Have you a combi boiler? Some combis have a simple control that alters the temp of the water during winter months (cold water is colder in winter).
On a cylinder system is the hot water fed to the cylinder via a three port valve off of the central heating circuit? The valve may be faulty and not letting the flow go sufficiently enough to the cylinder.
If fed via a two port valve the valve may again not be opening enough. There is generally a manual overide lever on the side. Try this in the fully open position and see what happens.
Check the thermostat on the cylinder. When ajusted low to high it should click. It may be faulty or set to low a temp.
If it is an immersion heater on the cylinder it may be a faulty thermostat or the stat is set to low.
BarryF, December 2009
