3x buffer tanks - number of heat exchangers & domestic water?

Hello,

I have the following question and hope for advice:

I would like to couple a Viessmann Verticell 340M (750L) with two buffer tanks.

The Verticell has a solar heat exchanger. There are 6 panels with a total of about 12 sqm available for this.

Furthermore, the storage tank has a stainless steel coil inside for heating 40 liters of domestic hot water.

I now have the following 2 questions:

1. 40 liters of domestic hot water seems too little for a household of 5 people? Should I connect an additional domestic hot water tank with about 150 - 300 liters? So that the water in the coil (in the large buffer) is heated and collects in the smaller domestic hot water tank?

2. Do the other two buffers need to have a heat exchanger? I don't think so, or does the heat exchanger in the Viessmann have a limitation here and not handle the power from all the solar thermal panels?

Many thanks and best regards

The intended setup is at first glance rather unusual; some more background information would be helpful, then there would certainly be more answers.

Basically, the domestic hot water preparation works in a buffer tank with a stainless steel coil in a flow-through principle. The 40L therefore presumably refers "only" to the capacity of the stainless steel coil. However, the hot water performance cannot be directly derived from this. The parameters for this are the capacity of the buffer tank (750L) and the system temperature of the heating water in winter without solar input.

The efficiency of the solar thermal system is influenced by the type of connection of the three planned buffer tanks. Why is a larger buffer tank not installed instead of 3 buffer tanks? Why three at all? Is the goal to store the solar input over a longer period? Essentially to manage the spring period without a heating connection using only solar thermal over a bad weather day? If so, the buffer tanks should be dimensioned very large.

Hello, since the primary energy source is wood, our house needs about 2100 liters. Due to the available room height, it is not possible to install one or two very large tanks, so it must be split into 3 tanks of 750 liters each.

Thank you for the additional information. With wood, the system temperature in the buffer in a separated storage configuration will presumably be around ~65 degrees, thus from my point of view this results in sufficient hot water performance for 5 people. I would definitely not add an additional domestic hot water tank. Only either one or the other, or the third option: fresh water station via heat exchanger.

The 3 buffer tanks can be connected in series or parallel. A conceivable configuration would be the following: only one buffer tank has a solar heat exchanger, the two additional buffers are connected in parallel. Parallel means connecting the heating water volumes of the three buffers at as many different layer levels as possible. The more connections, the better and faster the temperature levels can equalize across the different buffers. An optimal layering of the temperature levels is desirable. Further explanations on the topic can be found, for example, on the web under "Bosy Kaskadenschaltung Reihenschaltung".

Apparently, the vitocell 750 buffer tank only has a total of 4 connections at different heights – a little more would be better, and ideally the two additional buffers have the same connections at the same height levels. This point would concern me somewhat and I would look for a buffer with more connections. In the worst case, the additional buffers only "half" participate. Therefore, I would possibly prefer a tank in a special form with only 1500L rather than 2250L distributed across three buffers. If all 3 buffers receive a solar heat exchanger, this increases solar yield and would ensure more even layering. But of course, it also costs a little more.

Thank you for your reply. It would be ideal if I had 3x the exact same storage, as then the connections would all be at the same height and I would also have a total of 3 solar heat exchangers. Correct?

I thank you for your answer. So it would be optimal if I had 3x exactly the same storage tanks, since then all connections would be at the same height and I would also have a total of 3 solar heat exchangers. Right?

Yes, correct. Then the most equal temperature conditions prevail in all 3 buffer tanks.