Using a combi (or combination) boiler is one of the best ways to ensure that a good supply of hot water, while still reducing your energy bills. Combi boilers typically work on a tankless system that heats the water as you need it, so the production of lukewarm water at the start of running your taps should not be a surprise, but if the boiler continues to send out lukewarm water, you may have a more serious problem. Examine your boiler and hot water system to find out the reasons for the boiler not heating the water.
Check the Boiler and Central Heating
The first thing to do is to look at the boiler and your central heating system. If your central heating is working, but the hot water is not, examine the diverter valve, which switches your heat between radiators and taps. You may find that there is a stuck mechanism here which is preventing hot water from reaching your taps. You can repair the latter problem yourself by unscrewing the diverter valve (there are two screws at the back which need to be loosened). Open the valve and examine the mechanism. Remove any stuck pins, wash until clean, and then replace. if the valve is working correctly, you should check the thermostat on your boiler to ensure that it is not set too high, and then repeat. You may also find that turning off your boiler for a few hours, and then turning it back on, can help to reset it, and prompt it to start sending water back through the taps. If none of these attempts work, it could be a more serious problem with the hot water pipes, and you should call your boiler supplier to arrange a service visit.
No Hot Water
If the central heating system is not operating at all, then your boiler must be the cause of the problem. Your boiler supplies both radiator and water heating, so if both are having trouble, then you should check the temperature control on your boiler, ensure that the pilot light is on, test the water levels, and examine the boiler for other problems. You may also consider performing a blowdown, as this can help to release sediment at the bottom of the tank, and relieves pressure in the tank. After examining the boiler, check the hot water tank, and then trace the connections between them. Sometimes the problem can be in the storage tank, rather than in the boiler. The tank has a series of anodes in the system to prevent corrosion of the inside of the tank, but these can fail once they become heavily corroded. If you have tried the tank, and that is also not getting hot water, then you will need to call in a plumber to thoroughly check the hot water system.