MCS Design Compliance Calculator

MIS 3005-D requires the heat pump to provide at least 100% of the calculated design heat loss — it does not write a hard maximum into the standard. In practice installers keep the capacity close to the design heat loss because heavy oversizing causes short-cycling and a lower seasonal performance factor (SPF). This calculator uses about 125% (so a 10 kW pump for an 8 kW heat loss) as a good-practice upper bound, not a figure codified in MIS 3005-D.

MIS 3005-D requires a room-by-room heat loss calculation to BS EN 12831-1:2017 (the MCS Heat Pump Calculator is built to that standard). Whole-house estimates or rules-of-thumb are not acceptable. The calculation must account for the U-values of all building elements, ventilation and infiltration losses, and the design internal temperature for each room.

MCS 020 a) (mandatory for Permitted Development from 20 September 2025) requires the calculated heat-pump sound level not to exceed 37 dB LAeq,5min at the assessment position — 1 metre outside the centre of the nearest neighbouring habitable-room window or door, ignoring the façade effect. This is a fixed limit, not a background+5 dB(A) (BS 4142-style) calculation. The level is worked out from the manufacturer A-weighted sound power level (LWA) using LpA = LWA − 11 − 20·log10(distance), plus a directivity correction (+3/+6/+9 dB for 1/2/3 hard surfaces near the unit) and any barrier reduction (5 dB partial, 10 dB for a barrier that fully blocks the line of sight). The previous 42 dB(A) limit was superseded.

MCS does not set a graduated minimum SCOP by flow temperature. The one figure with an authority basis is SCOP 2.8: this is the Boiler Upgrade Scheme product-eligibility threshold (declared via MCS 026 / BS EN 14825), so a heat pump must reach SCOP 2.8 for the install to qualify for the £7,500 grant. The other targets shown here (for example 3.5 at 35°C flow) are indicative good-practice figures for a well-designed system at that flow temperature, not MCS requirements. Low efficiency usually points to poor design — high flow temperature, undersized emitters — which is why lower flow temperatures are designed for wherever possible.

MCS compliance is required for the Boiler Upgrade Scheme (BUS) grant of £7,500. It also ensures the system is correctly designed for efficiency, comfort, and neighbourly noise levels. Only MCS-certified installers using MCS-certified products can issue MCS certificates.