A Blower and Heat Strip Dangerous Mistake

Bryan shows how not to make a dangerous mistake when using a relay to create a blower and heat strip interlock.

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.

[vinnymac]

Perfect time to make this video, Bryan! I live in NC and much like you I mostly work on air handlers and this is all I've been dealing with since this little 2 day cold spell. Thanks for refreshing my memory!

[acrraideredward]

Always had some doubt on those relays. Thanks for the advice!

[bhagiraths]

Bryan, thanks for sharing the heat strip and its components in depth know how. You explained everything about interlocking blower and heat strip relays in such a good way. I am so happy to watch your videos. 👏👏

[LatinTurkNYC]

Never looked at it that way. It works when only calling for Heat, it works calling for only fan and it works calling for fan and heat together from the thermostat. I always looked at the relay in one way. Great video!

[robertsneed]

Wow, thank you very much. I was confused couple of days ago because I wasn't getting power to the fan motor. I saw the relays burnt underneath on the heat strip. Also my condenser wouldn't come on and I wasn't getting 24 volts to the thermostat nor to the contactor on low power.

[hvacjohnnie]

🤯The simple things like that are the most looked over and most valuable things to learn in this field of work. Thanks for sharing.

[awesomeaustin]

I learned this the hard way back in the day, thanks for sharing.

[bikerbobcat]

So helpful! Working in HVAC really got me to understand logic circuits and puzzles more. I've never even considered "backwards" relays, just the activating terminals and the NC/NO working terminals.

[jeffgrizzel]

I’ve always had trouble remembering how this system wires. Thinking in terms of turning the relay upside down is a perfect way to remember it. Great idea. Thanks.

[Stuart]

This was helpful. It might help to show the low-voltage wiring to the relay coils in another diagram as well.

[Barracuda]

In essence, the motor has the choice of power sources, from heater at terminal 2 or active relay term 3.
Will be passing this on to the apprentices.. thanks Bryan. I will pass your name on to friends in SoFla. 👍🏼

[dustyanderson]

Really appreciate the knowledge you share! Currently in school, your videos and podcasts have helped immensely. Thanks again!

[ykv]

What would be the benefit or drawback to interlocking the heating strips with something sensing the airflow directly (ie sail or static pressure)? Clogged filters or broken belts aren’t accounted for by the fan motor power. Even a locked rotor may allow the heat coils to be energized enough to be damaged.

[topher]

I always associate the common terminals with the common action. In this case we want the blower to run whether there is a G call or a W/aux call with G missing.

[Parabellum-X]

Brian, I am in HVACR school right now, and we have just started learning about Heating Systems. I was watching your video here, and, at 9:25, you said that (on a sequencer) between terminals 1 & 3 is Normally Closed, and between terminals 1 & 2 is Normally Open. However, the symbols on the side of the sequencer show the opposite! I am seeing a NO symbol between 1 & 3, and a NC symbol between 1 & 2!
Was that a slight mistake on your part? Are they actually the other way around, or is that correct? I'm so confused! Thanks!

[jacobframe]

Great video and information, thank you for sharing!

[kenbloker]

I don’t see how the heat strips would back feed through an open contactor but I understand what you’re saying.

[-JonnyBoy-]

I had a sequencer go bad in an electric furnace in one of my rentals. I went to fix it as they said the fan comes on but no heat. The sequencer was defiantly bad but it was also wired up wrong as in it would never have worked that way ever. Tenant must have messed with it before calling me lol and it took me an extra hour to trace the wiring diagram to figure out all the correct connections. (i'm not a furnace repair may by any stretch so that was fun)

[drg]

Got a call about this same thing, easy repair, moral, think it through before proceeding.

[rafaelcastro]

Wow thank you, much appreciated! Great explanation!