Using electric water heater

BUFF

coffeehound said:

BUFF as soon as the water in the tank reaches a point where the return water is >55 ℃, the boiler won't be condensing anymore.  It will vary by installation but perhaps mid-40s tank temperature?  Therefore a middle way could be to heat to 45 using gas, and then top-up to 60 using the immersion

Given that it's 55C return or lower for condensing & most people will heat a tank to 60-65C I agree that there is quite probably a point where the boiler may no longer be in condensing mode - I would have thought that it would have been at a tank temperature higher than 45C though. I admit that some quick Googling hasn't yet thrown up any hard facts & figures on it with a regular or system boiler +tank but I do know that whereas combis traditionally are noticeably less efficient at water heating than ch (mine is 80% v 92%) the latest combis are now hitting 90% efficiency for dhw as well - technology continues to move on.

& I think that it will still be over 85% efficient even if not condensing. The electricity would need to be 5.0p kWh for parity of cost.

coffeehound

Yes it would be interesting to test the tank temperature at which the return exceeds 55.  Dependent on a number of things like coil efficiency, flow temperature, pump speed, T-stat positioning.  At least the procedure would calibrate the process to the particular setup.

Edit - oh and it would give some leeway for reducing the flow temperature too, which is ofc a big one for heating efficiency.  Another thing is that as the temperature differential between the flow and the DHW gets ever smaller, the boiler's going to need to grind on longer and longer to get a degree increase, so it has to be beneficial to clip off that last inefficient phase.  I think Dolor's numbers show this.

QrizB

BUFF said:

Given that it's 55C return or lower for condensing & most people will heat a tank to 60-65C I agree that there is quite probably a point where the boiler may no longer be in condensing mode ... I think that it will still be over 85% efficient even if not condensing. The electricity would need to be 5.0p kWh for parity of cost.

If you believe the chart in this post:

https://forums.moneysavingexpert.com/discussion/comment/79033710/#Comment_79033710

... boiler efficiency with a return temperature of 100C will still be about 85%.

Yes it will be less efficient, but the effects are relatively small.