I'm having 5 zones, which might end up with more pipework than you could
imagine festooned on the cylinder. Most of the 2 port valves will be two
floors away and I don't want multiple runs of piping. So I suspect even if I
had decided to take the central heating off the bank, I would have valved
off a single tapping.
However, I might have required a bigger heat bank if I was to use it for
heating as well. I was also worried about heat stratification performance if
the central heating was forever stirring up the water.
At the end of the day, a decent condensing boiler can get 97% efficiency
without the use of a heat bank. I doubt that the heat bank could improve on
that much. Obviously, it is very useful if there are other heat sources
involved, such as solar or a solid fuel boiler, but there isn't and never
I intend to have a direct heat bank with the following:
- The top 2/3 of the cylinder, DHW section, will be heated to approx 75C
- A blending valve will be on the return to the boiler on the DHW side, set
to 53C promoting condensing efficiency, boiler efficiency in that it
operates at designed delta T, and rapid heat up of the stored water, with a
one boiler pass of all the stored water.
- The bottom 1/3 (or 1/4), CH section, will be heated to whatever a weather
compensator dictates. There will be an outside temp sensor and a sensor on
the cylinder. Another sensor will be in the living space dropping the
cylinder temp accordingly to room influences. If, for e.g., the outside
temp is 10C it may require a cylinder temp of 48C or less.
- From the boiler the flow will have a 3-way diverter valve that diverts
flow from the top cylinder section to the bottom and vice versa. The two
sections will operate on two temperatures. The DHW section set and the CH
section variable via a weather compensator.
- The top section takes priority, in that if the DHW cylinder stat calls for
heat the diverter valve takes all the boilers heat to then top section for
rapid heat up.
- The bottom CH section will have two zones run off this. One for upstairs
and one for down. This is easy for system balance. Each will have a
non-return valve and a Grundfos Alpha auto variable speed pump, variable on
pressure sensing to compensate when rad valves start to close.
So, when on CH the bottom section may only be 25-30C forcing the condensing
boiler into very low temps promoting high efficiency. In total 3 pumps and
one 3-way valve and an outside weather compensator.
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