[Danfoss TP5000Si][IDEAL ESP1 35] Heating won't turn off in 1 zone; replaced thermostats - no luck

[danielwtf]

Central heating

• 1 combi boiler in the kitchen IDEAL Logic Combi ESP1 35
• 1 thermostat in hall Danfoss TP5000Si
• 1 thermostat in upstairs bedroom Danfoss TP5000Si
• 2 motorised valves in upstairs airing cupboard Danfoss TYPE HPA2

Heating is not turned off
Both zones - but only 1 zone at a time - have had the issue where the heating is not turned off when the zone's thermostat reads that the desired temperature has been exceeded. The boiler continues to run, and the radiators stay on full constant heat. As an example, the bedroom zone has reached 27°C when the thermostat is set to not exceed 22°C.

2020
There are 2 motorised valves (Danfoss TYPE HPA2) in the upstairs airing cupboard.
In 2020, 1 was replaced as an attempt to fix the issue.
Shortly after, the issue returned and the heating engineer came back and swapped the bodies of the two stats over (Danfoss TP5000) which somehow stopped the issue from occurring so frequently.

Since then, the issue has occurred intermittently when heating is needed (Autumn to Spring) and much more frequently since December 2021.

In February 2022 the 2 mains-powered thermostats (Danfoss TP5000) were replaced with battery-powered stats (Danfoss TP5000SI) as an attempt to find if the stats were the cause.
The issue continues to occur in the bedroom zone

Both stats are brand new + new batteries, timings are programmed correctly, and wired based on instructions (permanent live > CO, switched live > N/O). Based on this, I believe the issue lies elsewhere in the central heating system.

Right now
As I type, the boiler continues to hum and the bedroom radiators are on full blast. The stat thermometer reads 24°C even though it is set to reach only 21°C.

Motorised valves differences
The two metal nubs on the motorised valves are currently pointed to AUTO
1 metal nub (right-hand valve) has great resistance when I attempt to move it to MAN
1 metal nub (left hand valve) has no resistance at all when I attempt to move it to MAN

I'm writing this as I have not found the issue described elsewhere online.

Please help me figure out if I can attempt more myself, or which kind of tradesman I should consult on the issue.

Happy to provide additional details where necessary. Thanks!

P.S. this is a new build by Barratt's homes

 

[CBW]

Sounds like the zone valve is knackered (the one without resistance).

 

[stem]

I've come across similar symptoms on 3 occasions.

1. The wiring to the two motorised valves was transposed, such that the downstairs thermostat was controlling the upstairs radiators and vice versa. When the upstairs heating is on when it shouldn't be try turning the downstairs thermostat down and see if it goes off then.

2. There was a frost thermostat installed. Frost thermostats are designed to override the daytime controls and bring on the heating regardless of the user settings. In this instance the frost stat was set too high and was coming on unnecessarily. This would be unusual for a new build though, as frost stats are traditionally used in outbuildings or areas with exposed pipes at risk of freezing.

3. Wiring fault. The motorised valve was getting a live supply from somewhere else other than the thermostat.

If the fault is readily apparent when a heating engineer comes to inspect it, it should be a fairly easy fault finding exercise. If the valve is open when according to the controls it shouldn't be, the first check is, is it receiving 230V across its L & N, if no, the valve is faulty as @CBW said. If yes, then the valve is doing 'as it's told to', and it's matter of tracing the live feed at the Motorised valve to find out where it's coming from. With a new installation and a convenient wiring centre, it shouldn't take long for a competent heating engineer or sparky to find the issue.

Switching components in an adhoc manner without carrying out tests and hoping to strike lucky is very rarely the best way to troubleshoot....as you seem to have found.

 

[danielwtf]

Thanks @stem & @CBW

You are totally right @stem, I naively hoped replacing the stats would be enough.

Really appreciate the time you took to read and reply to my issue - I will call a heating engineer first thing tomorrow and follow up with the result.

Thanks again

 

[danielwtf]

[SOLVED]

Heating engineer came this morning. Thankfully as he arrived, the issue was happening (heating was stuck on full blast in bedroom zone even though temperate had been met).
After my explanation of events, he performed troubleshooting, including:

1. Testing the wiring to the thermostats
2. Testing the wiring to the motorized valves
3. Testing motorized valve response to changes made from the thermostats
4. Replacing both motorized valve 'tops' (the electrical parts)
5. Replacing one motorized valve 'body' - he had to drain water out of the pipes to remove the actual valve

All of the wiring was behaving as expected.
Number 5 appears to have been the problem; engineer explained that the actual valve (metal nub that turns to switch off the flow of water and trigger the signal to the stats) had a tendency to seize up, but only sometimes which is why the issue is intermittent. He examined both bodies, but needed only to replace one of them.

It's an hour since his visit and all seems to be working so far - will update this thread again in future if the issue happens again.

Thanks again for input, gave me the push I needed to get a professional to sort it out once and for all.