Most economical way to run condensing boiler

EnglishMohican

Sorry, this is a hugely long post as it all seems to get quite complicated.

I am in the process of buying a new gas condensing, regular boiler and want to buy one that I can run economically. I am trying to make sure that I understand enough about what I am doing to choose the correct boiler.
My house is well insulated and we do not like bedrooms too hot. At an educated guess, we only need 2kw going to the radiators most of the time to keep the house warm enough. (Thats in the winter - nothing in the summer) With every radiator on and everywhere cold, then the radiators can dissipate about 7.5kw total. Most radiators will be off or very low and we have a gas fire in the lounge for pleasure, so 2kw typically.
There are TVRs on the radiators and once the room temperatures are right, then the TVR will restrict the flow to keep the temperature down.
The Worcester Bosch boiler we are considering controls the temperature of the water leaving the boiler. Lets say that I set it to be 70C to provide some drive to warm the DHW to about 60C quite quickly. I understand that when the radiators are cold and the TVRs fully open we will get maybe 30C drop and return water will be 40C - so the boiler will operate in condensing mode nicely. Great
A while later when the house has warmed up, the TVRs will start to shut down and we will only get maybe 10C drop so the return water will be at 60C and the boiler will not be condensing so not running economically. Not Great.
At some point the boiler will decide that the return water is so hot that it cannot modulate down far enough to avoid overheating the flow water and it will turn off. Thats OK providing it does not try to turn on again too soon as then it will be running doubly inefficiently- dry cycling and not condensing combined.
That could be improved by setting the boiler to deliver water at 50C - then the return water has to be cool enough for the boiler to condense all the time. It will still dry cycle but nothing I can do about that. BUT- then my DHW will not be warm enough.
Do you think I have understood how it all works correctly? What alternative schemes do other people use and does anybody know of a boiler that can have two set points, one for DHW and one for CH and choose the relevant set point - remember its a regular boiler - not a combi and the DHW set point could override the CH set point?

Norman_Castle

My Baxi duo tec combi 24HE has different temperature settings for heating and hot water.

EnglishMohican

I think lots of combis will do that but mine needs to be a regular boiler - or is your combi plumbed in to heat a hot water cylinder?

Norman_Castle

No hot water cylinder.

fezster

If you pair a vaillant boiler with a vaillant unvented, you can have different temperatures for heating and dhw. Similarly, for vented cylinders, vaillant do a kit to put a temperature probe in to achieve the same thing. Some people have also had luck using this probe in non vaillant unvented cylinders.

A common problem you describe, and no perfect solution, as the other thing to keep in mind is that cylinder stat is not recommended to be below 60, to prevent legionella.

bridgedino

It's already going to perform economically, they're the most efficient type of boiler

EnglishMohican

Saying that it is the most economical type of boiler may be true but is unhelpful.

It can be operated to be more economical or less economical depending on how you set it up. The problem seems to be that the 12kW and 15kW boilers I have investigated so far are pretty stupid things that cannot adjust their set up to compensate for the needs of the DHW system (highish flow temperature) as against the needs of the CH system (lowish flow temperatures). See the OP for the basis of that statement and the comment above about legionella.

I am asking whether anybody has found a solution to the problem be it a method of operation or a cleverer regular boiler that can have two set points and switch between them depending on which system (DHW or CH) is being driven at a moment in time.