SAP10.3 Oddities: WWHRS

We’ve had a few weeks to get under the bonnet of SAP10.3 to understand what a Future Homes Standard compliant home looks like, and we’ve uncovered a strange quirk which works against WWHRS.

To unpick what’s going on in the new SAP methodology, I met up with two of the biggest suppliers of Waste Water Heat Recovery Systems (WWHRS) in the UK: Showersave and ReCoup, at the Keystone Group offices in Derbyshire.

For context, WWHRS is a simple, tubular heat exchanger (usually vertical, but can also fit under a bath or shower tray). Cold, mains water flows across one side, and the hot wastewater from the shower flows along the opposite side. Heat energy that's normally wasted is transferred to pre-heat the incoming water for free, putting less demand on the heating system and using less energy.

A working demo of a Waste Water Heat Recovery system, including sensors that measure the water temperature through the system.

💰 Based on my own SAP data, homes with WWHRS typically see a reduction in hot water demand by 7-10%, saving £50-£80 a year (based on SAP occupancy, with an ASHP).

The WWHRS is hidden away with the rest of the plumbing, uses no electricity, has no pumps, requires no maintenance, and works consistently all year round.

So it should be a no-brainer energy-saver that would benefit all new homes and support a solution to comply with FHS.

When SAP calculates its Target Emission and Primary Energy Rates (TER and TPER), it uses a pre-loaded, notional specification which includes Waste Water Heat Recovery.

That doesn’t mean WWHRS is mandatory to pass the Future Homes Standard, but any design that excludes the devices will need to find improvements elsewhere to keep the numbers balanced ⚖️.

This is already the case with the current SAP10.2, which is why many housebuilders use gas boilers with WWHRS and PV as a preferred compliance route.

But where the SAP10.2 notional uses a high-efficiency gas boiler for heating, the new SAP10.3 notional uses an air source heat pump with WWHRS. Crucially, it’s not a very efficient ASHP.

So you don’t need to install WWHRS if you can offset its savings in another way. The problem with SAP10.3 is that most modern heat pumps operate at a much higher efficiency than the notional, providing ample energy savings to offset the need for WWHRS.

Adding WWHRS still improves the Primary Energy result - typically by 4-5 points, but will an even bigger pass on SAP be enough of an incentive for housebuilders?

Just because SAP isn't insisting on using WWHRS, that doesn’t alter the fact that they save energy, reduce bills and make your hot water setup more efficient.

This appears to be an unintended consequence of adding a low performing heat pump to the Notional, so will we find the same thing happens when we transition to the Home Energy Model? We'll be able to answer that in the coming months.

This isn’t the only oddity to come out of the new SAP10.3 – I’ll unpick more weird goings-on in future posts.

Thanks to Tony Gordon, Richard Kinloch, Ellis Maginn, Ian Steward.