Heat pumps run on dumb thermostats

A heat pump is the largest steady electrical load in winter, paired with a thermal mass that doubles as a battery. Price-aware scheduling captures spreads of 5–10x between cheap night hours and expensive mornings on the spot market — but only if the thermostat is smart enough to cooperate, which most aren't. The thermostat is the weak link.

Around half of new heating installations in Estonia today are heat pumps. The hardware caught on faster than the software did. In a modern home, the heat pump is the appliance that benefits the most from being clever about price. It is also the one most often left running on a fixed schedule from 2018.

Why does it benefit the most? Two reasons. It is the largest steady electrical load in winter, typically two to five kilowatts for hours at a time. And it works against a thermal mass — the house itself — that doubles as a battery. Heat poured into the slab and the radiators at 03:00 keeps the rooms comfortable until breakfast, by which time the spot price has already spiked and crashed.

The Estonian context makes this unusually rewarding. Elering data shows the day-ahead market regularly swinging from negative prices on windy nights to north of €500/MWh on cold weekday mornings. Static-tariff customers smooth that volatility into a flat, expensive average. Spot-aware customers, if they do nothing clever, eat the peaks raw. See Elering live prices for what a typical winter day actually looks like.

Smart control means three things in practice. First, price-aware scheduling: lean on the heat pump in the cheap night window, usually 02:00 to 06:00, and let the house coast through the morning peak. Second, gentle pre-heating: nudge the indoor setpoint up by one or two degrees during cheap hours so the house has thermal headroom to glide through expensive ones. Third, treat the domestic hot water tank as what it is, a 200 to 300 litre thermal battery, and finish heating it before sunrise rather than on demand at 08:00.

All of this rides on top of the manufacturer's weather curve, not under it. Outdoor temperature still drives base demand. Price awareness is an overlay. Modern units from NIBE, Bosch, Daikin, Mitsubishi, Panasonic and IVT expose either a cloud API or a Modbus / EEBus interface that lets an external controller shift setpoints by a degree or two without ever pushing the compressor outside its warranty envelope. Manufacturers like NIBE have published these integration paths for years; the question is whether anything is actually using them.

A heat pump on a fixed schedule, with no price input, is leaving most of its potential value on the table in a 2025 Estonian winter. The hardware is already excellent. The thermostat is the weak link.

Comfort is the part people worry about, and they are right to. A well-tuned controller stays inside a ±1 °C band around the resident's target. Nobody should feel the optimisation. If you do feel it, the bands are too wide or the pre-heat is too aggressive, and that is a tuning problem, not a fundamental one. The same logic applies to floor heating, where the slab's inertia is your friend: it stores heat for hours and releases it slowly, which is exactly the behaviour you want when prices are spiking.

Heat pumps also pair naturally with the rest of a flexible home. A rooftop array running into a midday surplus can dump that energy straight into the hot water tank rather than exporting it at near-zero feed-in tariffs, a pattern covered in the sister post on solar optimisation. A home battery, covered separately, handles the short fast peaks; the heat pump handles the long slow ones. They are complementary, not competing.

Volton Home, our retail supply, runs this for around six hundred Estonian homes. The customer connects the heat pump in the app once, the optimiser pulls tomorrow’s prices each afternoon, and the schedule is rewritten overnight. The reported average bill saving across the whole appliance mix is in the low thirties of a percent, and in a heated house most of that saving is the soojuspump pulling its weight.