Specialists in Low Energy and Heat Pump Heating Technology

Sizing the System

The system needs to be capable of being large enough to provide enough heat to keep your house at a warm enough temperature inside to be comfortable while the temperature outside is at what we call the design temperature.

Internal Design Temperature

MCS recommends an internal design temperature of 21C for living rooms, lounges and bedsits, 22C for Bathrooms and 18C for bedrooms, halls, landings, kitchens and toilets. We have based our suggestions upon maintaining the average temperature in the house at 20C.

External Design Temperature

The external design temperature is defined by MCS as the hourly external dry bulb temperature equal to, or exceeded, for 99% of the hours in a year. This will vary depending upon where you are in the country but our calculations are based upon -3C.

Sources of heat loss

You lose heat in your building from

  • Each of the four walls
  • The ceiling and roof
  • The floor
  • The windows
  • The doors
  • The thermal bridges where the elements above intersect
  • Natural Ventilation
  • Mechanical ventilation such as extract fans and cooker hoods

In general older buildings tend to be draftier and less well insulated so lose more heat while modern buildings tend to be better insulated and less well ventilated.

Measuring Heat Loss & Energy Use

We measure heat loss in KW (Kilowatts) and energy loss in KWh or (Kilowatt hours). In pre metric days we used BTU/hr (British Thermal Units per Hour) and British Thermal Units.

Converting between KW and BTU/hr or KWh and BTU by using a factor of 3,412. 1KW is equal to 3,412 BTU/hr and 1KWh is equal to 3,412 BTU.

If we take a typical house with a 9.0KW heat loss using an internal design temperature of 20C and external design temperature of -3C then it will require a maximum of 9KW or 30,708 BTU/hr to maintain the internal temperature for 99% of the year. Typically this will equate to a yearly heat requirement of around 20,000 KWh or 68,240,000 BTU.

The above house if using a heat pump with an average COP of 3.0 would require 6,667 KWh of energy to run which at 12p per KWh equates to £800 per year.

The above house using a gas boiler at 90% efficiency would require 22,222 KWh of gas which at 4.5p per KWh equates to £1,000 per year.

Selecting your Heat Pump

The best way to check on the heat loss of your house is for an MCS approved contractor to carry out a computer modelling exercise on it but you can get a rough idea of the heat loss by taking the total floor area and multiplying it by the figures in the chart below. This gives an instantaneous heat loss and normally installers would add 20% to allow for hot water and heating the building up from cold.

 Age of the building or construction standard Heat loss per square metre of floor KW/m2
 Old building 1960s or before 0.12 KW/m2
 1970s building 0.1 KW/m2
 1980s building 0.08 KW/m2
 1990s building 0.06 KW/m2
 2000 onwards 0.05 KW/m2
2010 onwards 0.04 KW/m2

These figures are only approximate and should always be checked using the MCS method by a trained contractor.

Example 1

So for example. If you have a 1980s house with 60 m2 downstairs and 80m2 upstairs for a total of 140m2 and this has not been upgraded with modern glazing and insulation then this will need 0.8 KW/m2 and so 140 x 80 is 11.2KW. Adding 20% would give us a heat pump needing 13.4KW at design conditions.

The problem with this example is that the largest single phase heat pump available only gives 12.0 KW at design conditions. We have the option of using a three phase model at 16.0 KW or using two heat pumps, either one for upstairs and one for down or mixing the two on a thermal break. Both options are possible but the cost will be high and it is possible that the building will not meet the requirements of the RHI.

The best solution would be to improve the building structure by extra insulation and improved glazing. That would improve the building to a modern class of 0.05 KW/m2 needing only 7KW. Adding 20% gives us a heat pump providing 8.4KW

This would be a 9KW TCAP model and installing one 9KW TCAP model is much cheaper than the two 9 KW standard models needed by the unmodified building.

Or to put it in terms of energy use. Before modification the building had 11.2 KW instantaneous requirement and over a year might have needed 25,000KWh of heat. With a heat pump this would have cost £1,000 per year to run or with a gas boiler £1,250. But with the modifications the instantaneous requirement is reduced to 7.0 KW and therefore the annual requirement drops to 15,556 KWh at a cost of £622 per year. In theory the heating costs are halved.

Example 2

A modern house built in 2007 has 60 m2 downstairs and 60 m2 upstairs. This is a total of 120 m2 and at 0.05 W/m2 this needs 6KW. Adding 20% we should select a unit giving 7.2KW and this will be a 9KW standard model.

Next Step

Once you have worked out what size of unit you need for your house then click here to continue