Efficiency comparison with condensing boilers
by Unknown - 13:43 on 01 December 2013
Efficiency comparison with condensing boilers
There is a Department of Energy/Energy Saving Trust report on an in situ study of condensing boilers. This reports on a study of actual boilers in use in households over a period of a year. I have placed a link to it on the links page. It is a fairly small study of 43 boilers, but it does give us some results on actual efficiency of condensing gas boilers (as opposed to measurements under ideal conditions) for comparison with my measurements of the efficiency of the Smart 120.
In the study, average efficiency was 82.5% (with a claimed standard deviation of 4%) for Combination Condensing Gas Boilers , and 85.3% (with a claimed standard deviation of 2.5%) for Regular Condensing Gas Boilers. The study notes that the apparent advantage for regular boilers is misleading because the difference arises from the inefficiency of supplying hot water during the summer, which was measured accurately for Combination Boilers, but not for Regular Boilers. This is because it was the output from the boiler that was measured and so there was no allowance for losses for Regular Boilers from transmission in the pipework and from the hot water tank during storage. For the Combination Condensing Boilers, there was a considerable diversity, 1 was below 70%, 2 74-78%, 7 78-82%, 15 82-86% and 6 86-90%. For the Regular Condensing Boilers, 1 was 78-82%, 5 82-86% and 4 86-90%.
For the Smart 120, my cumulative measurements over three months indicate an efficiency for the Smart 120 in the range of 75%-85% (albeit the bottom of the range was 73% measured over one month and the top was 90% measured over one month, with greater variations for single-day measurements). My personal opinion is that the actual efficiency is towards the top of my measurement range, but this is a matter for argument. In any case, it is important to remember that idiosyncracies of our system such as the height of our flue, the size of house that is being heated, and the zoning system we are using will all have an impact on efficiency.
As regards idiosyncracy, the study contains some figures for the size and age of properties in the study and notes that the study is biased towards relatively large properties because the owners of larger properties were, for one reason or another, more willing to take part in the study and it was easier to install measuring equipment in their properties. In order to assess bias, the Energy Savings Trust compares the stock of houses in the study with the stock reported in the English House Condition Survey Report (this, oddly, seems to include Wales) which the study assumes to be representative of the UK. In the study, just over 20% of the houses were pre 1918 (same as EHCS), about 14% 1919-1944 (18% EHCS), about 14% 1945-64 (20% EHCS), about 32% 1965-1980 (26% EHCS), about 7% 1981-1990 (8% EHCS) and about 11% post 1990 (10% EHCS). Thus the study was fairly representative of the age of housing in England and Wales. But in the study, about 2% of houses were less than 50m2 (about 11% EHCS), about 3% 50-69m2 (13% EHCS), about 24% 70-89m2 (30% EHCS), about 13% 90-109m2 (13% EHCS) and about 58% over 110m2 (21% EHCS). Our house appears to be an outlier. First, it is in the North of Scotland! Second, it was built around 1810 (although the study did include one 500 year old property). Third, it is 268m2 according to our Green Deal Assessment (this appears to make it an outlier as regards the study, let alone English housing stock, since the largest boiler in the study seems to have a rating of 42kw whereas our old oil boiler has a rating of 44kw and this seems about right for the house). Thus anyone reading this blog needs to make allowance for the idiosyncracy of our property and to recognise that the Smart 120 is about the right size for half our house and that we cope with this because we have zoned our house and only heat half of it at any one time.
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