ᐅ KfW 55 - Sole Heat Pump: Which Hot Water Storage Tank and Related Components?

Hello dear experts,

We have a new build planned in a few months, finally.

Here are the details:
Living area: 205 sqm (2205 sq ft)
Occupants: 3
Heating system: ground-source water heat pump with deep drilling (2x75 m (2x246 ft)). Most likely MTEC WPS 412 (max. 12 kW)
Heating type: underfloor heating with partial wall heating – which manufacturer would you recommend?
Controlled residential ventilation with heat recovery: Zehnder Comf. q350tr with enthalpy
Wall construction: 42.5 cm (17 inches)
Solar/photovoltaic support is not planned.
Hot water storage????

The hot water storage is still a big question mark here.
We are only being offered buffer tanks with 1000 l (265 gallons) at prices starting around 3000 euros.

However, I have read that buffer tanks can actually be counterproductive with geothermal heat pumps.

The only requirement here, since the build is part of the 10,000 houses program in Bavaria, is a hot water tank of at least 360 l (95 gallons) (minimum 30 l (8 gallons)/kW) with appropriate U-value/insulation.

What would be the best hot water preparation method and what size would you recommend?
I don’t want to spend thousands of euros just feeding the heating installer and even end up with higher electricity costs because the whole system isn’t properly coordinated, which unfortunately happens quite often even with supposedly "experts."

I would also appreciate any experiences with the MTEC WPS 412 or any suggestions (underfloor heating, etc.). It’s still possible to incorporate everything now.

Thank you very much in advance for your support. Happy Easter.

12kW seems completely oversized for the heat pump. What is the heating load of the house?

There are good storage tanks available starting from around 600€ (shwt, twl), branded products cost about 1000€ (Juratherm hdw). It is important to have a large heat exchanger of 2, ideally 3m² (21.5, ideally 32.3 ft²), inside the tank. Regarding size, the usual question is whether 200 liters or 300 liters (50, 80 gallons). A smaller tank has the advantage that the water stands for a shorter time (legionella risk), 300 liters is useful if there is a lot of bathing or frequent consecutive showers. You can best estimate this yourself. Bigger is not necessarily better.

Do not accept a buffer tank.

Underfloor heating: Have a room-by-room heating load calculation done with your desired temperatures. Based on that, plan the underfloor heating to allow a maximum flow temperature of about 30°C (86°F). Heating circuits should all be around 80 meters (260 feet). The bathroom should be planned carefully, possibly with occasional electric backup heating and/or embedding the underfloor heating in the wall. The goal is a low flow temperature for the entire house.

@boxandroof

Thank you. The 12 kW rating of the heat pump is only the maximum capacity. It is a modulating heat pump with a range from 4 to 12 kW. I took this information from the quote. For ground-source heat pumps, the range of available options is usually more limited than for air-to-water heat pumps. Often, you have no choice—the heating contractor installs whatever is included in their offering.

Regarding the hot water storage tank, according to the requirements of the 10,000 Houses Program, it should be at least 360 liters (95 gallons).

The bathroom will definitely be optimized with wall heating. Although my heating contractor said it’s not necessary, he still tried to sell me a towel warmer. And it’s not just one heating contractor—I’ve found they all just want to take your money. You invest tens of thousands of dollars and end up with electricity costs comparable to an old oil heating system.

Hausbauer20 schrieb:
[Whatever is included in the heating engineer’s program must be installed.
Regarding the hot water storage tank, according to the requirements of the 10000 Houses Program, it should be at least 360 liters (95 gallons) as mentioned..

The heating engineer should also install what he knows well; you should be worried if he installs something he isn’t familiar with. If it has to be 360 liters (95 gallons), then just take that. We use 230 liters (60 gallons) for two people, which has been sufficient so far. When our child grows up and it’s really the three of us, we’ll see.

Hausbauer20 schrieb:

The bathroom will definitely be optimized with wall heating. My heating engineer said it isn’t really necessary but also tried to push a towel warmer on me. And it wasn’t just one heating engineer. They all just want to take your money. Investing tens of thousands of euros and ending up with electricity costs like an old oil heating system.

We have towel radiators in the bathroom, and they work great; there’s no real downside. Towels dry before the next shower, and the bathroom stays comfortably warm at 24°C (75°F) without high energy consumption.
If your heating engineer doesn’t mess things up, despite your really large house of over 200 sqm (2,150 sq ft), you won’t have high heating costs thanks to the KfW 55 standard and ground source heat pump. That’s at least our experience with a 160 sqm (1,720 sq ft) KfW 55 home using an air-to-water heat pump. Just stay away from buffer tanks for heating and keep the flow temperature as low as possible. After moving in, adjust the heating curve for the house, and everything should be fine.

ares83 schrieb:
If you need 360 liters, then just use them.

The 360 liters are not necessary for the heat transfer, but just one of the chain reactions caused by the poorly designed 12 kW heat pump.
The builder might be very familiar with this heat pump, but above all, he will profit well from it, to the short- and long-term detriment of the homeowner.

Unfortunately, it is a mistake to just let the heating installer do whatever they want. They are not evaluated based on what makes sense for the customer in the long run; technically, it only needs to get warm enough. Good planning is not required for this. Instead, simply higher capacity and buffer tanks are sold because it is easier and more profitable.