Snow started falling in Denby Dale in the last week in November and lay on the ground right through Christmas. There was a ten day spell in early December when the temperature in Denby Dale never got above freezing, and it dropped right down to minus 18°C on the coldest night. At last, a meaningful test for the Denby Dale PassivHaus. How did it fare? Geoff Tunstall, the man who lives there with his wife Kate, takes up the story.
It all went just fine. I had to adjust the ventilation system a little, that’s all. The MVHR unit has a night temperature lowering setting which is set to run from 10pm to 6am and this left us with an internal temp of around 18°C in the morning, which we regard as a little chilly. But I changed the default setting and ran it on the day-time setting throughout the 24 hours during the cold snap and that did the trick. The internal temperatures went back up to 21°C. In fact, we overshot, and started to get too hot at one point.
Bear in mind, our gas boiler does three things: heats the hot water, runs the 3kW post heater in the ventilation system and heats a radiator and two towel rails in the bathrooms. In addition to this, we also have a gas hob for cooking. The post heater and the radiators were working throughout the cold snap, and our gas usage reflects this.
We burned just over 1600kWh of gas during December, which cost us around £100. But the months before and after the cold snap were more typical of Yorkshire winters, and in these months we used much less gas — 604kWh in November and around 650kWh in January. Our total gas usage for the five months from late August to late January was only 3394kWh (costing £210 including VAT), so that four-week cold snap accounted for nearly half the total we used. Our average monthly gas bill for the first eight months we have lived here including the cold snap is still only going to be £25 a month. In a more normal winter, it would be as little as £18. Our total electric bills for the first six months were just £108.
How does the performance of the Denby Dale house stack up against the key PassivHaus standard of 15kWh/m2/annum? It’s not a big house — it’s around 120m2 internal floor area, which translates to (120 x 15) to 1,800kWh heating a year. But (you cynics say) they burned nearly this much (1,600kWh) in December alone !
Yes and no. The PassivHaus standard is for space heating only, not for domestic hot water or for cooking, so you have to strip out something for these. Hot water probably accounts for about 250 to 300kWh per month in the winter (they have solar panels for the summer months), and gas cooking perhaps 20 to 40 kWh, especially over Christmas. You can see that the November and January space heating totals are probably around 300kWh a piece and that, had the entire winter consisted of months like these, the total space heating requirement would have easily come inside the 1,800kWh suggested by the Passivhaus standard. The fact that December in Yorkshire resembled what you might normally find in Russia or Quebec is by-the-by. The standard is not absolute, it’s relative to the location and the Tunstall’s home was designed as a Yorkshire PassivHaus, not a Russian one.
But do bear in mind, these are not “homes-without-heating”. Just not very much heating!