October 2011 — on why doing something properly means drawing it first

The Dutch gas grid is reliable, cheap by European standards, and completely outside your control. The price goes where it goes. The supply comes from wherever it comes from. The boiler it feeds is a black box maintained by people you call once a year and never quite understand what they tell you.

I wanted out of that arrangement. Not for ideological reasons — for the same reason I self-host my infrastructure: I want to understand what I’m running, control how it behaves, and not be at the mercy of a supplier whose incentives do not align with mine.

The project was to replace the entire heating and hot water system in our apartment with one that runs primarily on solar thermal energy, backed up by a wood stove, distributes heat via floor heating throughout, and stores enough thermal energy in a buffer tank to decouple generation from demand.

Before anything was drilled, cut, or ordered, I drew the system.

The system diagram

The design has four primary components:

Solar thermal collector — vacuum tube type, mounted on the flat roof. Vacuum tubes outperform flat plate collectors in the Netherlands climate because they work effectively at lower irradiance levels and in overcast conditions. The collector feeds heat into the buffer tank via a glycol loop.

Wood stove with back boiler — the backup heat source. When solar contribution is insufficient — winter, extended overcast periods — the wood stove heats water through a heat exchanger coil in the firebox and pumps it into the buffer tank. Not a pellet stove, not a heat pump: a wood stove. Simple, maintainable, fuel-agnostic.

Buffer tank — the thermal heart of the system. A large insulated tank stores the heat from both sources. The buffer decouples generation from demand: solar energy collected at noon is available for floor heating at 3am. The tank also integrates the domestic hot water coil — a heat exchanger inside the buffer produces hot tap water without mixing circuits.

Floor heating distribution — multiple zones, each independently controlled. Low-temperature floor heating is ideal for solar thermal systems because the source temperature required is lower than radiators, which means the solar collector operates more efficiently and the wood stove back boiler can contribute at lower fire temperatures. The floor itself acts as a thermal mass — it stores heat and releases it slowly, smoothing demand peaks.

Why these choices

Vacuum tubes over flat plate: the flat roof faces roughly south. In Dutch conditions (Amsterdam latitude, frequent cloud cover, low winter sun angle), vacuum tubes produce usefully more energy per unit area in the shoulder seasons — spring and autumn — which is where most of the heating load falls. In summer they overproduce, which is managed by shading. In deep winter they contribute less, but that’s what the wood stove is for.

Wood over heat pump: a heat pump requires electrical infrastructure, refrigerant maintenance, and a coefficient of performance that depends on conditions we can’t control. A wood stove is a fire. It produces heat at a rate proportional to how much wood you feed it, has no refrigerant, no compressor, no electronic expansion valve to fail. The back boiler — a heat exchanger coil inside the firebox — extracts useful energy from what would otherwise be radiated heat in the room, and puts it in the buffer tank instead.

Buffer tank over direct heating: without a buffer, solar thermal and wood stove outputs need to match demand in real time, which they don’t. A solar collector produces most energy around solar noon; heat demand is highest in morning and evening. A buffer tank stores the surplus and releases it when needed. It also allows the wood stove to run at its most efficient fire temperature regardless of what the heating zones are currently demanding.

Floor heating over radiators: underfloor heating at 35-45°C flow temperature works well with a buffer tank running at 50-55°C — there’s enough differential to drive the zones. Radiators would need 65-75°C, which is harder for solar thermal to achieve consistently and requires more wood consumption to sustain. Floor heating also distributes heat more evenly and is invisible — no radiators to work around.

The complexity that wasn’t obvious from the diagram

Drawing a system diagram takes an afternoon. Building what the diagram describes takes sixteen months.

The flat roof required three core-drilled penetrations through reinforced concrete: one for the flue, one for the solar collector pipes, one for the condensate and overflow drain. Concrete flat roofs on Dutch apartment blocks are not thin — getting through them requires a proper diamond core drill rig, water cooling, and patience.

The flue is double-wall insulated stainless steel, running from the wood stove location up through the ceiling, through the roof slab, and terminating above the roof with a rain cap. Every floor penetration is fire-rated and sealed. Every roof penetration is waterproofed to the EPDM membrane. None of this is optional — a flue that leaks combustion gases into the building or a roof penetration that lets water in are both serious failures.

The copper pipe distribution — from the buffer tank to the floor heating manifolds, to the domestic hot water tap points, to the solar collector loop connections — is all hard copper, soldered. Not push-fit, not press-fit. Soldered, because it’s a sealed system with glycol in the solar loop and elevated temperatures throughout, and push-fit connections on a system running at 80°C with glycol have a way of becoming a problem in year three.

The steel mounting frame for the solar collector was fabricated on site and welded — the collector needs to be at an angle optimised for Dutch winter sun elevation, bolted to a frame that can survive wind loading on a flat roof without penetrating the membrane.

The floor heating installation meant opening up the floors: excavating the existing screed, laying the loop pipes on a thermal insulation layer with steel mesh reinforcement, and pouring new screed to the correct depth for the pipe diameter. That’s not a weekend job. That’s weeks of dust, temporary living arrangements, and concrete mixing.

What the diagram is actually for

The system diagram isn’t just a plan. It’s the single reference that makes it possible to build something this complex without losing track of what connects to what, what the design intent was, and what the commissioning sequence needs to be.

When you’re standing in front of the buffer tank at 11pm with a soldering torch and three different circuits coming in from different directions, the diagram tells you which pipe is which. When you’re debugging why one floor heating zone isn’t heating correctly six months later, the diagram tells you where to look.

Every sub-system decision — where the check valves go, which side of the heat exchanger is primary, what the bypass arrangement is for the solar pump — is on the diagram before it’s in the building.

Design first. Build second. In that order, without exception.

The build is in the next article.