A lead technician diagnoses why a Bryant 604D unit's defrost cycle won't start: the timer was bypassed and a wire rerouted directly to the contactor. He walks through reading the wiring diagram, then covers HVAC math for airflow, BTU output, and electrical power calculations.
Pull up the wiring diagram and I'm talking through it to him. There's no wire on T1. That's what starts the timer. So it shows you right here, when a Y call comes in, it sends out a Y call. The Y call goes through the low pressure switch, the high pressure switch, comes back, activates T1. T1 starts your timer. And then puts it out on T2, which starts the compressor. Somebody had decided to take T1 and pull it out and put it right here. Yeah, and so the contactor had T2 and the T1 wire on it, so either one could bring it on. And so it'll never go into defrost, so the timer never got started. And so when I start talking to him, all right, it should be a wire on T1. He said, it's not a wire on T1. So we start tracing, tracing, tracing, and I said, well, there's a wire on Y1. He pulled Y1 off, the unit shuts off. I said, well, I have a feeling they took that wire and went straight to the contactor, and we traced it down. That's exactly what they did, because when they pulled Y1 off, Y1 should have never turned the compressor on by itself anyway. Yeah, well, now he left it through the pressure switches. Oh, okay. Yeah, but he just bypassed the timer. He bypassed— he went from here to here. Okay. And so the, the board knows I've got a Y call going out, but I don't have a Y call coming back. The board thinks that one of these pressure switches is open, and so that's why we would never get a defrost. And so the biggest moral of the story is if we can't find a wiring diagram— like, anytime there's an open terminal I know, I imagine what happened was somebody somewhere, may not have been us, bypassed it. And when we got there, it was probably summertime or something, and, uh, they just went straight to it. They could figure out what was going on. And we got there, we replaced the board exactly like we found it. And but whenever you see an open terminal, probably need to verify your wiring schematic to make sure that, uh, it's supposed to be an open terminal. Definitely after we replace it and continue to have issues, you couldn't even get it to short in defrost, could you? No. So whoever replaced it the first time never, you know, just run a short to make sure it's going to bring on the switch or reversing valve and do all the things it's supposed to do. So we also need to test it. That was my biggest thing is use. Use Google. So it was a 604D something another Bryant unit, uh, 604D install manual, and that pops up. Found the wiring schematic. Luckily that was the right one. Oh, Scott, for a residential unit, a two-stage, would a high-efficiency filter affect and cause high pressure lockout? It could. A lot of times you got ECM motors, and so those can overcome a lot of things. This is a commercial unit, so you got what you got. Okay. But residential, it could or couldn't, because you got ECM motors, they can get up to a 1-inch static pressure, you know, instead of a half-inch static pressure. But on any particular unit, we should be able to know what our temperature rise is. If I got a— here we go, learn some math today. I got an 80,000 BTU furnace. MBH. Alright. And it's 95% efficient. Right, so it's putting out 76,000 BTUs of heat, right? And it's on a 3-ton unit, so my CFM should be about what? About 1,200. All right, so 76,000 Divided by 1.08, 58.6. So if I've got 70 in, I should have 138.6 out. I've got 70 in and I got 160 out, I can do the other way. All right, if I got 70 in, 160 out, that's a 90-degree rise, right? So 76,000 divided by 90 divided by 1.08. I got 781 CFM instead of 1,200. I got a problem. Now, with gas, it's not an exact science because it may or may not be 95% efficient. It may be old, but it is a roundabout. But those of you who've been here before with electric heat, it is an exact science. And we remember that formula, right? No. Exactly. All right, so this is CFM times delta T times 1.08 equals BTUs. All right, that's how all this comes— this is how all this comes about. All right, the only thing that's different than electric is There is 3,415 BTUs per kW. How do we find out kW? Just look up there? No, we're going to actually measure it. kW equals amps. You can't read it. Amps times volts. Yes. And that equals, actually that's watts. So amps times volts divided by 1,000. Is Kw. Alright? If it's 3-phase, watts equals amps times volts times 1.73. And so, once I know my watts, I can figure out my BTUs and I can use this formula. And that on electric heat is exact. I mean, I'll say electric heat is 100% efficient. That's why I tell you guys, lean on us. Between Scott, Skip, and heaven forbid you ask me a technical question, they have the answers. You with me? So any problem you run into weak enough. Between the professional engineer brother, it was very rare to have a professional engineer who has actual install experience.
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