mor-flo dsi-6 heater boiling over

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BIGDOG

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Aug 2, 2005
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I have a Mor-flo DSI-6 water heater that won't shut down when the water reaches temperature. I have changed the limit switch and the thermostat, but the pressure relief valve kicks open and the heater stays lit. Does anyone have any suggestions, and where could I get an owner's manual for this unit?

The MH is a 1988 Corsair by General Coach. My next inclination is to change out the pressure relief valve.
 
won't shut down when the water reaches temperature.
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When you say "reaches temperature", do you mean normal temperature or extremely hot?

If the pressure relief valve is malfunctioning, it will bleed off hot water when the pressure rises, replacing it with cold water, and continue to try to heat the newly added cold water. The limit switch and thermostat which you have already replaced, each should have corrected the problem if it (they) was (were) at fault. If your gas valve has an integral thermostat, you might try turning the adjusting screw back and forth (hotter and colder) a few times to clean the contacts. A sticking gas valve could be the problem, but that's less likely. Try disconnecting one of the wires going to it while it's heating. If the flame goes out, the valve is working properly, and the problem is elsewhere. If it stays lit, you've got a bad gas valve.   
 
I searched around this forum last night and found the post about the expansion cushion in the tank. I drained off about a third of the water and re-lit the burner. Sure enough, after reaching temperature, the burner shut off and the unit works as it should. I still think the limit switch I replaced was pooched as the cut-off button wouldn't function, but it obviously wasn't the whole problem.

Thanks for the help.
 
Glad you were able to find the problem. Yes, the air cushion or 'head' is important, and must be replaced occasionally as you did, as the air is absorbed into the water over time. I might also suggest you install a good water pressure regulator to keep the system pressure within 40-60 p.s.i., which should be well below the opening pressure of the relief valve, and will protect your plumbing from excessive pressures you sometimes encounter with city water supplies.
 
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