ᐅ Vestaxx window heating – experiences?

Every time I read this topic, my resting heart rate noticeably rises. For me personally, as soon as I came across this thread while researching window heating, the system would have been completely removed from the project requirements. I still haven’t seen a clear and transparently understandable calculation. What I have seen are long blocks of text with lots of figures, which don’t always match 100% from one text to another. The forum allows uploading screenshots. Here, I would like to see clear data sets presented side by side, showing exact input parameters and results. I expect both from a university-educated managing director with 25 years of experience in electrical engineering.

Vestaxx GmbH schrieb:

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

And then there are always these outliers in between. How are we supposed to argue sensibly? I am left only with the assumption of lacking expertise or that facts are deliberately withheld or distorted for marketing reasons. Of course, you can measure a building’s consumption with a heat meter, including all distribution losses. What else would you measure with it? Or, to put it differently: The heat, including all transmission losses, remains within the building system and is thus used effectively. It serves to warm the building structure. Gross = Net. Why should I care about what actually reaches the human body?

I install a heat meter in the hydraulic system: gross consumption
I install an electricity meter before the heat pump including the pump (with monoblock systems, it’s one system): electricity consumption

Mixing both is what results in the seasonal performance factor (SPF) or COP or whatever.

Vestaxx GmbH schrieb:

And also the objection: the waste heat remains in the house ... unfortunately that’s not entirely true. The underfloor heating has a huge heat sink beneath it – the ground, with a consistent temperature of about 12°C (54°F). Delta T to the flow temperature is about 28°C (82°F) – so about 16 Kelvin difference. Combined with the floor area and the U-value of the floor to the ground, you can calculate the continuous heat loss to the ground. That loss is often even higher than the heat lost through the window glass.

In my opinion still irrelevant, as this is already accounted for in the gross/net consideration via the heat meter.

kati1337 schrieb:

You can cut out the variables since neither weather conditions nor heating behavior should change depending on the heating system. I don’t suddenly prefer 68°F (20°C) over 75°F (24°C) just because I have a different type of heating.
And a colder winter won’t happen because of that either.

With older buildings, I wouldn’t underestimate the impact of radiant asymmetry (cold walls, windows, etc.). Often, you really need 1-2°C (2-4°F) higher air temperature to feel comfortable. Hopefully, no one uses direct electric heating for that.

Vestaxx GmbH schrieb:

I suggest you simply provide appropriate values based on the parameters I mentioned, and I will create a straightforward overview calculation that is easy to understand for everyone. Then you can give your comments where you think something doesn’t fit. Just give it a try.

Alright. House according to the building energy law (since we want to save money), building footprint 9 m (30 feet) deep and 10 m (33 feet) wide with 130 m² (1,400 sq ft), gable roof at 30 degrees with ridge oriented east-west, resulting in one south-facing roof and one north-facing roof, located at 600 m (2,000 feet) above sea level in Upper Swabia. Large window front facing south because it’s modern. Electricity from local utility at 40 cents/kWh.

@Vestaxx GmbH - Andreas, since you are also in contact with many people (Fraunhofer, etc.), I would suggest having a master’s thesis done somewhere. It requires little effort (except for the author 😉) and provides a solid basis for discussion.

Here is a basic outline to consider:

150m² (1,615 sq ft) with 40kWh/m²a (12.6 kBtu/sq ft/year) → 6000kWh/year heating energy demand
10kWp photovoltaic system + 5kWh storage → approximately 15kWh/day in November, December, January, February. With 10kWh for household electricity including hot water, 5kWh/day remain for heating.
Electricity purchase price: 35 cents/kWh
Lost feed-in tariff: 8.2 cents/kWh

Direct electric heating:
4 months * 30 days * 5 kWh/day = 600 kWh from photovoltaic electricity
Remaining 5400 kWh to be purchased
Total cost: 5400 * 0.35 + 600 * 0.082 = €1939.20

Heat pump:
Electricity demand: 6000 / 4 = 1500 kWh
Remaining 900 kWh to be purchased
Total cost: 900 * 0.35 + 600 * 0.082 = €364.50

This results in annual heating cost savings of over €1500.

Assuming a heat pump including underfloor heating costs €25,000 more than electric resistance heating with a hot water heat pump, the payback period is less than 17 years.

kati1337 schrieb:

Huh? What does the house’s "net" consumption mean?

A thermometer can give a rough indication of that.

Who cares? What ultimately matters is the actual cost to me of keeping the house at a comfortable temperature.

You can exclude the variables since neither the weather nor heating behavior should change depending on the heating system. I don’t suddenly prefer 20°C (68°F) over 24°C (75°F) just because I have a different type of heating.
And it won’t make winter any colder either.

So? What are your promised numbers in a full cost comparison based on my example?

And you’re trying to fool someone here, thinking we won’t notice.

Why are you getting so hostile right away and assuming I’m trying to deceive you?
Unfortunately, you didn’t understand (or maybe I didn’t explain clearly enough — I’ll take the blame). 😕

So again, let me explain calmly:
“Net” means that everyone knows their seasonal performance factor (SPF) from the brochure and assumes they will generate their heat with that number — BUT that’s not accurate. Including losses, the seasonal performance factor might be around 2.8 or even much lower when it’s -10°C (14°F) outside (see current field test results from Fraunhofer in Freiburg, who have been measuring heat pump systems for 20 years).

A thermometer shows the temperature but not the losses. Therefore, consider the following: supply temperature at the heat pump outlet minus the floor surface temperature equals xy kelvin. This proves that heat (temperature) is lost on the way; otherwise, the heat pump outlet temperature would be the same as the floor surface temperature.

You care about the bottom line — very good! What would you say if your energy costs were lower than they are now? Would you be interested? If yes — watch the webinar (which was held for energy consultants, by the way).

Weather and heating behavior — Energy consultants use software that calculates your house’s heating demand based on many parameters. The program uses a 20-year average weather data set, which can vary — that’s the first uncertain variable. Heating behavior means that people heat differently and, for example, might ventilate a lot and lose heat through windows, or ventilate less 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 insights based on it and argue factually. Otherwise, I don’t enjoy giving factual responses, and I’m not into dealing with whining and insults. Thanks for your understanding!