heater for greenhouse.

I got a greenhouse from two dear old ladies last May for nothing but it took me up to February to finish rebuilding it. By the time the last pane went in, the weather improved but hasn't been all that bad this winter.
I bought a camping heater in B&M for just under a tenner as they were clearing stocks. It uses gas canisters at four for £3.99.
Although this is not a cheap way to heat a greenhouse, it is a reliable way and cuts down on the cost of having to bury electric cables, which incidentally must be spades depth or so underground.
As we are getting into milder weather (we still had hailstone yesterday in Manchester!) you shouldn't worry about heating the greenhouse unless to are in the middle of a blizzard but your seedlings should go in the greenhouse to avoid them struggling for light, or going leggy as it is known.
My best performers are always broad beans which I start first on a sunny window cill. These are now in the greenhouse as they a) need more light and b) need to get used to the colder air.
Hope this helps.

Don't forget it is always cheaper to heat only part of a greenhouse.

Consider adding bubblewrap as a form of insulation and maybe even use more as a sort of curtain, to separate the greenhouse into a heated section and a non heated section.

No need to heat everywhere if only a small section is being used for plants / seeds

A wee small heater in a coldframe too would be much more effective than the same heater in a big greenhouse .... and cheaper to run to.

If you want frost-free in winter, and perhaps overnight minimum of 10C from now onwards, Electric is most likely your cheapest (apart from the significant cost of getting an armoured cable from the house to the greenhouse)

However, in order to get the cos of an electric fan heater down you need to buy a really good thermostat, and that may well cost you £50 - but should save its cost in a season, or two at the most. The problem with cheap thermostats is that they often have a hysteresis of 10 degrees or more - that means you either overheat the greenhouse by 10C before it turns off, or under-heat it by 10C before it turns back on and all your plants are then dead

I have a very accurate thermostat and it keeps my greenhouse within 0.5C

Alternatives are bottled gas or paraffin. Both of these require you to light them on nights when you think it will be cold. Bottled gas heaters have a crude thermostat, so suffer from the same hysteresis problem as cheap electric thermostats. Paraffin heaters normally have no thermostat, so you light them on nights you think will be cold and they burn until you turn them off whether the temperature is cold enough or not. Both waste a lot of fuel, hence why electricity (more expensive per kWh unit) works out cheaper for occasional use. Gas and paraffin also generate a lot of condensation - not really a problem this time of the year, with sunny days, but in winter it is much more likely to lead to rot.

A fan heater will move the air about, and get the heat to the far corners, whereas a "flame" heater will be a lot hotter near the heater, and in the corners it may be cold - too cold? - for the plants. Putting a "flame" heater under metal staging can radiate the heat and warm the pots / roots, which can allow air temperatures to be a bit lower than the plants would normally like.

(By comparison, if you want to maintain a hot greenhouse in winter, say 10C, then it is unlikely that Electricity will be cheapest as the heater will be on for long periods of time.)

Insulation is the cheapest form of heating

Forecast for tonight is -3C to -6C

P.S. In the past I have put a small plastic poly-tunnel in my conservatory, and just heated that. A 4-shelf blowaway would probably cope with just some tee-lights in the bottom.

Or bring the plants in for the night - corridor or utility room will do, they don't need light at night

thank you everyone for your input. i do have a electric heater that kicks in if the temp drops below 5 degrees, but i was more interested in my seedlings that i have growing and they have started to get leggy because they are indoors. perhaps i should just get a propagator and keep my seedlings in there.
thank you for your suggestions.

Reading this thread and in particular Kristen's comment made me remember this:

Video: Heat your home office for 8p a day - Telegraph
Flower Pots & Tea Lights Heater

Not tried it myself, but, might be tempted to do an experiment in the greenhouse when I have more time. Tonight would be a good night to do it. But I got stuff to do!

Prob not enough heat for a whole gh, but, perhaps a sectioned off bit?

that's a good idea, you can buy a huge bag of tea lights from wilkinson.
i have succumbed to buying a electric propagator. but it probably won't be big enough for all my seedlings. so maybe i will give the tea lights a go.

Worth putting a tea light under a clay pot (to spread the heat) but it will only heat a very small volume - although it is helped if that container is inside, say, a greenhouse as the temperature outside, but within the greenhouse, is still going to be warmer than outside, and no cooling wind inside the greenhouse.

But ... to get it into perspective ... a Tea-light is 20 watts ... it takes 100 watts to raise a 10' x 8' greenhouse by 1C (rough figures). So if you want to raised the temperature in a greenhouse that size by 10C, e.g. on a frosty night, you'd need to be burning 50 of them ... and I suspect that will be more expensive than the electricity for a fan heater.

I use electric in the 6x4 at home, and propane for the 6x8 at the allotment. I get 2 months worth of heating from a medium sized gas cannister on the allotment, turning it off in the morning and back on at teatime through March and April.

Kristen was right about T lights, they suck. Firstly they dont last long, I bought the 8 hour ones which didnt last 8 hours, to give cover through the night you have to light them late. The heat they give out wouldnt warm a rice pudding in week. The only possible way to use them would be in groups of 4 in a very small enclosure.

Sarz how do you know when cylinder is nearly empty??? we are struggling with using 47kg Propane cylinders we never know how much is in them without waiting for them to run out completely then its a couple of days before we get replacements and its going to be cold for next few nights.

We weigh our propane and liquid carbon dioxide bottles once a week to prove how much is left in them.

Would hate to be 20 miles from base and run out.

Potty

Paraffin heaters always struck me as kind of pointless; the need to ventilate to prevent damp and subsequent rots, would surely also suck out the heat generated(?)
I have zero knowledge of propane, so won't comment on that.

Overall, I'd agree with Kristen.
An electric fan heater, on a separate thermostat, would be my preference for heating.
My GH is 8' x 12'. I have a Parasene 593 2kW Frost Fighter Greenhouse Heater: £27 including P+P from Amazon. It's only to prevent freezing, and circulate the air, which it does just fine. To try and actually 'heat' the space would probably require a bigger unit and use a lot of juice, but for what I want to grow that would be unnecessarily expensive.
I've barely used it this winter, its been so mild (if sodden).
(NB: Be careful of leaving cable on the surface overnight; my extension lead in the early days got chewed through by a badger or something. Its buried now, so no problems).

For seedlings, I would be inclined to keep them indoors a little longer, rather than look to raise the GH temperature. I think the GH will be naturally warm enough in a couple of weeks time to sustain growth. Expanding on what wbmkk said, for added peace of mind, you could get one of the plastic blowaways (3 or 4 tier), to put inside the main GH, with your seedlings in that? That'd save on the need to heat the whole structure.

In the meantime, you might be able to reduce the legginess by lowering the room temperature, and putting something reflective behind the plants to maximise natural light.

thank you all for these great suggestions.