ᐅ Heat Pump: Buffer Tank, Capacity, and Modulation

Hello everyone,
some background on my questions:
I am currently exploring the possibility of converting our house from a gas heating system to a heat pump. It might sound completely crazy for a house that is just under 7 years old, but this could be funded and financed as an “energy renovation project” together with a photovoltaic system through KFW 261. That could actually make it economically very attractive.
I am still in the evaluation phase and have started initial discussions with heating and solar experts.
This immediately raised some questions for me, because the first offers are quite contradictory to the principles I often see discussed here in the forum:

• Buffer tank (heating water) – Question: What exactly is the purpose of this and when does it make sense? So far, no seller has been able to clearly answer this, although the technical answer is simple: it should act as a hysteresis to reduce short cycling of the heat pump. The general idea is understandable, but I don’t really see how 100 liters (35 gallons) of buffer tank help when just the volume of my underfloor heating system already exceeds 200 liters (70 gallons), plus the large mass of screed acting as thermal storage. The current gas heating system works fine without a heating buffer tank.
• Heat pump capacity and modulation – the first quotes naturally come with absurdly oversized heat pumps because they only consider the total heated area of 270 square meters (2,900 square feet). When I point out that 12 or even 16 kW seems excessive, the answer is always: “It can modulate down from 3 to 12 kW.” That’s fine, but exactly this modulation is often warned against. So question: why is modulating down performance considered a problem? Is this the short cycling that people warn about? Technically, modulation means the entire system, including the compressor and all components, runs slower. But then the question arises how efficient it still is.

The insulation level of the house is about KFW 55 standard; however, the overall standard was not met back then due to the gas heating system. The heating demand is around 12,000-13,000 kWh for a total of 270 square meters (2,900 square feet). There is underfloor heating throughout, including the basement.
Best regards,
Andreas

The two questions are actually two sides of the same coin. Modulation has its limits, and if, for example, you only need a heating capacity of 1.5 kW during the transition period, a 3 kW heat pump is simply too large, causing the heat pump to cycle on and off. The buffer tank can help extend the cycles somewhat, but it cannot compensate for a significantly oversized heat pump. Modulation itself isn’t a bad thing, but heat pumps with a high maximum capacity generally cannot modulate down as far.

The buffer tank should also function as a hydraulic separator. The heat pump requires a minimum flow rate; otherwise, it will trigger a fault. For example, 50 liters per minute (l/min). If only half of your heating circuits are open at a time, the heat pump may fault because the flow rate is insufficient, even though heating output is actually needed. The buffer tank decouples the flow rates, ensuring the minimum flow rate is always maintained. The downside is that you need 2–4 K (3.6–7.2°F) higher supply temperature because the buffer tank always causes mixing of supply and return water.

You can also operate without a buffer tank. In that case, run without electronic room regulation (ERR), with low supply temperature for self-regulation and a well-adjusted heating curve to ensure the minimum flow rate. Additionally, or alternatively, use a bypass valve that opens when too many heating circuits are closed and the heat pump’s minimum flow rate is no longer guaranteed.

The buffer tank’s second function is to store heat during the defrost cycle. For this, you need sufficient warm water in the system. If your system volume is too small, you at least need a return buffer tank. However, with underfloor heating embedded in screed, this should not be necessary.

andimann schrieb:

„die kann doch runtermodulieren von 3-12 kW.

But it only modulates down to 3 kW, and probably only at low temperatures. At higher temperatures (when you actually need modulation), it will be around 4 kW. And you will likely need 4 kW for your house only at subzero temperatures, and 12 kW only when your 200 m² (2,150 sq ft) extension is completed. Refer to your current gas consumption and the thermal protection certificate.

andimann schrieb:

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 the maximum heat demand is about 12,000 kWh, which corresponds to approximately 5.5 kW heating load. If your house is about seven years old, this should also be documented in the thermal protection certificate in the building files. I would recommend installing a heat pump with a maximum output of 7 kW. Depending on the exact data in the building documents, a 5 kW unit could also suffice. My suggestion here: Wolf CHA 7 or the new Panasonic Aquarea L (I prefer monoblock systems). You will also need to replace the domestic hot water tank with a model that has a larger heat exchanger surface area (>2 m² (21.5 sq ft)).

Hello,

first of all, thanks for the replies. So the buffer is definitely not completely unnecessary. However, with some effort and careful planning, it can be avoided.

If I look at the "full load operating hours," the 12 MWh actually translate to about 5.5–6.5 kW heating demand. (I find values between 1600–2100 hours.) This suggests that a 12–16 kW system might be quite oversized.

A precise heating load calculation would probably be a worthwhile investment.

Best regards,

Andreas

andimann schrieb:

A precise heating load calculation would probably be well worth the investment.

This has already been done for the new build of the house and hasn’t changed since then. But you will need a room-by-room heating load calculation anyway if you want to apply for KfW funding.

You have a gas heating system that you can use to test this easily.
Maybe you can even check the maximum heating output during winter?

Otherwise, just see how low you can set the flow temperature.
And start performing a hydraulic balancing now.