Stress test your design for Overheating

I hate comfort complaints. Not the complaining bit – folks are nice – but I hate the failed expectations of comfort. This past summer I had several calls from clients living in certified Passive Houses. They say it’s too damn hot. What is going on here?

The Passive House Planning Package (PHPP) appears to predict the overheating pretty well. When I’ve gone back and tweaked the PHPP with the actual occupancy and how they’re operating the building. The overheating percentage seems to match well enough with what they’re experiencing. So that’s good, we can count on our tool.

Certification doesn’t guarantee summer comfort. I mean, it can’t. If certification allows any overheating and doesn’t require active cooling then by definition there will be times of dis-comfort. Additionally the certification criterion permits the flexibility of using night or additional windows ventilation and operable shading. But, if the occupants keep all the windows closed because they like it nice and quiet, or don’t close the blinds because they like the view, the building will overheat even more than expected. In order to allow the flexibility for the occupant who lives in the countryside to have the windows open or shading deployed the overheating percentage is dependent on occupant behaviour.

How do we know we’re doing the right thing? We should stress test our design in a standard way so we will know how bad it can get with a misbehaving occupant. You modify your PHPP file so that there’s no additional ventilation, (i.e. no night ventilation & no additional window ventilation), just the MVHR on typical settings with summer bypass if fitted. Then you set the minimum movable shading (i.e. delete those blinds). And then if the actual expected occupancy is higher than the certification occupancy, change it to what the actual expected occupancy is. Now, if you’ve got a building that’s less than 5% overheating at the stress test settings, then this building is ‘easy to keep cool’ even with a misbehaving occupant. It doesn’t mean it won’t ever overheat but a small amount of additional ventilation or shading would handle it. On the other hand, if it’s over 5% overheating, it is ‘likely to overheat’.

Now, you could argue and it is permitted by the certification criteria to consider this extra ventilation, or the client is going to operate this movable external shading in order to achieve the summer time thermal comfort but they also might not do those things. When I was in Munich at the Passive House International conference (April 2018), there were at least five presentations discussing overheating issues. One presentation by Alan Clark, on which this stress test is based; Two presentations on occupant survey and monitoring data, with people complaining with measured 1% overheating, and not complaining at 8%; Plus issues mentioned in Australia and in NYC. I’ve personally had overheating complaints in Queenstown, in Christchurch, both climates which do really well with night cooling.

In my presentation with Clare Parry at the South Pacific Passive House Conference 2017 in Christchurch one of my slides was titled ‘Don’t cook your clients’. I wasn’t joking; Well not much anyway. Your client has no idea how uncomfortable and for how long 10% overheating really is. It’s good practice to put in an active cooling systems. Heat pumps or A/C use very little energy to drop the temperature from 26°C to 22°C and provide heaps of increase in thermal comfort. Don’t make your clients hate you, if you can’t pass the stress test, put in cooling systems.

So in summary, stress test the building. To perform the stress test, you remove any additional ventilation, you set the operable shading to minimum and if the expected occupancy is higher than the certification occupancy, put the expected occupancy in. Then if you’re below 5% overheating you’ve got a building that’s easy to keep cool. If it’s over 5% overheating, and almost all buildings are, than it is likely to overheat. Put in a heat pump for cooling or at the absolute least put in the wire/ducting to add one easily later.

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