Categories: all aviation Building a Biplane bicycle gadgets misc motorcycle theater

I'm nearing the final phase of my garage refurbishment, which means that I now need to mud the drywall. Nothing fancy, just plugging up the cracks to complete its ability to resist fires. However, this means that I need it to be at least 55° F inside the garage, and to maintain that for roughly 24 hours while the mudding compound dries/cures.

Of course, the weather in Seattle just took an Arctic turn, and the first half of this week was entirely below freezing, with many (largely unfulfilled) threats of snow. However, it's still been in the 20s.

Clearly some supplemental heat was called for.

I have a 1500 watt milkhouse heater, though I know from past experience that it doesn't do a whole lot for even a one-car garage sized space in the winter. Imagining that it would help with a two-car space with an uninsulated roof (that's a project for a different day) was ludicrous.

So, I loaded up the website of my favorite big box store, and started trolling through reviews of 4000-5000 watt 240v heaters.

I noticed an odd trend: every heater would have a surprisingly large proportion of reviews that read roughly like this:

This heater is worthless. Turned it on, and it barely blows any warm air at all. Save your money.

After reading enough of these, along with some other reviews that
suggested maybe the complainers were plugging into 120v instead of 240,
I found my curiosity piqued. *Surely,* thought I, *if you plug
a 240v heater into 120v, you'd get half the performance, right?*

Then I remebered: Ohm's Law is a tricky bugger. What if it was a quarter?

Ohm's law explains the basic behavior of electrical circuits with resistive loads, such as our heater. It says that the voltage (in volts) is equal to the current (amps) times the resistance (ohms):

V = I * R

You can rearrange this to figure out any of the terms. So I wanted to know what the resistance of my chosen unit was. I knew it was a 4000 watt heater, that it ran on 240v, and that the spec sheet said it consumed 16.7 amps of current at 240v. Plugging those numbers in to Ohm's law to figure out the resistance, we get:

R = 240 / 16.7

R = 14.37 ohms

Cool. So now, what happens if we know the voltage and resistance, but want to find out the current? This is important because power (watts) is determined by the voltage times the current. This will determine if the power in watts is halved at 120v, or does something else.

So, calculator out, and we figure:

I = 120 / 14.37

I = 8.35 amps

Oooh, lookie there! It no longer draws 16.7 amps, instead drawing just half that amount. The resistance of the heater hasn't changed, but half the voltage actually pulls half the current. That means that indeed:

120 volts * 8.35 amps = 1002 watts

Compared to its full-voltage spec, the massive 4000 watt heater will only put out a thousand watts when plugged into a normal 120v circuit. Less than a hair dryer, and there are probably real-world inefficiencies that make the 240v heating element even less effective at 120v than the pure math would suggest.

And this is why you always check the specs of the thing you're buying, and don't just wire up a new outlet to fit the weird plug of this big appliance you just got (I can virtually guarantee you every reviewer who complained of lackluster performance made two trips to the store: one to pick up the new heater, and another grumbling and cursing to buy the funny-shaped outlet the new heater demanded, stupid heaters).

Posted at 15:22
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Categories: all aviation Building a Biplane bicycle gadgets misc motorcycle theater