Yellow water from new unvented cylinder

[MethodPH]

Has anyone experienced this before. Converted gravity hot water to new unvented cylinder (150l indirect evocyl) filled the cylinder up and had yellow water for about half an hour before it started to clear. Any ideas from anyone. I’m G3 registered. Was converted due to suspected split coil in vented cylinder

 

[ShaunCorbs]

Yep clearing the muck out of the pipes as it’s old gravity I normally pipe them up straight from the mains temp and give everything a decent flush / run then pipe up to the cylinder