If there is a secondary heat source on the same circuit as a domestic gas boiler and the gas boiler is the primary heat source, you are going to possibly have problems with overheating due to the fact the primary heat exchanger is so small, when the gas boiler goes off on the stat it just cuts the gas not always the pump, the boiler will have to dump the heat either by the fan running on or normally the pump running on, by adding a funky heat source you are "possibly" reducing the ability of the boiler to dissipate the heat, which would be very important if the safety overheat went and the pump ran on pulling in heat from the secondary source, this would be brown undergarment time. By fitting a secondary heat source you are moving away from what all manufacturers i know of would allow you to do with their boiler and you would be outside of IS813 as the gas boiler would not be fitted as per manufactures instruction. Gary.