Heat Pump Size Calculator Ireland

Use our heat pump size calculator for a quick estimate of the required heat pump capacity for your home. However, you should not choose a heat pump size based on this calculator alone. More detailed calculations must be done by a qualified installer before fitting, as accurate heat pump sizing is very important.

How to calculate heat pump size

You can use the heat pump size calculator above to quickly estimate the heat pump size for your home. The calculator is quick and convenient – great for initial estimates. But it’s not accurate enough to be used for the final design of a heat pump system.

The final design of a heat pump is much more complicated. That’s why you should always enlist the help of a qualified Technical Advisor as a first step towards your home energy upgrade. Call us on 0749710825 to learn how we can help.

What does heat pump size/capacity mean?

Heat pump capacity refers to the rate at which a heat pump can output heat. Typically, this is advertised in the product name. But be careful! A heat pump’s capacity depends on the specific temperature conditions that it’s used in. So check the datasheet for the capacity of the heat pump under the design conditions that you’ll be using. This may be different to the nameplate capacity.

What is my heat loss indicator (HLI)?

Your heat loss indicator measures the rate of heat loss from your house. This number will be high for a poorly-insulated house, and low for a well-insulated house. It’s calculated as part of the Building Energy Rating (BER) process, and also during a heat pump Technical Assessment.

Date Home BuiltHeat Loss Indicator (HLI)
Pre-2005Over 2
2005-2011~ 2
Post-2011Under 2
Heat loss indicators typical for Irish houses built during selected time periods

It’s generally not recommend to install a heat pump with a HLI above 2. Indeed, you can only claim heat pump grants for homes with a HLI of 2 or lower. So most Irish homes will need insulation upgrades before they can get a heat pump.

Why use an outdoor design temperature of -3 °C ?

Temperatures in Ireland occasionally drop well below -3 °C. So you might worry that a heat pump won’t keep your home warm on particularly cold nights. However, this is not the case. Heat pumps have an electric boost heater will kick in once the temperature drops below -3.

In Ireland, -3 °C is low enough that the boost heater will run only rarely. This saves energy, as the boost heater is more energy-hungry than the heat pump. On the other hand, using a lower design temperature would result in an oversized heat pump. So -3 °C is a sensible compromise which reduces the use of the boost heather without oversizing the heat pump.

House warmed by a heat pump on a snowy day
Below -3 °C, a heat pump will keep your home warm with the help of its electric boost heater

What should the indoor design temperature be?

The SEAI recommend different values depending on the type of room, as shown below:

Room TypeRecommended Design Temperature
Living areas i.e. – sitting room, dining room, play room18-21°C
Other areas i.e. – kitchen, hall, toilet, bedrooms16-18°C
Bathrooms (with shower)22°C

As part of a Technical Assessment, our Technical Advisor will make a detailed calculation of the heat requirement on a room-by-room basis. However, for a quick full-home estimate, we recommend something in the range of 18-21°C. The exact number depends on your personal preferred room temperature.

What happens if a heat pump is undersized?

There are two main problems with an undersized heat pump. First, an undersized heat pump will have to run its defrost cycle too frequently. This increases electricity consumption. Second, an undersized heat pump will have to fall back on the boost heater too often. This likewise increases electricity consumption and cost. Moreover, an undersized heat pump will wear more quickly than a correctly-sized heat pump.

What happens is a heat pump is oversized?

An oversized heat pump will cycle frequently. This increases both energy consumption and wear and tear. Cycling can be reduced somewhat with a large buffer tank. Variable-speed heat pumps also minimize the cycling problem. But even then, the heat pump will run very frequently at part-load, reducing efficiency.

Heat pump radiator under-sizing and oversizing

Under-sizing heat pump radiators is a bad idea. This is because a small radiator has to be hotter for the same heat output as a large radiator. In the best case scenario, as heat pump with undersized radiators will be more expensive to run. In the worst case, it won’t be able to keep your home warm.

On the other hand, oversizing heat pump radiators is generally harmless, or even beneficial. Bigger radiators give more heat at low temperatures. And these lower temperatures are great for heat pump efficiency. Just make sure the plumbing is adequate – heat pumps generally work with much larger diameter piping than oil or gas boilers.

Heat pump sizing table for houses in Ireland

The below table gives a general indication of the heat pump size for homes in Ireland. It assumes that Pre-2005 houses have retrofitted insulation to give them a heat loss indicator around 2.2. Meanwhile, it assumes that 2005-2011 houses have a heat loss indicator of 2, and post-2011 houses have a heat loss indicator of 1.5.

Built Pre-2005 (retrofitted)Built 2005-2011Post-2011
Small (~50 m2 / 1-bed)2.6 kW2.4 kW1.8 kW
Medium (~110 m2 / 3-bed)5.8 kW5.3 kW4 kW
Large (~180 m2 / 5-bed)9.5 kW8.6 kW6.5 kW
X-Large (~230 m2 / Large 5-bed)12.1 kW11 kW8.3 kW
Heat pump capacity table for houses in Ireland

Ready for A Warmer Home?

A Home Energy Assessment or Technical Assessment is the first step towards your home energy upgrades. Here’s why:

  • Independent advice from an SEAI-registered and qualified professional
  • Qualifies you to apply for SEAI grants, including the €6,500 heat pump installation grant
  • Grants up to €350 towards the assessment cost

The founder of Ecoplus Survyeors Limited, Dominic has worked with sustainable energy since 2019. He writes about energy-related topics, including heat pumps and solar PV. In his spare time, Dominic enjoys playing traditional Irish music and riding mountain bikes.