Only difference, depending on how long it takes you to shower, is time on. So only possibility, is something sticking, after prolonged heating time. 3 way valve, with pressure switch etc etc. is the usual connection for HW or Heating, so look to that 1st, possible sticking on, so will not allow heating, or other after, not returning to normal position, service kits I believe are available.
SD, December 2011
