ᐅ Heat pump, hot water storage tank, tankless water heater, heat recovery ventilation (HRV), underfloor heating, heating and cooling

Hello everyone,
I’m looking for advice on the design of our renovation project.
Key details: built in 1930, ground floor area 90 m² (970 ft²), upper floor area 40 m² (430 ft²), basement used only for storage, semi-detached house, extensive renovation planned (including basement ceiling, facade, windows, roof – all to qualify for BEG EM funding).

We are converting the entire ground floor to underfloor heating. On the upper floor, the plan is to use wall heating installed in the space between the rafters. According to calculations, after renovation the heating demand will be 5.3 kW. A air-to-water heat pump (Vitocal 252 – the 08 model) will be used. This heat pump does not have an integrated water storage tank.

The system will be combined with a hydronic fireplace, which delivers 2 kW of heat directly into the room and 8 kW to the water circuit. Additionally, one or more buffer tanks will be integrated to
a) store the heat from the fireplace,
b) use a heating element to feed in excess solar energy from a photovoltaic system during peak times,
and thus provide hot water for heating and domestic hot water.

Due to limited space in the basement, I will most likely need to place two buffer tanks side by side. I am targeting a volume of 800 to 1000 liters (210 to 265 gallons). To avoid having to constantly store hot domestic water during winter, I will integrate a tankless water heater (solar-compatible). The idea is to use the temperature level in the domestic hot water tank and only raise it by about 15 degrees Celsius (27 degrees Fahrenheit) if needed, preferably less.

This is the basic theory, which I have discussed with the heating engineer. However, I want to fully understand the system and I’m stuck on some points, so I hope for your support.

1.) What is the best way to connect the buffer tanks? From my point of view, a series connection seems suitable, with the domestic hot water tank with the DHW loop as the rear, warmer tank, is that correct?
2.) Where is the best place for the heat pump to feed in? Ideally in the middle, right? But where would the middle be with two tanks?
3.) Where should the hydronic fireplace connect?
4.) Up to what temperature in the buffer tank does it make sense to operate the heat pump?

An additional thought, which is not a priority: There is the possibility of a cooling function in summer. Does it make sense to consider the buffer tanks in this regard or would that only complicate things further? Otherwise, in summer it could be possible to have one cold and one hot buffer tank: one for domestic hot water and one for heating water.

Thank you very much for your interest!

Havanna86 schrieb:

Thank you for your interest!

Wow – I’ve rarely seen such a complex, cumbersome, and expensive solution.

I can’t answer your specific question, but I’d like to suggest considering a 5 kW monoblock for heating and hot water. A fireplace could be added just for aesthetics.
With the amount of money saved—likely a smaller five-figure sum—you could treat yourself to a new ID3 and charge it with rooftop solar panels, which would also fully support the heat pump for hot water in summer and partially support heating.

P.S. Cooling through underfloor heating is widely overrated!!!

Havanna86 schrieb:

According to calculations, after the renovation, our heating demand will be 5.3 kW.

Per minute, per hour, per day, per 24 hours, in summer or winter?
Such a complex system for a single-family house—was your HVAC consultant involved?

kW is power, so it is independent of time 😉
But what is probably meant is the heating load at the standard outdoor temperature.

My advice would be to properly size the hydraulics, omit the buffer tank, omit the chimney, and omit the heating rods.

KarstenausNRW schrieb:

Wow – rarely have I seen such a complex, complicated, and expensive solution.

Well, the system was developed taking many factors into account.
The instantaneous water heater to provide immediate hot water and to keep the heat pump always operating within its optimal range.
The electric backup heater to ensure safety on cold winter days and to further reduce heating costs.
The buffer tanks, because the electric backup heater alone makes little sense without a buffer, and to optimize the use of solar gains from the photovoltaic system.

Certainly, all of this costs money, but the goal is to reduce expenses in the long run.

KarstenausNRW schrieb:

I can’t really answer your specific question, but I’ll just suggest a 5 kW monoblock for heating and hot water. A fireplace for the aesthetics.

RotorMotor schrieb:

My advice would be to properly design the hydraulics, omit the buffer tanks, omit the fireplace, omit the heating rods.

I think we have moved beyond the point of generally modifying the system. Ultimately, this comes down to a matter of preference. Either invest a significant amount now and reduce running costs over time, or install a cheaper system now and accept somewhat higher operating costs in the long term...

As I said, for me, the most important current question is really the optimal integration of the buffer tanks, and this is where my background knowledge is lacking.
I would greatly appreciate any valuable advice here!