Welcome to Sunline Owners Club and congrats on your T-299SR. That is a nice layout.
To add to what Steve said, this pic is off my 2004 T2499. It should be very close to your 2003. The bottom has a 3 way valve and the top has a brass check valve screwed into the top of the tank. Due to the angle of the pic, you can't see the brass check
A few questions first.
1. When the system is filled with water, does water come out of the sink faucets on the hot side? at full force like the cold. If the answer is "No" then water is not going into the HW heater or if a dribbling is coming out then flow is partly blocked and still not going into the heater.
If the answer is "yes" then it appears you have the bypass valve flipped wrong. The bypass is on the bottom of the heater in the white Tee fitting. Flip the lever 90 degrees the other way. If I recall right, (not sure) the lever handle end points to the way it allows water to flow. Meaning for water to fill the heater, the lever is horizontal. To bypass, the lever is vertical.
2. Assuming no water or very little is coming out of the hot water faucets, this points to the discharge check valve being stuck closed or almost closed. There is a brass check valve in the top outlet port of the heater and can and has been known to freeze closed. My son had his freeze up last year. It is a classic issue. Most times after long periods of no use, sediment gets stuck in the valve and the plastic plunger gets jammed. While some water may go into the heater, no air can come out and the heater will only partly fill and then no water or very little comes out.
If it is no. 2, then bleed all pressure off the system and out of the hot water heater. Open the saftey relief valve outside or pull the plastic drain plug. Make sure the HW heater is turned OFF on either electric or gas. You do not want to burn out your electric element with no water in the heater. Then take apart the top fittings on the top discharge of the HW heater and unscrew out the brass check valve. The plastic plunger and O ring in side will come apart to be cleaned or it is so bad it needs to be replaced.
The check valve is part of the by pass kit. If you are stranded in the middle of no where and need water, the check can be eliminated or the plunger removed but you will no longer have the bypass feature. The intent of the check valve is to stop water going into the heater when the bypass valve is flipped.
Hope this helps and good luck. Please report back with what you find out.
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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