PHPP Plug-in: peak heating load

Exterior of Max Fordham House. A modern mews house covered in ivy.

We’re big fans of Passivhaus, and the energy modelling tool PHPP. We’ve developed a plugin to try and make it even more useful, and thought we’d share it.


Passivhaus offers two certification options for heating:

  • specific heating demand, in kWh/m².a, measuring the amount of heat supplied to the space over an entire year

  • peak heating load, in W/m², measuring the maximum heat power supplied to the space on a cold day

There are pros and cons to both approaches. Some of the arguments for peak load include:

  • it doesn’t incentivise passive solar design, which tends to increase the risk of overheating in summer

  • for a future renewable-based electricity grid, peak power draw may be more important than annual consumption

  • it’s less sensitive to orientation

Recently, there's been increased interest in designing to peak heating load in the UK, spurred on by work from Nick Grant and Alan Clarke. Nick presented this work at both the international and UK Passivhaus conferences, with the paper and presentation both available online.

PHPP has a graph to display the heat gains and losses for the space heating demand approach, which is a very useful design tool to help understand the design, including which heat transfer mechanisms are significant or trivial.

There's not currently an equivalent graph for peak heating load.

As we thought it would be useful, we've made a plugin for PHPP that generates the peak heating load graph, which can be copied in to a PHPP (designed for and tested on PHPP 9.6a). It follows the same conventions as the space heating demand graph, with the tweak that it splits ventilation into infiltration and mechanical ventilation, as shown in the example graph at the bottom of this page.

It's available for download here.

Please feel free to use it, and let us know if you have any feedback, either positive or negative.


Colour bar chart showing heating load for most extreme conditions.