ᐅ Air-to-air heat pump in a KfW 40+ house – Switching to a ground-source heat pump just before the building permit application?

Hello everyone,

We are building a prefabricated house next year. Basically, all the prefabricated house manufacturers we have spoken to recommended the air-to-air heat pump from Proxon. We are just about to submit the building permit / planning permission application, so in principle, we can still change everything.

Key data:

- 200 sqm (2153 sq ft) KfW 40+ prefabricated house with timber frame construction, 120 sqm (1292 sq ft) on the ground floor, 80 sqm (861 sq ft) on the upper floor
- Plus a basement, partly for storage, partly possibly to be converted later into living space; currently no heating concept for the basement
- Plot size is 1,000 sqm (10,764 sq ft), of which 300 sqm (3,229 sq ft) in the rear area is an orchard
- Photovoltaic system still in planning, but I will cover the entire roof; 10 kWp should easily fit; battery storage is not worthwhile, so I will keep it minimal to meet the 40+ standard

Technical data I calculated:

- Envelope surface area 540 sqm (5809 sq ft) (roof 140 sqm (1507 sq ft), ground slab/ basement ceiling = 166 sqm (1786 sq ft), walls 234 sqm (2519 sq ft))
- Heated gross volume 530 m³ (18,714 cu ft)

- Transmission heat load 4318 W
+ Ventilation load 243 W
+ Domestic hot water load 400 W
= Total heating load 4961 W

I have researched a lot, and the reports on the air-to-air heat pump vary. Our situation is actually suitable for it, but we would need to find a solution for the basement.

I am worried about dry air (which seems unavoidable, so I will have to come up with something) and very high electricity costs, or that the system will not keep our 200 sqm (2153 sq ft) house comfortably warm.

My current preference would be to switch to a brine-to-water heat pump with either a horizontal collector or a loop trench collector, partially as a DIY project, since we have 1,000 sqm (10,764 sq ft) of land we could use.

- What would you recommend? As far as I understand, everything other than the air-to-air heat pump will ultimately be combined with underfloor heating. I currently have underfloor heating in my rented apartment and cannot get used to it – it is extremely slow to respond. Are today’s systems better? This experience was one of the main reasons I was initially attracted to the air-to-air heat pump.
- Can I simply lay the loop trench collector around the basement? Then it would be deep, and since the excavator will already be there, I could just have him dig a bit more (it’s only about 2 m (6.6 ft) deep).
- Will switching to another heat pump significantly delay the building permit / planning permission process?
- Are all heat pumps (air-to-air heat pump, air-to-water heat pump, brine-to-water heat pump) equally suitable for KfW 40+ standards?

Thank you for your support!!

At the moment, there are still some things mixed up.
An air-to-air heat pump is definitely not the same as underfloor heating. I probably wouldn’t use an air-to-air heat pump (high energy consumption, dry air).

Current underfloor heating systems are always very slow to respond. What exactly bothers you about it?

I would indeed consider a ground-source heat pump to be ideal if you have the space, time, and/or budget. Otherwise, an air-to-water heat pump.

Whether the switch is possible and what it would cost should be clarified with the prefabricated house provider.

A trench collector would also be my favorite option. There are useful calculation tools available (Google can help). With a 1000m² (12,000 sq ft) area, there should be enough space. However, this does not change the inertia of the connected single-family house. DIY work is quite common with trench collectors, but you should first check with the plumber whether they will connect the rest. Otherwise, you might end up with pipes in the ground that serve no purpose.

It’s better to discuss the entire planning and your current considerations with the builder and their plumber to gain more clarity. The energy consultant, who is required anyway, should (and must) provide ongoing advice!

Hello,
you will have to address the issue of dry air anyway, even if you switch to a ground source heat pump. The reason is: for a passive house standard 40+ you necessarily need a ventilation system, and unless it has an enthalpy heat exchanger, it is this system that causes dry air, not the air-to-air heat pump, which comes after the air heat exchanger.

The Proxxon only manages about 2 kW through the built-in heat pump (the rest is through electric heating elements), because it extracts heat from the (small) exhaust air flow, resulting in a very poor efficiency. This essentially becomes a direct electric heater. Many well-insulated houses end up using 6,000 to 7,000 kWh per year of electricity just for heating. To me, this is a total contradiction: you build a 40+ house, but then have very high electricity consumption for the air-to-air heat pump. If you install underfloor heating with an air-to-water heat pump in the same house, the electricity demand is much less, under 2,000 kWh per year. Choosing an air-to-air heat pump is basically a bet on electricity prices staying "low" in the coming decades. Whether that will be the case in future winters, especially without coal power plants and with gas plants facing high CO2 taxes, is questionable.

We are also building a timber frame house and decided on underfloor heating combined with an air-to-water heat pump. A ground source heat pump did not make economic sense. The latest air-to-water heat pumps have a high efficiency with underfloor heating, with a seasonal performance factor of 4.7 at our location.

There were also two additional reasons why we decided against an air-to-air heat pump.
1) We rent out a flat. The experience is that air-to-air heat pumps do not always reach the target temperatures, and some rooms can remain too cold, which would cause conflicts with tenants.
2) As mentioned above, if your heating load is 5 kW and this is delivered solely via the air stream, you either need a very high air volume flow (which leads to drafts and noise) or the supply air must be very hot. We did not want either option.

First of all, thank you for your feedback.

Indeed, the fact that the underfloor heating is so slow to respond is currently bothering us a bit. Within the family, there is the idea that individual rooms can be heated more quickly with the air-to-air heat pump, if desired.

I have now been informed by the builder that the type of heating system is specified in the building permit / planning permission, so unfortunately I have to make a quick decision :/

How can I find out whether a ground-source heat pump (brine-to-water) or an air-to-water heat pump would be more cost-effective for me? As mentioned, we also plan to rent out the property; in the basement, using an air-to-air heat pump in the house would likely have required an additional infrared heater on the ceiling.

The major change now is switching to underfloor heating (different floor structure, etc.) as well as the need for ventilation (central ventilation with ductwork or decentralized controlled residential ventilation). Whether and how this is possible should be clarified by your builder at short notice. Whether it will be an air-to-water heat pump or a ground-source (brine-to-water) heat pump is actually secondary at this point, since the units look and operate almost identically (the only difference being that the air-to-water heat pump has an outdoor ventilation unit, while the ground-source heat pump requires either a borehole or a ring/area collector).

In short: currently determine if switching to underfloor heating and controlled residential ventilation (I would personally prefer central) is feasible, and if so, what the cost will be.