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I found an article online from somebody who built a rig consisting of a number of pc fans mounted under a radiator, blowing upwards. A temperature sensor ensured the fans would kick in as soon as the radiator reached 30 degrees Celsius or higher. The idea was that by pushing air upwards through the radiator it would boost the convection effect, allowing the radiator to heat up the air more efficiently, which resulted in the author being able to save on gas by being able to turn down the central heating's boiler.

I think it's an interesting idea, and was wondering if anyone else had tried a similar technique. Reading up on the practice I found a number of people expanding on the theory by using pulse width modulation to gradually increase the fan speed based on heat necessity, but I'm kind of wondering if that's worth the extra parts, time and complexity.

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Use a ceiling fan instead

For the reasons you discuss, ceiling fans are often installed and used in winter to draw air upward, from this answer on diy.SE:

This directs the relatively warm air above the fan outward and then downward, making the rest of the room feel warmer.

But building it yourself is cooler more awesome!

The cheapest PWM fan I could find on Digikey at a reasonable DC voltage is $8 USD. But if there's a university/college/school anywhere near you, it may be possible to find old PCs and salvage the fans from those. (My university, University of Wisconsin, has a warehouse with bins of non-working PCs where you can pull out parts for cheap.) Typically anything with four wires will work with PWM control.

Adafruit sells a custom micro-controller with PWM control--the Metro Mini 328--for just $12.50 in the US. For a bit more, an Arduino Nano will also do the trick.

After that you'll need a dc power supply, a thermistor (depending on the fan--some will include this), and assorted wires and connectors. I'd estimate $10 to $30 for all of this stuff.

Budget

Assuming you're able to scrounge at least a few of the parts you need, I estimate you could do this for $30 per radiator if you're going to make at least four or five for your home.

Savings?

Doing a bit of back-of-the-envelope engineering, if you use four fans per radiator like the one I linked, and it's running for six hours per day on average for six months of the year, your electric usage per radiator will be about 10 kWh.

If we assume your electric cost is $0.15/kWh, and the system will last at least five years, then the annual cost will be about $8 per radiator per year. So as long as it saves you this much in heating costs per year, then it's a win!*

In reality, two or three fans would probably do, and they'll probably last longer than five years.

Other things to consider:

  • If your electricity supply is from renewable sources, then by using electricity to reduce your need to burn gas for heating, you're saving the planet.
  • While you're doing all of this work, you'll be burning calories, which will enable you to lower your thermostat and reduce heating costs.
  • The time you spend doing this is inconsequential, because it's fun! and you'll be learning stuff!



*Figuring out how much this will actually save you per year will either require lots of work, or lots of assumptions, or both. I'll let someone else tackle that part of the problem.

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Getting the heat out of the radiator isn't the only important thing. It's also important to get the heat to where it's felt, otherwise it's essentially being wasted. As cool as they might be, small fans may extract the heat from the radiator more efficiently, but unless they can circulate the warm air more evenly throughout the room the warm air may just gently rise to the ceiling and be wasted. This effect will be stronger if for taller ceilings or poorly insulated ceilings.

Given that wall mounted radiators are intended to radiate heat (hence the name) horizontally towards the centre of the room, if the proposed solution causes them to be somewhat cooler then the heat radiated will be reduced. The net effect may actually be that the room feels cooler.

Maybe you could increase the efficiency of the radiation effect by insulating the back of the radiator panel? This could be done by covering the wall-facing side with aluminium foil (shiny metal surfaces radiate less heat). Bear in mind though that this solution may cause the radiator to be hotter unless you turn down the boiler. Running hotter could void a warranty, cause fire safety issues or be a risk for small children or animals.

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