I'll also note that as this is an older property, the wiring *may* be TRS - rubber. If it is, it is probably past end of life and should be replaced ASAP, especially on a hot water circuit.
Hot water is one of the longest-running loads in a house, often running at full current for hours at a time. It's really punishing on anything that's even near the edges of what it's rated to.
We don't generally run 1.5 at 16A any more unless it's in a commercial installation where there's no insulation near the cables.