The HWC Mystery - only reaches 50c despite a tested thermostat...

[Prothed]

Background - One morning about two weeks ago I found we had hot water in the cold water tank, so I ran off the cold (now hot) water and let the cold tank refill to try and diagnose the reason. But the cold tank has stayed cold ever since after that with nothing coming back from the hot cylinder via the feed pipe or overflow.
However, since then the HWC temperature will not rise above 50-55c despite the thermostat still being set at 65c.
It is a 200l cylinder with a 3kw immersion fed from excess solar power with a 4h boost at night.
I assumed the thermostat had failed, but after fitting a new one (tested to switch at 60-65c in a pot of hot water) there is no difference - the hot water heats until about 50c and then the controller says 'Hot' and switches off the power.
This morning, in desperation, I bypassed the themostat and lo-and-behold the water carries on heating. (I switched off when it got to 70c).
I have no idea whether the hot cold water incident and the HWC switching off early are connected, or just a coincidence, but I'm running out of ideas on what the problem might be.
Any suggestions welcome. Thanks.

 

[John.g]

Have you a (top mounted) dual immersion?
when the stat operates at "50/55C", are the first few liters of water at 60/65C and then reduces?.

 

[Prothed2]

Sorry for the delayed reply. The forum blocked my post, saying 'spam or inappropriate content'. wtf?
Anyway, Yes - The top IH is only operated manually for a boost. The bottom IH is fed from the iboost controller which feeds the solar power during the day and a timed boost at night. This house has a recirculating hot water system but the valve in the return pipe to the cylinder is off (closed).
No - the thermometer is attached to the top outlet of the cylinder and it doesn't change as hot water is drawn (and the water from the tap doesn't seem to change after it starts running).
I'll try and post a picture of the setup.

 

[prothed2]

Upper IH manually switched only for boost if required.
Lower IH fed by topmost controller (excess solar power during day with boost during night).

 

[prothed2]

(No it didn't...? I set up another account just in case...)
The initial temperature doesn't seem to change when the flow starts.

 

[John.g]

Does the top IH reach 60/65C?.

 

[ShaunCorbs]

Your not logged in it keeps them in a mod approval queue if not logged in approved your original one

 

[Murdoch]

If the water in the cold tank is too high, it’s possible that the overflow from the hot tank can “circulate“ water - meaning your cold tank can be heated “by mistake “

as for the 50 degree issue, can you post a photo showing ”hot”?

 

[prothed2]

If the water in the cold tank is too high, it’s possible that the overflow from the hot tank can “circulate“ water - meaning your cold tank can be heated “by mistake “
as for the 50 degree issue, can you post a photo showing ”hot”?

The strange thing is that the 'circulation', whatever caused it, only happened once and hasn't recurred.

The solar controller display will say 'Water heating 1.7kW' or similar when the sun is out and the water is heating, but when the IH thermostat opens and the display shows 'HOT' then the thermometer on the top of the cylinder will only have reached around 50c.
(I could get a photo of this but it would take a bit of faffing around).

 

[John.g]

If the diverter sees or think it sees excess solar generation then it changes from mains supply to solar PV so maybe some gremlins in the controller?.

Just saw your post now.

 

[John.g]

The strange thing is that the 'circulation', whatever caused it, only happened once and hasn't recurred.

The solar controller display will say 'Water heating 1.7kW' or similar when the sun is out and the water is heating, but when the IH thermostat opens and the display shows 'HOT' then the thermometer on the top of the cylinder will only have reached around 50c.
(I could get a photo of this but it would take a bit of faffing around).

Are you sure that the (bottom) IH stat has actually opened though at < 60/65C, also ensure that the mains is supplying the lower IH as under cloudy conditions the power supply will be cycling between mains and solar until stat satisfied.

 

[prothed2]

If there is no sun, or you switch the kettle on, the controller just says 'Not heating' since it only diverts excess power to the IH. The mains power is only switched on by its boost timer at night.
When I bypassed the thermostat the water just kept heating until it reached 70c and I switched off the power (!)

 

[John.g]

Does the boost timer at night also only heats to 50C?.

 

[prothed2]

Does the boost timer at night also only heats to 50C?.

Yes.

 

[John.g]

Suggest swapping top/bottom IH stats and see what happens.