I set mine to 18.5, but I draught-proofed every door and, more noticeably, put plastic film taped over each window - inside and outside: already double-glazed windows became 4x so with the addition of two extra air barriers. Cost maybe €25 for seven glass doors/ windows. Bathroom temp inside 19C, behind the first layer of added plastic 17.5C, behind the glass 12.5C, behind that 10C. I also have wooden blinds - draught-proofed and lined with bubble-wrap - temp was 7.5C and actually outside was 5-6. Add: warm shower when I got home, or got up, sensible warm clothing, and whilst my neighbours thought my plastic looked crappy I was happy with my results. I also send in over-read meter readings thru the year: late december i told the truth...gas bill was actually -€30 ie €0. Waiting for the electric bill.