Not exactly sure what type of water heater bypass you have. Depending on the model year, Sunline used older style bypass valves and new style. And then, a prior owner could have changed something after it left the factory.
"Assuming" your 97 has the new style Pex piping water heater bypass, it will look like this at the back of the heater. This is the standard way Sunline did these with a 1 lever valve setup.
On the bottom where the cold water goes in, is a valve with a lever on it. Yellow arrow pointing to it. The lever is horizontal to allow cold water to flow into the water heater. If you flip that lever vertical (90 degrees), the lever then is pointing up and water will flow up the vertical pipe that connects the bottom blue tube (cold) to the T at the top and then flow to the red tube (hot). By flipping the lever vertical on this style setup, the heater is bypassed from filling with water or antifreeze. Cold water will no longer enter the bottom of the heater.
At the top of the heater in a brass fitting screwed into the tank directly. It is a check valve. (hard to see in the pic as the white elbow covers the view). This pic shows the check valve better, but not the bottom lever valve.
The check valve if it is working, only allows water to flow out of the heater and not into it. So when the bypass is turned on at the bottom, the water or antifreeze will rise up the vertical pipe but cannot go backwards into the top of the heater as the check valve prevents it. This 1 one lever valve lever system with a check valve at the top is the most common method Sunline used. But not all of them.
The other newer method is to use 2 of those lever valves, one at the bottom and one at the top. In this case, the bottom valve is still pointing vertical for bypass (pointing up) and the top valve is turned vertical, (pointing down) and the water heater is bypassed. There is no check valve in this setup.
It would look something like this. Here is the heater out on the bench. Both levers on this pic are horizontal (in line with the flow) of the water. It is "not" bypassed in this pic.
Here is the same heater, installed. Just it's hard to see the bottom valve in this pic. This pic also shows the heater "not" bypassed,
Hope this helps. Ask more questions as needed and ideally post a pic of what your looks like so we can better tell you.
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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