Over the course of the last two years the Tipperary Energy Agency (a non-profit organisation) has been retrofitting houses with air source heat pumps (ASHP) with great success under the Superhomes programme funded by SEAI’s better energy programme. It has been supporting homeowners and housing agencies retrofitting homes since 2004, and we had come to the conclusion on reading the data from the EU SERVE project (retrofitted 350 homes from d2 to b3) that the retrofit was not deep enough and the homes still relied on too much fossil fuel to reduce costs sufficiently for householders or allow Ireland reach our climate targets.
Why heat pumps? — cost, comfort and the environment
However, based on the heat road map for Europe that shows heat pumps are a core technology for decarbonising heat,and considering that 20% of Swedish homes are heated by heat pumps, it is clear that heat pump technology works, even in cold climates. This view is endorsed by many industry experts. So, why should someone install an air source heat pump to heat their home, and what are the key considerations? There are three reasons – cost, comfort and the environment — writes Paul Kenny, Chief Executive Officer, Tipperary Energy Agency.
First of all, I’d like to dispel some myths:
• Heat pumps (the majority of the Irish market players use R410a) work down to minus 20ºC;
• Ireland isn’t that cold, with average winter temperatures of 7ºC and the mean daily minimum above 2ºC all year round;
• Heat pumps work really well at 7ºC air temperature and 35ºC flow temperature (typically COP of 4.5 in the lab, and over 4 in real world applications);
• There is no need for a back-up immersion or boiler. We do generally ensure a high-efficiency stove is installed in our retrofitted buildings, but we find most people don’t use them with cheap even heat from the heat pump;
• Radiators are not radiators, they are really convectors, and they put out heat at all temperatures above the room temperature they are located in. So, if the boiler used to run for six hours and now runs for 24 hours, the flow temperature versus room temperature can come down by 75%, eg 60ºC to 30ºC (room at 20ºC);
• Heat pumps can heat water to 55ºC, and a top-up heating cycle using an existing immersion heater for legionella control uses a few kWh per annum when required.
The methodology employed by Superhomes is to design and install ASHPs into radiators that are oversized in comparison to typical radiators, i.e. low-temperature radiators. This allows a higher heat output at lower flow and return temperature. The design of the emitters allows the heat pump run at about 31ºC, 27ºC return at 7ºC external temperature. The heat pumps are commissioned to be “always on”, thereby maintaining a steady indoor temperature at the desired set point.
Therefore, the heat pump only needs to replace the energy that is lost from the building fabric – typically 2-3 kW at 7ºC. The resultant impact on the heat pump is that the required output per radiator is generally only 150-300w and minimises the flow temperature (maximising efficiency), resulting in typical heating (not hot water) performance of between 3.3 and 3.6 average co-efficient of performance throughout the heating season.
Using an average delivered energy cost of 11c/kWh (40% night and 60% day rate, bonkers.ie 14/01/17), this delivers heat at a little over 3.1c/kWh. Compare this to natural gas (86% efficiency and standing charge €92 split of 15MWh) of 6.4c/kWh, and oil (59c/l) at 9.2c/kWh delivered into the house. The ongoing heat cost is one third of oil and half that of gas. For those knowledgeable in energy price predictions, the likelihood of oil and gas rising versus electricity is likely to continue.
Hot water heating cycles typically rise from 30ºC flow temperature to 58-60ºC flow temperature and do have a lower co-efficient of performance than heating, typically about 2.4-2.6 over a season. This, usually completed at night for the bulk of heating (80% night (6.6c), 20% day (14c)) results in a net heat cost of 3.25c/kWh, similar to heating, and similar margins below the alternate fossil fuels.
In conjunction with the installation of an air source heat pump, and steady interior temperatures, air leakage must be reduced, ideally to an air change rate of 3-5 air changes per hour under 50 pascals of pressure, corresponding to an average rate of 0.15-0.25 air changes from infiltration in typical conditions.
Once this is achieved a designed ventilation system must be used. In the case of Superhomes, demand control ventilation is employed. This designed mechanical extract system ensures a steady, low and controlled flow of fresh air into the dwelling.
The impact of this commissioning to maintain a constant temperature in the dwelling has a number of “symptoms”. Steady air temperatures encourage walls to rise to a more even higher temperature, thereby lowering the radiative heat loss from people to surrounding surfaces and adding to the feeling of comfort. This also increases the interior temperature at thermal bridges, thereby increasing the dew point of condensation, and lowering the likelihood of condensation, mould and ill health. Coupled with the ventilation system, almost all the surveyed participants in Superhomes report that they have noticed a significant reduction in condensation.
Finally, the carbon performance of homes utilising heat pumps versus oil and gas should be understood in the context of steadiliy-decreasing carbon content of electricity. It is currently 467g CO2/ kWh of electricity, 205 for natural gas, 257 for kerosene, 229 for LPG. Forecasting this to 2030, it is, in the absence of peat and coal thermal plants and with increasing renewable electricity, likely to be below 300g/kWh CO2. Utilising an average heating and hot water COP of 3.2 (this is being achieved on an annual basis in Superhomes houses) we can see that the carbon per net kWh of heat from a heat pump will be 145 in 2015 and 90g/ kWh in 2030, versus natural gas (86% efficient boiler) at 238, and 266 and 299 for LPG and kerosene heating oil respectively. So, this equates to a 39% and 58% cut today per net kWh and a 60-70% cut by 2030.
Without getting too technical, this also puts the carbon emissions of the individual houses into the European emissions trading scheme, which moves them from the state’s carbon balance sheet and also, in theory, in a cap and trade marketplace, pushes out higher polluting carbon-intensive electricity sources.
In a new build situation, the marginal cost of installing a heat pump, appropriate cylinder and potentially larger radiators versus gas + connection or oil + tank is likely to be similar in cost to that of the photovoltaics required with the gas or oil boiler for compliance with Part L of the Building Regulations. The savings will ensure that even outside of compliance, investment will be returned in the first three to five years at worst.
The insulation or buffering from energy price increases is also worth some peace of mind. In terms of retrofit, the economic case is slightly less generous. The catch is that the cost of a retrofit of this nature – including the airtightness measures, the ventilation system and the heat pump – is unlikely to be less than €15,000. Over the next 20 years this is about €3.75 per heating day, gobbling up about 50% -70% of the savings. If we take energy price inflation into account, using the last 15 years as an indication of the next 15, this is likely to break even in 10 to 12 years. A 35% SEAI grant, available within the Superhomes programme, will bring this to seven to ten years.
So, economically home-owners will not win or lose in the short-term, but environmentally and from a comfort point of view, they will be significantly better off, as will their children going forward.