A heat store has continual heat loss all the time it is kept hot! Its not always the most efficient way to use a boiler either.
Some people say that it allows a condensing boiler to condense because of stratification. Closer inspection shows thats not the case unless the boiler can operate with a flow return differential of 60°C which is never the case.
Tony
This is really totally and utterly incorrect. I advise the OP not to take any notice of it.
1. It is the most efficient way to use a boiler. It is at optimum operation for most of run time with full flow through the heat exchanger, which is important in a small tubed boiler. Inefficient and harmful boiler cycling is eliminated.
2. Even when the heat bank setpoint is set to 80C the boiler will be condensing at least 80% of re-heat. More when set to 70 to 76C.
3. Thermal layering which heat banks provide in DHW and boiler re-heat, provide a cool spot at the bottom of the cylinder. A DHW plate heat exchanger will put 20C into the bottom of the cylinder. The boiler only re-heats when a large proportion of the cylinder is cooled and the bottom section will be
very cool. The boiler re-heats in one long efficient burn.
30C and below is
very efficient when using condensing boilers.
Take no notice of this, "closer inspection".
4. A heat bank also acts as a CH buffer which has many great advantages.
A normal cylinder also has continual heat loss when it is hot too. Heat banks tend to be better insulated than DHW cylinders.
The worst scenario is directly heated radiators and a Part L cylinder heated via a coil. Boiler wear is maximised and efficiency lowered in this setup. If TRVs are fitted all around and they all close up, the auto by-pass opens giving a direct short cut from flow to return dropping efficiency like a stone.