1 Feb 2011

How Denby Dale weathered the cold snap

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!

21 comments:

  1. That cold period was also a clear-sky period of well above average solar incidence. It's a pity that PH isn't equipped conceptually to make significant use of such solar opportunity - it puts a very low, really defensive limit on solar gain, for fear of uncontrolled overheating. Far from requiring extra fuelled heating, that cold period could have required reduced, or no fuelled heating, if only PH could do that trick.

    ReplyDelete
  2. Tom,

    That's nonsense. Not only was the cold spell NOT typified by clear sunny days (the Alpine ski resort climate) but Denby Dale has an unusually large south-facing glazed screen area, ideally suited to picking up any passive solar if it's there to be had.

    There's nothing in PassivHaus that says you can't have large south-facing glazed areas, you just have to make adjustments elsewhere. In Denby Dales's case, there is an electrically-operated blind system that is designed to keep the sun out in summer.

    ReplyDelete
  3. Well, in Devon mid December was definitely days on end of high pressure cold clear sunshine.

    Big south windows PH-style don't do it - as soon as the sun goes in the gain radiates straight out again to the cold surroundings. It's a technique that guarantees that you only collect the current solar average. In deep winter that's usually more loss than gain, and in summer is massive over-gain.

    PH has nothing to say about ways of collecting that in deep winter harvest the very ample solar peaks and reject the troughs, often on a minute-by-minute timescale - and in summer harvest as little as you like, without resorting to blinds.

    ReplyDelete
  4. 1600kWh in one month is pretty high for a state-of-the-art house, even allowing for the cold weather. How many heating degree days were there in December? By contrast, I live in an ancient relatively un-insulated house, no optimization for solar gain and I used only 2000kWh for a total of 739HDD18 - and this in a 179m2 house with 90m2 of heated basement. Average temp over the whole month was -5.8C.

    Saying the energy was not really for heating is a bit disingenuous. I wonder why so much energy was required?

    Paul

    ReplyDelete
  5. Paul,


    Aren't you using GSHP? With a CoP of 3+? Doesn't that distort the calculations somewhat?

    And the point about the multiple uses of gas at Denby Dale isn't "disingenuous" - it's critical. The famous PH target is for space heating only, and doesn't include DHW. In a normal winter month, Denby Dale will be using the majority of its gas to heat DHW.

    ReplyDelete
  6. Maybe I was exaggerating a bit by saying "disingenuous" - but I think there's more to it. Passivhaus levels of energy usage claim they have low space-heating requirements but, really, it's all the "waste" heat from lighting, appliances and so on which allow this claim. The point I was making is that one can achieve a similar total energy consumption through some adroit application of technology. Maybe there's a trade-off somewhere between extreme insulation and efficient heating equipment as the latter is required even in passivhaus designs when the weather is unseasonably cold for extended periods

    I suspect that climate change will lead to more extremes of weather in Northern Europe than just a milder climate.

    On a tangent,I think fostertom is on the right track with his thermal storage ideas that have enough capacity to bridge extended cold periods.

    Oh, and speaking of DHW, my annual water heating consumption is of the order of 2000kWh - low because the GSHP provides a pre-heat. Since I know I can't get by without heating, it seems a fair trade-off - especially in the summer when the airconditioning function also dumps heat into the DHW.

    As always, different climates require different trade-offs. Definitely an interesting topic.

    ReplyDelete
  7. Tom, a window is well equipped to make good use of passive solar in winter - compared to say a solar hot water panel it is far better insulated since the glazing is triple, argon filled and low emissivity, and the heat loss is from an absorber at the lowest possible temperature to be of use to heat the house - ie it is at room temperature.

    I've seen the peak watts going into my solar thermal on sunny winter days and thought "this looks good, a few more hours and we'll get a tank of hot water" - and then the sun goes down. In the UK, especially in the north, the days are just so short that even if you have the kW you don't get the kWh. (And changing the clocks to European time isn't going to make this any better!)

    ReplyDelete
  8. Good stuff, Paul and Alan.
    Paul, part of the "thermal storage ideas that have enough capacity to bridge extended cold periods" package must be that the solar collectors that feed the storage must operate at extremely low flow/return temp e.g. 20C flow, 17C return, in order to drop the collectors' gain/loss threshold low enough that collection can happen on as many days/hours as possible. That way, the effective length of the "cold periods" that have to be bridged, is reduced, so storage need be less. In fact without such low flow/return temps there's very little chance of nett-collecting anything worthwhile in UK Dec/Jan.
    Alan, so as I say above, make the "solar hot water panel" into "an absorber" as low in temp as the room interior would be, and by all means front the panel with triple glass. Such a panel's advantage, over a room interior/window as collector, is that the panel can be shut down, 'collection' cease, and thermofluid drain back out of it, the instant the sun goes behind cloud. That way, all the hesat you've collected (and transported safely away to storage) while the sun shone, isn't immediately re-radiated back out when the sun goes behind cloud, as is the case with a room/interior/window.

    ReplyDelete
  9. Tom,

    just one flaw in your logic about solar gain being re-radiated out of windows. Low-e coatings on glass allow short wave IR into the house but block long wave IR from escaping. This is essentially the greenhouse effect. The sun's IR is at a temperature of several thousand Kelvin, but the warmed objects in the room are at a few hundred K. The same argument could be used for low-e coatings on the glazing covering solar panels as well.

    ReplyDelete
  10. Exactly - what goes for coatings, multiple panes etc on windows goes for (or can go far) solar collectors as well. But the coatings and panes don't fully 'block' escaping long wave radiation - they only resist and reduce it to varying degrees. A top-tech 3G coated window will still let plenty of the gain of 10mins ago escape, when the sun goes in. That's what happens in a PH building, so in PH solar gain is relatively ineffective when you need it, in winter - but a big overheating threat in summer.

    ReplyDelete
  11. Hi Mark, we met in 2006, at Nottingham.
    There seems a simple way to settle this, for the Denby house family to have a gas engineer fit a gas meter on their boiler, to give a separately readable consumption.
    I have a GSHP with a meter to distinguish its consumption from the rest of the house. 4kW of PV roof above also muddies the calculations for consumption, so the GHSP meter provides an Absolute reference. it includes the Heating, DHW and the circulating pump for the underfloor heating. The GSHP display tell me how much heat is spent on DHW, and the pump has known hours and consumption, so I can calculate the heating Precisely.
    Kilowatt hours can be misleading if you compare Gas and Electricity, so I am glad that the Govt seem to be using CO2 emission (10 kg/m2/year) as the standard to aim for.

    ReplyDelete
  12. Hi again, Mark, I just looked at my December 2010 figures for my own house. Our GSHP used 763.88 kWh in that exceptional month (degree days were 499 [base 15.5] for the month in Nottingham).
    Of that 50 is the underfloor heating circulator and 65-70kwh is the hot water, leaving the heating to be approx 645 kWh.
    As I have solar charging of the boreholes, the deep ground temperature was, amazingly, still above 10º by new years eve.

    ReplyDelete
  13. I should have mentioned that it is also easy to fit a Water Meter to the Hot Water pipe leaving your main tank, so you know exactly the volume of DHW used, weekly or monthly.

    ReplyDelete
  14. Hi Mark, I did a little write up about your write up, on my Solar blog -
    http://chargingtheearth.blogspot.com/2011/02/mark-brinkleys-blog-denby-dale.html

    ReplyDelete
  15. There's nothing in PassivHaus that says you can't have large south-facing glazed areas, you just have to make adjustments elsewhere. In Denby Dales's case, there is an electrically-operated blind system that is designed to keep the sun out in summer.

    ReplyDelete
  16. Thanks Fasulent - I posted a few days ago a discussion between Technology and Insulation, a topic that Mark and others mention above.
    http://chargingtheearth.blogspot.com/2011/04/insulation-technology-and-embodied.html
    Of course you need both, and first you need insulation, but I dont go as far as Passivhaus extremists in saying that the house would need no heating at all!

    ReplyDelete
  17. Mark, is it possible to tweak some of the display features on the blog, for example, for comments to articles, it shows the TIME of day of the posting, but not the DATE, and surely, the date is the more important, especially if we are discussing weather and the seasons.

    ReplyDelete
  18. Good point David. As you see, I have adjusted the settings.

    ReplyDelete
  19. The cold period last year was not at all sunny in Cambridge either. It was notably cold + grey at the same time, and there was very little solar gain to be had.

    ReplyDelete
  20. 1600kWh in one month is pretty high for a state-of-the-art house, even allowing for the cold weather. How many heating degree days were there in December? By contrast, I live in an ancient relatively un-insulated house, no optimization for solar gain and I used only 2000kWh for a total of 739HDD18 - and this in a 179m2 house with 90m2 of heated basement. Average temp over the whole month was -5.8C.

    ReplyDelete
  21. I've no idea what the degree days were for Denby Dale in Dec 2011. But is the comparison relevant? Did you keep your house at 21°C throughout December?

    ReplyDelete