Some facts first.
I live in a dormer bungalow.
I have mobility issues so have not been in upstairs bedroom for about a month
Four weeks ago my son and his partner stayed here for the weekend and slept in the dormer bedroom
He left the blind almost fully down when leaving
Over this very cold spell, I have been increasingly aware that my central heating, on from 7 am to 10pm set at 19°, has been struggling to get to temperature, only reaching 16 or 17° according to the thermostat in the hall and my downstairs bedroom has felt rather nippy over night. I put this down to the weather and was considering leaving the heating on overnight to stop the temperature dropping so much.
Today my sister came from the US to stay with me for Christmas and when she took her suitcase upstairs, found that the velum roof window was open at the bottom, partially hidden by the blind! I sat on the stairs to look and could feel a cold gale blowing down into the hall.
I realised that for four weeks my thermostat has been affected by this open window and the heating had been on probably full time from 7 am to 10 pm trying to heat up to 19°. Obviously had there been no freezing wind, the heating would have been switching on and off as it should do! Grrrrr,
So now my darling son is in big trouble and going to get the blame when I get my next bill! It’s a good job I love him and can see the funny side - note to myself to drag myself upstairs after I have visitors to check that all is well in the bedroom!
How stupid am I not to have realised that something was not right!