SAP10.3 Oddities: Exhaust Air Heat Pumps

To comply with the Future Homes Standard, most new homes will require a heat pump.

I expect the majority will choose air source, some ground source, and inner city sites could opt for ambient loops.

Apartment complexes might look at Exhaust Air Heat Pumps, but it seems the new SAP10.3 calculator isn’t so keen on those.

🐇 I've been down a SAP rabbithole to find out why.

Advanced ventilation systems can also provide heating and hot water to dwellings. SAP10.3 isn't great at modelling multi-purpose systems.

Exhaust Air Heat Pumps provide a single solution for ventilation, heating, cooling and hot water. They are usually designed to work with warm air emitters (similar to air conditioning), but can also be paired with radiators.

As with most heat pumps, you’ll need a white metal box of wizardry in a cupboard, but you don’t need the external fan or boreholes. Instead, a network of ducting runs through the dwelling to connect the various fans and emitters, with two main intake and exhaust vents usually poking through the roof.

Just like other breeds of heat pump, they produce more heat than the electricity they use, making them a high-efficiency fossil-fuel-free solution for both new homes and renovation projects.

But, modelling EAHP systems in SAP10.3 to meet the new Part L targets isn’t as simple as it should be.

It’s no secret that SAP struggles with certain technologies. It’s happy with a gas boiler, it just about copes with ASHP, but EAHP pushes SAP to its limits.

This isn’t a specific issue with the latest 10.3; it’s also a problem with the current 10.2. But these new stricter targets have pushed this quirk of SAP into the centre-stage.

SAP understands that EAHP provide high efficiency heating, so that part performs well.

SAP maybe doesn’t calculate the hot water efficiency as generously as it could, but that’s not usually a problem.

What is a problem is how SAP treats the ventilation part. The Notional (which is used to calculate the Target Emission and Primary Energy Rates), is based on a decentralised MEV ventilation system. This means the energy demand for pumps and fans in the notional is very low.

I won’t bore you with the details of SAP Tables 4f, 4g and 4h (even I find these pages dull!), but you can look them up to see how SAP assumes the energy demand on whole house ventilation systems, and how it applies ‘in-use’ factors (another way of saying ‘assumptions’) which lower the claimed efficiency and makes results worse.

This means the combined energy demand of an EAHP system is often higher than the notional, which can cause the Primary Energy to fail.

EAHP systems with integrated heat recovery (MVHR) are more likely to comply with SAP10.3 without issue.

When SAP assessors try to move away from an ASHP in 10.3, things get difficult quickly. We know the Home Energy Model will be far more accurate when dealing with EAHP, as well as other ‘emerging’ alternatives, such as thermal batteries and smart cylinders.

Whether ‘accuracy’ translates to ‘easier to pass’ remains to be seen.