I'm re-modelling my bathroom to remove the hot water cylinder. I currently have a single pipe 'S' Plan system running on gravity feed hot water and pumped CH. I installed the CH system back in 1986 when money was tight. The original Glow-Worm boiler was replaced about 8 years ago with a Glow-worm 18hxi condensing boiler and works extremely well. Radiators heat up well, no obvious signs of starved rads. When I piped it originally, I sized the pipes as 22mm to the first few radiators dropping to 15mm I think then back to 22 on the last run from the last radiator. The boiler never cycles or locks out. The pump is running on mid speed. I've been quoted for a new combi boiler. The plumber knows the system is single pipe but did have a few concerns about return temperature. Is this a problem if the current boiler copes well.