ᐅ Heat Pump – Domestic Hot Water Production

Hello everyone,

there is a topic that’s been on my mind. I only have some theoretical knowledge and don’t fully understand it yet, so I wanted to ask you. Maybe I’m already on the right track, but I’m not completely sure.

If I build a KfW 40 house and install a heat pump, I usually have underfloor heating.

The heat pump will probably heat the heating circuit based on demand. Do I also have a buffer tank here, or is it basically just the water in the heating circuit?

How does hot water generation work with a heat pump?
Is it common to have, let’s say, a 120-liter (32-gallon) hot water storage tank that is heated? Is this usually included with the system, or do I need an external buffer tank?
Or is it done without a large intermediate buffer, with the heat generated “online”? I suspect this might not work well since the heat pump is not usually sized that large.

Can the heat pump handle this in winter at all, or is it necessary to have an electric heating element switched on? The hot water should be heated to about 45°C (113°F), and in winter we can have temperatures down to around -15°C (5°F), so that would be a temperature difference of 60°C (108°F). I imagine that puts a lot of strain on the system.

Maybe you can help explain this a bit. Despite having quite a bit of knowledge about house building and heating, this part isn’t entirely clear to me yet. But maybe I’m already more or less right.

Thanks for your experience and knowledge.

Best regards,
Specki

Ideally, the heat pump is connected directly to the underfloor heating system, as the screed acts as a much larger heat storage compared to the few hundred liters often installed. This is controlled via the heating curve, so you can do without individual room thermostats or disable them after moving in.

The domestic hot water is stored in a tank, either a hygienic storage tank with a heat exchanger or simply 100–300 liters (25–80 gallons) of hot water heated on demand.

This works fine even in winter; we haven’t experienced really cold conditions yet. After one year, we’re still adjusting the system and seem to need quite a bit of heating – but it is a very large house. The annual electricity bill should arrive soon, and I will share an update then.

If you are going for KfW 40 standard, consider upgrading to 40+ right away. The main difference is the addition of photovoltaic panels and storage. Photovoltaic panels make sense anyway, and the battery storage is effectively free through government incentives linked to the PLUS standard.

Buffer tanks should be avoided if possible. Inside my heat pump unit, a domestic hot water tank is installed. When needed, a 3-way valve switches between the hot water and the underfloor heating.

Producing hot water should not be a problem even in the coldest winter. It just takes a bit longer, and the seasonal performance factor decreases significantly.

So, we are a household of four and the house will be relatively small, around 120–125 sq meters (1,292–1,345 sq ft).
Since we really need very little hot water, a small storage tank should be sufficient. It would be ideal if it’s integrated directly into the heat pump. Which model do you have, @michert?

Photovoltaics will definitely be installed on the roof anyway. It will probably be a KFW40+ house, as you mentioned, since the storage basically comes for free with the additional 5,000 euros. The storage should then be kept as small as possible. They just aren’t economical yet… On my current house, I’m installing a 24.5 kWp system, but without storage.

Okay, so heating basically without storage, that’s what I thought—that the heat pump manages that "on demand." And for hot water, then a small storage tank. It only needs to cover more than a bathtub fill (my wife uses that once a week). Otherwise, only short showers are taken sparingly, and except for one big clean-up once a week, no hot water is really needed.

fragg schrieb:

Hot water is stored either in a hygienic storage tank with a heat exchanger, or simply in 100–300 liters (26–79 gallons) of hot water that is heated as needed.

Hygienic storage tank is clear, but what exactly does “simply” mean here?

Thanks already, it’s becoming clearer.

Regards
Specki

If the bathtub has standard dimensions, for hygiene reasons choose the smallest possible heat pump-compatible storage tank with a large heat exchanger of 2–3 m² (21.5–32.3 ft²). A 200-liter (53-gallon) tank is more than enough for your needs; this way the water doesn’t stay in the tank too long. There aren’t any significantly smaller heat pump storage tanks available. I would try to avoid circulation. Alternatively, a fresh water station is an option, but in my opinion, it’s not necessary. However, read up on Legionella and make your own decision.

Integrated storage tanks are small enough.

Very important: no buffer tank. Even if it’s been mentioned already.

If you have a choice when selecting the heating system, research ground loop collectors (horizontal trench collectors) or alternatively a Panasonic 5 kW (Heisha) unit. Both options are more cost-effective and better than what a general contractor or builder might provide.

Edit: In the Allgäu region, a ground loop collector is preferable if possible.

Circulation is out of the question anyway. Everything is planned with short pipe runs. In my opinion, circulation systems are pointless in a single-family house. When renovating my current two-family house, I didn’t install one either.

200 liters (about 53 gallons) should be more than enough; I would even go as low as 150 liters (about 40 gallons). I believe that in our current two-family house, with 4 adults and 4 children, we have only installed a 210-liter (about 55 gallons) tank. (Okay, we have a gas boiler, so reheating is quite fast.)