Turning a Diesel Block Heater On Based On Time and Temperature

I’m a tightwad. Few who know me would disagree. So it kind of irritates me that I have to put my truck on a block heater in the winter. I measured the heater a few years back with a Kill-A-Watt meter:


As I recall it was using 1800W and it ran as long as it was plugged in – there is no thermostat. That’s a lot of energy if I just leave it plugged in all the time.

I quickly learned that many people would put the block heater on a timer and start heating the block just several hours before the morning commute. I did the same thing using my trusty Intermatic Timer:


I programmed the timer to turn on at 5AM and off at 10AM. This system worked pretty well, but it still bugged me that the heater would periodically run when it was kind of warm and the block heater really didn’t need to fire up at all.

So I found this 115v thermostat that turns on only if the temp falls below 35 degrees:


I plug the timer into the electrical outlet, then the thermostat into the timer, and finally the truck into the thermostat. That creates a logical and:

if (time >= 5AM and <= 10AM) and temp <=35 then HEAT MY BLOCK!

Total cost of equipment is $25. I never tried to calculate the payback, but given it is consuming 1800W, I would expect it to be pretty quick.


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2 Responses to Turning a Diesel Block Heater On Based On Time and Temperature

  1. Sean Straw says:

    It only occasionally frosts (and gone by late morning) where I live, so I have zero first hand experience with the need for a block heater. That doesn’t mean I won’t share my uninformed opinion anyway…

    Seems to me, either tapping the engine coolant sensor and using that, or affixing an NTC to the side of the block would allow a uC (which could be powered from the vehicle battery, and using sleep, would be an insignificant drain) to sense the block temperature and drive a triac to switch the block heater on and off. in this way, it’s the vehicle engine temperature which engages the block heater, not the ambient temperature over near the outlet. Once your engine is above freezing due to the block heater, the thermostatic outlet isn’t going to have changed temperature much, so the outlet style switching could run your block heater the entire duration even though 30 minutes of 1800W is likely going to bring the block up to temp well enough unless you’re in a snowdrift.

    Barring using a uC, putting the thermostatic switch on the end of an extension cord so that it is inside the engine bay might improve it’s efficiency somewhat – I can’t help but expect that an 1800W heating element running for several hours will eventually radiate heat from the block. But as I said, I’ve got zero experience with the need for these.

    Perhaps you’re not the type to get up early on a weekend, but play along for this scenario: you get up early on a Sunday and run an errand. Yay, your vehicle starts easy because the block heater was running since 5am. You return a short while later (fresh fried donuts! I’ll take an apple fritter please) and dutifully reconnect the block heater, since you may not be going back out today, but you’d really like the vehicle to be ready to go come Monday morning. Now, even if the garage is cold, the block heater shouldn’t need to run for the remainder of the morning heat cycle, because the block is already hot. Likewise, if it’s deathly cold all day on a weekend and you go to head out in the evening, will the block be warm when the heater cut off earlier that morning, or will you have to crank and crank, despite having a block heater?

    Expected components:

    NTC (or other temperature sensor, but it should be able to handle the block temp when the engine is hot)
    uC (attiny85 is more than sufficient here)
    Triac, such as a BTA16-600 – needs to handle the current of your block heater.
    opto-isolator, MOC3010 or MOC3020 – you’re driving a resistive load, so a device with zero crossing isn’t significant to the design here.
    power supply / regulation for the uC (down from your 12V automotive supply, or use batteries, or a wall-wart – if I went with a wall-wart, I’d size the enclosure large enough to include it, and tie it into the supply cord coming in to supply the heater – you unplug that one external cord from the extension cable, and everything is OFF)
    Possibly an RTC module if you care about running heat only between certain hours, but I suspect that a device sensing the block temp will provide sufficient improvement in overall efficiency that it won’t be as big an issue to limit it to just certain hours – though the extension cord could still be plugged into a timer switch.
    current limiting resistor for the opto
    small plastic enclosure for the uC controls.
    AC wire out to connect to extension cable
    AC receptacle (you could nab one from an old metal enclosed power strip, where each outlet is an individual module, not molded in like cheap plastic strips are now, and not a household dual plug, though you can find single plug receptacles).

    Added benefit to having the logic onboard: you can plug in when you’re elsewhere, and it’s still efficient. I presume you don’t pack up the timer and thermoswitch.

    • Dan TheMan says:

      “if it’s deathly cold all day on a weekend and you go to head out in the evening, will the block be warm when the heater cut off earlier that morning”

      That is really the crux of the problem – it takes several hours to heat the oil so you have to try to mind-read. Even with my new system, the truck regularly gets heated and not used or used cold (and I really, really hate the sound of a cold diesel starting – it just sounds like money is being wasted in worn parts).

      Most guys use their trucks daily so if they know the truck will be used the next morning. And once the truck has been started, it is usually good for the rest of the day (at least around these parts).

      It’s guys like me, who work at home, or like my buddy that has a diesel tractor he uses to plow snow. He tells me funny stories about waking up at 4 in the morning, seeing snow and running outside half-naked to plug the tractor in (he used to leave it plugged in all the time until I told him he was using 1800 watts continuously).

      I do wish the manufacturers would at least put a thermostat in so once the oil is warm enough, it cuts off. As I recall the CAT 2MW generator for our data center did that. Of course it costs as much as a small house.

      I like your idea of sensing the block temp myself and adjusting the heat accordingly. But, alas, after spending years maintaining my own vehicles, I now detest having to even open the hood. Not even the enticement of building electronics would probably get me under the truck!

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