A heat dump is a heat-rejection circuit — typically a radiator, fan coil, pool loop or buried pipe coil — that a solar thermal controller uses to discharge surplus collector heat once the storage tank is fully charged. By keeping the circuit below stagnation conditions, where flat-plate collectors can reach 160–200 °C, it protects the glycol, seals and expansion vessel from repeated thermal stress.
How does a heat dump work?
When a solar store reaches its maximum permitted temperature, the controller normally just stops the pump — and the collector then climbs toward its stagnation temperature, typically 160–200 °C for flat plates and higher for evacuated tubes. A heat dump gives the controller a third option: instead of stopping, it diverts flow through a rejection circuit — a radiator mounted in a garage or on a shaded wall, a fan coil, a swimming pool heat exchanger or a buried ground coil — and dumps the surplus until collector temperatures fall back to a safe band. The collector keeps operating at moderate temperature, the fluid never boils, and the system rides through the surplus period without stress.
When do you need a heat dump?
When a system regularly produces more heat than it can store, and cannot protect itself another way. The classic triggers are a high solar fraction design (collectors sized for winter that heavily over-produce in summer), buildings that empty for weeks in summer — holiday homes, schools, seasonal hotels — and systems whose glycol, components or controls are not rated for repeated stagnation. Systems that reach stagnation only a handful of days a year generally manage without one, because a properly engineered circuit is designed to survive occasional stagnation. The decision tree is worked through in Do I need a heat dump on a solar hot water system?
What can be used as a heat dump?
| Rejection option | How it rejects | Notes |
|---|---|---|
| Oversized radiator / convector | Passively to air | Simple, silent, no extra power; mount where waste heat is harmless |
| Fan coil unit | Forced air | Compact, high capacity; needs electricity and makes some noise |
| Swimming pool heat exchanger | Into pool water | Turns "waste" into useful heat — the best dump is a load |
| Buried ground coil | Into the soil | Invisible; capacity depends on ground conditions |
The rejection capacity should be matched to the collector field's peak output — on a clear summer day a flat-plate array delivers on the order of 600–700 W per m² of aperture, so a 6 m² field needs roughly 4 kW of rejection capacity to hold temperatures down.
What are the alternatives to a heat dump?
Several, and they are often better. A drain-back system empties the collectors whenever the pump stops, making overheating physically impossible. Steam-back designs let a controlled steam pocket push fluid into the expansion vessel and simply wait out the stagnation. Controllers offer holiday functions and night re-cooling, which circulates hot store water through the collectors after dark to shed heat. And the most elegant fix is upstream: sizing the array honestly against summer load, or steepening collector tilt to favour winter sun. These strategies are compared in How do I stop my solar thermal system overheating in summer?
Does a heat dump waste the value of solar heat?
Only heat that had nowhere else to go. A dump circuit activates exclusively above the store's cut-off, so it discards nothing the building could have used that day. The engineering view is pragmatic: repeated stagnation degrades propylene glycol — accelerating the fluid replacement interval — and stresses seals and membranes, so occasionally discarding a few kWh is cheaper than replacing fluid and components. Wherever possible, route the "waste" somewhere useful, such as a pool: manufacturers including Solimpeks size collector fields and stores together precisely so that genuine surplus stays small.
Frequently asked questions
Is a heat dump mandatory on solar thermal systems?
No. Well-designed systems with correctly sized arrays, stagnation-tolerant components and adequate expansion volume can safely ride out occasional stagnation. A dump becomes worthwhile when surplus is frequent — high solar fraction, summer vacancy, or components not rated for stagnation.
How big should a heat dump radiator be?
Match it to the collector field's peak output, roughly 600–700 W per m² of aperture in strong sun. A 4–6 m² domestic array therefore needs about 3–4 kW of rejection capacity to keep the loop out of stagnation.
Can the heat dump work during a power cut?
A pumped dump circuit cannot — no electricity, no circulation. This is a real limitation: systems relying on active dumping should still tolerate stagnation, or use inherently fail-safe protection such as drain-back, where the collectors empty by gravity when the pump stops.
Where should a dump radiator be installed?
Anywhere the rejected heat is harmless or useful and frost-safe practice is observed: a garage, plant room, north wall or fence line are common. Keep it accessible, and pipe it so the diverting valve fails to a safe position.