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Electric heating - total wattage or watts per element?

Electric heating - total wattage or watts per element?

Good morning.

I feel like this should be fairly straightforward, and I'm currently testing it, but I had a question about electric heating. Is heat output determined by total watts consumed, or by wattage of the individual elements? Heating is I^2R, so I'm thinking more current per element should result in more heat. But part of me says that watts is watts.

I came across a scrapped 600v 10kW unit heater and have rebuilt it as a garage heater. Most available garage heaters are pretty much junk, and these old industrial ones are built pretty solid. I've removed the failed control transformer, replaced the contactor and fan, and have configured the elements (there were 9, but one had open-circuited). It runs at 240v, and for the last few years I've had it configured for about 4800w. Each element is around 10 ohms, and I had two in series paralleled with two in series, for total resistance of around 10 ohms. I figured it would be less stressful on the elements this way. (I know that calculates to about 5700W, but the elements aren't exactly 10 ohms, and I measured 20.2 A)

I wanted to add a programmable thermostat, and the only line voltage one I could find is rated for 3000W max, so I need to reconfigure it. I'm shooting for around 20 ohms total. My options are to just use 2 elements in series, or to parallel two sets of 4 in series. Both should total the same resistance and draw the same current. In the 2 element in series, each element should consume 1440W for a total of 2880W. In the 4 paralleled with 4, each should consume 360W for the same total.

My question is, will the total heat output be the same? It should I think, but I'm not 100% certain.

Any thoughts?

RE: Electric heating - total wattage or watts per element?

The simple answer is yes.
The more correct answer is:
Slightly more heat with the lower wattage per element or cooler running elements..
Not enough to make a difference to your thermostat as in all cases the heat output will be slightly less than the value calculated with the measured 10 Ohms.
As the element heats up, the resistance typically rises as described by the temperature coefficient of the element material.
The higher R means less I2 in the hot element in the I2R equation.
Elements at around 10 Ohms: That would be 24 Amps on 240 Volts.
The measured current of 20.2 Amps equals almost 12 Ohms. The increase in resistance may be a reflection of higher resistance at higher temperature, a wrong assumption of the actual voltage, or both.

"Why not the best?"
Jimmy Carter

RE: Electric heating - total wattage or watts per element?

Add a contactor to the main heater circuit, switch the contactor with your programmable stat. You can pick up contactors for next to nothing on eBay. Include a fuse for the control circuit!

RE: Electric heating - total wattage or watts per element?

I totally agree with Scotty. I'd NOT dream of using a line voltage thermostat on that monster.

I'd get two contactors and a thermostat that has two stage heating. Most programmables do now. If the temp is two degrees below setpoint close both contactors to run the whole heater. If the temp is less than 2 degrees turn on only one contactor.

Keith Cress
kcress -

RE: Electric heating - total wattage or watts per element?

Nice idea on the two-stage heating Keith.

kw1 - if you plan to leave a powerful heater unattended, consider adding an independent supervisory 'stat with a manual reset, wired to drop out the line contactors if the air temperature reaches, say, 30°C. You don't need a fire in your garage.

RE: Electric heating - total wattage or watts per element?

Quote (OP)

I've removed the failed control transformer, replaced the contactor and fan,
An 10 kW industrial unit heater reconfigured for 2880 Watts may not be that bad, but the high limit is a good idea regardless.
low1, why don't you control the contactor with the programmable thermostat?

"Why not the best?"
Jimmy Carter

RE: Electric heating - total wattage or watts per element?

Thank you, everyone.

Regarding why I don't add a contactor, there is one there and it's been in use for the last few years, but it's coil is 240v. I don't believe a low voltage programmable thermostat would work with a line voltage contactor. I'm looking at tracking down a 240/24v transformer and a relay to drive the contactor (with a low voltage programmable thermostat to drive the relay), but I'm afraid it's getting needlessly complicated at that point, with more potential points of failure.

I had originally tried to use this line voltage programmable thermostat to drive the contactor, but it requires a minimum load of 1.25A, and it just kept dropping out and cycling. So that's when I decided to reconfigure. Interestingly, in my not-so-scientific testing, 2 in series "feels" warmer than 4 in parallel with 4. The former blows warm air, the latter blows not-very-warm air. My explanation for it is that the single series circuit has a higher current flowing through it, and so with i^2r, the 2 elements get warmer than any of the 8 would. So it seems to me that the individual wattage (and therefore heat) of the elements matters more than total wattage. I was surprised by this. I might actually get a thermometer to get some real numbers, but the air coming out of the heater was noticeably warmer with the 2 in series.

RE: Electric heating - total wattage or watts per element?

Use the thermostat to drive one string directly and use the contactor to drive the rest of the elements.
With the lower current you are getting a lower temperature spread over more area.
Same Watts but less efficient heat transfer at a lower element temperature.

"Why not the best?"
Jimmy Carter

RE: Electric heating - total wattage or watts per element?

Find a line voltage thermostat without a minimum current rating and use it to drive the contactor?

RE: Electric heating - total wattage or watts per element?

Waross, that is something that is definitely possible and I'm not sure why I didn't consider it before. Thank you. At the moment, my ~2600W unit heater is doing a good job of maintaining the temperature in the garage. I'm not sure what will happen when it gets to -40, though.

If someone can point me to a programmable thermostat that will work on 240V control voltage, preferably that uses a relay and not a triac, that would be capable of operating a contactor, I'd love to have one. I haven't been able to find such a thing, hence my current set-up.

Thank you everyone for the replies.

RE: Electric heating - total wattage or watts per element?

You didn't search very hard, or maybe at all? Some Honeywell line thermostats use a relay output.

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