ᐅ Air-to-Air vs. Air-to-Water Heat Pump for a KfW55 House – Importance of Cooling Function

We don’t really mind shading; we already do it completely in our current apartment. This way, we can maintain around 25°C (77°F) in our KfW55-certified apartment. The real challenge is more with the oven and dryer, as they naturally release extra heat into the living space each time they are used.

In a house, you even have the advantage of always having one or two directions that don’t need shading. So this shouldn’t be a problem. Also, underfloor cooling probably doesn’t make much sense if it only lowers the temperature by 2–3°C (4–5°F).

However, being able to cool down from 25°C (77°F) to 22°C (72°F) would already be very welcome and is definitely better than no cooling at all. Unfortunately, the budget for more isn’t available right now.

KatlarOne schrieb:

Have you planned for a centralized ventilation system?
We are currently leaning towards an air-to-water heat pump with centralized ventilation (still unsure about including a cooling function) and photovoltaics.

Yes, a central ventilation system with both supply and exhaust is planned. Photovoltaics are definitely planned in the long term; whether we install them right at the start depends on the overall construction costs. If there is room in the budget, we would include photovoltaics from the beginning; otherwise, we will retrofit later.

kati1337 schrieb:

Our situation is a bit special because of working from home – but in summer, with two running computers and four monitors, the office temperature quickly rises above 30°C (86°F) without cooling. The rest of the house also heats up significantly due to the window areas, even with good insulation.

This is also our concern. We both work as programmers, often from home, and a bit stereotypically spend a lot of our free time on the computer as well. Even in our very cold old apartment, you can already feel the difference – it’s quite comfortable in winter but noticeably hotter in summer. I dread to imagine how this affects a well-insulated house. Reducing the temperature by 2-3°C (4-5°F) through underfloor heating cooling seemed rather ineffective in proportion, even though it certainly helps somewhat.

On the other hand, as jrth2151 said:

jrth2151 schrieb:

If we had unlimited money, we would definitely go straight for photovoltaics plus air conditioning.

Unfortunately, costs are also a factor. In the end, we will probably have to bite one of the bitter pills – either spend more money on a proper air conditioning system or take the risk that the cooling function of the underfloor heating system won’t be sufficient. At least, with the responses in this thread, we are now more confident that it will likely be an air-to-water heat pump rather than an air-to-air heat pump.

kati1337 schrieb:

I often see the opinion here that you have to keep all the shutters closed in summer without exception. To some extent, I get that – without shading, it’s hardly possible in a new build. But really closing EVERYTHING all summer? Then you’re just sitting like a hobbit in a cave. To me, that’s not good living quality.

It’s enough to shade the sunny side. With clever planning, it doesn’t necessarily have to be dark. We have around 10 meters (33 feet) of floor-to-ceiling windows in our open-plan area, but one faces east, two south, and one west. This way, we can shade around the building without it feeling dark. Almost every room has windows facing two directions. It’s not quite as effective as in the open-plan area, but still works pretty well. Our bedrooms are on the south side. This happened naturally, but it has the advantage that shading there during the day doesn’t really bother us. The home office is on the northwest side, so I can manage there quite long without shading. The concrete ceilings absorb quite a bit of heat during the day, and at night we air it out again. The real test with a week of humid 35°C (95°F) weather is still ahead, but I’m quite confident despite the EH40 building envelope and large windows.
I also know it differently, for example from an office. There you really sit in a cave after lunch. And although the windows are large, the one that can be opened is more like a loophole. So you can’t really air out the heat in the morning. Multiple powerful computers and people in one room are also a problem. 100 watts per person, 300 watts per computer, plus peripherals... you quickly end up with about 1 kilowatt of heat output in the room.

jrth2151 schrieb:

The cooling function of the heat pump costs only about €500 (around $550) extra, so that would be the first option.

At €500 (around $550), I’d go for it and then see how often you need it. For us, they quoted €4,000 (about $4,400), which was too much for me.

From real-life experience and operation over several years: the underfloor heating and cooling system simply works. I’m absolutely glad I insisted on it, and my wife regularly thanks me for it during the summer. Yes, exactly—the woman who hates cold feet more than anything. Do it or don’t, I don’t care, I don’t earn anything from it. But when I hear “it doesn’t help anyway,” I have to say: oh yes, it does.

Regarding shading: well, you do what you can. The two children’s rooms have large south-facing windows—so when the kids leave the house in the morning, we just lower the blinds. The stuffed animals then lie in the dark. Yes, the skylight in the guest bathroom also has a blind closed. That doesn’t bother me at all. I designed the ground floor south side so that in summer no direct sunlight shines on the doors; hence, the blinds stay up and it is brighter than you might want.

In short: underfloor heating and cooling works wonderfully—within the limits the system naturally has. When it’s 34.5°C (94°F) outside, I consistently have 22.0°C (72°F) inside, and that’s exactly what I want. Shading does NOT mean living in darkness.

WilderSueden schrieb:

At 500€, I would go for them and then see how many you actually need. Here, they were charging 4k for that, which was too much for me.

I agree. For us, it was 2k, and we preferred to invest in the proper climate control.

nanella schrieb:

These are also our concerns. We both work as programmers, often from home, and stereotypically spend a lot of our free time at the computer as well.

Are you us? 😀
No joke—it's the same for us. Both in IT, both fully remote, and during free time we’re usually gaming rather than watching TV. Don’t underestimate the heat buildup in a new build. We experienced this for two years in our first house. It was a KfW55 standard, but the office was so hot even during winter that we had to keep ventilating constantly or, if it was noisy outside, run the air conditioning. In December. 🤨

In summer, you won’t survive without AC. Above all, though, working in a well-tempered room is much more comfortable. The cost of air conditioning depends a lot on how big you size the system. In the first house, we had a smaller outdoor unit and three split units: one for the entire upper floor, one for the office, and one for the living/kitchen area. That was enough for the house size—about 150m² (1,615ft²). We paid around 5k directly through the construction loan back then.

The new house is a bit bigger. We took a big hit to the wallet when the plumber told us it would be about 14k. 🤨 But we went all in on a part of our budget buffer. I’d say we will easily reach our target of 1.5°C (2.7°F) difference, unless something major happens. It won’t get any cooler than that. I don’t know how it is for you, but for us spring basically didn’t happen this year. We heated into May (currently still in an old building, mind you) and just two weeks later we were setting up the mobile air conditioner in the office again.

So the investment in AC was worth it for us, as I believe we will need it frequently in the future. We’re saving elsewhere, where it’s purely about aesthetics. I hope I’ll care less about my metal railing when I’m sweating less in August.

nanella schrieb:

Even in our very cold old building apartment, you can already feel the difference—it’s quite comfortable in winter, but in summer the room clearly gets hotter. I can hardly imagine how that works in a well-insulated house. Cooling of 2-3°C (4-5°F) through underfloor heating seemed rather useless to us, even if it certainly helps a bit.

You’re pretty much spot on. I’m currently temporarily living in an old house because the new build isn’t finished yet. It’s poorly insulated. The office is under the roof, and it’s been almost unbearably hot in there the entire June. The rest of the house is still okay.

Before this, I lived in a well-insulated new build for two years, and by the end of May we already had the problem: Outside it was a comfortable 20°C (68°F), but inside it was 28°C (82°F). We hadn’t realized that shading was necessary. It doesn’t take strong heat outside; the sunlight coming through the windows is enough. Getting the heat out of the house is difficult.

WilderSueden schrieb:

I also know it differently, for example from offices. After lunch, you really sit in a cave. The windows are huge, but the operable parts are just small slits. So in the morning you can’t ventilate the heat out. Several high-performance computers and people in one room are also a problem. About 100W per person, 300W per computer, plus peripherals… you’re quickly at 1 kW of heat output in the room.

That’s reality at our home too. The office isn’t as large as a company office, we’re two people, and there’s a lot of hardware running. I’m not even sure if 300W is enough to cover what our computers produce.