Why cold wash water causes grades
Cold water cleans won't kill coliform bacteria. Heat melts fat and fuels the chemistry, and most alkali detergents need heat to work. Alkali breaks down fat and protein, but only in hot water. Fat melts at around 48C, so the wash needs to stay above 55C the whole way through.
Start at 85C going into the tub and dump at 55C or above. That gives you a 30C working zone for effective cleaning. If the temperature drops below 55C mid-wash, fat can re-stick to the surface. More chemical won't fix a 45C wash.
How to confirm it
Check the temperature of the water going into the tub, and again at the dump. You want 85C at the start and no lower than 55C at the end. If either number falls short, that's your problem.
What to do now
Fix the hot water supply first. Often it's a blown element.
Once it's fixed, run a bomb wash through the plant and the vat, with a high dose of alkali followed by a high dose of acid. A bomb wash is the right call after you've found what caused your grade, and when water temperature has been low for multiple washes.
Run the alkali first and then the acid, never both at once. Wear gloves and safety glasses when you handle them.
How to stop it coming back
Watch the temperature through the wash. If it drops below 55C, stop and reheat.
To reduce the drop, preheat the plant. Dump the first 50 to 100 litres of hot water before returning flow to the tub.
Give the wash contact time. Most farm systems give 60 to 90 seconds, and the best results come from at least 8 minutes of recirculation.
Keep the jetters flowing and fix any air leaks. Moving water does about 40% of the total clean.
If water temperature has been low during previous washes, give the milk line a full clean as well.
Get the full numbers
The Dairy Handbook chapter What makes a dairy plant wash actually work? has the numbers and how to hold the heat. How do you bomb wash a milking plant? has the full wash.