Re: Domestic c/h with Vaillant ecoTEC plus 837 boiler. For the past 12 months or so, boiler water pressure has intermittently dropped from the normal operating pressure of 1.8bar to 0.3bar overnight, indicated by fault code F75. I would re-pressurise the system and monitor pressure for a couple of weeks noting stable pressure then forget about it. A couple of months later, I would wake up to no-heat and/ or no-DHW and re-pressurise again. About the 3rd occassion I called out my local installer/service engineer and advised him that there had not been any slow reduction in water pressure. He did a visual search but could not find water leak. The Vaillant service engineer was then called out and replaced faulty water pressure sensor which was 'gunged up'. 2 weeks later, same problem, woke up to no heat, water pressure at 0.3bar, code F22 this time. My local installer called out, visual search for water leak, none found. I then agreed to him adding C-Tec Miracle Seal to the system. The product information states that it is "100% guaranteed not to clog the system in accordance with application instructions." However I have a radiator with TRV set for frost protection, which, by coincidence, won't now heat up when TVR operated to call for heat. I have checked for air-lock and bled the rad, no air or aerated water released, just lots of clear water....no sign of the coloured sealant nor sealing of the 'leaking' bleed valve!. Before calling out my service engineer again, I am hoping this forum's experts could help with my queries viz:- Q1 - Does anyone know of a case where adding the leak sealant can cause problems?. Q2 - Why hasn't the sealant 'sealed' the bleed valve?. Q3 - What is the likely reason for the rad not heating up ?. Q4 - What other causes of loss of water pressure in Vaillant boilers are regularly recorded?. Looking forward to your responses....many thanks.