Most correctly sized domestic solar hot water systems do not need a heat dump: certified collectors are designed to survive stagnation, and a right-sized array saturates the tank without chronic summer surplus. A heat dump earns its place when the array is deliberately oversized for winter or space heating, when the property stands empty for weeks each summer, or when the designer wants to avoid stagnation events entirely.
What does a heat dump actually do?
A heat dump is a deliberate place to throw away surplus solar heat — typically an oversized radiator, a buried pipe loop, a fan coil or a swimming pool circuit — activated by the controller when the store is full and the collectors keep producing. Its purpose is to prevent the system from entering stagnation: the state where circulation stops, the fluid boils out of the collector, and temperatures climb toward the collector's stagnation temperature, which for a good flat plate sits around 200 °C. Stagnation is not an explosion risk in a properly built system — the expansion vessel and safety valve are sized for it — but repeated stagnation ages the glycol, stresses seals and takes the system offline until it cools.
When is a heat dump genuinely needed?
Three situations justify the extra hardware. First, deliberate winter-oriented oversizing: an array sized to lift December coverage produces a large, unavoidable summer surplus — the trade-off explained in summer vs winter solar coverage. Second, predictable absence: a holiday home or a family away every August leaves zero draw-off for weeks, so the store saturates on day one and the array stagnates daily thereafter. Third, solar space-heating combi systems, where the array is sized for shoulder-season heating load and summer surplus is structural. Outside these cases, a domestic DHW system sized for 50–70% annual solar fraction reaches stagnation rarely — a handful of peak days a year — which certified hardware tolerates by design.
What are the alternatives to a heat dump?
| Strategy | How it works | Best for |
|---|---|---|
| Correct sizing | Array matched to demand; surplus days are rare | Every new design — first resort |
| Stagnation-tolerant design | Certified collectors, correctly sized expansion vessel, solar-grade glycol | Standard pumped systems |
| Holiday function / night cooling | Controller circulates at night to radiate stored heat from the collectors | Occupied homes with occasional absence |
| Drain-back design | Fluid empties from collectors when pump stops — stagnation becomes harmless | New systems in freeze- or surplus-prone sites |
| Heat dump | Active surplus rejection to radiator, ground loop or pool | Oversized arrays, empty properties, combi systems |
Night cooling costs a little pump energy but needs no extra hardware; a drain-back system solves overheating and freezing with the same mechanism, which is why it is the elegant answer for holiday properties. The broader overheating toolkit — including controller settings and shading — is covered in how to stop solar thermal overheating in summer.
If I fit a heat dump, how should it be done?
Passively safe is the design rule: the dump should work even when the electricity fails, because power cuts on sunny days are precisely when stagnation strikes. A thermostatic valve diverting to a gravity-fed radiator meets this test; a mains-powered fan coil alone does not — it protects against a full tank but not against a dead pump. The dump circuit must sit on the solar primary side, sized to reject something close to the array's peak output, and it must open only above the tank's target temperature so it never steals useful heat. A swimming pool, where one exists, is the happiest version of a heat dump: the "waste" heat becomes free bathing comfort — see can solar thermal heat a swimming pool.
The honest summary
Treat a heat dump as a consequence of a sizing decision, not a default component. A Solar Keymark-certified collector — Solimpeks' Wunder series among them — is tested to survive stagnation, and a system designed around realistic demand will spend almost no time there. Decide the array size first, be honest about summer occupancy, and let those two answers tell you whether surplus rejection deserves hardware, a controller function, or nothing at all.
Frequently asked questions
Is stagnation dangerous for a solar thermal system?
Not acutely, in a properly built system: the expansion vessel and safety valve are sized to handle it, and certified collectors are tested for it. The damage is cumulative — repeated stagnation degrades the glycol and stresses components — so frequent stagnation is a design fault to fix.
What is the holiday function on a solar controller?
A mode that cools the store by circulating fluid through the collectors at night, radiating heat to the sky so the tank has capacity for the next day's sun. It reduces stagnation during absences without extra hardware, at the cost of a little pump energy.
Can I use my swimming pool as a heat dump?
Yes — it is the best possible one, since a pool absorbs large amounts of low-grade heat usefully. The controller simply diverts surplus solar heat to the pool exchanger once the hot water cylinder reaches its target temperature.
Do evacuated tube systems need a heat dump more than flat plates?
They are more prone to surplus, since evacuated tubes retain high efficiency at high temperatures and reach higher stagnation temperatures. Oversized tube arrays are the classic case where active surplus management or careful sizing matters most.