Combination boilers produce constant hot water for as long as you want, leave the hot water tap open and you should have continuous hot water. The hot water takes priority over the heating, so should you run the tap for over an hour the heating will cool down, once you close the tap the heating will kick in and start heating up the radiators.
If you are experiencing problems with the hot water from your combination boiler like the water goes hot and cold, then there are a couple of things it maybe. The first one and the main fault of combination boilers is the heat plate exchanger. The plate exchanger gets blocked with sludge, this is due to the system not having been flushed out and protected with inhibitor to stop sludge build up.
The hot water is heated up rapidly through the plate exchanger, the way it works is, the water that comes out of your tap is not the same that's in your radiators. The plate exchanger has two side to it, one being the water inside the boiler and the other from your water mains. When you open a tap for hot water, the boiler moves the internal diverter valve over for hot water only. The water inside the boiler is now circulating within.
As you can imagine it heats up very quickly, it passes through the plate exchanger to heat the water that is passing through it on the other side. The best way to describe it is, heat up some water say by the kettle, empty it into a pot, you now have hot water in your pot, now get a bottle with cold water and put it in the pot, eventually the cold water in the bottle will heat up to the same temperature as the water in the pot. The only difference with the combination boiler is the water is heated rapidly.
Now if the plate exchanger is partly blocked with sludge, the boiler will start to get to temperature a lot quicker, and shuts down, as it cools it will relight and this is why you get hot and cold water at the hot water tap. 90% of all problems with the hot water on combination boilers is due to the plate exchanger being blocked with sludge.
Other problems are sometimes due to a faulty thermistor that senses the water temperature, or partly blocked filters.
Going back to the heat plate exchanger, if you have this problem with the hot water, you can have the exchanger replaced, but that won't cure the problem, because it will return. If you have this problem with sludge, then the rest of the system is infected as well. The only way to cure it for good is to have your heating system flushed out with a cleaning agent, and then an inhibitor added to protect the boiler and system.
Another good prevention is to have an inline filter fitted to the return pipe of the boiler. The best one on the market in my opinion is the MagnaClean, it stops any sludge entering the boiler, it collects within the filter, and it's also a good way of adding the inhibitor to the system.
When inhibitor is added it will protect against sludge build up, but it also needs to be checked each year and topped up if needed. This should be done when you have an annual boiler service, which may become compulsory very soon. Government are thinking of making it compulsory that every gas boiler in the Uk should have an annual service, just like the car MOT.
You know your system has inhibitor in it, but did you know if you drain down the system or even just a radiator for decorating, you must add more inhibitor when refilling, otherwise you could cause sludge build up again and damage your boiler.
If you think you have this problem with your combination boiler with the hot water, speak to your local gas safe boiler engineer, who will be able to advise you on how to solve this problem and stop it returning.