Interesting thought I currently have a 28kw combi, 1 bathroom, 2 up 2 down semi. I'm g3 and gas safe. I fit unvented and replacement boilers all the time. I'm thinking about replacing my combi with a small 120 litre unvented cylinder in the loft. An unvented cylinder usually draws about 3-5kw per hour, and heats up in about 30-45mins I reckon the new cylinder will lower my gas bills? Thoughts..