Heating a mountain eco-community without burning wood

Every infrastructure decision at Tepla Gora starts with the same tension. The site is at 800 metres in the Ukrainian Carpathians, at the end of a rocky mountain road. Whatever we design has to work in a harsh continental climate: heating season from October through April, design temperature minus twenty, weeks of overcast skies, eighty to ninety percent humidity in winter. And whatever we design has to be buildable on a mountain with limited vehicle access.

This article traces how we worked through the heating system for Unity Hub, a three-story net-zero community building currently in design. The process involved a basement that could not be built, a wood boiler we decided not to rely on, and a geothermal option that turned out to be surprisingly practical for a site where almost nothing is straightforward.

No final decisions have been made. We are publishing the thinking as it develops, because the tradeoffs are instructive even if the answers change.

Where to put the engine room

Before choosing a heat pump, we needed somewhere to put it. A building like Unity Hub needs a centralized equipment room: inverters, batteries, a heat pump, a buffer tank, circulation pumps, manifolds, and possibly a backup boiler with chimney. The original plan called for TeploHub, a separate insulated wood building near the workshop. Simple, proven, easy to build.

Then someone asked: what if we put all of this in a full concrete basement under Unity Hub?

On paper, the basement was perfect. It solved everything in one structure. Energy equipment, heating, ventilation, forty cubic metres of water storage in IBC tanks, greywater recirculation, general storage — all in one frost-protected underground space. No separate building, no buried heating pipes between structures, no heat transfer losses. Unity Hub would become the infrastructure hub for the entire community, distributing heat, power, and water to the hostel and future buildings from its own basement.

We spent weeks developing this idea. The functional advantages were obvious. The cost comparison was surprisingly close, because a full basement eliminates two or three separate construction projects (the equipment building, an underground water reservoir, and the buried pipe runs between them). The foundation was already planned as concrete pillars, so the step to a full basement seemed incremental.

The contractor's reality check

Then our building contractor did what contractors do: he checked whether the idea survived contact with the actual site.

Concrete delivery was the problem. A stationary concrete pump, the kind that pushes mixed concrete through hoses to where you need it, weighs seven tons and is not self-propelled. It cannot reach Tepla Gora. A concrete mixer truck can get within four to five hundred metres of the building site, but no closer.

Without mechanized delivery, all concrete work is manual. For a full basement requiring roughly eighty to a hundred cubic metres of concrete, that means 3,300 batches with a small on-site mixer, 260 tons of material moved by hand, eight to nine thousand wheelbarrow trips to distribute it across the foundation, and all of it lifted to 2.2 metres height for pouring columns, beams, and the ceiling slab. Our contractor said what any honest contractor would say: workers will refuse this job.

This killed not just the basement but any large-scale concrete project on the site, including the underground water reservoir that would have been needed in the original TeploHub plan too.

The theoretically optimal solution, everything integrated in one place, zero duplication, means nothing if it cannot be built. The right infrastructure for a mountain site is the infrastructure that can be built on a mountain.

We went back to TeploHub. Wood frame, carried up in pieces, assembled on site. Its concrete floor slab needs only five or six cubic metres — easily done by hand. The building grew from the original fifteen to twenty square metres to about thirty, because we added storage for ten glamping tepee tent sets that the community needs somewhere dry and lockable. Equipment in one half, storage in the other, with a fireproof partition between them.

Choosing the heat source

With the building sorted, the next question: what kind of heat pump?

The default answer in Ukraine is air-to-water. Widely available, many brands, local service, simple to install. You mount an outdoor unit on a pad, run refrigerant lines to the indoor unit, connect to the buffer tank, and you are heating. For most of the year, a modern air-to-water unit with an EVI compressor runs at a coefficient of performance around three to four, meaning three to four kilowatt-hours of heat for every kilowatt-hour of electricity.

But "most of the year" is not the problem. The problem is December through February.

At Tepla Gora in winter, air temperatures sit between minus five and minus twenty for weeks. Humidity runs at eighty to ninety percent. This is bad for an air-source heat pump. The outdoor coil ices up constantly. The unit runs defrost cycles, briefly reversing to melt the ice, which costs energy and reduces effective output. At minus fifteen with high humidity, COP drops to 1.5 to 2.0. At that point you are spending almost as much electricity as direct electric heating.

The standard solution is a backup wood boiler. Wood is essentially free at Tepla Gora. The surrounding forest produces deadwood that falls naturally, and collecting it is ecologically beneficial. A fifteen to twenty kilowatt boiler with a water jacket connects to the same buffer tank as the heat pump. When conditions get harsh, the boiler takes over.

This works. Many mountain communities heat this way. But when we listed the implications honestly, they added up:

Someone has to load the boiler. In cold periods, that means twice a day. At six in the morning, in minus twenty, someone walks to TeploHub and loads firewood. This is a community building, not a farmstead. There is no dedicated caretaker.

Firewood management is not zero effort even when the wood is free. Harvesting, transporting, splitting, stacking, drying. A hundred or more hours per year.

Wood burning produces particulates. PM2.5 from a wood boiler chimney near a building that contains a children's room, massage rooms, and a gym is a real health concern, not a theoretical one. Mountain valleys are prone to temperature inversions that trap smoke near the ground.

A building that regularly burns wood for heating does not meet the spirit of net-zero, even if the building envelope is excellent.

And our positioning as an applied research lab for sustainable buildings sits oddly with a system that relies on daily wood burning through winter.

None of these concerns are fatal on their own. Together, they made us look harder at the alternative.

The ground under our feet

A ground-source heat pump extracts heat not from air but from the earth. At one and a half to two metres depth, ground temperature in the Carpathians stays between eight and twelve degrees year-round — regardless of whether it is minus twenty or plus twenty-five on the surface. The heat pump always sees the same stable source. No defrost cycles. No capacity drop. No humidity problem. COP stays at four to five through the entire heating season.

The usual objection is cost. Ground-source systems require either deep boreholes (fifty to a hundred metres, drilled by specialist rigs) or horizontal ground loops (pipes buried in trenches across a large area). Boreholes are expensive. At Tepla Gora, they also raise a concern we take seriously: the community did not want to drill deep into the mountain for water supply out of concern for aquifer impact. Drilling for heat uses the same technique and raises the same questions. The boreholes are closed-loop and sealed, so the actual environmental risk is minimal with proper installation, but the concern is real and we did not want to dismiss it.

Horizontal loops avoid all of this. The pipes sit at one and a half to two metres — well above any deep water-bearing formations. No drilling, no aquifer interaction. The installation is basic excavator work: dig parallel trenches, lay polyethylene pipe, backfill. The pipe contains food-grade propylene glycol antifreeze in a sealed loop. Nothing enters the soil.

The catch is land area. A horizontal loop needs roughly forty to fifty square metres of open ground per kilowatt of heat extraction. For fifteen kilowatts, that is six to seven hundred and fifty square metres — a patch of meadow about twenty-five by thirty metres. It cannot be paved or built on afterward (the ground needs rain and snowmelt infiltration), and no deep-rooted trees can grow directly above (roots can damage pipes at that depth).

Two things made us look at this seriously.

First, Tepla Gora is purchasing an excavator for general site development. The trenching for a horizontal loop is exactly the kind of work this machine does. No specialist drilling contractor. No mobilization fee for equipment that comes once and leaves. The excavator is already there.

Second, the site plan showed open ground in exactly the right location. South-east of Unity Hub, between the building and the area where the septic system is planned, there is roughly six to eight hundred square metres of open meadow. The septic installation requires excavation in the same area. Combining both projects into one excavation campaign — dig the ground loop trenches and the septic pit in the same weeks, with the same machine — is an obvious efficiency.

TeploHub itself sits in the middle of this area, but the loop pipes are routed around its footprint, not under it. That way future pipe access (unlikely, since polyethylene loops last fifty-plus years) does not require demolishing the building. The building still helps the loop by insulating the surrounding ground from frost.

The cost surprise

We expected geothermal to be significantly more expensive than air-source. The European pricing data we started with suggested the heat pump unit alone would cost twice as much, plus the ground loop installation on top.

Ukrainian market pricing tells a different story. Local manufacturers like GeoSun produce ground-source units at prices well below European brands. Turnkey geothermal installations, including vertical borehole drilling, advertise at under ten thousand dollars. With a horizontal loop and your own excavator, the ground loop cost drops to roughly one to two thousand dollars (pipe and antifreeze, the excavator is already paid for). The total system comes in at eight to fourteen thousand dollars.

An air-source heat pump plus wood boiler, the supposedly cheap option, costs eight to twelve thousand dollars when you add the boiler, chimney, combustion air ducting, and firewood infrastructure.

The price difference is small. In some configurations, the geothermal option is the same cost or cheaper. And it comes with much better winter performance and none of the wood-burning downsides.

What about the PV array?

In a separate analysis, we designed a twenty-kilowatt vertical bifacial PV system optimized for winter production. With an air-source heat pump running at COP 1.5 to 2 in winter, that array size was driven partly by the need to power an inefficient heat pump during the months when solar production is lowest.

Geothermal changes this. At COP four to five, the same heating output requires half the electricity. A fifteen-kilowatt array with geothermal provides roughly the same winter energy coverage as twenty kilowatts with air-source.

We considered reducing the array to save two to three thousand dollars and tens of metres of solar fence. Then we thought about what the extra panels buy: the ability to almost never run the wood boiler. With twenty kilowatts of PV, sixty kilowatt-hours of battery storage, a geothermal heat pump, and two grid connections as backup, the boiler becomes emergency-only equipment. Loaded once in autumn, checked monthly, fired up maybe zero to five days per year when the power goes out or the heat pump needs service.

The two to three thousand dollars in extra panels is cheap compared to a hundred hours per year of firewood management, the health impact of wood smoke near occupied buildings, and the credibility cost of daily wood burning in a building that calls itself net-zero.

The system as it stands

Nothing is final. We have not selected a specific heat pump model, have not run detailed ground loop calculations, and have not confirmed soil thermal properties with a test pit. The cost figures are market research, not quotes.

But the direction is becoming clear.

A horizontal ground-source heat pump as the sole heating system, with COP four to five through the entire Carpathian winter. Twenty kilowatts of vertical bifacial PV optimized for winter production. Sixty kilowatt-hours of battery storage. Two grid connections as backup. A wood boiler in TeploHub that runs only when something fails — not as a regular part of winter operations.

What makes this work is a set of site-specific factors that happened to line up: an excavator already on site, clay soil with good thermal conductivity, open land in the right location, a septic excavation that can absorb the loop trenching at marginal cost, and Ukrainian market pricing that closes the gap with air-source.

We will update this analysis as we get real quotes and refine the design. The ground loop layout needs to be finalized before the excavator arrives for septic work, and that is the real deadline.

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This research is part of Unity Hub energy system design at Tepla Gora. Related: Designing autonomous sanitation for a remote eco-community, Designing autonomous water supply for a Carpathian eco-community, Designing a winter-optimized PV system for a Carpathian eco-community.