Vestaxx window heating - experiences?

Every time I read this topic, my resting heart rate rises noticeably. For me personally, as soon as I discovered this thread while researching window heating, the system would have been completely removed from the specifications sheet at the latest. I still haven’t seen a clear and transparently comprehensible calculation. What I have seen are long flowing texts with lots of numbers that don’t always match 100% from one passage to another. The forum offers the possibility to upload screenshots. Here I want to see clear data series presented side by side, where input parameters and results can be clearly read. I expect both from a graduate managing director of a company with 25 years of professional experience in electrical engineering.

Yes, that’s all correct – but people can only see the consumption figures, not what the house really consumed net.

And then these outliers keep appearing in between. How are we supposed to argue reasonably? I am left with the suspicions of either lacking expertise or that facts are deliberately suppressed or misrepresented for marketing reasons. Of course, you can determine the consumption of a house including all distribution losses with a heat meter. What else should you measure with it? Or put differently: The heat including all transmission losses remains within the house system and is therefore sensibly used. They serve to warm the building structure. Gross = net. What does it matter to me what reaches the person?

I install a heat meter in the hydraulics: gross consumption.
I install an electricity meter before the heat pump including pump (in monoblock units a system anyway): electricity consumption.

Mixing both = annual performance factor/COP or whatever.

And also the objection: the waste heat stays in the house ... unfortunately not quite true. The underfloor heating has a huge radiator underneath it -> the ground with, say, a continuous 12 degrees C. Delta T to the flow temperature of, say, simply 28 degrees C -> results in a delta T of 16 Kelvin. In conjunction with the floor’s base area and the floor’s U-value to the ground you can calculate the continuous loss power to the ground. And that is often even higher than the losses through the window glass.

imho still irrelevant, since it is already accounted for in gross/net consideration via the heat meter.

You can cut out the variables, since neither weather nor heating behavior should change depending on the heating system. I don’t suddenly find 20°C better than 24°C just because I have a different type of heating. And I won’t get a colder winter from it either.

In old buildings, I wouldn’t underestimate the radiation asymmetry (cold walls, windows, etc.). You often really need 1-2 °C higher air temperature to feel comfortable. Hopefully, no one uses direct electric heating there.

I suggest you simply propose appropriate values based on the parameters I mentioned, and I will make a simple overview calculation that everyone can understand. Then you can give your comments where you think it doesn't fit. Just give it a try.

Alright. House according to the Building Energy Act (since we want to save money), building envelope 9m deep and 10m wide with 130sqm, gable roof 30 degrees with ridge running east-west accordingly 1 south roof and 1 north roof, at 600m above sea level in Upper Swabia. Large window front facing south because modern. Electricity from the basic supplier at 40c/kWh.

- Andreas, since you are also in conversation with many people (Fraunhofer etc.), I would have a master's thesis done somewhere. It is little effort (except for the creator ;)) and you have a good basis for discussion.

I'll throw this basis into the room:

150m² with 40kWh/m²a -> 6000kWh/a heating energy demand
10kWp photovoltaic + 5kWh storage -> ~15kWh/day in Nov, Dec, Jan, Feb. With 10kWh for household electricity including hot water, 5kWh/day remain for heating
Electricity purchase price 35ct/kWh
Price of lost feed-in tariff: 8.2ct/kWh

Direct heating:
4 [months] * 30 [days] * 5 [kWh/day] = 600 [kWh] from photovoltaic power
5400kWh remain as purchase
In total: 5400*35 + 600*8.2 = 1939.2€

Heat pump:
Electricity demand: 6000/4 = 1500
900 remain as purchase
In total: 900*35 + 600*8.2 = 364.5€

So you save over 1500€ per year on heating costs.

Assuming a heat pump including underfloor heating costs 25000€ more than a window heating including [BWWP], the payback period is not even 17 years.

Huh? What do you mean by the house's "net" consumption?

A thermometer can tell that relatively well.

Who cares? What ultimately matters is what it costs me monetarily to have a temperature in the house where I feel comfortable.

You can cut out the variables, since neither weather nor heating behavior should change depending on the heating system. I don’t suddenly find 20°C better than 24°C just because I have a different type of heating.
And I’m not going to get a colder winter because of that either.

Namely? What are your much-promised numbers in the full cost comparison based on my example?

And you’re trying to baffle someone here and THINK we don’t notice.

Why do you have to get so snappy right away and think I want to baffle you all with something?
Unfortunately, you didn’t understand it (or I didn’t explain it clearly enough – I’ll look for the mistake with myself). :confused:

So again and very matter-of-factly:
Net means that everyone knows their annual performance factor from the brochure and thinks that they generate their heat with this figure – BUT THAT is not true. Including losses, the annual performance factor is maybe 2.8 or even much lower when it’s -10 degrees outside (see current results from field tests by Fraunhofer in Freiburg, which have been measuring heat pump systems for 20 years).

A thermometer shows one temperature but no losses. Therefore, please consider the following: supply temperature at the heat pump outlet minus floor surface temperature = xy Kelvin. That is proof that heat (temperature) has been lost on the way. Otherwise, the heat pump outlet temperature would have to be reached there.

What ultimately comes out interests you -> very good! What would you say if your energy costs were lower than now? Would that interest you? If yes -> watch the webinar (which was held for energy consultants, by the way).

Weather and heating behavior -> The energy consultant calculates the heating demand of your house with his program using a variety of parameters. The program is based on a 20-year average weather, which can vary -> 1st uncertain variable. Heating behavior means that people heat differently or, for example, ventilate a lot and thus throw energy out the window or ventilate little to keep the heat inside longer.

Full cost accounting -> before you get personal again and accuse me of cheating the readers here, please educate yourself -> watch the webinar and then come back with the insights gained from it and argue factually. Otherwise, I don’t enjoy answering you factually, and I don’t tolerate whining and insults. Thanks for your understanding!