Wednesday, July 1, 2009

Definite Purpose Contactors

Ok so most of you know that I have been having problems with my furnace as of late. I kind of figured that I just did not know what the heck I was doing but it turns out it was a little bit more than that. I have figured out the big mystery in the furnace.

Definite Purpose Contactors are the poor man solution to the Mercury Relay Switch. What both of these pieces of equipment do is pretty much the same. They are the big switch in the electrical path that either feeds power to the elements(what heats up the furnace) or what turns it off. It is in laymans term a light switch. ON/OFF. Instead of you turning it on and off there is a little computer that opens or closes the switch. It does this to either heat up or cool down the furnace by cutting or powering up the elements.

The problem is DP contactors are loud. They sound like the loud click your air conditioning unit outside makes when it turns on. If you really wanted a real world example of a furnace contoller look at your thermostat in your house. It is basically the same thing but instead of controlling the heat it is controlling the cool! To hot, CLICK it turns on the air and it gets down to 76, CLICK it turns it off. Same thing.

Back to the DP contractors(pictured above) the furnace that I have actually uses a DP contactor that is used in large Air units. They might last 7 years in an air unit but in a furnace...well not so long. In a furnace it is turning on and off about 10 times as often. Thus is breaks ten times as fast. They are rated for only so many on and off cycles.

What I have found out is that this particular DP unit has almost a 1 to 1 ratio with my element life. Two times I have burned out my elements from use. This is normal. Both times inside of a week the DP contactor died. Both times the elements went first, followed by the DP contactor going out about a week later. So to save myself and anyone else that is using this kind of furnace I will start replacing the DP contactor every time I replace the elements. Either that or I am going to wire in a Mercy Relay Switch(pictured above) which is good for about 10 million cycles and makes no sound at all. To be honest I kind of like the click it lets me know everything is working like it should.


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