Condensate drainage

An air-source heat pump produces water outside. Rain can enter the casing, and moisture freezes onto the outdoor coil in cold, damp weather. During defrost, that ice melts and can leave the unit quickly.

The drainage design has to collect this water and discharge it safely without creating ice on a path, damp at the building, blocked outlets or damage to the unit. MIS 3005-I requires condensate from an air-source heat pump to be discharged safely to a suitable drain or soakaway.

The amount is not a fixed product figure

Defrost water varies with outdoor temperature, humidity, run time, coil size and the way the heat pump controls defrost. A summer observation or a generic litres-per-day claim cannot size the drainage for a cold, damp spell.

The design should use the manufacturer’s drainage instructions and allow for both defrost discharge and rain entering the base. Some units drain from one connection; others shed water through several openings across the base pan. A small pipe attached to one hole is unsuitable if the appliance is designed for open-base drainage.

This water is not the acidic flue condensate produced by a condensing boiler. That does not make every discharge location acceptable: uncontrolled water can still freeze, cause damp or carry dirt and treatment residues.

Choose the discharge point first

A suitable route may lead to a purpose-designed soakaway or to an appropriate drainage connection. The correct choice depends on the unit instructions, soil, building drainage, local conditions and the applicable building rules.

The discharge should not:

• run across a path, step, pavement or vehicle area
• collect beside foundations or against the wall
• flow onto neighbouring land
• erode a slope or wash out the unit base
• block an air brick, damp-proof course or ventilation opening
• rely on a rainwater arrangement that cannot accept it
• introduce a route for refrigerant into the building or drainage system contrary to the appliance instructions

Connection to a foul, surface-water or combined system is not interchangeable. The installer should establish which system is present and whether a trapped, air-gapped or otherwise protected connection is required. Approval can be needed for changes to drainage.

Soakaways

A small local soakaway can work where the ground absorbs water and its position will not affect foundations or boundaries. Its size and construction should reflect the ground and expected discharge rather than a universal crate or gravel-pit dimension.

Heavy clay, a high water table, a paved garden or an already waterlogged area may make infiltration unsuitable. A hole filled with loose stones immediately beneath the unit can also freeze, silt up or allow water to sit against the base.

The finished arrangement needs an inspection and cleaning route. Geotextile, graded stone or a proprietary component may form part of the design, but the important result is free drainage under the foreseeable winter condition.

Pipe route and frost risk

Where pipework carries the water, it should normally be short, adequately sized and laid to a continuous fall in accordance with the manufacturer. Unnecessary bends, traps, flat sections and exposed narrow pipe increase the chance of freezing or blocking.

Insulation can slow freezing but does not rescue a pipe that retains water. Trace heating is sometimes specified for an unavoidable exposed route; it then needs suitable electrical protection, controls and maintenance. Neither should be added as a stock detail without checking the appliance and drainage design.

An indoor route can reduce frost exposure, but penetrations must remain weather-tight and the connection must not create odour, backflow or a path for a flammable refrigerant leak. The selected heat pump’s instructions take priority.

Base, clearance and ice

The heat pump should sit on the base or brackets specified for it, with the required clearance beneath and around drainage openings. Anti-vibration feet should not be arranged so that a base tray is trapped against the slab.

The supporting surface should remain stable when wet and should not settle into the drainage area. Wall brackets have the same water problem as ground mounts: defrost water still needs a controlled route and must not fall onto a path or stain the facade.

An ice sheet beneath the coil suggests that water cannot get away, the base clearance is wrong, the drain is frozen or the defrost system needs investigation. Chipping ice near the coil, fan or refrigerant pipe can cause damage. The safe response is to isolate only as instructed and arrange competent inspection.

R290 and other refrigerants

Some outdoor heat pumps use R290 propane. The manufacturer defines a safety zone around openings and parts from which refrigerant could escape. Drains, gullies, ducts, air bricks, electrical equipment and low points can be restricted within that zone because leaked gas can travel or collect.

There is no safe generic exclusion-zone dimension for every R290 product. The outdoor-unit position, base and condensate route must be designed together from the instructions for the exact model. A drain detail copied from a non-flammable-refrigerant unit may be inappropriate.

Condensate or a water leak

Clear water beneath the outdoor coil during or after defrost can be normal. Continuous water from a heating pipe connection, falling system pressure, visible glycol staining or water from inside the building is not explained by normal defrost.

The homeowner should be shown the expected discharge point and system pressure. A leak from the sealed heating circuit needs attention because it can reduce frost protection, introduce air and stop the system.

Commissioning checks

The HPA commissioning checklist records that defrost and condensate provision has been made. A useful site check goes further and confirms:

• every intended base opening or drain connection is clear
• water reaches the selected discharge point
• pipework has fall and support and does not retain water
• the route cannot create a slip or ice hazard
• foundations, damp-proofing and neighbouring land are protected
• R290 safety-zone restrictions have been followed where applicable
• the arrangement remains accessible for cleaning

Photographs before covers or landscaping conceal the route should be included in the handover file.

Maintenance

Leaves, silt and ice can block an arrangement that was sound when installed. The owner should keep the area around the base clear and look for changed discharge behaviour during cold weather. Maintenance instructions should say which grating, trap, pipe or soakaway can be inspected without opening the appliance.

Pouring boiling water, adding salt or antifreeze, or forcing tools into the unit can damage components and contaminate drainage. A repeated freeze-up calls for correction of the route or appliance fault.

Related entries

• Outdoor unit siting
• Monobloc ASHP units
• Defrost cycle behaviour
• MIS 3005 heat pump standard