Heat Pump Size Calculator for Ireland

NB: The heat pump power at the required design temperature (generally -3 Celcius in Ireland) may be different to the nameplate capacity. Check the datasheet for the power at design conditions.

How to Calculate Heat Pump Size

You can use the heat pump size calculator for a quick initial estiamte of the heat pump size for your home. NB: It's *not* accurate enough to be used for the final design of a heat pump system.

Make sure to get a qualified installer or designer to do the final design for your heat pump system.

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 should the indoor design temperature be?

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

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.

Under-Sized and Over-Sized Heat Pumps

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 if 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 over-sizing

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, a 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.

What Is My Heat Loss Indicator (HLI)?

In simple terms, a Heat Loss Indicator (HLI) tells you how quickly heat escapes from a building. Lower is better. Here’s what you need to know about it.

Your heat loss indicator is an estimate 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.

What determines your heat loss indicator?

• Insulation: The better the insulation, the lower the heat loss
• Air tightness: Airtight homes retain heat better than draughty ones
• Heat recovery ventilation: This recovers waste heat from stale air as it leaves your home, resulting in a better HLI
• Shape of the home: A compact, cube-shaped home retains heat better than complicated layouts with more wall area
• Sheltered sides: Mid-terrace houses and apartments retain heat better because several of their sides are pressed against other heated spaces, rather than exposed to the outside

How to find your heat loss indicator

Provided your home has an up-to-date BER Assessment, here are the steps to find your HLI:

1. Go to the SEAI BER Database
2. Enter your electricity meter number and click search
3. Download the Advisory Report
4. Check the first row of the recommended upgrades table on page 3
5. This will show you both your current HLI, and the HLI you could achieve with the recommended insulation upgrades

If your home doesn’t have an up-to-date BER (last 10 years), then you won’t be able to find your Advisory Report on the SEAI database. In this case, you’ll need to get a BER Assessment done. Alternatively, if you have lots of spare time, you can do an unofficial BER Assessment yourself for free, using DEAP software.

If you’re serious about getting a heat pump, then it may be worth getting a Technical Assessment, which will determine your HLI, and more.

Heat Loss Indicator formula

The heat loss indicator is the estimated heat loss per square metre of living space in your home, per °C of temperature difference between outside and inside (W / m²·°C).

For example, if it’s 20 °C inside, 5 °C outside, and your 100 m² house is calculated to lose 2,800 Watts of heat, then your HLI would be:

HLI = 2,800 ÷ (100 × (20 − 5)) = 1.87 W/m²·°C

HLI and heating demand

You can get a quick estimate of the annual heat requirement (in kWh) using the formula below:

Annual heat (kWh) = HLI × floor area (m²) × heating degree days × 24 ÷ 1,000

For example, a house in Ireland might have a HLI of 2.3, a floor area of 112 m², and 2,196 heating degree days in an average year. In this case, the annual heat requirement would be:

2.3 × 112 × 2,196 × 24 ÷ 1,000 = ≈ 13,600 kWh per year

Estimating HLI based on building age

HLI accuracy

Heat Loss Indicators are calculated as part of the Building Energy Rating (BER) process. The BER process is useful, but far from perfect. Here are some sources of error:

• Human error on the part of the BER Assessor
• Use of estimates, rather than actual measured data, for U-values
• Thermal bridges are generally estimated, not calculated
• The assumption of proper construction methods is not always accurate
• Evidence of construction details is often not available