Questions about central heating/hot water tank

[ASmidgeofMidge]

We have a fairly old gas boiler and a hot water tank in an airing cupboard. I think our system is gravity fed (cold water tank in loft). Our hot water tank has the option to be an electric immersion heater (although we rarely use this). There is a thermostat on the top of the tank - does this serve only the immersion part of the system? ie is there a separate thermostat to control the hot water temp when it's being heated by the gas boiler? Where would this thermostat normally be located?

Sorry - very boring question but hoping someone can help...

The immersion heater has its own thermostat under the round cap. If there is a rectangular thermostat strapped to the side of the cylinder it will be to control the pump, three port valve and boiler.

Have you recently tried the immersion heater to check it works?

What colour is your cylinder?

Many apologies for delay in returning. Cylinder is yellow and has a round cap on the side, as well as a Honeywell branded thermostat-looking thing at the front

yellow foamed cylinder is fairly old and not up to modern insulation standards, also it means this design of cylinder will take longer to heat up from the boiler than a modern one. Not worth changing it unless it goes wrong or is too small, but it would be an advantage to fit a red insulating jacket round it to save energy, and put rigid foam lagging on the hot pipes.

Round immersion cap on the side is good, as it means that all the water above the cap will be heated by the immersion, so more hot water than most. Test it before winter. Is there one on the top as well? Look for a 2-way switch if yes. Immersion heater thermostats and their elements are service replacable parts and do not last as long as the cylinder.

As you have a Honeywell thermostat strapped tpo the side of the cylinder, you almost certainly have a fully pumped system, which is quite effective and modern, and easy to service. The main difference would be that a more modern condensing boiler would use less gas, and a more modern cylinder would heat up in about 20 minutes. Nothing to be concerned about, though, and no need to spend thousands just for a change.