Temperature inside CONA homes

The main reason we asked for some volunteers to accept temperature sensors in the home was to track when people were using their woodburners and how that might change from day to day. But the data from these sensors reveal a whole lot more information if you care to look and we’re just starting to look now. The sensors themselves were thermocouples provided on loan by BRANZ the building research institute. They consist of a probe which was attached to the top of the firebox and a logger placed nearby.

So far I have looked through data from 9 of the 14 homes we surveyed. In a moment, a taste of some of the things we can work out from the indoor temperature data. But first, a quick look at the outdoor temperatures.

Below is a 'box plot'. All of the data from every days is combined and sorted into hours. The middle 50 % of all data lies within the box (the short horizontal line is the average). The top and bottom 25% are represented by the dashed lines. This chart shows that, on average, overnight temperatures were around 4 to 5 degrees but actually ranged from -3 to 12. Temperatures peaked at 2pm at 11 degrees on average. 

Now back to the homes. One of the first things we look for was whether there were days when the woodburner was not used. This is very helpful as it gives us something to compare with. All but one of the 9 homes had at least one no-burn day.

Without woodburning it is clear that the home still warms up in the day and cools at night. In the chart below I have combined all of the data from all of our houses for the whole campaign together.

Comparing this chart with the previous one you can immediately see that they look different. In brief temperatures are higher indoors than outdoors at night and peak slightly later in the day (4pm rather than 2pm). You can see on non-burning days that, on average, the temperature is lowest in the homes around 5am at about 12 degrees. At the same time the average outdoor temperature was 4 degrees. In fact, on average, on days without woodburning our homes were about 10 degrees warmer than outside from about 9pm to 7am. During the daytime that difference drops to a minimum of 1 or 2 degrees around lunchtime. When woodburning is absent the home is at its warmest, on average, at around 4pm.

There are three reasons why an apparently unheated home is warmer than outside. The first is supplementary heating – probably electric. A few of our homes occasionally had a little extra heating. This is easy to spot in our data if the heater is on a timer – we see a clear jump in temperatures at the same time every day. If you look in the chart above you see a little 'bump' in the morning due to non-wood heating. The second reason is that the sun warms the home. The amount of warming obviously depends on how sunny it was that day and we can see that in the data too. It also depends a little on the materials the home is made from – especially how many windows it has and how they are oriented with respect to the sun. The third reason homes stay warmer than the air outside is through other objects or activities which release heat, like cooking, electrical devices, and even warm bodies, and the fact that the fabric of the home itself stores heat and then slowly releases it. We call this ‘heat storage’ and is the reason why the indoor temperature peaks later than the outdoor temperature, and it takes longer for the indoor air to cool down.

On days with woodburning, unsurprisingly the pattern looks different. The average dawn temperature was 18 degrees (rather than 10 degrees), and the temperature peaked around 11pm at 23 degrees on average.

The size of the boxes also tells us something. The boxes are large in the morning meaning a bigger range or difference between homes, than in the evening. This is telling us that whilst all of our homes used their woodburner in the evening, only some used it in the morning, or used it less.

When occupants started their wood fires indoor temperatures rose, but at differing rates. Some homes warmed at a rather sluggish 2 degrees per hour, whilst in some it was over 10 degrees per hour. Sometime just before or shortly after midnight the air in each home started to cool. In most homes we could see that the cooling was faster when the outdoor temperature was lower. There was some variation in how fast each home cooled ranging from half a degree per hour to 2 degrees per hour. This meant that by dawn some homes had completely lost all of the heat from the woodfire (although the effect of storage of solar heating was still present) whilst a few still retained a little fire heat from the previous evening. In some homes we saw a dip in temperatures in the morning. This may be due to doors being opened letting some of the stored heat out.

Finally, a quick look at the total number of homes using their woodburner on any given day (chart below). Through late August it was clear that every one of our homes was using their woodburner every day. However, on the 2^ndSeptember and again on the 9^th one or two homes temporarily stopped burning. Both days had significantly warmer outdoor temperatures than the previous days. On the 15^thSeptember the afternoon outdoor air temperature ‘soared’ to 13 degrees (compared to 8 degrees the day before). All but one of our homes didn’t use their woodburner that day or the following (equally warm) day. This is the first glimpse at one of our major research questions - when and how do people decide to stop (or start) woodburning.