Tag: electric heat

In residential air handler/fan coils it is common to use a high voltage interlock between the blower and the electric heat strips to ensure that the blower comes on whenever the heat is on.

The problem is that it CANNOT work the other way around where the heat comes on with the blower.

Heat strips are generally going to draw 20+ amps depending on the voltage and KW rating which means you CANNOT power them through a typical relay like a blower relay or board which are generally rated for 15 amps or less.

The way the interlock is wired is really quite obvious but is easy to forget because it’s the reverse of what we are used to with a relay.

In short, we connect the blower to the “common” terminal on the relay, L1 power to the normally open (n.o.) terminal and the load (out) side of the heat strip contactor /relay/sequencer to the normally closed (n.c.) terminal.

This diagram from Carrier shows the blower connected on the common terminal and constant power coming in on black to the normally open terminal from the right side of the transformer primary.

Using this 90-340 relay as an example, the blower would connect to 1, power to 3 and the heat strips to 2.

I made a video on it as well if you need it. The result is that the blower runs with the heat but the heat doesn’t run with the blower.

— Bryan

I started working as a tech when I was 17 years old, fresh out of tech school. My first winter out on my own I went to a service call in an older part of Orlando, a part of town I had never worked on before. It was an especially cold Winter that year, and the service call was for insufficient heat.

 

When I arrived, I found the system was a really old GE straight cool system. After testing the system, I found the system had a 10kw heater, but only 5kw was working. After a closer look it was discovered that 5 KW of the heat was disconnected. This was no problem for me; wiring was always my specialty! I grabbed some #12 stranded and had that puppy heating in no time.

Problem….

#1 – It smoked like a chimney and set off every alarm in the house

#2 – Once I got the doors and windows open and the smell cleared out as best I could it got me thinking… How long has it been since that second 5kw was connected?

When I looked closer I saw that the feed wire going to the air handler was only #10… then it dawned on me.

The REASON they had one-half of the heat disconnected was because the breaker and wire size were only rated for 5kw. Why did they a 10kw you might ask? Likely it’s what they had on the truck and they figured if they disconnected one-half it would be safe.

Lessons to learn –

#1 – Never assume that a system was installed properly, to begin with and keep an eye out for proper feed wire size.

#2 – Don’t use improperly rated heat strips or other rated parts and simply make an “alteration”. When the next technician arrives he likely won’t understand what you did. At best you confuse him, at worst you kill him.

— Bryan

P.S. – We released a new podcast on circuit boards today, you can listen here

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