Basically, this is a function to incorporate improved spf and heat exchanger efficiencies for hrv systems that perform better than the SAP default of 2w/l and 65% recovery.

I was looking at this for a project and for example the using the appendix q spreadsheet and the data for the greenwood airvac mvhr 90, with a spf of 0.77 and recovery of 80%, the reductions to the predicted electrical use are approximately 50%.

option one, was install the HRV and accept the extra cost for this and the extra PV required to offset the electricity use, or two don’t bother and accept Code Level 5 as Level 6 is a mythical crock. Certainly from what I have seen, the stamp duty exemption doesnt seem to be enough to cover the costs to meet the treasury and/or Code requirements for true zero carbon.

What was interesting was that for these dwellings, 5no. terrraced units, (3 mid, 2 end of..) with U-values throughout of 0.12-0.14 and glazing at 0.7-0.8, it was still necessary to include MVHR in the SAP to acheive the HLP requirements. But of course, this pushes the DER up, due to increased electricity consumption. This was in turn made even worse by the fact that the heat source being offset by the HRV was biomass.

So all in all, it didn’t make financial or even practical sense. It was Hobsons choice to achieve Code Level 6: Include HRV to achieve the HLP