved to the downstairs zone.

While the system is at fault.The zone valves need checking for defective switching using a multimeter and checked for let by via observation of heat movement.

Depending on the zone valves and the installation method,testing will vary.

 

[AVR2]

I swapped the stats over at the weekend and the problem stayed the same, so I guess that points to the zone valves? Although I suppose there's still one more thing to try, which would be to replace the button batteries inside the stats (that I didn't know existed until I popped them off the wall). I'd already replaced the AAs that seem to drive the display, so maybe the replacing the button batteries will force a full reset of the stats which *might* help...?

 

[stem]

If the problem remained with the upstairs radiators despite swapping the stats over, then fault is not with the stats. Time to start checking elsewhere. The zone valves as suggested by old& bold would be my next port of call.