Do you know if you have a hot water bypass valve for winterizing and do you know if it is turned to force "All" water thru the heater? If it is part way it can both fill the heater and mix cold with it at the same time.
If the bypass is set to the correct position, tell us how many seconds it took with the water running to get hot water the 1st time? Did it purge cold for about 5 seconds then just get 1 2 second slug of hot and back to cold? AND was only the hot water faucet on at the time, no cold at all?
About how long did the HW heater run the 1st time to heat up? 5 minutes, 10 minutes, 20 minutes? Or how long?
We are “assuming” you have a 2 knob faucet when this happened and not a single lever faucet. Or did you?
Pending what you come back with you may be experiencing a bypass valve partly not in the full HW only mode and maybe a stuck discharge check valve on the top of the HW heater.
Hope this helps
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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