Posted Thursday, April 17/08 in Mods & Tests

OK, so spring may not the best time to be talking about block heaters. Then again, the planet has two hemispheres, after all. So while it may be finally warming up here, somewhere somebody may be plugging in a block heater right now...

Plus, who says block heaters are only for sub-zero use? I use mine three seasons of the year. (And the only reason I don't use it regularly in the summer is because I'm lazy.) Toyota's hybrids (like the Prius) employ their thermos-like tanks to preserve coolant heat for efficiency gains whether January or July.

The question this post addresses is: how long should you plug in a block heater? In other words, what's the shortest time needed for the maximum temperature rise?

Overview

• Why use a block heater?
  
• Test details: two block heater styles tested
  
• 300 watt warm-up time
  
• 800 watt warm-up time
  
• Combined 300w + 800w warm-up time
  
• Observations

Why use a block heater?

I've written about block heaters before, so I won't go into much detail. From an efficiency standpoint, the big reason is because engines use less fuel when they're fully warmed up - as demonstrated in the cold start idling warm up experiment. In addition to efficiency benefits, they also decrease cold start engine wear and deliver heat to the cabin sooner.

Test details: two block heater styles tested...

The purpose of the test was simple: to measure the amount of time it takes to warm up the Firefly's 1.0L / 993cc engine from a cold soak (over night) condition with two different block heaters, separately and then combined. The block heaters in question are:

• A 300 watt external bolt-on element style heater. This is the OEM block heater available for this engine. I installed it in December, 2005.
• An 800 watt inline tank style coolant heater. I installed one in April 2007 because I wanted faster warm-up and higher temperatures than I was getting from the OEM unit. It's mounted inline the heater core return hose and uses convective circulation.

Both heaters use standard (North American) household power, 120 volts.

Temperature readings were taken from a ScanGauge every 15 minutes.

Note: the engine was started and idled for 30 seconds every 15 minutes to mix and circulate the coolant past the temperature sensor to get an accurate reading. This was necessary with the 800w inline heater due to "localized" (uneven) heating of the coolant. Both "pre" and "post" idling temperature readings are noted on the graph. The same method was used for the 300w heater measurements, even though there was negligible difference between the before/after idling temperatures with that heater.

300 watt warm-up time...

800 watt warm-up time...

1100 watt warm-up time (300w + 800w heaters running simultaneously)...

Observations...

• Regardless of the wattage, about 75 minutes appears to be about the time the temperature rise starts leveling off, give or take 15 minutes.
• Anything beyond 2 hours is a waste of power.
• With the 1100w combination, 84% of the total potential warm-up occurs in 45 minutes, 93% in 60 minutes.
• The ideal plug-in time will vary between vehicles depending on the mass & heat dissipating properties of different size engines. This data may translate to other small engines, but anecdotally, I've read that 2.5 to 3.5 hours are required for a 3.3L V6 engine to reach a stable temperature with a 400 watt heater.
• Of course the only way to know for sure what's best for your engine size & block heater power is to try it yourself!

Resources ...

• In praise of the lowly block heater - MetroMPG.com
• Mini-experiment: cold start fuel consumption and warm-up time - MetroMPG.com
• Discussion of this experiment at EcoModder.com
• Heat engine block for efficiency? - more discussion on this topic @ EcoModder.com