More and more modern appliances are coming equipped with the provision to add solar assisted hot water and some have a setting and/or a sensor which produces a delay before the appliance will fire the gas burner for DHW. This allows for the situation where the incoming water is already warm enough preventing the appliance firing unnecessarily. The diagram below shows a typical system layout for such a setup.

This should be considered when working on this type of system as the the appliance may not respond to a DHW demand in the expected way. This should also be taken into account when fault finding on an appliance with this capability, as if the setting to activate it were enabled accidentally it may either delay or prevent the DHW working.

This sort of installation can also affect DHW temperature rises and DHW flow rates so cylinder pipework and controls would also need to be checked as well as the appliance itself.