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Tuesday, October 23, 2018

Exploring efficeincy of electric heating my bedroom

This spring I moved into the smaller bedroom of my house so I could rent out the much larger bedroom full-time.  I had been renting the small room out on airbnb previously, but only during the warmer months .  While the large bedroom has an adequate Rinnai propane heater, the small room has no permanent heater.  As winter is approaching I have started using a small 1500W heater made by Pelonis.  

I bounght this neat little unit back in the 1980s.  It contains a ceramic disc heating element that was a spin-off from NASA space shuttle technology.  The clever feature of this unique heater is that it can continuously vary it's output in response to room temperature.  The temperature sensor is embedded in the power cord a few inches from the heater.
A simple knob lets you set the temperature, and a switch allows automatic (thermostatically controlled) or manual setting.  The manual setting just stays at a fixed power level.  Output power can range from 350 to 1500 Watts.

In use what happens in the automatic mode is that as the room approaches the set temperature, the heater and fan speed dial down across a continuous range.  The issue I have is that the fan can produce some odd harmonics at some speeds that can be annoying - it sounds like a jet engine winding down.

Recently I came across this clever programmable wireless thermostat that can switch power to a room heater plugged into its switching module.
wireless programmable thermostat
So I bought a couple of them (click image for details) since they were relatively inexpensive and appealed to my geek nature.  I like the programmable features that let you set up to 8 programs for specific days and time of days.  Of course actually programming it is very tedious, but I figured it out eventually. (And while the picture shows Centigrade, it can be set for Fahrenheit).

The obvious application was for my bedroom, so I could program the heater to set back during the day and warm up the bedroom at night before I go to bed.  As an interim solution I had been using a programmable timer to turn the heater on and off in its automatic mode, but this left the room too cold during the day if I wanted to take a nap.  I like these $10 timers from Harbor Freight (click image for details).
https://www.harborfreight.com/digital-timer-95205.html
Like the thermostat, it takes time to figure out, but can be used to set multiple times of on/off cycles per day and has a manual override button.  

So I decided to test the new wireless thermostat with the Pelonis heater and compare it to the timer that would activate the heater in it's auto mode only at night.  With the wireless thermostat, I set the heater to switch on at full power of 1500 Watts and let it cycle on and off based on the temperature sensed at the thermostat on my bedside table.  This means that I would not hear the annoying whine of the heater at lower speeds, but it would switch off for periods which is better, and while loud when running it is less irritating.

So being a geek, I decided to see which mode is the most efficient from a power standpoint.  In theory using this heater to maintain a fixed temperature in the room should be the same either way.  But the devil is in the details.  I set up a data logger to record outside temperature, room temperature, heater power and outside ambient light (because why not!).  All the sensors are ones that I custom built for other projects.  I also used a Kill-A-Watt meter to record cumulative power.
Test setup with datalogger, sensors and Kill-A-Watt meter

 Here's what I learned.
click image to embiggen
On day 1 the wireless programmable thermostat switched the heater on and off and used 5.2kWh.  On day 2 the heater used its internal thermostat and used 6.5kWh.  Note that at the left end of the chart the wireless thermostat was activating the heater periodically to maintain the set-back temperature of 65F.  As the winter gets colder, this will be a bonus.

The outdoor temperatures were roughly the same averaging in the mid 30s, so it's a fair comparison and clearly the the external thermostat wins and will save me about 1kWh (at the utility rate of 15cents) a day in energy costs while also reducing my carbon footprint.  Maine's electric power is almost 40% renewable, and my solar power system provides much of my energy in the winter anyway so the carbon footprint aspect is relatively moot.  I have found that I prefer the sound of the heater switching on and off and sleep better not hearing the whine of the reduced power mode.  The only drawback is the wide temperature swings of over 5 degrees, but I can handle that.

At some point in the future I may explore using a different heater like an oil filled electric radiator because it should be much quieter.





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