is there a separate stat on the hot water and central heating because when my central heating is on it just runs constantly,yet when the room stat is turned down it works perfectly on the hot water going off and on

peter j, April 2017

To answer your question - as asked...
Is there a separate stat on the hot water and central heating?

Yes and no - there are two - one to set the room temperature and another on the hot water storage tank to set the water temperature.

There will also be another control on the boiler that sets the temperature of the heated water that gets pumped around the system.

There is also an electro-mechanical valve that diverts the heated water to either the radiators or the water tank and sometimes to both at the same time. Under some circumstances this valve assembly sends a fire-up command to the boiler - this may be your problem.

A competent service engineer will be able to solve the dilemma for you. When you call him be sure to describe the boiler make and model.

Good Luck..., April 2017

No idea. You have told us nothing about your system. Make and model of the various parts, etc., whether or not you have a thermostat on the hot water cylinder, etc.

Maybe the boiler is firing continuously because it can only just keep up with heating demand, for some reason or other. DO the rads need bleeding? Is the pump doing its job properly?

Find out the make and model of boiler, search for and download a manual and check that out - look for whether there is the facility to connect a room stat and a hot water stat.

Have a look on the cylinder and see if there is a cylinder thermostat. Don't know what one looks like? Do a Google search for "Hot water cylinder thermostat" and you should find plenty of examples. Even switch to Google image results.

Do a bit of research and understand what goes on in a heating system and why.

acp, April 2017