Hot water too hot

[srobo2]

Hi, our mains water was off for 4 days due to several bursts in the water main. When the water was off, we tried a few times each day to see if it was working, however the taps were sucking air in.
Once the mains water was restored, our hot water has been scolding hot. I don't know if it is a co-incidence, or if it is linked (somehow) to no mains water. (we currently turn the oil boiler on manually for a short period of time each day, rather than rely on the timer so the water doesn't get toooo hot..

The cylinder is around 12 years old (oso indirect, mains pressure). Initially, I thought it was the thermostat, however there are 2 thermostats - one for the boiler side and another for the 3kW electric immersion heater. Today, I left the boiler on for 40mins and the water is extremely hot. I then turned the thermostat right down on the immersion heater and switched it on - yup - the immersion turns on and keeps heating the water.

Are the 2 thermostats likely to be linked somehow? (we have a solar diverter for the electric immersion to use up excess pv energy, although I have it turned off until the fault is fixed). If the boiler side thermostat has failed, would the separate thermostat for the immersion not tell the immersion not to turn on?

I am struggling to get a plumber/heating engineer to come and help (which one of them would be better in this instance?)

The central heating itself works properly - turns on/off with the house thermostat (and it doesnt heat the water when only heating the house)

When the water heater is on, a small amount of (hot/warm) water is dripping out via the tundish.

Looking at the instruction manual, I have tried to 'recharge' the air gap in the cylinder - opening the commissioning valve, but no water comes out. Water does run into the tundish if I open the temperature/pressure relief valve and the check/expansion valve.

Any ideas on what it might be? I am not a plumber/heating engineer, but I thought that the 2 thermostats were independent of each other and its surely very unlikely that both failed at the same time?
(also - if I forget to turn off the boiler one day, are there likely to be any further problems other than scalding hot water - will excess water/pressure escape through the valves/tundish or am I looking at a major failure & leak. It's not easy getting someone out for a look at the problem)

 

[ianmcd]

you need to replace the immersion heater thermostat, this MUST be identical to the one you already have, you can not mix and match them, turn the Immersion off at the fused spur until you can replace it

 

[CBW]

I’m confused a little, are you using both the boiler and immersions for heating the water? Is the hot water too hot from when the boiler has been on?

 

[srobo2]

Just to clarify - when the immersion is off and we heat the water via the oil boiler, the water just gets hotter and hotter. Today, I tried the immersion heater (with the oil boiler switched off to see if that helped to rule out the thermostat (on the oil boiler side of things). Despite turning the thermostat right down on the immersion heater (colder than the water in the tank was at that moment in time), the immersion still turned on and started heating the water.

(I don't use both boiler and immersion heating at the same time - it was an 'experiment' to see if the immersion recognised that the tank was very hot already)

you need to replace the immersion heater thermostat, this MUST be identical to the one you already have, you can not mix and match them, turn the Immersion off at the fused spur until you can replace it

The thermostat on the immersion and central heating side are currently the same. If the fused spur for the immersion is off, the oil boiler heats up the tank too much. If the oil boiler is off, the immersion heats up the water too much.

It seems like a coincidence that both thermostats have failed at the same time. If (for example) the thermostat on the oil heating side of the cylinder had failed, surely that wouldn't affect the immersion thermostat (when the oil boiler is switched off?)

Is it possible that during our 4 day water outage, air got trapped in the system somewhere, or one of the valves got stuck in the wrong position? There was a lot of air bubbles in the water for a few days after it did come back on again - almost like fizzy/sparkling water.
Could it be something other than a thermostat?

 

[CBW]

How do you know the immersion was heating the water? If it’s getting hotter just from your boiler, then it’s likely a failed motorised valve.

 

[srobo2]

Hi, I have battery storage at my house. When I turned on the immersion (as an experiment this afternoon), the battery inverter showed me that the immersion was drawing 3kW. I left it for 20mins, just to see if the immersion would just go on for a few minutes before the thermostat switched it off - it didn't as it kept drawing 3kW from the batteries. There was nowhere else the energy could have been going for that length of time.
I'll try again tomorrow, with the immersion turned right down to 40C - if I turn on an immersion when the water is hotter than the thermostat on the immersion, how long would you expect the immersion to switch on for before 'tripping' off again. Would it be immediate, or take a few minutes for the immersion to start heating up before sensing that it is already too hot? (I have very little knowledge about this!)

Lets forget the immersion for a minute as my biggest concern is the boiler side of things - what would be the symptom of a failed motorised valve? Is there an easy way for a relative novice to test the valve? My system appears to have 2 motorised valves. On one of them, when I turn on the actual house central heating, a small lever moves across to 'H' then back again when switched off. On the other motorised valve, there isnt anything immediately visible.

(Thanks)