Simple thermostat for shop heater?

[Willyou]

I recently installed a 5000 w heater in my shop hanging it from the ceiling. It required 10 ga wire and a 30 amp 220 v circuit. It also has an internal stat. However, I bypassed it so I could install a line stat on the wall. If you go this route, be aware that working with heavy ga wire is a pain. Be sure to get a large box for your stat as all those large ga wires will take up a lot of room and be difficult to curl up in there. Also, be sure to get a 4 wire (of course) stat. One more point; when you get your stat, look carefully where the wires (color wise) are connected. Some are wire so that you will end up with red connected to black on the load side; and sometimes red to red and black to black (can be confusing. learned the hard way).

If your stat is to be a long distance from the panel and heater, what daddo said might be a better way to go

[®smpr_fi_mac®]

Yes, there's a wiring diagram that's easy to follow and simple instructions for bypassing the internal thermostat; you simply attach the thermostat's control wires to a block on the back of the unit.

I've worked with 10AWG wire and have seen 8AWG. Yeah, it can be a pain! The heater will only be about 15' from the sub box, with the thermostat in between.

I'll have to use it on the lower setting once the shop is warmed up; my sub box is only 60A.

[Willyou]

Maybe my experience with a line thermostat will help you with yours. I am experiencing strange behavior (to me) from my line thermostat. I believe the temperature control inside it is a bi-metal strip. As such, I can't explain this behavior. If I first turn it on at a temperature of 45 degrees, it will run for a while until the temperature reaches perhaps 55 degrees and then cycle off. About 5 to 10 minutes later it will cycle back on and run for about 20 minutes and then cycle off again with a rise it temperature of maybe 3-4 degrees. It will continue to cycle on and off this way with small increments of temp increase. I have not yet let it go beyond around 70 degrees. By that time I'm ready to quit.

The thermostat I have does not have any numbers on the dial. So, I don't know exactly what temp I'm setting it at. I just picked a spot in the "Comfort Zone" and leave it there. My first thought was that my heater was shutting itself down intermittently, It does have a safety overheat switch. However, I have checked with a volt meter and the thermostat is controlling the power (you can hear it click when it does).

It seems to me that the thermostat should allow the heater to run until the temp reaches what is selected on the dial before it shuts off and then cycle periodically to maintain that temp. Am I wrong?

BTW, I am checking the actual temps during the cycles with a glass bulb type thermometer hanging next to the thermostat.

[TDKPE]

Maybe my experience with a line thermostat will help you with yours. I am experiencing strange behavior (to me) from my line thermostat.

My 240V baseboard heater in my basement office does the same thing.  The thermostat is in one of the ends, so I suspect it gets a bit extra heat from the structure and lack of air flow through that little chamber, and once off, it reacts to the cooling after a while and closes again.  It will stabilize after a while, and I have it marked (like yours, no marks other than 'comfort zone' or something like that) so I can put it there, but if it's too chilly down there, I'll set it higher and let it run, then turn it down to the mark to maintain.

[Roly]

My 240V baseboard heater in my basement office does the same thing.  The thermostat is in one of the ends, so I suspect it gets a bit extra heat from the structure and lack of air flow through that little chamber, and once off, it reacts to the cooling after a while and closes again.  It will stabilize after a while, and I have it marked (like yours, no marks other than 'comfort zone' or something like that) so I can put it there, but if it's too chilly down there, I'll set it higher and let it run, then turn it down to the mark to maintain.

 It may have a small heater (anticipater) circuit in it that prevents if from overrunning the set temp too much.      My question is does he need a line voltage thermostat ?  I would imagine that a 7 kw heater would have a relay built in and just dry contacts to operate it.    8 ga wire would be very large for a thermostat.  Roly

[Willyou]

TDKPE,
Hmm? That is interesting. I really don't mean to hy-jack this thread. I hope this is useful to the OP. I did think about the possible delay due to air circulation. So, I set up a small fan and the other end of the room to run continuously. This helps, but doesn't prevent the continuous cycling with small temp increase steps.

Roly,
Your thought about an "anticipater" circuit is interesting. Except, in my case, the temp is not getting anywhere near the goal temp before it cycles off and then on again. If the OPs heater is anything like mine (it is a bit larger) the internal thermostat uses line voltage and was easy to bypass in order to put in one on the wall more conveniently located.

I'm not happy with the way mine works and the OP might not be happy with the same if it works the same way. For both of us, it might be good to know if a different type of thermostat would work better.

[TDKPE]

 It may have a small heater (anticipater) circuit in it that prevents if from overrunning the set temp too much.      My question is does he need a line voltage thermostat ?  I would imagine that a 7 kw heater would have a relay built in and just dry contacts to operate it.    8 ga wire would be very large for a thermostat.  Roly

While there are 30A line-voltage thermostats out there (not cheap, either), I agree that running 8-gauge to it (40A circuit) would be tough, especially if disconnecting both poles.  That's a lot of big wire.  The thermostat in my 36" baseboard heater (750W, 240V) has a set of 22A DPST contacts.  I don't know what's in the OP's unit, nor do I know what unit it actually is as a search turned up no 7kW wall-type heaters.

Cheapest thing for him to do is . . . nothing.  Just turn it up high until the temp is where he wants it, then turn it down to maintain, which (I'm guessing) would prevent the stop/start problem.

[®smpr_fi_mac®]

My mistake. It's actually rated at 7500 watts. I'll get a picture of it and specs later today.

Yeah, simply using the thermostat on it is the easiest and cheapest route, but as it calls for a required mounting height of 8', that kinda makes operating the thermostat dial a PITA.

It's a ceiling mount unit, with a U bracket that gets bolted to a stud in the ceiling. With a 12' ceiling, I'll instead fab a bracket out of dimensioned lumber to hang it from the wall.

Who knows. I may just put a heavy duty switch inline and use that to turn it on and off.

By the time I figure out what I'll do it'll likely be spring!

[pprobus]

While there are 30A line-voltage thermostats out there (not cheap, either), I agree that running 8-gauge to it (40A circuit) would be tough, especially if disconnecting both poles.  That's a lot of big wire.  The thermostat in my 36" baseboard heater (750W, 240V) has a set of 22A DPST contacts.  I don't know what's in the OP's unit, nor do I know what unit it actually is as a search turned up no 7kW wall-type heaters.

Cheapest thing for him to do is . . . nothing.  Just turn it up high until the temp is where he wants it, then turn it down to maintain, which (I'm guessing) would prevent the stop/start problem.

Perhaps the heater I have has a defective built-in thermostat because it can vary wildly, I can set mine at a setting (no numbers), come in tomorrow and its 53 degrees, say, the following day, without touching the thermostat it's 73 degrees, same temperature outside both days.  Mine is a 3 stage, manual select, 3K, 4K and 5K Watt heater, it is permanently mounted and hardwired, so its not like I move it around which could affect the temperature swings of the thermostat.  With my heater, there's no factory option to bypass the thermostat.  When I ran the circuit for it, I put a line voltage thermostat inline and turned the thermostat on the unit all the way up.  Everything worked good last winter I was able to set the temperature on the remote thermostat and it kept the shop within about 5 degrees of the setpoint, but about a week ago (right before this thread was created) I went into my shop and smelled a burnt plastic smell.  Could not find out where the smell was coming from until the heater kicked on and a spark/flame came out the top of the thermostat and the shop went totally dark.  Popped the 30 amp breaker for the heater, popped the 100A breaker for the shop panel and popped the 100A breaker for the circuit that feeds my shop, in the house. 

Not sure if the amp draw of the heater was too much for the thermostat, the specs called for <21 amps ( something like 20.3 amps or 20.5 amps or something like that) at the high setting, the thermostat I bought was good to 22 amps according to its specs and I assumed there was some factor of safety built into it.  The only thing I can think of is that due to voltage drop from the source (approx. 100' from house to shop, then another roughly 30' after all the bends etc, from the shop panel to the heater) the heater ended up pulling more than the 20.x amps it is spec'd to pull and actually pulled over the 22 amp limit of the thermostat. 

Anyway, not trying to steal the thread, I have been lurking to see recommendations that I might be able to apply to my heater, I only spoke up because unless my heater's unique, the built-in thermostat is not very good or reliable for set and forget.  While I'm deciding my next step, I took the line voltage thermostat out of the circuit and am using the built-in thermostat and have to adjust it almost everytime I go in there.  At this  point, I am leaning toward getting a heating relay from Ebay and have the relay turn the heater on and off and the thermostat turn the relay on and off. 

Paul

[TDKPE]

The only thing I can think of is that due to voltage drop from the source (approx. 100' from house to shop, then another roughly 30' after all the bends etc, from the shop panel to the heater) the heater ended up pulling more than the 20.x amps it is spec'd to pull and actually pulled over the 22 amp limit of the thermostat. 

That load is mainly resistive (as opposed to inductive), so a very long wiring run with large voltage drop will result in the heater actually allowing a lower current in the wiring and breakers.  Total resistance is higher, so total current is lower, for the same source voltage.

Sounds like the thermostat simply let go.
  Time for a heavy line-voltage relay and better thermostat.