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I'm trying to determine the heating cost savings between two different u-values. Does this look correct, it's a crude approximation based on a fixed temperature difference (the difference is higher than the average would be in reality) but does it hold up, is the maths correct?

Work out the difference between the two window U-values (1.7 – 1.4 = 0.3Wm2K)

Multiply that number by the window area (0.3 x (2 x 2) = 1.2)

Work out temperature difference inside and outside(18DegC – 2DegC = 16)

Multiply the temperature difference by the U-value area (1.2 x 16 = 19.2)

Divide that number by 1000 to convert it into kilowatts per hour (19.2 / 1000 = 0.0192kWh)

Multiply that by hours in a day that the heating is on (10) and the number of days in a year that the heating (180) is on to get your annual kWh (0.0192 x 10 x 180 = 34.56kWh)

Multiply that figure by my current cost of a kWh of gas (34.56 x 12.3p =425.08 per year)

So the annual cost saving of moving to a 1.4 u-value sliding door would be £4.25 per year. But the 1.4 u-value sliding door costs several thousand pounds more, so it isn't worth it.

Rohit Gupta
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boat234
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  • Your numbers sound plausible, but I know nothing about u-values, so just a comment. Look at it from the other direction: Total heating cost per year is what? £1,000? 2,000? Let's say it is 2,000 - sounds like a lot to me. By any reasonable back-of-the-envelope guess, I would expect changing one sliding door to make a difference of at most 10% in total heating cost (probably a *lot* less). So let's say worst case 200. That's a tiny fraction of "several thousand" and therefore not worth it. – manassehkatz-Moving 2 Codidact Apr 18 '23 at 18:15
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    In a high school physics sort of way that all makes sense. In reality if its 2C outside during some parts of the day it presumably isn't over 18 the rest of the day? You turn your heating entirely off for 14 hours, the house will cool off, partly through this door, and have to be warmed up again. If you use an average temp dif over all 24x180 hours of heating, and multiply by the entire period not just 10 hours a day you'll get a better result. But you'll still come to the same conclusion. Intuitively, thousands of pounds would be hard to recoup for such a marginal improvement. – jay613 Apr 18 '23 at 18:27
  • It is the same idea as buying newer large appliances to get better electrical efficiently and save money on your electric bill. The newest appliance I have is from the 90s, some are older. I will save a few dollars per year if I bought new ones. As for your door, part of the savings will also be in increase comfort near that door. Something that would be hard to put money signs on. – crip659 Apr 18 '23 at 18:53
  • What is your ACH50 number? Not really worth spending / worrying about insulation values until your air sealing is done well. – Fresh Codemonger Apr 18 '23 at 19:09
  • I will look at the calculations later. But another argument that I have used for myself is that I intended to stay in this house until I died. So I invested when I could afford it, to reap the benefits when I retired – Rohit Gupta Apr 18 '23 at 21:10
  • Thanks for your inputs, they are really appreciated and give me the confidence to go with the much cheaper option. I'll be having a new sliding door going into a newly created hole in the wall in an existing building. I believe that I'm correct in saying that it has to comply will building control regs which say that it should have a maximum 1.4 u-value or it can an Energy Performance Certificates (EPCs) C. Apparently EPC C maps onto u-values at around 1.7. So I'll go with the 1.7 option due to the huge price gulf and the limited effect the 0.3 difference will have. Thanks again. – boat234 Apr 19 '23 at 19:59

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