Very little hot water

I have an unvented system (megaflow 170 litre tank) whereby the central heating system is gravity fed. The system is set up so that I can turn on the central heating and the hot water separately. When the hot water is on the bathroom radiator is designed to come on too (unfortunately so do the radiators in 3 other rooms which is another issue that needs resolving!). My problem is I'm getting very litle hot water. I've noticed that the hot water return from the megaflow even after 2 hours of running gets only lukewarm. The system is 4 years old and used to work o.k. I did have a new pump fitted about 18 months ago by a very experienced plumber but again, the system has worked o.k since. One point to note, however, the system will not refill from the header tank and has to be done by attaching a hosepipe from the mains.

Sounds like the feed pipe connection from the header tank is blocked and there is insufficient water in the system - hence thermal transfer is hindered in the cylinder coil.

Check if there is water in header tank and whether connection is blocked (normally where it tee's in to system).

There is normally an air bleed point on the cylinder pipework - check if air or water is expelled - (if nothing suspect blockage)

Radiators heating up when only hot water on is due to diverter/mid-position valve letting by on heating circuit or mutiple return pipework connections from radiators

Thanks. I'm home at the weekend so will take a look. With regards to your comment about the radiators and 'multiple return connections', why should this be, is it poor design?

All the radiator return pipes should converge into a single return pipe, this then joins the main return back to the boiler (together with the cylinder return).

If this is not done, when the hot water is selected you can find some radiators will get warm.

The return water from the cylinder takes multiple paths back to the boiler - reverse flowing through the incorrectly connected radiators.