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Sequencer relays having gone bad are probably 
the most common reason for this. Each one of the heat strips feeds through these 
sequencer relays as their control mechanism. If one sequencer or a set of contacts 
on one fails, then one or more heating strips will not work.


Each one of the above pictures is a sequencing relay.  A delay occurs before each set 
of contacts close and a delay before the contacts open again. One in the picture has 
one set of contacts, another has two, one has three and the other has four sets of 
contacts. The contact is made through the terminals that are opposite each other on 
a horizontal plane. Each set is numbered 1 through 5 (if it has that many contacts).

The terminals at the bottom are for the 24 VAC control and the stack or stacks above 
it is a set of contacts. The ones that have three, four or five contacts are actually 
two or three relays setting beside each other on a common base, but are sequenced so 
there is a delay after each one makes contact before the others make. 

You can control a 5 KW heat strip with each set of contacts or control a motor. 
Sometime it is wired to bring on the first strip AND the fan at the same time on 
the same contact.
Some units will bring in the strips with contactors like this one. This is less common 
and may require several contactors and they have no delay factor.  They are heavier duty, 
though, and will probably last longer than the sequencer relays.

Burned wires can be anywhere and may be the problem.  
Check for these carefully and repair any that you find.

			TIP: If a wire in burned at a terminal of a relay, 
			control module or other sensitive locations, chances are the part will have to be 
			replaced.  If you replace the wire and not the part, the part may fail later.

Burned or broken heat element strips.
			electric heat strips
The heat element wires are a special cadmium/nickel compound and are, of course, 
isolated from touching anything.  230 VAC is applied to the ends and the wire glows 
hot --- NOT RED HOT!  It doesn't need to be red hot.  If enough air flows over it, 
it will be black hot.  Glowing will only occur under the insulators. If these strips 
are allowed to glow without air flowing over them, they will quickly burn in two. 

You can buy replacement re-string kits to replace the wire strip and thermal links 
for just 10 to 13 dollars.  You will have to replace the framework also if the 
insulators are broken and this can get much more expensive.

Low air flow will cause the strips to glow too hot and the thermal 
protectors may be cutting in and out to prevent overheating.  Check the filter, 
grills and fan for airflow problems.

Thermostat anticipator setting is too low.  Check the section on 
Thermostats (from the menu at left) for a detailed description of the setting. 
It may need to be raised to allow longer cycles.

Tripped circuit breakers or blown fuses to the supply voltage may be 
keeping some heat strips out but still allow others and the fan to run normal.
link back to the heating fix page