Rainwater harvesting for heat pumps

Harvested rainwater is not normally a practical replacement for the groundwater used by an open-loop heat pump. A roof collects water intermittently, while a heat pump needs a dependable thermal source during the periods when heating demand is highest. A storage tank can shift the timing of supply, but it cannot create more annual water or more heat.

The question often confuses three different systems:

• an open-loop ground source heat pump that abstracts and discharges groundwater
• a rainwater system supplying non-potable uses such as toilet flushing or irrigation
• a closed water or brine circuit using a tank as part of a thermal store or heat exchanger

They have different hydraulic, water-quality and regulatory requirements. Connecting them together is not a normal shortcut.

Compare annual collection with source-water demand

Annual rainwater collection is estimated from roof plan area, local rainfall and a collection coefficient that allows for losses. Heat-pump source-water demand follows from required heat transfer, water temperature change and operating hours.

Those two calculations should be compared before discussing a tank. A larger tank can carry water from wet periods into dry periods, but once its stored volume has been used the continuing demand is limited by new rainfall. Heating demand and rainfall also do not necessarily coincide.

This is why a universal statement such as “a house roof supplies one per cent” is not appropriate. Roof area, climate and heating demand vary. The durable conclusion is that an ordinary domestic catchment is generally far smaller and less dependable than a continuously available open-loop source. If a designer claims otherwise, the annual water balance and worst-period storage model should make it evident.

Water temperature and usable heat matter too

Water volume alone does not establish a useful heat source. The energy available depends on how far the water can be cooled without freezing or breaching equipment limits. A tank exposed to winter weather may be colder just when the building needs most heat.

Repeatedly taking heat from a fixed tank without an adequate way for heat to return will cool it towards freezing. At that point the system has become a thermal-store design, and the engineer must model tank temperature, recharge from air, ground, solar or incoming rain, ice formation and heat-exchanger performance. Occasional rainwater top-up does not solve an energy imbalance.

If water is discharged after one pass, the scheme needs a continuous volume and a lawful discharge route. If it is recirculated, it is a closed source circuit and needs suitable freeze protection, materials and controls. Calling both arrangements “rainwater harvesting” obscures that difference.

Water quality and system separation

Roof runoff can contain sediment, organic matter, bird fouling and roof or gutter contaminants. Filters suitable for toilet supply do not necessarily protect a heat exchanger from fouling or corrosion. Water chemistry, materials, filtration, cleaning access and stagnation all need assessment.

Rainwater systems must also remain correctly separated from wholesome mains water. Any mains top-up, overflow and backflow protection should comply with the applicable water-fittings and building requirements. A direct cross-connection between a rainwater tank, mains supply and source circuit is not an acceptable substitute for that separation.

Using groundwater and rainwater alternately adds controls and sampling without removing the need to prove the groundwater abstraction and discharge route. Under the current English exemption for some open-loop ground source schemes, the abstracted water must not be used for another purpose or have substances added. A combined scheme cannot assume that exemption continues to apply.

Where rainwater may still be useful

Harvested rainwater may have a separate role at the same property, reducing mains-water demand for approved non-potable uses. It may also form part of a specifically engineered thermal system on a large site with an unusually large catchment, suitable storage and a credible heat-recharge mechanism.

Those are project-specific designs. They should show:

• monthly collection and demand, not just annual totals
• storage level through the critical heating period
• minimum source temperature and freeze behaviour
• source-pump energy and heat-exchanger performance
• water-quality controls and cleaning access
• overflow, discharge, mains separation and environmental permissions
• what happens during drought, prolonged cold or equipment failure

For an ordinary home choosing a heat source, a proven air source or properly designed ground or water source is usually more defensible than trying to make the rainwater tank perform two unrelated jobs.

Related entries

• Open-loop GSHP
• Ground source heat pumps
• Shared ground loop systems
• Electrical supply for heat pumps