MediumRoastSteam If you have a tank, most likely there’s a thermostat control on the tank itself in which you set the temperature of the hot water as you please.
It’s not the same though, that thermostat on the tank just tells the boiler when to stop. The one on the boiler tells it how hot to make the water that circulates in the HX tube. At which point you should experiment with how long the hot is on to give a decent tank hot fill. This is because you cannot rely on a tankstat set to say 60, as it will never turn off….
The efficiency of a condensing boiler is higher when the outflow temp is lower and the return temp is definitely as low as it can be (I think above 50C and most boilers won’t condense, loosing 20-30% efficiency
Turning the HW on at the same time as heating in many boilers results in a lower return, although you still have to work out how long to heat the HW for. My boiler has a single thermostat for outflow temp. So it’s quite important to me to have heating and hot water on simultaneously, to get the coolest return I can.
In fact for my boiler, I want the heating to come on for a while, as the return will be cool as the house warms up….then add the hot water in for it’s allotted time, which helps keep the return temp low. Maximising boiler efficiency.
Oh the reason for 55C is legionella, 55C Kills Listeria in about 2 hours, 60C to 65C kills it in 15 minutes…but really it’s not a risk to have 55 in a domestic environment. Especially if the tank is only 70% hot with a lower cooler layer…lower than 55 is probably not great.
Obviously with a Combi, Legionella is not an issue, what might be is a very low temp for showering ,might mean full hot and reduced flow…usually it’s OK though and 40C will give a more than hot enough shower.
