Air conditioning systems used for heating
What You Need to Know About Air Conditioning Systems Used for Heating
The information below is provided for customers who are purchasing an air conditioning system with the intention of using it for heating. It explains how outdoor temperatures can influence the performance of your equipment, and what to expect in various conditions.
Air-to-Air Heat Pumps
The system you have been quoted for is an air-to-air heat pump. You may already be familiar with air-to-water heat pumps from media coverage, given the government’s drive to replace gas boilers with low-carbon heating alternatives.
Both technologies work on the same fundamental principle – the key difference is where the heat ends up. With an air-to-water system, heat is transferred into water and distributed via underfloor heating or radiators. With an air-to-air system (such as a wall-mounted unit, cassette, or ducted system), heat is transferred directly into the air inside the building.
In heating mode, the system draws in air from outside and passes it over the outdoor unit’s coil, which contains refrigerant. The refrigerant absorbs heat from the outdoor air as it evaporates. That energy is then released indoors as the refrigerant passes through the indoor unit’s coil, warming the air inside the building.
It’s important to understand that as outdoor temperatures drop, there is progressively less heat energy available in the outside air. This means the system has to work harder and takes longer to heat the space. In particularly cold conditions – generally around +5°C and below – the system may struggle to maintain a comfortable indoor temperature on its own, and supplementary heating or a longer pre-heat period may be needed.
Defrost Cycles
When a heat pump has been running in heating mode for an extended period, frost and ice can accumulate on the outdoor unit. This is completely normal. To deal with this, the system will periodically enter a defrost cycle, temporarily reversing the refrigerant flow to use warmth from the indoor air to clear ice from the outdoor coil.
During this cycle, you may notice cool air blowing from the indoor unit. This is not a fault – it is simply an unavoidable part of how this technology works. On systems with a wired controller, the defrost cycle is usually shown on-screen. On systems using an infrared remote, there is no visual indicator, so this can sometimes cause confusion.
The defrost cycle is brief and self-managing. Once complete, the system will return to normal heating operation.
Heating Spaces with High or Vaulted Ceilings
Warm air naturally rises, which can be a challenge in buildings with high ceilings. You may notice that the upper areas of the space feel warm while the lower, occupied zones remain cooler. This is not specific to air conditioning – it applies equally to gas, electric, or any other heat source.
Typically, the upper void fills with warm air first, and as heating continues, the lower areas begin to warm up too. However, this process takes longer in very cold weather, or where the building structure (such as an all-brick or all-concrete construction, or poor roof insulation) allows heat to escape quickly through the fabric of the building.
In these situations, we strongly recommend the use of destratification fans. These fans gently circulate the warm air pooled at ceiling level back down into the occupied zone, significantly improving comfort and reducing the running time – and therefore the energy costs – of the heat pump.
Heat Load and Heat Loss Calculations
Unless your quotation specifically states otherwise, equipment sizing has been based on a combination of industry rule of thumb, any known equipment loadings, and extensive practical experience in the field.
In the majority of cases, this approach produces perfectly suitable results. A full thermal modelling or heat-loss calculation carried out by a building services consultant can cost in the region of £500 per area – an expense that is often impractical to absorb, particularly at quotation stage. Where a more precise assessment is required, this can be arranged and costed separately.
Outdoor Unit Drainage and Freezing
During the defrost cycle, water will drip from the outdoor unit as ice melts from the coil. Where available, we fit a bung kit to the outdoor unit, which seals the base drain holes and provides a managed drain outlet. In cases where a bung kit is not available for the specific unit, we can supply and fit a drip tray beneath the unit at additional cost.
In either case, the collected water is routed via a 22mm overflow pipe to a suitable discharge point – typically a soakaway or open drain. Where this isn’t feasible, and with the customer’s agreement, discharge can be directed elsewhere (e.g. to a pathway or grassed area).
This arrangement works well for the majority of the year. However, in freezing temperatures, water in the drain pipe can freeze and block the outlet, causing water to overflow the tray or spill from the base of the unit.
To prevent this, trace heating tape can be fitted to the drainage pipework at additional cost. This is a more involved installation and not all customers require it. Most opt not to have it unless the water spillage could create a slip hazard for staff, customers, or visitors. Please get in touch if you’d like advice on whether this is appropriate for your installation.