Hot radiators, but not water with system boiler

Easy one to answer, call for hot water only, place your hand on the heating pipe leading directly to the top side connection of you hot water cylinder, does it begin to get warm? If so then check the bottom pipe (return) from the same cylinder, if that after a period of time is not becoming warm then you have proved that the hot water cylinder coil heat exchanger is preventing heat flow due to debris/sludge build up! Copy and paste link below in to your browser, as it illustrates your 'S' plan heating system and my above checking method: http://wiki.diyfaq.org.uk/index.php?title=File:S-Plan-Water.gif Please let us know how you get on?

Could be , however , unlikely. I would suggest opening valve manually and feel with your hands if there is any difference in the surface pipe temperature of the uppermost pipe feeding the cylinder from the boiler. Most actuators have a manual operation facility , so this should be simple. If you operate the actuator and the pipe gets hot , this will indicate incorrect wiring of the controls/actuator. I came across a fault similar to this some time ago and found the expansion/breather pipe from the cylinder to the feed tank in the loft completely blocked - removed blockage - hot water restored. However , if you have water running freely at hot taps - whatever temperature - there is some form of impediment of the heating to the tank. There should also be an air vent at the uppermost heating feed to the tank - try venting this with actuator in manual operation.

Firstly, check that you have a power supply to the 2 way zone valve when you have selected for hot water at the programmer, if so, check that the 2 port valve opens and closes when the cylinder stat is turned up and down. If these checks give positive results then I see 2 possible causes for no hot water. 1, air lock on the coil inside the cylinder. 2, partial blockage in the pipework between the 2 way valve and the return pipework common tee. If it's an air lock then selecting hot water only on the programmer and closing down the by pass valve might force feed and start working, if it turns out to be a blockage of sludge or debris, then a Powerflush is needed or to trace the blockage and remove manually to resolve. Hope this helps. Regards, Andy