Geeky #EnerPHit thread alert!

The Passivhaus annual heating demand target of 15 kWh/m2 is well known. Less well known is the alternative target of 10 W/m2 peak heat load (on the most challenging winter day in the weather file). #Passivhaus #Retrofit

1/17

This target was initially because 10 W/m2 is ~the maximum amount of heat you can deliver through the supply air without recirculation, if you want to use the ventilation system to also supply the heating. You don't have to, but some Passivhaus buildings are heated this way.

2/17

Serendipitously this target now acts very well as a proxy for buildings that, when we #ElectrifyEverything will 'tread lightly' on a future, largely renewable, electricity grid (HT @Greengaugenergy, more on this here: https://ggbec.co.uk/treading-lightly-on-the-grid).

3/17

Many practitioners (I think @PassiveHouseBB switched me on to this) feel it's a better measure of how 'future ready' a building is than the 15 kWh/m2a target. Why? Because, in a largely renewable grid, peak heat load will likely be bigger challenge than total energy demand.

4/17

A building that has an annual heat demand of 18 kWh/m2 and a peak heat load of 10 W/m2 uses 20% more heating energy than a building of the same size with an annual heat demand of 15 kWh/m2 and a peak heat load of 12 W/m2. Both are Passivhaus.

5/17

But the one with the lower peak heat load will likely be easier to heat with renewables.

Whats more, designing to peak heat load removes the incentive to oversize south facing windows (which can reduce annual heating demand but usually increases peak heat load).

6/17

This means we can size windows according to daylight, summer ventilation, aesthetics (inside and outside) and budget.

7/17

Large south facing windows are sometimes called 'free heating'. As @ecominimalnick has shown (sorry I can’t find the link), because windows are more expensive than walls, increasing south window size is actually an expensive way of providing heat.

8/17

Having modestly sized windows makes summer comfort much easier. This is already a problem with many new buildings, increasing the fabric performance of the building without good design exacerbates it. Problem will worsen as #ClimateCrisis advances. http://www.highlandpassive.com/index.php/do-low-energy-buildings-have-an-overheating-problem/

9/17

Having a lower peak heat load also, I suspect (haven't done any analysis of this), makes the building a better candidate for use as a thermal battery, slightly over heating when renewables are abundant so you can comfortably ride out most lulls. http://www.highlandpassive.com/index.php/heating-with-wind/

10/17

Halfway-through-thread summary: the 10 W/m2 peak heat load target was originally for MVHR heating but now acts as a good target for a future-proof building, even if not heated that way.

11/17

So where am I going with this? What’s it got to do with #EnerPHit?

Well, for EnerPHit there is no target for peak heat load, only for annual heat demand (25 kWh/m2 for the UK climate).

12/17

That target is really challenging, & oversizing S windows can make a big difference. For example, on my own house project (design iteration 3 pictured), I'm currently sitting at 27.9 kWh/m2a. If I max out the south window sizes I can just get below the magic 25 kWh/m2a.

13/17

. @ecominimalnick's logic about increasing the size of south windows being an expensive form of heat applies +++ to retrofit, where the cost comparison is usually larger windows v adding insulation & airtightness to more m2 of existing structure, rather more m2 of new wall.

14/17

This drop in annual heat demand comes with an increase in peak heat load (from 11.2 to 12.1 W/m2) and makes it harder, but still doable, to meet the overheating criteria.

This design could now reach EnerPHit standard, but it's objectively & subjectively a worse building.

15/17

Why? It'll be less comfortable for much of the winter ( http://www.highlandpassive.com/index.php/why-do-i-feel-chilly/), it'll be less comfortable in the summer, it'll cost a lot more to do and, imho, it would look rubbish.

16/17

If peak heat load is as important to the future performance of buildings as many of us think it is, would it not make sense to also have a peak heat load target for EnerPHit? I think so.

17/17

Postcript: There is of course the component method of EnerPHit certification, which allows a building to be certified even if above the annual heat demand threshold so long as all of the elements meet certain performance criteria, but this is onerous in other ways.