Air-to-air heat pump in KfW 40+ house - switch to brine-water heat pump just before building application?

Hello everyone,

we are building a prefabricated house next year – basically, all the prefabricated house builders we talked to recommended the air-to-air heat pump from Proxon. We are about to submit the building application, meaning we can still basically change everything.

Key data:

- 200 sqm Kfw 40+ prefabricated house with timber frame construction, 120 sqm on the ground floor, 80 sqm on the upper floor
- Plus a basement, partly utility cellar, partly possibly later conversion to a living cellar, currently no heating concept here
- Plot of land is 1,000 sqm, of which 300 sqm in the back part is an orchard meadow
- Photovoltaic system still in the planning stage, but I will fully utilize the roof, 10 kWp should easily fit, battery storage is not worthwhile, so I will keep it minimal so that it fits with 40+

Technical data I have calculated:

- Envelope surface 540 sqm (roof 140 sqm, base plate = basement ceiling = 166 sqm, side walls 234 sqm)
- Heated gross volume 530 m³

- Transmission load 4318 W
+ Ventilation load 243 W
+ Hot water load 400 W
= Total heating load 4961 W

I have researched a lot now and the reports on the air-to-air heat pump vary. Actually, our case is suitable for it, we’d only need to find a solution for the basement.

I am concerned about dry air (that's probably a given, I’ll have to figure something out) and very high electricity costs or that the system won’t properly and comfortably heat our 200 sqm house.

My current favorite would be to switch to a brine-water heat pump with surface collector or trench collector partly as DIY work, since we have 1,000 sqm of land we could use.

- What would you recommend? As far as I understand, everything except the air-to-air heat pump would end up being underfloor heating. I currently have that in my rental flat and can’t deal with it – extremely sluggish. Are today’s systems better (this experience was one of the main reasons for the air-to-air heat pump)?
- Can I simply lay the trench collector around the basement? Then it would be deep, and the earthworks guy is already there, or I could have him dig a bit more (it's only about 2 m deep).
- Does switching to a different heat pump still significantly delay the building application?
- Are all heat pumps (LLPW, air-water heat pump, brine-water heat pump) equally suitable for KfW 40+?

Thanks for your support!!

Currently, there are still some things mixed up. A [Luft-luftwärmepumpe] is definitely not underfloor heating. I probably wouldn't install a [Luft-luftwärmepumpe] (high consumption, dry air).

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

I would indeed describe a [Sole-Wasser-Wärmepumpe] as ideal if you have space, time, and/or money. Otherwise, [Luft-Wasser-Wärmepumpe].

Whether the change is possible and what it costs, you should clarify with the prefab house supplier.

Ring trench collector would also be my favorite. There are nice calculation tools (google helps here). There should be enough space with 1000m². But that doesn't change the inertia of the connected [FHB]. Self-performance is rather standard with ring trench collectors, first talk to the plumber to see if he will connect the rest. Otherwise, you'll have pipes lying in the ground without any use. Better to discuss the entire planning and your current considerations with the [BU] and his plumber to get more clarity. The energy consultant, who is necessary anyway, will (must) provide accompanying advice!

Hello,
you will have to solve the dry air issue one way or another, even if you switch to brine. Reason: For a 40+ house, you absolutely need a ventilation system and it is the ventilation system (if it doesn’t have an enthalpy heat exchanger) that causes the dry air, not the LL heating, which comes after the air heat exchanger.
The Proxxon provides only ~2kW via the built-in heat pump (the rest via heating wire), since it extracts this from the (low) exhaust air flow and therefore also operates with a very poor efficiency. This basically becomes a direct electric heater. The network is full of such cases where well-insulated houses then require 6-7000 kWh/year of electricity just for heating. And for me, that is a total contradiction: you build a 40+ house and have a high electricity demand for the LL heating (air-to-air heat pump). If you build underfloor heating with an air-to-water heat pump in the same house, the electricity demand is <<2000 kWh/year. Air-to-air heat pump is basically a bet on the future that electricity prices will remain "low" in the coming decades. Whether this will stay so in future winters, when there are no more coal power plants and gas power plants have to pay heavy CO2 taxes, is questionable.
We are also building a timber frame house and have decided on underfloor heating and air-to-water heat pump. A brine heat pump did not pay off. The new air-to-water heat pumps have a high efficiency with underfloor heating, annual figure at our location is 4.7.
But there were also two other reasons why we decided against an air-to-air heat pump.
1) We rent out an apartment. And since the feedback is full that air-to-air heat pumps do not always reach the set temperatures, or some rooms are too cold, we didn’t want trouble with the tenants.
2) If you, as written above, have a heating load of 5kW and transport that only through the air flow, you either have a very high volume flow (it will draft and be loud) or the blown-out air will be very hot. We didn’t want either.

First of all, thank you for your feedback.

Indeed, the fact that the underfloor heating is so sluggish bothers us a bit at the moment. Within the family, there is the idea that with the air-to-air heat pump, you can warm up individual rooms faster, if desired.

I have now received the statement from the BU that the type of heating system is specified in the building application, so unfortunately I have to decide quickly :/

How do I find out whether a brine-water heat pump or an air-water heat pump is more cost-effective for me? As mentioned, we also want to rent out, and in the basement, with the use of an air-to-air heat pump in the house, that would probably have meant an additional infrared heater on the ceiling.

The major change now consists of switching to underfloor heating (different floor structure, etc.) as well as the necessity of ventilation (central (= piping) or decentralized controlled residential ventilation). Whether and how this is possible, the BU would have to tell you at short notice. Whether it will then be an air-water heat pump or a ground-water heat pump is currently actually secondary, since the units look and operate almost identically (the only difference: with air-water heat pumps there is this ventilation box outside, with ground-water heat pumps instead a borehole or a ring/area collector).

In short: currently clarify whether switching to underfloor heating and controlled residential ventilation (I would prefer central) is possible, and if so, what the cost would be.