Evohome HW BDR not switching on

[yiangosliv]

Further to solving the problems related to configuring HW along with Heating Valve (for radiators) [see https://www.wordpress-1219309-438749...ng-as-expected ] , I am now facing another problem. When I configured the system, I switched on the heating (radiators) takes 2-3 minutes for HR92 to initiate and then BDR91 fires up. While the heating (radiators) BDR91 was on I switched on the HW and the 2nd BDR91 switched on instantly. after 5-10 minutes I switched off HW and 30 mins later the heating was off (as expected). Next day, when I tried to replicate the same scenario the HW BDR91 does not seem to fire up. I have even checked the system parameters (which i also uploaded as attachments) Can someone please have a look and advise why the HW BDR91 might not be switching on? Furthermore i d like to understand what the percentages mean in the system summary.

Thank you
HW Boost on.jpgOverview (HW is on).jpgStored HW parameters.jpgSystem Summary.jpg

[DBMandrake]

Your hot water configuration looks very strange - why have you only got it set to 40C ? This is far, far too low to protect against Legionella bacteria, especially with a 7 degree differential.

I have mine set to 54C and 5C differential and even that is technically "too low" from a bacteria point of view. (I find anything higher too hot at the tap however)

The way you have it configured, the hot water relay will not switch on until the temperature drops below the set point minus the differential - eg 33C. If it is between 33C and 40C it may or may not switch on depending on whether the temperature was most recently heating or dropping.

Finally, why the 10 minute hot water overrun ? Normally you would only want 1-2 minutes at most. This feature is to allow water to circulate through the cylinder after the boiler is switched off when the hot water target is reached. However hot water overrun does not work properly unless you use a separate boiler control relay - if you only have heating and hot water relays and boiler control is set to none you should set the hot water overrun to zero minutes or you may find the hot water temperature overshoots, as it will simply cause the hot water relay to remain on for X minutes after the target is set as it is not able to separately turn the boiler off when there isn't a dedicated boiler control relay.

[yiangosliv]

Your hot water configuration looks very strange - why have you only got it set to 40C ? This is far, far too low to protect against Legionella bacteria, especially with a 7 degree differential.

I have mine set to 54C and 5C differential and even that is technically "too low" from a bacteria point of view. (I find anything higher too hot at the tap however)

The way you have it configured, the hot water relay will not switch on until the temperature drops below the set point minus the differential - eg 33C. If it is between 33C and 40C it may or may not switch on depending on whether the temperature was most recently heating or dropping.

Finally, why the 10 minute hot water overrun ? Normally you would only want 1-2 minutes at most. This feature is to allow water to circulate through the cylinder after the boiler is switched off when the hot water target is reached. However hot water overrun does not work properly unless you use a separate boiler control relay - if you only have heating and hot water relays and boiler control is set to none you should set the hot water overrun to zero minutes or you may find the hot water temperature overshoots, as it will simply cause the hot water relay to remain on for X minutes after the target is set as it is not able to separately turn the boiler off when there isn't a dedicated boiler control relay.

The reason I have it at 40 is because my sensor is at one end of the HW cylinder (i did not wanted to cut the insulation to place it where Honeywell recommends, therefore i placed it somewhere where the cylinder manufacturer advised me (that i wont have any heat loss. Even if the sensor reads 40 by experience i know that the actual HW temp is much higher in reality. But your advise is well noted, I will increase that to 60 or 55 with a diferential of 10. Regarding the the HW overun i will set it to 0 minutes as you have advised. Any ideas though why this had not worked? The HW temperature was 23 and according to the configuration, the HW BDR would not fire up unless the temperature was 33-40 degrees.