I spotted this dark stain on my wood floor around 3 weeks ago, last time the underfloor heating was on was in December. Yesterday I switched it on again and it seems to be getting bigger. The stain is dark, no cupping or crowning, no sign of water or moisture. Any ideas whether it's a chemical reaction or an actual leakage please.The flat is 8 years old and the wood plank is in front of the door where the water cylinder and manifold are. Thank you.