“Can we open the window yet?” : monitoring and managing home temperatures during a heat emergency

The UK experienced its hottest temperatures on record on Monday and Tuesday last week (18 and 19 July 2022), exceeding 40°C for the first time in several parts of England.

This seemed far-fetched even a couple weeks ago – but here we are, and here we appear to be, until we take radical steps to reduce the amount of CO2 and methane in the atmosphere. In the meantime, we can continue to expect our summers to get dangerously hot, more frequently and for longer periods of time.

Our house was built in 1956. Over the last decade, we’ve converted the attic to bedrooms and a small bathroom, extended the ground floor, fitted double glazing and installed cavity wall insulation. This all makes the house more energy-efficient by preventing heat loss through the walls, window and roof when it’s cold.

This is great in the winter, but it’s not helpful in a heat crisis – the thick brick walls absorb the sun’s energy during the day, and hold onto it in the evening. And my home office is in one of the top floor attic rooms. I do most of my work from home so I need to be able to carry on working when it’s hot – and it gets incredibly hot in the attic.

So faced with two kids (school closed due to high temperatures) and me at home, I wanted to keep temperatures inside as cool as possible, and use the technology available to me to help.

We followed three principles during the heat crisis:

  • Close the windows and doors until it’s cooler outside than inside (otherwise we’re just warming the house further)
  • Close all of the blinds to limit the amount of sunlight entering the house (the greenhouse effect)
  • Use a portable aircon to keep the upstairs study habitable, only for the periods that the room was in use.

Enter Home Assistant.

Home Assistant (HA) is a mature open-source project to monitor and control a wide variety of internet-connected devices at home. Amongst other things, I have our central heating system, lighting and electric car integrated into the platform. The central heating system is switched off for the summer, but all of the radiator valves contain thermometers, meaning that a room-by-room reading of temperature is possible. Using an outside temperature feed from a weather service, it was straightforward to create this dashboard.

This snapshot was taken late afternoon, and already provides helpful information:

  • The ground and first floor rooms (Living Room and Danny’s Room respectively) stayed bearable throughout the worst of the heat, by keeping sunlight and hot air out. Fans and ice pops helped us stay comfortable.
  • The top floor rooms (Upstairs Study) temperature started to climb more steeply. The aircon unit was operating between 11:00 and 16:30 which kept the room just about usable, although between around 13:00 and 16:30 it couldn’t really cope and the temperature continued to drift upwards.
  • Once aircon was switched off, and the room abandoned to its fate, the temperature shot up quickly, hitting 30°C after around 90 minutes.

So, closing the windows and blinds, and selective aircon worked. But the outside temperatures stayed very high until the late evening. So when to open windows? Again the Home Assistant graph helped.

We’re looking for the point where outside temperatures are less than inside temperatures. On Monday, for the top floor (A), this was 21:30; for the first floor (B) it was 00:30 and for the ground floor (C) this was 02:30. Temperatures outside started to shoot up at dawn, so everything needed to close again at 06:30.

Home Assistant can also be used to send alerts, flash lights, switch on the aircon and other automated actions but for now, I’m using this dashboard manually. And it helps to allow the kids to access it themselves, instead of continually asking when the windows can be opened.

Air conditioning

I knew I’d have to remove heat from the Upstairs Study to keep it habitable. Last summer I bought a portable aircon unit, with an exhaust hose and a window seal kit so the waste heat could be vented through the velux window. This summer I added some reflective foil to the outside of the window to bounce more of the sun’s energy back into the sky.

None of this is pretty. It’s also noisy, and we’re pulling power from the grid at a carbon-intensive time of day. It’s also expensive to run.

But, it did work. I had a zoom call in the room between 14:00 and 16:00, and by directing the output of the aircon straight at me, it was bearable. Even as temperatures outside soared towards 39°C, the aircon brought the temperature down from 30°C to 27°C inside. Electricity costs on my current tariff were around £0.40 for two hours of cooling. The noise in a small room was significant – a decent mic and noise-cancelling headphones were both necessary – and not cheap.

Lessons Learned

We’ve dangerously destabilised our climate, and even if we take extreme steps to decarbonise now, days like Monday and Tuesday are going to be more frequent and more intense – I can’t see this reversing in my lifetime. So we need to adapt. British houses, with their chunky brick walls and dark roofs and poor internal airflow are going be harder and harder to keep cool.

Home Assistant can help us to monitor temperatures real-time, so we can learn how to manage the heat more effectively. But this is a long way away from being financially, or technically accessible to most people.

Bill of Materials

There are cheaper alternatives to using the central heating system to measure temperature, but costs for a 3-radiator system are shown here as that’s what I had already.

Home Assistantfree
Raspberry Pi hardware for Home Assistant (limited availability due to global chip shortage)£100
Honeywell Evohome central heating system (3 radiator valves)£380
Cost to monitor house temperatures£480
NETTA Portable Aircon£370
Window seal kit£18
Radiator foil (for outside of velux windows)£19
Noise cancelling headphones£210
Directional USB microphone£100
Cost to keep the upstairs study habitable£717

Not cheap at all.


3 responses to ““Can we open the window yet?” : monitoring and managing home temperatures during a heat emergency”

  1. Sarah Harbour avatar
    Sarah Harbour

    Bit more technological than our set up. We sacrifice the top floor in weather like this (one bedroom which is usually the guest room/training room and shower room). We actually leave the Veluxes open as without that those rooms would actually get hotter than outside and the bit of airflow up through the house helps a bit! We have an outside thermometer in our back garden and one in the living room (onto the back garden) and judge window openness from that. We’re pretty much screwed over though by direct sunshine on the back garden which is a patio, and the block paved carpark out the back of the house, which is surrounded on all 4 sides… Back garden hit 39.9 despite a large parasol blocking out the sun from the back of the house (and 1/3 of the garden) and the honeysuckle that we’ve grown up the back of the house to try and keep the brickwork from heating up.
    Still, pretty pleased to have kept the living room at about 24 on the first day of the heat with windows and blackout linings in the curtains – got caught out by the temperature not dropping quickly enough to ventilate overnight when we needed to sleep rather than stay up to make sure no-one walked in through open doors and windows…

    I will disagree with you about the insulation causing a problem – it keeps heat out in summer as well as in during the winter – an extreme example of this would be a church with thick walls that are insulated by their very nature! The main problem we have is the brickwork/patio itself heating up due to the direct sun – that radiates heat out of the back of the house when the sun goes down and keeps the air temperature out of the back of the house hotter than out of the front!

    1. Had I not needed to, I would have have sacrificed our top floor too – and same with the velux. Good work to have kept your downstairs to 24C, given all of that hard standing outside. We’re lucky that we can vent the ground floor through velux in our kitchen ceiling, means that things can be left open overnight.

      And thanks for calling me out on the insulation point. I was conflating the big chunky brick walls (which do heat up given that level of solar radiation and then store the energy) with insulated walls – but you’re right, the insulation helps. I’ve re-worded the post to reflect that.

  2. Rannasha avatar

    You can save significantly on the cost of this setup if you don’t need the smart radiator valves. Simple temperature measurements that can integrate into Home Assistant can be had for less than 10 Dollar/Euro/Pound with a tiny bit of DIY. More convenient options are available between 10 and 25 of your favorite currency-unit.

    To measure and report temperatures, fetch an ESP8266 microcontroller board with wifi as well as a DHT22 temperature/humidity sensor from Aliexpress or a similar website. Install the ESPHome addon in Home Assistant (the addon is developed by the HA developers), which allows you to quickly configure the ESP8266 board for the temperature sensor. You power the thing with a micro-USB charger, of which most people have plenty (and they’re cheap otherwise). It’s not pretty (although you can 3D-print a case for the thing) and requires a bit of tinkering, but it’s cheap.

    Finally, there are also ready-to-go temperature sensors, such as the one from Shelly (uses wifi, no hub needed) or Sonoff (uses Zigbee, hub or coordinator needed) as well as a bunch of other brands, that can integrate easily into HA.

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