This is an electrical question, but I want to explain the application first:
I have an LED fireplace that I'm putting in to replace a wall heater. I wired it to run through the wall thermostat, but when the thermostat turns the fireplace on it just turns on the lights, then you have to manually turn on the heat. Since I'm already messing with the wiring I'm trying to figure out a way to have the thermostat just turn on one of the heating elements and the fan (so the lights aren't turning on and off every time the thermostat turns on), then also wiring the fireplace through it's main switch so you can manually turn it on anytime with the heat and fan included. I've included a picture of the wiring diagram, I drew in the secondary wiring for the wall thermostat. My first question is, the heating element and fan-motor are both supplied 120v, so if I wire my 120v line directly to them they shouldn't burn out without a resistor, right? Second question, if the thermostat and manual switch are ever both tuned on, it shouldn't add any amperage to the heater and fan since they're both coming from the same source wires, right? Third question, do you see any risk of frying anything else on the boards if I connect the wiring this way? Thanks in advance for the help!

 

If you bypass the power switch with a thermostat, the thermostat will probably not survive.

Normally a thermostat is used to control some other device such as a relay to then switch the power.

 

If you bypass the power switch with a thermostat, the thermostat will probably not survive.

Normally a thermostat is used to control some other device such as a relay to then switch the power.

Thanks for the response. This is a wall mounted thermostat that's made for resistance heaters like this. It can do up to 8.1A and 1000W@120VAC so it should be able to handle the fan and one of the heating elements.

 

Should be good to go then.

But why isn't the board controlling power to the heating elements. It looks like it was made to? Also in this wiring setup the board is still able to control power to the heating elements and fan even if the thermostat is turned off.

I'm assuming the heating elements are controlled by relays. If the heating elements were controlled by let's say IGBT's this would have been a different matter.

 

It is bad practice to bypass the main power switch with a thermostat, since that could leave the circuit live when a user would expect it to be 'off'.
The fireplace already has a temperature sensor. If this is sensing room temperature (most likely), then why do you need the wall-mounted stat?

 

Should be good to go then.

But why isn't the board controlling power to the heating elements. It looks like it was made to? Also in this wiring setup the board is still able to control power to the heating elements and fan even if the thermostat is turned off.

I'm assuming the heating elements are controlled by relays. If the heating elements were controlled by let's say IGBT's this would have been a different matter.

I am leaving it so the board is controlling the heaters and the fan so someone can manually turn on the fireplace and it fully works as it was designed to. And yes the board controls them with relays.

I'm also connecting the thermostat to one of the heaters and the fan to automatically control the minimum temperature in the room.

 

It is bad practice to bypass the main power switch with a thermostat, since that could leave the circuit live when a user would expect it to be 'off'.
The fireplace already has a temperature sensor. If this is sensing room temperature (most likely), then why do you need the wall-mounted stat?

The bypass power (through the thermostat) is coming from the same supply wire as the power through the mains switch, so if the circuit breaker is off for the one it's off for the other. Let's say someone thinks the thermostat is the only power supply to the fireplace and decides to work on it without turning the breaker off (which they should never do), then the mains switch would also be off and there wouldn't be any power to the fireplace still. The only time this could be an issue is if they think the mains switch is the only power to it and rely on it being off to work on the fireplace without flipping the breaker, then the thermostat drops below the set temperature and kicks on making the wires hot. But since this is going to be inset in a wall with a thermostat directly above it, it will be pretty obvious that the thermostat controls it.

It does show a temp sensor on the diagram, but there is no way to set the temperature on the fireplace itself. It's all manual on/off and adjustment. This is the only source of heat in a bedroom so it needs to have a thermostat on it, and when it turns on through the mains switch it doesn't supply heat until you push a button. So this is my solution so I don't have to deal with returning it and finding another that MIGHT work. Also I really like the idea of it being able to heat the room when it needs to without turning on the lights since it's in a bedroom.

 

Those three rectangles shown are relays?

If that is the case I would change the bypass to the other side of the main switch, so it simply bypasses the control board for those two loads.

Then you could have manual control for full power and automatic low power.

 

The first thing that jumped out at me is the fan and one of the two heater elements being wired together. If the heater is meant to come on before the fan does then with the way you've drawn it - the fan will come on immediately when the heater comes on. The problem there is also that the relay might not handle the heater AND the fan load. The fan relay, if it is meant to keep the fan on for a period of time after the heating element has shut down, will want to power the heater as well. It's just my guess, but I'd think the fan relay is not going to be big enough to carry the load of the heating element.

I predict excitement.

 

Another striking detail is the stepper motor. Not knowing what it controls, I can't guess what outcome may result there. But if it controls a flue vent, and you bypass the heating system you might have carbon monoxide leaking into the living space. Now, I know an electric heater doesn't produce CO gas. Perhaps the LED's are for effect only, making it LOOK like there's flame in the box. So I may be wrong on the CO assumption. But that motor does something. And something may be important.

Then there's the keyboard. What will happen if you back feed Line power? You might be bypassing the transformer - again, I don't know. So my advice - good or not good - is don't do this. Others have pointed out other dangers as well, such as the thermostat could end up powering a line unexpectedly when you believe it to be off. A shocking, an injury or even a death could occur. There's a reason why electricians have to go through an apprentice period of time before they become journeymen. And I'm not even an apprentice. I would not be attempting this modification. But that's me.

 

Good point on the fan relay, the OP should check to see if in fact there is a pre-on or delay-off, if so check the contact ratings and be sure they can handle the load, if fact I would check just for safety reasons because the element relay could fail.

As far as back feeding the board...I can't see any likely hood of that even if they are SSRs.

The stepper motor is likely to be part of the special effects, but only the OP knows for sure.

 

Those three rectangles shown are relays?

If that is the case I would change the bypass to the other side of the main switch, so it simply bypasses the control board for those two loads.

Then you could have manual control for full power and automatic low power.

That's actually not a bad idea. The schematic as-drawn has the potential to back-power through the relays if both the mains switch and thermostat are on then the mains switch is turned off. I was thinking I would have to add in another relay powered by the thermostat to select between the two circuits, but there might be a way to avoid this by moving the jumper to the other side of the switch and leaving it on. I'll have to look into if this solves the back-power problem and still gives me all the functionality I want.

 

The first thing that jumped out at me is the fan and one of the two heater elements being wired together. If the heater is meant to come on before the fan does then with the way you've drawn it - the fan will come on immediately when the heater comes on. The problem there is also that the relay might not handle the heater AND the fan load. The fan relay, if it is meant to keep the fan on for a period of time after the heating element has shut down, will want to power the heater as well. It's just my guess, but I'd think the fan relay is not going to be big enough to carry the load of the heating element.

I predict excitement.

The fan and heaters kick on and off at the same time. And the idea was to bypass the relays with the thermostat, but in a very specific situation the relays could be back-powered by the thermostat leg, so I was thinking I need to add in another relay to switch between the board and thermostat powering the fan and one heating element, this would prevent what you're describing too.

 

Another striking detail is the stepper motor. Not knowing what it controls, I can't guess what outcome may result there. But if it controls a flue vent, and you bypass the heating system you might have carbon monoxide leaking into the living space. Now, I know an electric heater doesn't produce CO gas. Perhaps the LED's are for effect only, making it LOOK like there's flame in the box. So I may be wrong on the CO assumption. But that motor does something. And something may be important.

Then there's the keyboard. What will happen if you back feed Line power? You might be bypassing the transformer - again, I don't know. So my advice - good or not good - is don't do this. Others have pointed out other dangers as well, such as the thermostat could end up powering a line unexpectedly when you believe it to be off. A shocking, an injury or even a death could occur. There's a reason why electricians have to go through an apprentice period of time before they become journeymen. And I'm not even an apprentice. I would not be attempting this modification. But that's me.

Yeah it's electric...no CO. The stepper is just for the "flame" speed, not a safety issue. Yeah it'll obviously be bad if I back-power anything on this board, which should never happen unless both the thermostat and main switch are on then the main switch gets turned off. Because of the application I don't ever see this happening, but because it's a possibility I'm planning on adding another relay to select between the two power sources to eliminate the potential of back-powering.
Thank you for your concern, I fully understand the potential hazards of doing this. I know you have a lot of question marks seeing this but I know what all I'm working with. A well-designed heater is be safe from the factory and a well-designed modification can be just as safe. That's why I'm on here getting other professional eyes on it to catch anything I miss, and this will be fully tested before it ever goes into a wall. I appreciate your feedback though.

 

I think the risk of something bad happening because of the back feed is very low. (The board probably has a fuse which you will most likely be bypassing and thus the safety of your board is somewhat compromised) and besides the back-feed would only be possible if the board was actively trying to power the heater (thus it's on) or when the heating from the board was on, the thermostat was on and then the power was removed by the main switch. Then the board will shutdown the next time it thinks it has reached its setpoint and not power on again. It's like it's finishing its task. For your mental well being the relay would be nice but most likely not necessary.

 

OK, well, I'm glad you're confident in your design work and engineering. One more thought of concern just came to me - that's if the heaters are 220 powered. If you jump Leg 1 to Leg 2 you will be dead shorting the two legs. In that case, again, I predict extreme excitement. Possible fire and a chance of injury as well.

Here in America (don't know where you are)common house wiring has two hot legs; L1 & L2. Since you're a mechanical engineer I'll assume you Probably know this. Nevertheless, for the sake of those who may follow this thread and not know as much - let me continue. The two legs are split with the Neutral line. So from L1 to Neutral you get 120VAC. From L2 to Neutral, 120VAC. BETWEEN L1 & L2 you get 240VAC. Years back while on an extended assignment I rented an apartment with electric baseboard heat. It was 240VAC powered. That meant it used L1 and L2 as a power source. Your schematic indicates it's 120VAC powered and shows no evidence of there being more than the one live line (L1 or L2). Your schematic is primarily showing the control board, so I hesitate to assume there's only one line and one neutral. Ground is there to take away any stray voltage or should there be a short circuit to something, ground is meant to direct the current away safely while tripping the circuit breaker.

I'm probably incorrect, but I prefer to error on the side of caution. And here's what I worry about: Back feeding L1 into L2 will prove disastrous. Like I said "extreme excitement" for sure. And probably a bewilderment of why things went so wrong so fast. But based on your credentials I won't assume to tell you you're wrong. I don't know that. I do know that I have no certainty you know what you're doing. Plenty of people have come here mis-representing themselves. So with a sense of responsibility I'm taking it up to caution you - or anyone else who may follow this thread without the necessary years of practical experience or knowledge.

I wish you well. And many years of participation here on AAC. And if no one said it - welcome to AAC.

 

OK, well, I'm glad you're confident in your design work and engineering. One more thought of concern just came to me - that's if the heaters are 220 powered. If you jump Leg 1 to Leg 2 you will be dead shorting the two legs. In that case, again, I predict extreme excitement. Possible fire and a chance of injury as well.

Here in America (don't know where you are)common house wiring has two hot legs; L1 & L2. Since you're a mechanical engineer I'll assume you Probably know this. Nevertheless, for the sake of those who may follow this thread and not know as much - let me continue. The two legs are split with the Neutral line. So from L1 to Neutral you get 120VAC. From L2 to Neutral, 120VAC. BETWEEN L1 & L2 you get 240VAC. Years back while on an extended assignment I rented an apartment with electric baseboard heat. It was 240VAC powered. That meant it used L1 and L2 as a power source. Your schematic indicates it's 120VAC powered and shows no evidence of there being more than the one live line (L1 or L2). Your schematic is primarily showing the control board, so I hesitate to assume there's only one line and one neutral. Ground is there to take away any stray voltage or should there be a short circuit to something, ground is meant to direct the current away safely while tripping the circuit breaker.

I'm probably incorrect, but I prefer to error on the side of caution. And here's what I worry about: Back feeding L1 into L2 will prove disastrous. Like I said "extreme excitement" for sure. And probably a bewilderment of why things went so wrong so fast. But based on your credentials I won't assume to tell you you're wrong. I don't know that. I do know that I have no certainty you know what you're doing. Plenty of people have come here mis-representing themselves. So with a sense of responsibility I'm taking it up to caution you - or anyone else who may follow this thread without the necessary years of practical experience or knowledge.

I wish you well. And many years of participation here on AAC. And if no one said it - welcome to AAC.

Thanks for the welcome and the caution again. It's all 120V, only 2 wire with power, neutral, and ground. And for your peace of mind, I grew up doing construction with my dad including wiring so have plenty of experience and familiarity with household circuitry, I took several electrical classes when I was working on my engineering degree, and am currently working in the irrigation industry where I regularly work with very high powered well and pump motors as well as the power that controls everything from center pivots to automated filters. In short I am VERY familiar with the potential hazards of working with electricity, and I also know enough to know what I don't know, which is why I'm on here getting extra eyes on the plan to double check everything. Also this will be thoroughly tested before it ever goes in the wall.

 

I think the risk of something bad happening because of the back feed is very low. (The board probably has a fuse which you will most likely be bypassing and thus the safety of your board is somewhat compromised) and besides the back-feed would only be possible if the board was actively trying to power the heater (thus it's on) or when the heating from the board was on, the thermostat was on and then the power was removed by the main switch. Then the board will shutdown the next time it thinks it has reached its setpoint and not power on again. It's like it's finishing its task. For your mental well being the relay would be nice but most likely not necessary.

Yeah the only situation where this can be a problem is if the thermostat kicks on, then someone manually turns on the fireplace, then manually turns on the heat (it doesn't turn on when the fireplace turns on), AND THEN decides to turn it off before the thermostat reaches the set temp and hasn't kicked off yet, then it would back-power through the board. This would definitely fry the thermostat because it's only rated for 1000W and it would suddenly be running both heating elements. What I don't know, and is one of my main questions, is if the circuit board would survive the back-powering. The board is fused, but that won't protect against back-powering, just overpowering.
Overall this is a very unlikely scenario, BUT since it is a possibility I need to plan for it because of the potential dangers if it happened.

 

If the thermostat can't handle the load then you need that relay.

Getting power through the relais wouldn't be a big deal, unless the single relay can't handle the total load aswell. You will probably bypass the fuse and maybe some filtering and overvoltage circuitry, but if you are only doing it for short periods of time it should be OK.
This is a bit of guessing though, a picture of the PCB could give more info.

 

I also know enough to know what I don't know

Same here.