Forum user dismisses solar thermal heating: 12 kWh vs 300 needed

Solar thermal for heating: 12 kWh versus 300 required

A 140-square-meter house spread over two floors, located in a village in northern Spain, is cold and damp. It is eighteen years old and relies on a communal oil boiler that requires priming every time it is started. The owner only visits during spring and summer. The question that peine the discussion was whether a solar thermal energy installation could prevent the home from dropping to 8 degrees instead of 15 when arriving on a winter weekend. The first calculation posted in the conversation shut the idea down immediately.

The problem: 140 cold, damp meters used only half the year

The house is not made of stone, but it behaves as if it were. Its owner describes a humid environment with no visible stains on the walls and ceramic flooring on the ground floor that takes forever to dry even when windows are open. His theory is that this humidity keeps the interior cool in summer and makes raising the temperature difficult in winter.

The stated goal was not living there in January, but something more modest: using sunlight hours to remove the chill from the house so that upon arrival, one would need to push the thermometer from 15 to 20 degrees rather than starting from 8. A reasonable aspiration. And, according to a participant's calculation, impossible with panels.

Why doesn't solar thermal heat a house in winter?

The calculation used to close the debate assumes a dwelling that is not poorly insulated and consumes an average of 100 kWh per day just to maintain temperature. Due to its age and location, this figure would easily triple. That means about 300 kWh daily.

On the other side of the scale is the available winter radiation: roughly 300 W per square meter for eight hours a day, being generous. With 10 m² of solar thermal collectors—which is already a lot—and 50% efficiency, the daily input remains at 12 kWh. The ratio is one to twenty-five. No inertia tank can bridge that gap.

Some argue that solar thermal works as support, but only with low-temperature emitters: underfloor heating at around 40 degrees, not conventional radiators. And underfloor heating is new construction work, not an improvised renovation.

Where it does make sense: hot water and high consumption

Field experience shared points in another direction. In a private home with panels installed eight years ago, heating water is free, backed up by a gas heater for cloudy spells: in all that time, they used only one and a half butane cylinders. The big leap is in hospitality: a hostel with 70 rooms using panels for hot water significantly reduced bills.

Against this, the objection is scale. Domestic hot water is a small, predictable consumption; heating a cold house in winter is not. One user states their hot water cost with gas is around two euros per month per person, including hot showers.

Insulating inside or outside: the unresolved debate

Here the conversation split in two. One camp defends internal insulation—drywall backings with wool on walls and ceilings, thick carpets, sealing doors and windows—because it is cheaper, heats only the occupied volume, and requires no facade permits. With the home well wrapped, the heating system becomes secondary.

The opposing view holds that external insulation is technically superior, although admitting it is absurdly costly for a weekend house. A third, more radical path denies the premise directly: some recall that in Castile, people have lived for centuries in adobe houses with a wood stove, because adobe does not insulate, it accumulates. The complete case, with investment figures versus annual consumption, is what never quite closed.

Wood, diesel, and coal: the real bill for each system

Cost comparison dominates the second half of the exchange. One testimony puts the last winter with a coal boiler at 1,200 euros plus seven or eight butane cylinders; switching to diesel jumped the bill to 2,200 euros in the first year, amid soaring fuel prices. On the opposite side, a house of over 400 meters in Canada, keeping 26 degrees constant with natural gas, stayed at 70-80 dollars a month.

Insulation explains that difference better than fuel. For sporadic use, the proposal gaining trinc is the simplest: a closed fireplace or thermo-fireplace with outlets to two or three radiators, wood as fuel, and no construction work. And to avoid arriving at a frozen house, a programmer that starts the heating hours before, leveraging thermal inertia and turning it off before leaving.

The humidity nobody fully explains

One loose thread remains unresolved. The owner worries if renting an industrial dehumidifier for a weekend is enough to bring the house to reasonable levels. The most plausible explanation given is that discomfort comes not from water vapor, but from surface temperatures: with walls, floors, and ceilings at 7 degrees, the air can be warm yet still feel cold and damp until that mass warms up.

With those numbers, the technical conclusion is that solar thermal does not sustain heating, internal insulation helps, and compartmentalizing floors—closing the staircase with a sliding panel—is on the table. What remains unanswered is how much of that cold is real humidity and how much is simply walls that do not heat up.