What are the benefits of Air Source Heat Pumps (ASHP) over Ground Source Heat Pumps (GSHP)?
There are a number of benefits when using an ASHP. Capital cost is lower as the ASHPs are normally self-contained units without additional heat transfer requirements such as ground pipe work. This also means that installation costs are lower as the ASHP simply needs to be sited in an appropriate position and connected to the mains supply electricity and building hot water circuit; whereas a GSHP will require significant ground works, either bore holes or deep furrows. This type of work is not always possible on existing buildings due to limited access and is always costly. New technology has enabled ASHPs to achieve similar seasonal efficiencies to those traditionally associated with GSHPs.

What is the best type of system to connect a heat pump to?
For optimum efficiency an under floor system provides the best results, as it requires a lower flow temperature and the heat pump can be programmed to provide the water at the required temperature with no need for a mixing valve. Any system will benefit from using a storage tank rather than a direct feed, which will allow the heat pump to raise the water to the correct operating temperature more quickly as there is no direct heat loss from the storage tank during the start up period.

What is the lower limit operating temperature?
-20C is defined as the lower operating limit but, the heat pump will not switch off at this temperature. If the heat pump is expected to operate below -5C for long periods of time then a boost heater should be fitted to act as a backup. At -20C the maximum supply water temperature will be 45C.

What controls will the heat pump work with?
The heat pump is fitted with a controller that is pre-programmed with all of the operating set points and alarms and simply requires an input from an external clock/thermostat. Any standard type of central heating controller will work with the heat pump but the best solution is to use a controller with a number of on/off options for each day to take advantage of any special off peak tariffs available.

What are the primary maintenance requirements?
The heat pump should be serviced once a year by a qualified technician and the following items checked:

  • The unit structure for signs of corrosion or damage
  • Panels are securely fastened and vibration free
  • All electrical connections are secure and that there is no damage to any of the wiring
  • All water connections are tight and the system water pressure is correct
  • Pipe insulation has not deteriorated or come loose
  • The air path to and from the unit is clear
  • Condensate drain pan and pipe are clean and clear
The unit should be switched on, and the following checked:
  • The controls are working correctly
  • The water pump is free and operating correctly
  • The unit fan is operating
  • The unit raises the water to the correct operating temperature

What are the siting requirements of the unit?
Sufficient space is required for airflow into the machine and adequate clearance is needed at the front of the heat pump to prevent cold air re-circulation. Access is also needed for service and maintenance. If possible the unit should be sheltered from high winds to improve efficiency by reducing the fan power requirement. Units are best placed in a south facing aspect to gain maximum benefit from solar gains.

Can radiator and underfoor systems be combined?
Yes, but if used in this way the heat pump return water should be set to suit the radiator system and a mixing valve used to reduce the under floor supply to the design temperature. On all retro installs it may be necessary to resize certain radiators as all heat pump systems generally need larger than normal output surface areas.

How long does it take for radiators to get up to temperature?
This will depend upon a number of factors. A heat pump will raise the water passing through it by between 5oC and 10oC depending upon the water flow rate. Getting the radiators up to temperature will depend upon a combination of the rate of temperature rise across the heat pump, the amount of water in the system and temperature of the standing water in the system when the unit switches on. As a rule of thumb the heat pump will raise the water temperature at a rate of 1oC per minute therefore the time taken to achieve the maximum operating temperature will be the difference between the standing temperature and the required operating temperature in minutes.

What is the maximum number of radiators per heat pump system?
The number of radiators is less significant than the total capacity of all the radiators added together. This should not exceed the expected heat pump output at the design condition; i.e. if the heat pump output is 6kW at the design outdoor condition then the radiator capacity should be 6kW based upon the water supply temperature of 50C.

How often should the water temperature be boosted to raise it to its operating temperature?
In most conditions the heat pump will start and raise the water temperature as expected. In periods of very cold weather where there is going to be significant reduction in the system standing water temperature to below 15C then a boost heater should be fitted in the return water side to help the heat pump start up and to prevent it from going straight into de-frost. This will work automatically when required.

How should Domestic Hot Water be heated?
The heat pump should be used to pre-heat the water before additional heating is added to raise the water to the desired temperature. If connected in parallel with a solar heating system the heat pump should be set to operate if the solar cannot provide adequate heating. DHW systems should be designed to comply with current regulations.

Can the units be hidden behind fences, bushes, etc.?
Units may be installed behind barriers or even inside enclosures, garages or sheds. The important consideration is ensuring that adequate airflow is available and that the discharge air cannot be re-circulating back onto the inlet as this will result in a continual lowering of the air temperature reducing the efficiency of the unit.

How does the heat pump compare with other heating appliances?
Sound levels inside the house are generally lower than those that would be expected from a gas or oil boiler with a fan assisted flue and will not cause a nuisance provided the installation guidelines have been followed correctly. Noise levels outside may be 2-3dB higher than those expected from a condensing boiler flue and this should be considered when selecting the position of the heat pump.

Can the heat pump cool as well as heat?
Yes, however if cooling is required an alternative range of units is available to provide both heating and cooling. Cooling may be used with under floor heating or with fan coil units, but is not recommended with radiators as even though the minimum water temperature can be controlled, on certain days, depending upon conditions, the radiators could sweat and cause water damage.

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