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Old 03-23-2020, 02:41 PM
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Sky Campbell Sky Campbell is offline
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Originally Posted by Cecil McKenzie View Post
In my experience mercury relays have always failed on. I have melted paperweights and multiple vases melted together as evidence. When i started using my small Denver unit I had a couple failures with the MDRs failing in the off position which surprised me . After I started my furnace this year it ran fine then failed in off position. I could tap the relay and it would work again but then fail again. I replaced it with a relay I bought 8 years ago. It was different from many I had used in that it had brass bars connecting the two sides of the coils top and bottom. The manufacturer said this was to make them synchronized.

i was pleased since it ran much quieter than the old one. About 5 days later i noticed the furnace was at 1972 F when it said it was holding at 1950F. I am pretty sure it had been higher and then become unstuck.

When I talked to Denver they said that they thought solid state relays always stick on. I talked to Brice recently because I knew he used solid state relays. He said he though his have failed sometimes in the on position and sometimes in the off. Denver suggested a fail safe of some type to keep furnace from over heating.

These considerations have prompted me to seek advice from this board.

The only possible explanation for a mdr to fail on would be if you used a normally closed relay. That is the power flows through the relay until it is energized then the connection is cut.
Denver uses normally open relays as we all should for heating applications.
Understand how a mdr works. It is a tube with mercury inside. There are contacts at the top and bottom when it is energized a plunger is forced into the liquid displaced to make the connection. There is no arching and no contacts to wear out. It is in theory impossible to stick in the on position. They are rated in the millions of cycles and are the most reliable type relay you can use.

Solid state relays have a tenancy to fail and short on the output causing the heater to stay on as the circuit is welded together.
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Last edited by Sky Campbell; 03-23-2020 at 04:45 PM. Reason: Backasswards
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