Thermostatic Cartridge

If you have thermostatic type shower and this is running cold or perhaps with lukewarm or no hot water at all, it is quite likely that the thermo core or thermostatic cartridge  is in need replacing or cleaning.

Should your hot water system be providing hot water to the rest of the property and to the pipes feeding the shower, then this is another good indicator that the thermostatic cartridge is failing.

Thermostatic showers require balanced pressure on the hot and cold supplies at all times. If you have a Combi type boiler fitted, then a PEV - Pressure Equalising Valve should also be installed. Not having a PEV can cause a thermostatic cartridge to fail. With Gravity type boilers a twin-impeller pump can be fitted to obtain the required balance pressure. 

In all instances, do consult a local plumber or other advisor first, for verification of your no hot water issues.