I'll leave off combifying the boiler and combining with heatbank output, though. With 180L I do not envisage running out of water at all, so it is probably a lot of complexity for little gain.
I didn't specify them, so I assume they're not there. I'll take a look, though. I decided I didn't want to do it that way. I wasn't convinced that I would get higher SEDBUK through the heat bank, due to the higher condensing boiler temperature compared to running the CH directly offsetting the expected short high power burn savings. It would also be a more complicated system with yet more pumps required.
I also have no intention of using the immersion as electrical backup central heating!
Complexity? a plate (no moving parts), 3-way valve and stat? What was the difference in price from a 140 litre to a 180 litre heat bank? Probably the cost of the ext plate and 3-way valve. The point is, if you have all that power available in the boiler use it to good effect.
complicated
With a heat bank you can specify the tappings for each zone. In short, two zones 4 tappings. The zones can all come off the heat bank. All you need is a pump and check valve for each zone. A pump is about the price of a
2-port valve. The heat bank is a neutral point so better balancing.
The condensing boiler will work efficiently enough.
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 will be.
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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