DHW turns on at 15oC even when switched off

[daddypew]

I have Evohome setup (ATC928G3000) around the house for a number of years and have used the "Stored Hot Water"(DHW) function to control our HotTub heating (no DHW cylinder).
Hot tub is set up with a 2 port zone valve via BDR91 Wireless Relay Box and CS92A Cylinder Thermostat with a Swimming pool heatexchanger to heat the water.
Everything works as it should with Evohome DHW set to 40oC it controls the tub temperature when I need it heated.
However, when I turn the DHW heating off, the temperature of the tub will drop over the next few days to approx 15oC then the boiler and BDR91 switch on and tub starts to heat up for 30-40 mins . How do I stop this behaviour to allow the tub to drop much lower in winter (I'll add some secondary automation to prevent freezing)
Is there a setting I can change to prevent this ?
Thanks

[img]20231203_081848.jpg[/img]

[gazza77]

I can't provide an answer, however we have a similar set up for hot water provision, and it's turning itself on whenever the temp falls to 16c. That happens irrespective of timed control and can be either when the heating is off or on.

[gazza77]

Follow up comment, I'm wondering if it is the frost protection on the boiler, as it only happens over winter and our boiler is in an outhouse.

[Richard]

I have Evohome setup (ATC928G3000) around the house for a number of years and have used the "Stored Hot Water"(DHW) function to control our HotTub heating (no DHW cylinder).
Hot tub is set up with a 2 port zone valve via BDR91 Wireless Relay Box and CS92A Cylinder Thermostat with a Swimming pool heatexchanger to heat the water.
Everything works as it should with Evohome DHW set to 40oC it controls the tub temperature when I need it heated.
However, when I turn the DHW heating off, the temperature of the tub will drop over the next few days to approx 15oC then the boiler and BDR91 switch on and tub starts to heat up for 30-40 mins . How do I stop this behaviour to allow the tub to drop much lower in winter (I'll add some secondary automation to prevent freezing)
Is there a setting I can change to prevent this ?
Thanks

[img]20231203_081848.jpg[/img]

As the hot water kit is designed to protect hot water cylinders, using it 'out of scope' means it is going to do this as evohome is frost protecting the 'cylinder'.

[daddypew]

As the hot water kit is designed to protect hot water cylinders, using it 'out of scope' means it is going to do this as evohome is frost protecting the 'cylinder'.

Seems a feasible answer, thankyou.
Shame as the function works perfectly otherwise.
I dont suppose there's a way to totally disable the HotWater control temporarily ?
that way I can choose when to have it active and when not
Thanks

[Richard]

No, because otherwise you would disable the frost protection when its used correctly? This system wasn't designed to heat hot tubs...

[daddypew]

No, because otherwise you would disable the frost protection when its used correctly? This system wasn't designed to heat hot tubs...

So, my particular problem now solved .... I realise not in scope of normal Evohome setup, so not recommending this, but just for information ...
I've added a parallel resistor to the temperature sensor to offset the actual temperature, in my case 39KR so now temperature can drop to approx 5oC before frost protection kicks in. Obviously the temperature reading is now incorrect, at 40oC actual will read approx 43oC but solved with minor change on target setpoint.