Could someone help answer the following 2 questions 1) Why does a drop in pressure on a combi system affect h/w operation. I recently drained system down to change some rads. Turned c/h off, left h/w on timed programme. As the c/h is a separate sealed system I didn't think this would affect h/w operation. Had to leave c/h off for a day and h/w stopped working (boiler lockout?). Anyway, altered rads, refilled system and now all ok. Just puzzled as to why h/w was affected when I thought in theory I hadn't touched this. Is it simply that h/w operation requires 1 bar reading to operate too? 2)Have been getting dirty brown water coming out of bath hot tap from combi. If bath not used for a couple of days then dirty water comes out of hot tap and lasts for about 10-15 secs then goes clear. No other taps affected. Why is this happening? Hope my queries are clear. Many thanks.