The Efficient Home Forum

Hi all

Why does my hot water go past the settings I have given it ? It’s set at 43 and has gone up to 59
I have the hot water prob in a pocket in the hot water tank cylinder .
I am using CS92A Wireless Tank Thermostat .
I am puzzled at the fact that there does not look to be any fail safe ?
Any help out there please.

I'm not sure why that's happening, but setting the hot water to 43c seems very low. Stored hot water should be a much higher temperature (60c) in order to prevent the development of bacteria such as legionella in the tank.

Also, have you checked the settings in the control panel? There's a temperature differential setting in the Stored Hot Water section that can go up to 10 degrees - appreciate your post indicates more than that but perhaps that combined with some boiler overrun could account for it.

Are you sure the zone valve for the hot water is wired correctly in conjunction with your evohome system?

stewbie60 wrote:Hi all

Why does my hot water go past the settings I have given it ? It’s set at 43 and has gone up to 59
I have the hot water prob in a pocket in the hot water tank cylinder .
I am using CS92A Wireless Tank Thermostat .
I am puzzled at the fact that there does not look to be any fail safe ?
Any help out there please.

Could be something outside of evohome causing the issue, installation issue(s) or faulty zone valve for instance?

I saw this after recently adding DHW control to our already existing CH Evohome system.

As per other threads I've done an imperfect retrofit to our S-Plan system, taking the system demand / boiler control BDR to the CH 2-port valve and the new hot water BDR to the DHW 2-port.

This means that when the DHW valve is energised it will also fire the boiler (in addition to the CH / system call). I kind of like this since it provides a bit of resilience if the "main" BDR fails or goes into failsafe mode for some reason. No doubt others will frown on it .

The problem arises when the Evohome DHW has overrun set. Evohome things it has turned off the boiler and is still letting a "European style" DHW circulating pump (or boiler overrun) dump residual heat into the DHW cylinder. But because the DHW 2-port is still firing the boiler it actually gives 10 minutes (or whatever is configured as overrun) of active heating to the cylinder - causing significant overshoot.

Although I haven't tried this there would presumably be overshoot in a properly wired configuration (ie no boiler trigger from valve) if the DHW 2-port was left open via overrun while the CH circuit was calling for heat. This might be prevented if DHW priority suppressed the CH call for heat until the end of overrun as well as during actual heating.

I've now set the overrun to zero, and DHW reaches intended temperature before stopping.

OptiMiser wrote:The problem arises when the Evohome DHW has overrun set. Evohome things it has turned off the boiler and is still letting a "European style" DHW circulating pump (or boiler overrun) dump residual heat into the DHW cylinder. But because the DHW 2-port is still firing the boiler it actually gives 10 minutes (or whatever is configured as overrun) of active heating to the cylinder - causing significant overshoot.

The overrun is there for when you use evohome on OpenTherm. When charging the cylinder, the boiler is told by the OpenTherm Bridge to run at a significantly higher temperature to recover the cylinder. The problem comes that the space heating might only have only been running at 45 degrees C. The overrun time allows the boiler heat exchanger to cool to at least the cylinder temperature before evohome swaps back over to doing space heating, to minimise the amount of excess energy that goes into the space heating circuit.

So, if you are just using evohome on a non-OpenTherm installation, the overrun can be set to 0 minutes.