ᐅ Is a natural stone heating system practical when combined with a photovoltaic system?

Hello, I am currently trying to figure out which heating system makes the most sense.

I have a quote for a heat pump with geothermal drilling plus underfloor heating for 32,000 euros.
An air source heat pump is about 10,000 euros cheaper, so 22,000 euros, but the seasonal performance factor (SPF) of 3.5 is rarely achieved in practice and realistically drops to about 2.8 (energy inefficient).
Ventilation with heat recovery is included in the price, costing about 6,000 euros.

I plan to build a timber frame house with a U-value of 0.14 and triple glazing; the building envelope meets KfW 55 standard. The living area is about 130 m² (1,400 ft²).

I also had a consultant who recommended a natural stone heating system since the building requires little energy.
The natural stone heating would cost around 9,000 euros.

Until now, I was reluctant to consider this option, but looking at follow-up costs and maintenance expenses, the natural stone heating combined with a photovoltaic system becomes attractive.
Assuming an energy consumption of 40 kWh/m² per year, this results in 5,200 kWh without hot water.
That would be about 1,200 euros in electricity costs without a special tariff, based on 0.23 euros per kWh (relatively close to gas prices).

If the actual consumption is 60 kWh/m² per year, that would be 8,500 kWh without hot water, so 1,800 euros in electricity costs (which is expensive).

If I invest the costs of drilling and underfloor heating, approximately 16,000 euros, into a photovoltaic system that produces 8,000 kWh of electricity per year,
then the monthly heating costs would be zero, although loan interest, repayment, and other costs would obviously apply.

Advantages of this heating system are no maintenance costs and a very long lifespan.
The photovoltaic system incurs monthly meter fees, and I still need to inquire about maintenance.
Of course, the photovoltaic system’s performance decreases over time; after 20 years it should still perform at about 85%, and by then smaller, more affordable solar panels will also be available.

The total investment is also around 30,000 euros, with running costs and maintenance close to zero per month.

Is this an overly optimistic calculation, or does it sound reasonable?

Hello,

first of all –

Radiant heating in the form of natural stone heating is, in my opinion, the best type of heating. It is healthy, comfortable, has a one-time installation cost, and requires no maintenance or follow-up expenses. Combined with photovoltaic panels, it is almost perfect... even in older buildings, provided they have solid construction.

Mellomacher schrieb:

... although the annual performance factor of 3.5 is usually not achieved and in reality drops to 2.8. (Energy guzzler).

That is not true! I always wonder where such sweeping statements come from... and how unpleasantly they spread across the country.

Mellomacher schrieb:

I am planning a timber frame house with a U-value of 0.14 + triple glazing, the building envelope reaches KfW 55. Approximately 130m² (1400 sq ft) living area. I had a consultant recommend natural stone heating because the building requires little energy.

I hope this is the system your Austrian neighbors use; all other systems are third or fourth choice.

Mellomacher schrieb:

... If the real consumption is 60 kWh/m² per year, that amounts to 8,500 kWh without hot water, so 1,800 Euro electricity (that is expensive)

In my opinion, you will not achieve this because...

Mellomacher schrieb:

Is this a rough estimate or does it make sense?!

... you want to build a single-family house using timber frame construction; however, natural stone heating works like the sun. All solid objects, including your body, are warmed and “store” the heat generated over a certain period, depending on solid building materials. Timber frame cannot do this – the material simply has, explained in layman’s terms, no storage potential – which is why I also think your worst-case energy cost will be higher than €1,800 per year if you choose this system.

Natural stone heating combined with photovoltaic panels is unbeatable – healthy and affordable – when installed in a solidly constructed house. By the way, it is also cost-effective without photovoltaics if the utility’s heat pump tariff is applied and it is used in a KfW 70 or higher efficiency house.

Kind regards

Hello €uro,

€uro schrieb:

What are these strange numbers or units?

Someone selling natural stone heating systems usually prepares the "heat demand calculation" themselves by breaking down each individual room into its components (U-values for exterior/interior walls, windows, floor, ceiling, door) and calculating the expected heat demand (Q-watts) per room based on these values. The totals for each room are then added up – depending on the use, a higher (bathroom) or lower (utility room) heat demand per square meter is applied; the sum results in the calculated "heat demand" for the single-family house.

It is a tedious task – often, the U-values for components of older single-family houses are not easy to find; that’s why I always commission a planning office for this task, even though I can do it myself. I prefer to have this kind of verification at hand. You may be surprised, but the calculated heat demand regularly matched reality, and the estimated costs from the energy supplier also aligned almost exactly, with only minor differences. Essential for this manual method is carefully asking the clients about their personal comfort preferences; otherwise, there can be an unpleasant surprise.

Kind regards

Hello Bauexperte,

Bauexperte schrieb:
...Someone who sells natural stone heating systems usually creates the "heat demand calculation" themselves,..... the total then results in the calculated "heat demand" for the single-family house.

You also seem to have problems with SI units. Heat = energy (kWh)! Power => (W; kW) Heat demand is equal to energy demand, not power demand!
How heating loads are determined in the EU and Germany (national annex) is specified in standard 12831. If sellers of natural stone heating systems do not adhere to this, that is their issue.
The sum of room heat loads does not automatically equal the standardized heating load for the building. It seems these sellers also do not consider ventilation heat losses.

Bauexperte schrieb:
...It may surprise you, but the calculated heat demand regularly matched, and the calculated costs with the energy supplier were also accurate, down to a few pennies.

Concrete, verifiable (not anonymous) examples?

Best regards.

Hello €uro,

€uro schrieb:

It seems you also have problems with SI units.

That may be... after all, I’m not an engineer.

€uro schrieb:

Heat demand is equal to energy demand, but not power demand!

I never claimed otherwise; I only wrote that—after a specific calculation procedure—the heat demand, sorry, energy demand, is known.

€uro schrieb:

Concrete, verifiable (not anonymous) examples?

Certainly.

Best regards

Hello Bauexperte,

Bauexperte schrieb:
...I never claimed that anywhere; I only wrote that – after a certain calculation procedure – the heat demand, sorry, energy demand, is known.

Unfortunately, that is incorrect! The method you described results in the room heating load (power (W; kW)) or the specific values per square meter, without taking ventilation heat losses into account, which is absolutely necessary.
How does the originator of this method arrive at the annual energy demand (heating)?

Bauexperte schrieb:
...Certainly

I would be very interested in seeing these results. You can surely provide me with the numbers (calculation, actual energy consumption).

Best regards