Get a plumber to look at the HWS, especially if it's gas! So is it a instantaneous or is it just a Storage HWS?
If it's instantaneous and someone else is the house uses the hot water, then this can cause issues with the water going hot and cold. They don't have a great enough flow rate to heat water and supply numerous fixtures at once - although they are getting better. If the water saver shower head is leaking and the user has 'full on' showers than this might be another cause of the issue aswell.
If it's a storage HWS then it sounds like it might be the Tempering Valve that is playing up. Basically the tempering valve will limit the temp to about 50 degrees, but if it ever plays up or fails it cuts the hot water to protect the user - hence only cold water coming out.
I'm not very experienced in HWS's but I think it's gotta be one of the issues I just listed. Can't think of anything else atm.
If it's instantaneous and someone else is the house uses the hot water, then this can cause issues with the water going hot and cold. They don't have a great enough flow rate to heat water and supply numerous fixtures at once - although they are getting better. If the water saver shower head is leaking and the user has 'full on' showers than this might be another cause of the issue aswell.
If it's a storage HWS then it sounds like it might be the Tempering Valve that is playing up. Basically the tempering valve will limit the temp to about 50 degrees, but if it ever plays up or fails it cuts the hot water to protect the user - hence only cold water coming out.
I'm not very experienced in HWS's but I think it's gotta be one of the issues I just listed. Can't think of anything else atm.
