Eating the Heating

One thing any greenhouse owner has to think about is what will be grown in the greenhouse and, if plants will need a minimum temperature, how to keep it warm. I’d mention that, at the moment, I have no insulation on the greenhouse – I wanted to see how things panned out over a winter before deciding what, if any, type to get.  The main glazing is pretty much airtight though there are two louvre panels which are obviously not. The sliding door has brushes to seal its edges against the main framework.

It may be that all you need is frost protection – keeping the temperature inside the greenhouse to about 3-4 degrees C. But if, say, like me, you’re going to grow a lot of tender annuals then remember that begonias, for example, grow best at a minimum temperature of 15-18C in their formative stages. If you need the greenhouse to germinate seeds, some need a minimum temperature of over 20C to germinate successfully. Though it’s a bit expensive to heat a whole greenhouse to that temperature unless you’re germinating LOTS of seeds at once; a propagator or window sill in the house is slightly cheaper!

I’m not a fan of electricity in greenhouses; indeed I’m wary of it outdoors, going into multi-stage protection overdrive with any mains-powered stuff which ventures outside the door.  So I ruled electrical heating out completely.

Diverting for a minute, though..... Both paraffin and gas heating generate moisture. If whatever you're keeping in the greenhouse needs to be kept dry - succulents for example, then electricity may be your only option.

If you ignore the tiny little paraffin heaters that could just about keep an outside toilet from freezing, there are three main types:

 

Single burner 0.3kW paraffin heater
Single burner 0.3kW paraffin heater
Dual burner 0.6kW paraffin heater
Dual burner 0.6kW paraffin heater
Timorous beastie 2.5kW paraffin heater
Timorous beastie 2.5kW paraffin heater

In my previous 5′ long greenhouse I used two paraffin heaters (a single and a double burner) to get a decent temperature during the young plant growing season but I accepted that these two together couldn’t economically keep the temperature high during the cold winter months. The double burner deliver frost protection.

I refer to the two small ones as single and double burner but each burner (the bit below the upright pipe) usually has two wicks. Some people suggest that you can reduce paraffin consumption by only lighting one wick in each burner but this is actually dangerous. In any event, the rated heat output is only achieved by lighting all wicks.

The big red heater is capable of delivering proper heat to a fairly large greenhouse but it drinks paraffin as it there were no tomorrow.

With paraffin currently costing about £1.70 a litre, it’ll cost you about £5 to run a single burner heater for a week and £10 to run a double burner. For that you get frost protection in a small and mid-sized greenhouse respectively. Red beastie delivers a hefty 2.5kW of heat but will run for only 16 hours on a 4-litre fill so needs to be refilled twice a day. For a full week, it’ll guzzle  £71 worth of paraffin.

These figures are for constant burning, of course. Which is what you’ll get if you are not prepared to nip out to the greenhouse when the temperature falls and again when it rises. There are no thermostats. These heaters will churn out their allotted measure of heat until the tank runs dry. And they will churn out smoke and soot unless you get the wicks adjusted just right (you can’t turn the heating up or down, only on or off). And if the tank runs dry, the wicks will quickly dry out. You need to fill the tank and allow them some soaking time before you’ll be able to light them. And you need to let the heater cool a bit before refilling – paraffin has a low flash point – so all in all every fill-time will involve a period without heat. And you need somewhere safe to store your paraffin!

This time round, I decided to go for gas. Propane to be exact.

Propane 1.9kW heater
Propane 1.9kW heater

Compared to the three paraffin heaters (£39, £49 and £79 typically from smallest to largest), this cost a bit more (£139 typical though I bought it with the new greenhouse for less). But, for my money, I get a thermostat and that makes all the difference.

My greenhouse is 8′ long. The heater sits about 2′ in from one end and the gas bottle’s at the other end, about 5′ away and connected by a hose. The thermostatic detector sits in the middle, protected from the sun of course and low down (because low down is the coldest part of the greenhouse). That’s safety and thermostat logistics taken care of.

A tank of propane costs me £27 and I’ve got two; the second sits outside in the open air behind the shed, out of sight; no special storage required. If one runs out, it takes about a minute to swap the tanks over and I have heat again. If a cold spell is forecast, I can gauge how much gas is in a tank by lifting it and can swap tanks over if necessary before the active one empties. My local gas supplier doesn’t charge me a tank deposit so I always have the option of getting a third if I want. When the weather warms a little I can swap tanks again to use up what’s left of the original tank. If I wanted to be really clever I could buy a dual regulator and connect two tanks at a time for auto switch-over. But I’m stingy plus I’d lose more floor space in the greenhouse.

And unlike paraffin heaters, the thermostat controls the heat output, only turning the burner on if the temperature drops below its current setting. So no repeat trips back and forth to light and extinguish. In the current cold weather (-2C at night) and with the thermostat set to 6C because of what I have in the greenhouse, a tank is covering about a month of heat.

The mathematically able amongst you will have worked out that the additional cost of the gas heater over the red paraffin one (£60 difference) has been recouped in the first month of use (£27 gas compared to £250-ish if I’d gone for paraffin). The equation will distort a little when I warm the place up a bit for my begonias but gas will still work out oodles cheaper and a lot more convenient.

So if you want anything more than simple frost protection, and don’t need the dryness of electricity, I’d suggest you go for gas. You get more flexibility, less need to continually check the greenhouse and it’s cheaper. Even if you have to pay a deposit on the gas bottle, you’ll get that back in the end.

You may be able to avoid paying a deposit on the gas bottle if you source your gas bottles from, say, a garage that regularly services your car or a local petrol station from which you regularly buy petrol. Country living has lots of advantages even if your local Tesco is 8 miles away and you have to forget Waitrose!

Only minor caveat is that there are a few different types of connections on gas bottles. Make sure you get the right one (for example most greenhouse heater regulators connect to Calor orange bottles but not to Calor blue ones).  Greenhouse heaters will usually be sold with a hose and the right regulator which securely “screws” rather than clips to the bottle. Attaching and removing the regulator to/from the bottle can be fiddly but it’s a LOT SAFER than the clip-on type which shouldn’t be used unattended! You’ll need a screwdriver to assemble the hose bits securely and a large spanner to tighten the connector to the gas bottle. Being lazy, I bought an extra spanner which I could leave in the greenhouse to save continually digging through my tool case.

Prices quoted in this post are typical purchase prices as at February 2016. Paraffin heater tanks usually hold 4 or 4.5 litres. Paraffin is usually sold in 4 or 5 litre plastic bottles. The gas bottles I use are 13Kg capacity. There are smaller bottles but the gas works out more expensive and they obviously won't last as long.

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