I have an indirect water heating system and the hot water from the tank is running into the central heating expansion tank, it does not fill up and go down the overflow so I caan only assume that the central heating water is leaking into the hot water tank and going out of the overflow into the expansion tank and the expansion tank is compensating for the loss of central heating water. 1. Am I right in assuming this to be the problem and 2. is it fixable or do I have to buy a new tankl 3. Do I just leave it ?

Are you getting your heat from underground?

Sorry doug, been away, My hot water is heated by an oil fired boiler with a circulating pump for the central heating, the same boiler heats my hot water by circulating the central heating water through a coil in the hot water tank (HWT) and so heats it up. The HWT has an overflow (I suppose in case the water boils?) which feeds into the central heating system expansion tank and then overflows outside if it overfills. When my central heating is switched on there is a constant trickle of water into the expansion tank, the level of water in the expansion tank never increases so the flow into the expansion tank must equal the water loss from the central heating system. I'm pretty confident that the coil within the HWT is leaking, just want to know if it is repairable?

I would think it is too costly to repair.

Disconnect lines and pressure check to be sure.

If water from your heating system is leaking into your domestic hot water, it could be back feeding into your potable water. You need to get it fixed now. How much? That depends on how old the system is, can you buy any parts you need, and how easy it is to remove and repair/replace the coil. If you are not comfortable with removing the coil yourself, better have a HVAC professional look at it.

Just cap off the lines that went to the domestic water tank and install a conventional water heater, should cost approx' the same to operate and allot less to install.

I think however, something is wrong with your understanding of the system, the expansion tank for for boiler should be about 15 psi and the pop-off is rated at 30. I think you are calling the overflow the pop off. If this is a low pressure steam boiler, it may truly have an overflow, so the boiler doesn't flood, but since you said you have a circulating pump that can't be.