Your ΔT would be expected to be around 20°C for a condensing boiler or 5°C for a heat pump. However you should check with the manufacturer of the appliance for further details.

The specific heat capacity is based on the heat transfer medium being deployed. For plain water, we'd expect around 4.2 Joules per °K per kg of fluid. However, mix in Gycol at 25% dilution and this could drop to 3.8 J/K/kg. Once again, you may need to check with manufacturers about how the SHC may be affected.