If the original heater controls are all original from 1983, considering an all new heater may prove to be a longer term better value.
The labor to install just a tank as mainah stated, will be more than just installing new heater. While you may save some on reusing the old parts at the initital install, often times once you distrub something that old, the gas valve, the burner, the saftey relief valve, it does not want to seem to work correct again for the long term. So when it stops working with no warranty at all, you have to pay again for a new gas valve or controls as you have a brand new tank. End result is it can cost more later than an entire new heater with a fresh warranty from the get go.
I know money is always tight and understand that position, the realtity can be if they are 1983 gas controls and they still work, your lucky. How many more years will the seals and other elastomers in that old valve keep working? 1 year, 5 years, 2 months?
Part of this is, how long will I keep this camper? 6 months, 3 years, 6 years? How long may play a part of this decision too.
Good luck and 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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