Size Matters (When It Comes to Heat Pumps)

Sometimes it is said that heat pumps only work with underfloor heating systems.

Sometimes it is said that heat pumps only work with underfloor heating systems.

This is not true.

Heat pumps work very well with any emitter system provided the system is correctly designed.

What is true is that a heat pump will be at its most efficient when the temperature of the water it heats is kept as low as possible.

Heat pumps are considerably more efficient than other conventional central heating systems.

Underfloor heating systems generally operate at lower temperatures hence they are often paired and produce high levels of efficiency. But, the UFH system also needs to be designed correctly.

Lets talk about boilers

Boilers have traditionally come in a very small range of sizes — It has been the habit of the plumbing/ heating industry for many years to use boilers which have a heat output of 20kW+ on even the smallest of properties as they have been sized to provide instant domestic hot water.

Table 1: https://www.boilerguide.co.uk/articles/what-size-boiler-needed

Unfortunately, in the UK we’ve also been subjected to drafty homes.

So what does that mean?

The shocking fact is that most of the UK’s near-30m homes have whopping great big (carbon-intense) boilers chugging away, heating up the air that is flowing out of our homes.

We are quite literally throwing money out the window.

But worse than that, our homes account for around 15% of the UK’s greenhouse gas (GHG) emissions through their use of oil and gas for heating and hot water.

Heat Pumps

Conventional boilers and emitter systems operate at around 70–80°C, heat pumps however have a flow temperature (the temperature water flows around your emitter system) of around 35–50°C.

This has led to a somewhat misleading term of heat pump radiators being called ‘oversized’, when in actual fact they’re accurately sized as part of the survey process.

It’s worth highlighting at this stage the following;

A) The 2021/22 Part L uplift and 2025 Future Homes Standards will set targets to ensure there are significant fabric improvements (insulation levels) to new build homes

This will decrease the energy (heat) demand for that property and energy efficiency will be a primary focus of this.

B) When moving away from gas boilers (2025 — see link to FHS above) and looking at other low-carbon heating solutions such as heat pumps, a full design calculation should be done of the property to determine its true heat loss.

A heat loss calculation generally looks at the following information; property location (for weather data), U and G-values, property size and occupancy (quantity of bedrooms and bathrooms), internal design temperatures, air change rates (for ventilation), emitter type and flow temperatures.

As you can see, a lot more information is needed when sizing a heat pump versus when selecting a boiler (ref: Table 1). The average (domestic) air-source heat pump available in the UK ranges from 3 to 12kW — this is a truer reflection of a domestic property heat demand. Note; when considering a heat pump system you must also account for the requirement of a domestic hot water cylinder (tank) as heat pumps heat stored water as opposed to producing instantaneous hot water. For further reading on this CIBSE have a wealth of information available.

Embodied Carbon

This section deserves its own dedicated article which I will do in a follow up piece.

What is embodied carbon and how is it estimated?

Embodied carbon means all the CO2 emitted in producing materials. It’s estimated from the energy used to extract and transport raw materials as well as emissions from manufacturing processes.

The embodied carbon of a building can include all the emissions from the construction materials, the building process, all the fixtures and fittings inside as well as from deconstructing and disposing of it at the end of it’s lifetime.

So why mention it now?

For two reasons;

1. Moving away from gas boilers and carbon-intense heating systems is an absolute MUST. We are in a climate crisis and we no longer have the luxury of feigning ignorance.
   Moving to solutions such as heat pumps is great — but in itself it is not enough. We must understand the whole life carbon, not just the operational carbon of these products.
   The embodied carbon of a heat pump should show us a significant saving from a gas boiler — but to achieve net zero* we need to measure and take account of all data.
2. Changing heating systems is exactly that. In most cases we will be changing the whole system, not just swapping a boiler for a heat pump — this means the calculations for the embodied carbon must reflect all of the changes and take into account any additional materials. The added level of responsibility this will create will thereby ensure that we minimise waste throughout all aspects of the supply chain — ideally, we would fully transform our model to a circular approach**.

CIBSE have produced a technical document (under peer review) and a webinar with more detail on this: https://cibse.org/knowledge/knowledge-items/detail?id=a0q3Y00000K8ePVQAZ

*What does net zero mean and is it enough (?) will be another separate discussion in a future article.
**Climate Change and the role that the built environment has within that is a very complex, multi-faceted topic which could never be explored in enough detail — but the (one?) upside is that it gives me plenty to think and write about.

In summary, previously the whole heating system has been ‘over sized’ whilst our properties have been poorly insulated — our focus now must be on energy efficiency (fabric first) and the correct sizing of our systems as we move to decarbonise heating across the UK.