Global Warming Effects

Any use of fossil fuel creates harmful CO2 emissions. Electricity is mostly produced by burning fuels, and the process of generation is inefficient. Electricity is therefore not a good form of energy for heating. However, the energy advantages of heat pumps can, in many cases, more than compensate for this, and makes them score well with respect to CO2 emissions.

Vast amounts of heat energy are used by heating our houses, and it is always advantageous to minimise the heat requirements by normal conservation measures like insulation, this is certainly the best and simplest first-step. Sadly, we cannot all live in super-insulated eco-houses, so some sort of heat energy input is required.

The graph compares the carbon dioxide emissions from common heating systems (gas, oil and electricity) to that of heat pump systems. The COP (efficiency) for a typical Ground Source heat pump with radiators is 3, but if well designed underfloor is used the efficiency can be 4.

Notes and assumptions.
Gas condensing boiler efficiency 90%
Oil boiler efficiency 85%
LPG condensing boiler efficiency 88%
Electricity 100% efficient at point of use
Off peak electricity efficiency 85%, (may vary from 70% in old building to 95% in highly insulated house
(Note, the efficiency for off-peak heating is less than 100% since unnecessarily high temperatures at night are inevitable. This varies from 60% in a badly insulated house to 95% in a super-insulated house. We have used 85% in this example)

As can be seen, the carbon dioxide contribution from a good heat pump system can be one third that of oil and about half that of gas. Direct electric heating, especially storage heaters, scores very badly with respect to global warming.
Wood burning is likely to show the lowest CO2 figures but this depends on how it is burnt, and where the wood is sourced from.

Some heat pumps incorporate a normal electric back-up heater to cope with the coldest periods in the winter. This is more the case with air-source systems since the period of highest heat-demand also corresponds with the time when there is minimum heat available in the air. Therefore, on the coldest day the electricity consumption for some heat pumps will increase many fold, putting a strain on the electricity supply grid. It would be better to use boilers, or ideally, wood stoves as a back-up. It should however be notes that the total annual contribution by back-up heaters is surprisingly small.

The COP (coefficient of performance) of a heat pump is the ratio of input to output.
Fuel figures taken from www.johnwilloughby.co.uk

REFRIGERANTS (the working fluid)

Gone are the days of refrigerants affecting the ozone layer. They are now all 'ozone-friendly'. However, most refrigerants do have a high global warming potential, meaning that they are far more potent than the gas - CO2.

The refrigerant fluid within the heat pump is very unlikely to leak (when did you last hear of a refrigerator leaking its refrigerant?) but should it do so, it could detract from the CO2 savings. The following information attempts to give a balanced view of the issue and concludes that the savings made during the heat pump's working life, far outweigh any negative effects caused by loss of refrigerant.

The following table lists the Global Warming Potential (GWP) of various gases. This gives a figure of how damaging the gas is, as compared to CO2.

Refrigerant	Global Warming Potential
Carbon Dioxide, CO2	1
Hydrocarbon, HC	7
HFC R407C, R410A, R134A	1,900

Inside a typical 8kW heat pump there is approximately 2kg of HFC refrigerant. This should stay intact within the unit for its whole life. It is a legal requirement to recover the refrigerant charge at the end of the heat pumps life. This can then be used in another heat pump, or destroyed.

In this worse case scenario, we have assumed that the refrigerant is lost via a leak during a 10 year period.

The first five columns show the total CO2 released to atmosphere over the 10 years of running each system. The saving in CO2 that you would make if you have a heat pump would be the difference between the heat pump column (purple), and the fuel it replaces to the left.

The common HFC refrigerants (shown in pink) have a relatively small effect compared to the vast amount of CO2 produced by using energy over a long period.
Like your domestic fridge, a heat pump is unlikely to lose its refrigerant charge over its life, so this column could prove to be very small in reality.

Graph taken from expected consumption for a year 2002 regs. house.
10,500kWh space heating, 3,600kWh. hot water.

Whilst this might show that HFC refrigerants are acceptable, it is obviously preferable to use an Hydrocarbon (HC) refrigerant if possible. If heat pumps become widespread, then benign refrigerants like hydrocarbons should ideally be adopted.

We have added wood to the graph to indicate how good logs are as a back-up for a system.

Notes and assumptions
Gas condensing boiler efficiency 88%
Oil boiler efficiency 85%
Electricity 100% efficient at point of use
Heat Pump COP 3.8 (i.e. 2.1 kW input for 8kW output).