Winter Design Day Temperatures

[Mark Siddall]

I've been looking for typical winter design day temps for Scotland and Southern England. The best that I've found to date is a BRE document suggesting -5C for the UK. Does anyone have any better guidance? (Referances to supporting docs appreciated)

Mark

[Peter Wilkinson]

I've always thought that -5C is an inadequate level. We regularly get below that in winter and for a number of days at a time. I've always insisted on using -10C as a minimum but not found any supporting data for UK to back this up. Common sense to me. Others may have.

[David OLIVIER]

Peter, do you live on a mountain top?! Mid Wales tends to be like that since the height can reduce the annual temperature by 2 or 3 deg K compared to sea level. For inland Scotland, the temp. should definitely be lower than -5 degC. For southern England and coastal Wales I think it's fine.

PHPP comes with weather data for only five UK sites and this info. doesn't include the design temps. ASHRAE claims to have UK weather data for 50 sites possibly including the design temps and sell a CD. I'm interested in getting this too.

Mark if you have time you may be able to do some web searching re. hardiness of garden plants (!!) which is loosely based on the local design air temperature. The USA classifies its landmass into about ten different Zones, all based on the design temp., and there may be European charts available.

David.

[Mark Siddall]

Very clever David. I wouldn't have thought about using the hardiness of plants as an indicator for design temps, but it makes a great deal of sense.

The "Meteonorm" weather data software is meant to be good for a whole range of weather related info and is capable of stochastic modeling of any site location (achieved by interpolating the data from known sites). The problem is it comes with about a £400 price tag....ouch!

Mark

[Mark Siddall]

The plant hardiness test seems to present more demanding (lower) min average temps than standard MET data. Having referred to the sites below it would seem that the UK min mean winter temp is about +1C (MET Office) with an absolute minimum mean of -5C, but the min design day using plant hardiness data is -7C to -12C depending upon region.
If this all sounds a little confusing, it's down to how you can read the maps.

Hardiness of garden plants for Europe (just click on the UK for a larger image):-
http://www.uk.gardenweb.com/forums/zones/hze.html

And a more detailed version:
http://www.trebrown.com/hrdzone.html

The MET Office min. mean annual temp:-
http://www.metoffice.gov.uk/climate/uk/averages/19712000/tmin/17.gif

The MET Office min. mean winter temp:-
http://www.metoffice.gov.uk/climate/uk/averages/19712000/tmin/16.gif

Full searchable monthly and annual MET Office stats measures over 1971-2000
http://www.metoffice.gov.uk/climate/uk/averages/19712000/mapped.html

The only thing that is a little perplexing is the fact that both the MET Office data and the Hardiness of Garden Plants data are derived from the same records and both seek to present the mean winter temperatures.

On this basis how can the conclusions of the two reports/maps conflict so greatly?

Mark

[Mark Siddall]

I may have the answer to my earlier question:-

The UK Plant Hardiness Zones have been calculated using the USDA method. According to “Plant Hardiness Zones for Australia” (http://www.anbg.gov.au/hort.research/zones.html) the USDA hardiness map is derived from “the average, over ten or more years, of the very lowest temperature (the absolute minimum) observed for each year for each meteorological station.” i.e. take the coldest recorded temp in each location each year and then find the ten year (or more) average.

Where as the MET Office data is probably the derived from the mean of the min monthly temps between 1971-2001.

It’s no doubt as a consequence of this methodology that the Plant Hardiness figures tends to make places look very cold!

Mark

[Mark Siddall]

Eric Roberts of Zero Energy Design suggested that I look at CIBSE Guide J. The Guide states:

“Design temperature has a large influence on the capital cost of building services systems, and some influence on running costs. No single design temperature is given for a particular location; rather, a range from which the designer can select an appropriate design temperature in consultation with the client (bearing in mind the previous sentence).”

It goes on to say….

“For buildings with low thermal inertia, earlier editions of Guide A recommended design temperatures such that, on average, only one day in each heating season had a lower mean temperature. Similarly, for buildings with high inertia, a design temperature was recommended such that only one two-day period in each heating season had a lower mean temperature.”

Thus the Hardiness of Plants data would mean that you are designing to THE worst average day and as a consequence the services plant would be oversized and the costs incurred would be greater than necessary.

Having had a quick scout through Guide J it seems that over eight sites –3C to –6C is the range for which min temps in the UK will, on average, be exceeded less than 1 day per year (based on a data set between 1976–1995.)

For the efficient (and cost effective) sizing of plant check the local Met Office data.

Mark

[David OLIVIER]

Mark


The USA tends to use 97.5%, 99% or 99.5% design temps..; i.e. the outside temp. falls below that temp. for a small propn. of the heating season. An old edition of the ASHRAE Handbook had design temps for several 1000 sites worldwide but I think you have to buy a CD now to get full list (possibly 10,000 sites now incl 100s of UK sites). It was useful and provides a sound basis for sizing heating systems. From memory as I said southern England and Wales are roughly -5 degC but high land and inland sites have lower temps.  see below. I can get you ASHRAE stuff at member's price, if any use.

The Met. Office says Newport, Shropshire (1hr from here) has a record low of -26 deg C. This is well below its design temp. which is probably about -10 degC. If CIBSE is encouraging smaller heating systems than the design temp. implies it seems to be going for reduced (heating) plant cost at expense of a risk of greater discomfort. (Or maybe by suggesting a discussion with clients it's attempting to reduce members' liability; the UK has quite a lot of undersized heating systems due to our poor-performing thermal envelopes, not due to engineers' errors.)

The zones for (garden) plant hardiness are probably based on long-term averages; the mid 20th C records suggest even colder temps. than the last 10-20 years. But the economic consequences are less; it's cheaper to replace a half-hardy plant after an extreme winter than it is to enlarge a heating system. It would seem eccentric if the UK is using temperature data which puts the survival of garden plants above the comfort of people!

Incidentally I think this has limited relevance to Passivhaus, Gold or Silver (masonry) buildings, due to their slow thermal response. Strictly speaking one should use higher design temps. for buildings with very long time constants and the Russians apparently do so - I've no time to search for more info. but you may have - if so good luck.

David.

[Mark Siddall]

David,
Is there not something to be said for thermal mass helping a building to ride out the colder spots, thus reducing the equipment requirements? The CIBSE document suggests that lightweight and heavyweight buildings can ride out different durations of extreme weather. If the cold weather duration is less than one or two days depending mass type then the whole system (mass + mechanical system) should be able to provide adequate comfort. As plants do not have the benefit of such mass and insulation (unless they are indoors) they have to be more robust by nature or a cold snap could kill them off.

Re: The Newport example duration is what matters if this was for "one hour" then -26C is not such a great concern as it would be if it were for "one day." CIBSE are certainly passing the buck to engineers in determining what the design temp should be whether this means that the systems will be undersized is another matter (I've only ever heard of engineers oversizing their kit to avoid PII claims.....thus leading to a life time of expensive and inefficient plant operation. As this type of equally poor design is easily overlooked, i.e. no POE, they can get away with poor design and no PII claim!)

Amory Lovins has noted that a PH (and the RMI office for that matter) looses just 0.5C per day without heating “under eclipse conditions,” (didn't mention "design day") if one accepts this premise (I have no calcs to suggest otherwise) comfort should not be undermined as a heating system will obviously help to ride out the cold weather.

Perhaps the plant hardiness map is not suited to being used for sizing mechanical systems due to the thermal mass issue. (Though it may remain useful for other aspects of comfort/building design i.e. assessing down draft from windows.)

Mark