Heat pump: buffer tank, capacity and modulation

Hello everyone,
about the background of my questions:
currently I am dealing with the possibilities of converting our house from gas heating to a heat pump. Sounds completely crazy for a house that is almost 7 years old, but it could be funded and financed as an "energetic refurbishment project" together with photovoltaics via KFW 261. And then it can actually be economically really fun.
I am still in the evaluation phase and having the first conversations with heating and solar people.
Directly questions arose for me because the first offers are quite contradictory to the mantras I always read here in the forum:

[*]Buffer tank (heating water) – Question: what exact purpose is it supposed to have and when is it useful? No seller has been able to answer me this so far, although the technical answer is simple: it is supposed to work as a hysteresis so that the heat pump cycles less often. The basic idea is quite okay, but how much 100 l buffer tank helps me if the filling of my underfloor heating alone is already over 200 l plus the countless tons of screed as storage mass does not really open up to me. The current gas heating also works without a heating buffer tank.
[*]Heat pump power and modulation – the first offers, of course, come with absurdly oversized heat pumps because they only read 270 sqm heated area. To my remark that I find 12 or even 16 kW somewhat excessive, the answer is always: "it can modulate down from 3-12 kW." Fine and good, but exactly this is always warned against. So Question: why is modulating down the power so bad? Is that exactly the cycling that is always warned against? Actually, the technical definition of modulation would be that the entire system with compressor and everything runs slower. But even then the question arises how efficient that still is.

The house is insulated somewhere in the KFW 55 range, but did not reach the standard back then because of the gas heating. Heating demand is between 12,000 and 13,000 kWh for a total of 270 sqm area. Underfloor heating everywhere, including basement.
Best regards,

Andreas

The two questions are actually two sides of the same coin. Modulation has its limits, and if you have only 1.5kW heating load during the transition period, for example, 3kW is simply too much and the heat pump cycles. The buffer tank can extend the cycles somewhat, but it cannot compensate for a significantly oversized heat pump. Modulation itself is not a bad thing, but heat pumps with high maximum power usually cannot modulate down that low.

The buffer is also intended to serve as a hydraulic separator. The heat pump requires a minimum volume flow, otherwise it will fault, e.g. 50 l/min. If now only half of your heating circuits are open, the heat pump may fault because the throughput is not there, although heating power is actually needed. The buffer now decouples the volume flows, ensuring the minimum flow rate is always guaranteed. The disadvantage is the 2-4k more in the flow temperature you need, because due to the buffer there is always a mixing of flow and return flow. You can also operate without a buffer. Then simply without ERR with a low flow temperature for self-regulation and a well-adjusted heating curve to ensure the minimum volume flow. For this or alternatively, a bypass valve. It activates when too many heating circuits are closed and the minimum flow rate for the heat pump is no longer guaranteed.

The second function of the buffer is as a heat storage during the defrosting process. For this, you need enough warm water in the system. If your system volume is too small, you at least need a return flow buffer. However, with underfloor heating in the screed, this should not be the case.

„die kann doch runtermodulieren von 3-12 kW.

But only "just" down to 3 kW and probably only at low temperatures. At higher temperatures (when you need modulation), it is then immediately 4 kW. And you probably only need 4 kW with your house at below zero degrees and 12 kW only when you have completed the 200 m² extension. References to the current gas consumption and the energy performance certificate.

Im Bereich KFW 55, erreichte den Standard damals aber aufgrund der Gasheizung nicht. Heizbedarf liegt bei 12000-13000 kWh bei insgesamt 270 qm Fläche.

So a maximum heat demand of 12,000 kWh, i.e. about 5.5 kW heating load. If your house is 7 years old, this should also be in the energy performance certificate of the building file. I would have a maximum 7 kW pump installed. Depending on the exact values from the building file, a 5 kW system might also suffice. My tip here: Wolf CHA 7 or the new Panasonic Aquarea L (I prefer monoblock). You will also have to replace the hot water storage tank with a model with a larger heat exchanger surface (>2 m²).

Hello,

first of all, thank you for the answers. So the buffer is by no means completely absurd. However, it can be dispensed with with a bit of effort and love of interpretation.

If I exceed the "full utilization hours," the 12 MWh indeed result in about 5.5-6.5 kWh heating load. (I find values of 1600-2100 h) Thus, a 12-16 kW system seems quite oversized.

Presumably, an accurate heating load calculation would be well-invested money there.

Best regards,

Andreas

Presumably, an exact heating load calculation would be well-invested money again.

That has already been done during the new construction of the house and has not changed since then.
But you need a room-by-room heating load anyway if you want to use the KfW funds.

You have a gas heating system with which you can try that out well. Maybe you can even read the maximum heating capacity there in winter?

Otherwise, just see how far you can lower the flow temperature. And already do a thermal balancing now.