From Reische on another forum:
Posted this in another thread but thought you guys might be interested:
So in the hot-side system the thermostat is placed pre-radiator: coolant flows in from the side (GREEN/RED) and, depending on it's temp, coolant is directed either up to the radiator to be chilled or if the coolant is not hot enough, the thermostat stays closed and the coolant is directed down through the bypass to recirculate. Of course the thermostat will often run partially open, directing a little coolant to both paths.
In the cold-side system coolant flows the opposite direction and the thermostat is post-radiator: Unlike a hot-side thermostat, the bypass control (brass bottom) is not attached to the upper assembly/diaphragm and does not move when the thermostat opens. It maintains contact so the bypass coolant can only flow up through the inside of the pipe, keeping the heat motor in direct contact with the bypass coolant temperature so the thermostat can function properly at all times. When the bypass coolant is hot enough the thermostat opens (bringing in chilled coolant from the radiator - BLUE); this also pushes the heat motor further down into the pipe which closes off the holes in the pipe for the bypass (RED) and shields it from the chilled coolant. However a small amount of bypass coolant is allowed to flow across the heat motor at all times so it can still monitor and respond to changes in the coolant temp.
A hot-side thermostat can function in a cold-side setup to a large degree but there are some drawbacks: When the thermostat opens some of the chilled coolant will mix in and expose the heat motor to cooler temps then it should be seeing to operate properly. Cooler temps then force the thermostat to start closing prematurely and once this happens, the hot bypass coolant will then start to make it open back up... and the cycle repeats. Another issue is the thermostat will always struggle to stay fully open because as soon as the bypass is completely shut off, the heat motor will no longer be exposed to hot coolant, forcing it to close again. The LMS/Even Flo thermostat attempts to address this by placing 4 holes in the bypass valve but then you always have a good amount of coolant recirculating through the bypass instead of being directed through the radiator as it should be.
In my own real world testing with a hot-side thermostat in a cold-side application I found the primary drawback was the way it handled changing conditions. You could see a nice stable temp cruising down the freeway but as soon as you exited and started slowing down to a stop the temp would immediately spike up and it took a little time for it to re-stabilize and cool back down.
