Why hot water is the energy guzzler to watch out for when designing an NZEB house

The greatest challenge we have when it comes to new builds in Ireland is how to provide hot water efficiently. Here’s what you need to know.

In this article we cover:

  • Space heating needs in a Near Zero Energy Building (NZEB)
  • Hot water needs in an NZEB house
  • How heat pumps can supply one or both
  • System sizing

New houses are requiring less and less energy for space heating, thanks to high levels of insulation and airtightness, yet our hot water needs are increasing.

Compounding this trend is that the water temperature required for space heating in a modern home (around 45dgeC flow temperature) is much lower than for hot water, which is around 60degC. This means you need more energy to produce hot water than space heating.

For an NZEB house where the space heating load might be just 4 kW, all you’re likely to need in theory are two high efficiency electric panel radiators. But then the question is, how efficiently can you produce hot water?

Currently most heat pumps will supply hot water to an efficiency of about 160 per cent, so for one unit of electricity you’re getting 1.6 units of hot water. This seemingly low efficiency is due to integrating the hot water function with space heating.

As soon as you start combining functions into a single unit, such as heating with hot water, or even systems that integrate heating, ventilation and hot water, there are engineering compromises. Each function can be individually maximised, but when combined, efficiencies will fall for one or some of the functions.

I think it’s better to let an appliance do one job, and do it well. Heat pumps that only supply hot water can reach efficiencies as high as 343 per cent. In terms of heat pump technology, air to water heat pumps dominate the selfbuild market because they’re a one box solution and are suited to our mild Irish weather patterns. Ground source heat pumps are losing market share because of the capital cost of ground works and the combined heat pump products, that include ventilation, tend to be installed in very low energy homes.

To qualify that remark, each design is unique – if you have a cube for a house versus a sprawling design of the same surface area, you will require more energy for the sprawling house.

This is why it’s vital for self-builders to understand that heat pumps are not an off-the-shelf product; they must be designed for the building, sized accordingly, as soon as the design is finalised including specification for insulation and airtightness. 

As a manufacturer, we would do a first principle heat loss calculation, at -2degC outside and +21degC inside, to help with system sizing and we also check the installation (commissioning) where any of our products are installed.

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