Q. What is the valve assembly?
A. It simply controls the flow of water to the heater cores. The valves are vacuum actuated.
Q. Where is the valve assembly?
A. The two hoses which go through the firewall above the washer fluid reservoir, lead to the heater core inside. The opposite ends of those hoses lead to the valve assembly. (Sorry, I did have photos, but I lost them in a computer crash)
Q. Do I add coolant concentrate, or 50/50 mix?
A. The total cooling system capacity for a 2000 van with rear heat is 16.5 quarts (15.6 L), or 13.5 quarts (12.8 L) without rear heat. (Other years could be different, so check your manual) The strength of your mix will depend on the climate you live in. I chose a concentration of roughly 50%, because it will never be -40°C here. Remember: water cools your engine, and the green stuff keeps it from freezing, so don't put more than you need to. Half of 16 L is 8 L which is exactly two jugs of full strength antifreeze, and it just happens to fit nicely into an empty radiator! Then you just top up with water. If you buy 16 L of premix, how are you going to get it all in there? Concentrate is also better value for your money. When you buy 50/50, you're just paying for water! I guess 1 + 1 = 2 is just too much math for some people. Ah well you know what they say ... a fool and his money are soon parted.
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The specific heat capacity of water = 4.19 kJ/kg-˚C The specific heat capacity of pure ethylene glycol = 2.38 kJ/kg-˚C And for the 'mericans: The specific heat capacity of water = 1.0 BTU/lb-˚F The specific heat capacity of pure ethylene glycol = 0.57 BTU/lb-˚F
(The following chart is for ethylene-glycol based antifreeze. The freezing point profile may vary for other types, and you should consult the product label.)