A Blower and Heat Strip Dangerous Mistake

When heat strips run on a heat pump system, we want the blower to run as well. A G call controls the blower, and a W or W2 call usually brings on the heat strips. So, we want the G call to happen during a W or W2 call but NOT the other way around; a blower relay is not rated to handle the current of the heat strips, and we don’t want to run the heat strips when we simply don’t need to bring them on.

Stack sequencers, on the other hand, can typically handle the current of the heat strips.

To create an interlock, you will need a relay. There may be specific blower relays, but a 90-370 or 90-340 relay will suffice. It’s best to think of the 90-340 relay as being upside-down when you’re interlocking the blower and heat strips. In the case of the 90-340, we would connect L1 to point (contact) 3 or 6, heat strips to point 2 or 5, and the blower to point 1 or 4. In the case of a Carrier relay, we would connect the blower to common, normally open to the power supply, and normally closed to the heat strips.

When power goes through the heat strips in the event of a W call, it enters through the contactor from L1 and goes through the fusible links, goes through the heat strips, comes out through the other side, passes through the thermal limits, goes to the L2 side, and goes back to the power supply on the other side.

When we attempt to use a relay to interlock the blower, we might be tempted to attach L1 to common. However, if we connect the heat strips to the normally closed contact, it will either run all the time, cause a dead short, or not run at all. Therefore, reading the relay from left to right isn’t useful. If we turn the relay upside-down, however, the new left-to-right orientation is really right to left. We’d connect the blower to common, and L1 would connect to the normally open contact.

If the W call forces the heat strips to come on, the power will feed through the normally closed contact to the blower. However, if we just get a G call, the relay coil will be energized, which will close the contacts between L1 and the blower to bring on the blower; it will open the normally closed contacts to the heat strips, so the power won’t backfeed to the heat strips.