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.

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 is my heat loss indicator (HLI)?

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.

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

What should the indoor design temperature be?

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

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, 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 higher flow rates 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.