Need more engineering help

[MrNate]

Ok, so I was working on a wiring diagram for the HLT - I'm converting over to electric heat. I was hoping someone could take a look at this and let me know if I did it right, if I'm missing any critical components, etc.

Here are my parameters:

- Line voltage
- Thermostat controlled (I have a Honeywell break-on-rise aquastat)
- "Heat on" indicator/warning lamp
- Warning buzzer when setpoint is reached

Sorry about the poor schematics. This isn't my strong point.

[Red T]

Your heating element is only 120v? If it is than it looks ok. A two pole switch would work for the "master and alarm". And the relay is rated for the heater load? That are the only things I can think of, by the diagram you provided.

[MrNate]

120, 121... whatever it takes.

:D

Seriously, I was surprised by that too. I saw them in home despot, so I picked one up to see how well it would work.

I haven't bought the relay yet - that's the peice I'm trying to figure out now. Do I just need to figure out how many amps the heater draws and get a relay rated for that or higher?

Thanks for the help!

[Red T]

Yes, the amps (load) will determine which relay you will need. If you don't have a properly sized (rated) relay, you will only use it once!
At a second glance of your schematic I see that you are running full load thru the stat. I would look at that too to see what the load rating is. If it is not the same or greater, of the heating element, you may want to reconfigure the wiring. I could probably help you with that too.

[MrNate]

Ok, great... I will check that out. Thanks again for the help, I will repost once I get the load rating off all the components.

[MrNate]

Ok, so I looked at the stat and the heater. Heating element is 2000w, stat says: Full Load 8w; Locked Rotor 48w. I have no clue what that means, but it sounds like it needs to be reconfigured.

I'll take you up on that offer now.

[Red T]

Yes it does need reconfigured, and I could, can, help you. I just need a day to draw it out and then send it to you. I'm working overtime, so I might not be too prompt. My appoligies. I will help you though.

[MrNate]

No worries, whenever you can - I appreciate the help.

[Mill Rat]

Originally posted by MrNate
Ok, so I looked at the stat and the heater. Heating element is 2000w, stat says: Full Load 8w; Locked Rotor 48w. I have no clue what that means, but it sounds like it needs to be reconfigured.

I'll take you up on that offer now.

OK. Watts = Volts x Amps. If your heater is 2000 watts at 120 volts (check the colt rating on your heater), then your amp draw is 16.7. If your heater is really rated for 240 volts, it'll draw half of that, 8.3 amps.

It is odd that your stat would be rated in watts, not amps (check it). The "locked rotor" applies only if your hitching up a motor to it. (The fact that it's specifying a LR draw is a big clue that those are amps, not watts). The only number you need to be concerned about is the 8 amps (I'm assuming amps from here on in). So if your heater is really rated 2kw at 120 volts, the stat won't do it alone, because it will draw twice what the stat contacts are rated for. Get a power relay with contacts rated at 20 amps and put it in between the stat and the heater. If it's a 240 volt heater, then 8.3 out of 8 is probably acceptable since your heater is a resistance load, and it sounds like your stat is made to control a circulating pump motor. An inductive load like a motor puts a lot more hurt on a set of contacts than a resistive load. Most good designs, though, only operate equipment at 80% of its rating, so even if your heater is a 240 volt model, the relay is still a good idea of you want that stat to last a good long time, but that's my own $.02.

[MrNate]

Yes, that should be amps and not watts. Good call.

Despite that brain fart, I promise (stick a needle in my eye) that this is really, REALLY a 120v element.

I will do another wiring diagram in a minute. Thank you for the help, I'm glad I posted this before I went ahead and tried it.

Oh, one more thing - If I'm using 10-2 to wire this, do I need to worry about resistance in those load calculations? Or is it negligible on such a short run?

[MrNate]

Here is my latest attempt at a wiring diagram. Will that work? I took a look through McMaster and found a 120v SPDT power relay rated at 30a (which I assume is the contact rating and not the coil rating). Part number is 7384K21 if you wouldn't mid checking that out.

Also, since I probably won't be running a dedicated circuit from the box, I guess I will have to make sure whatever I plug into is a 20a. Presumably the master switch will need to be 20a too, but I'm not sure about the buzzer or buzzer switch. Seems to me that they wouldn't need to be rated at 20a, but like I said - i just don't know for sure.

[Red T]

Nate,
Your latest drawing looks good. You should be fine to use #12 AWG for the wiring though. And it is easier to work with! 12 gauge wire will hold your load just fine without any loss, or added resistance. I did notice that the buzzer circuit is on a normaly open contact on the relay. That means that it will only go off when the heating element is off. Is that what you want? And you could get away with #14 AWG for the buzzer and related wiring. Even easier yet to work with.

[MrNate]

Yeah, that's exactly what I want. Basically what I'm looking for is to turn the master switch on, turn the buzzer on, set the stat to the desired temperature, then turn my attention elsewhere. Stat hits limit, element goes off, buzzer goes on and stays on until I flip the switch or element kicks on again.

Good news about the 12ga, because I have some lying around.

Thanks again for the help. I will order this relay and buzzer and keep you posted.

[Red T]

Sounds like you are one the right track Nate, hope to see how you make out. By the way how did you make the drawings? What program?

[MrNate]

Drawings were done in MS Visio. Not the best, but it suits my purpose well enough, and it came with Office.