Combination Boiler being awkward!

[Kat535]

My 3yr old Sime combi boiler hates me! Despite having hot water everywhere else in the house about 50% of the time it fails to provide hot water for the shower. You either get none or it will fool you into getting in and then will douse you with cold water! It's worse when the heating is off or when you've recently flushed the toilet. Pressure does keep dropping in the boiler but I keep it topped up, can't find a leak and there's no air in the system. Has anyone got any ideas? Please!

[stanleyman]

You say that you have hot water everywhere else, will it fill a bath ok? If so it is not a boiler problem. Could be the shower thermostat is faulty or the water supply pressure and flow is poor causing the shower thermostat to shut off the hot for safety when you flush or use another tap. Make sure all your mains supply valves including the water boards are fully open.

If the pressure is always dropping in the boiler then there must be a leak or possibly a faulty expansion vessel causing the pressure to be relieved. This could also be related to the shower problem if the boiler shuts down through lack of water. You may have more than one problem so I would suggest getting a heating engineer to look at it and set it up properly.

[JudgeJ]

My experience of using a combi-boiler (or a multi-point water heater) is that they are not the best devices to use to supply a shower with hot water.

When you use a mixing device (as in a shower control) you inevitably effect the water pressure in each of the two inputs. Turning on a cold supply whilst the domestic hot water (dhw) is being supplied by the boiler will cause a pressure drop in the dhw input and could cause the boiler to shut down. This is so even if the cold you turn on is from an indirect supply (i.e. from the static cold water tank) as the tank will begin to fill as you drain it, with the same effect on the dhw input pressure.

As stanleyman says, pressure dropping in the boiler, whilst not directly related to the dhw supply, could cause problems and should be checked out. The pressure is actually that contained within the sealed heating system. If you have to keep topping up the system it could indicate a either leak or a failing expansion vessel.

I gave up trying to use my combi-boiler to run the shower and invested in a self-contained electric model.