Very low wattage electric heater?

A 20 Watt light bulb?

Light bulb, or electric pipe antifreeze tape.

If you have thermostatic control then the actual power is not that important.

Any use?

Yeah - I was going to say that. Was going to suggest something like a yogurt maker, but I think they do pretty much use a 20W lightbulb.

Look for a "terrarium heater" in eBay or in a pet shop. Used by reptile fanciers to keep their lizards and snakes warm. Basically a simple heated mat.

You can probably get one with an internal stat but I'm not sure they'll adjust temp downwards as far as you would like.

Also search "vivarium heater". Same meat, different gravy.

A small light bulb wired with an old c.heating roomstat.

mark

Thanks. I had meant to say in op that I have considered using a light bulb, which would do the job and is the cheapest method. I would prefer the box was was not illuminated (even though it would not be visible externally) but I could mount a bulb within a 'tin can' arrangement. Biggest concern with using a bulb is reliability. I will give this a try and see how it goes. It'll only cost a bit of time as I have all the bits required to hand.

I agree, but he'll have Hansen complaining that some of the energy is wasted as light!

Thanks Adam, looks very useful and I'll keep it on record. For now I'm going to try a lightbulb and see how it goes through the winter.

Nice try, but if he blacks out the windows it'll all get absorbed and converted to heat.

Buy three 7W 1kOhm resistors and connect them in series across the mains.

e.g. Rapid 62-8268

P=v^2 /R = (240 x 240)/3000 = 19.2W

or do the same with 25W resistors, e.g. Rapid 62-8128 if you don't want to run the 7W resistors quite so close to their limit.

You'd need to work this out, but for my Father's seed-growing thingy I used

Putting a bulb inside a tin leads to very short bulb life. I tried this last year for a bird egg incubator. It is incredible how hot the tin gets, and how fast the bulb burns out. One bulb lasted less then 4 hours, a couple of others lasted less than a day. I then tried the small candle type bulbs. These were slightly better, though they still popped with alarming frequency, in the end I ended up with a grapefruit tin, a normal bulb only half in the tin, to dissipate some of its heat. This lasted for the few days I needed it, but the downside was the constant light, which I didnt really want. Alan.

This is a very good use for incandescent lamps. Wire two 60w in series, then they are seriously under-run and last forever. That should give you about

Two in series (seriously under-run) are very reliable.

Soil heating cable? 4m is about 25W.

Electric animal heating pad - about 12W

And if you don't want to do two bulbs in series to get extended life etc, then stick a silicon diode in series so that you run it half wave.

Best thing is real anti-freeze tape. It does your control problem for you.

Otherwise (like the anti-condenstation heater in my chisel cupboard), a big wirewound resistor.