Advice on a greenhouse heater

[Richie88]

Hi
after getting loads of great advice on putting a greenhouse up on the forum
which we have now named steve austin [gentelmen we can rebuild him] looking for advise on the best way to heat a 6x4 polycarbonate greenhouse tried paraffin heater but to hit and miss thinking about electric type which one could anyone advise me on
greenhouse is in the garden so easy to run from a outside socket
cheers
richie

[roitelet]

Electricity could be rather expensive. Polycarbonate doesn't hold the heat well. You would need to double line it with bubble wrap in order to retain the heat. You need to think about the economics of heating a greenhouse.

I use a gas heater but not until the 1st of March at the earliest. This in a poly tunnel with an end sectioned off with bubble wrap an I use about a bottle and a half of gas. That costs around £25 a year.

[Potstubsdustbins]

It really depends on the type of polycarbonate like Roitelet points out if it is single skin it really does not hold the heat well.

If how ever it is 4mm or greater twin wall it is probably the best insulated green house glazing you can get. I have an 8 x 6 4mm twin wall GH and use an electric convector with a decent thermostat, heat goes on in mid Feb and cost about £10-00 per season.

[Richie88]

thanks for the replies
the greenhouse is the 4mm twin wall so may go with the bubble wrap and convector heater seem to be able to pick one up for about 20 quid
anyone tried electric tube heater are they any good
cheers richie

[Urban]

Ive got a little electric fan heater in my greenhouse, it doesn't cost that much to run if you have a decent thermostat, and the best thing is the moving air helps you stop getting any fungal diseases

[Richie88]

thanks for that urban sounds like a good idea as in a 6x4 space is limited

[Kristen]

anyone tried electric tube heater are they any good

not in my opinion. If they are X watts then that is the heat they produce. There is nothing magical about them in that regard. If you had a fan heater, also of X watts, it would cost the same to run and produce the same amount of heat.

The problem, as I perceive it, with tube heaters is their slow response time. When the thermostat kicks in they take a little while to heat up, and when the thermostat clicks off they have quite a bit of residual heat that continues to heat the greenhouse. So the net result is a high "hysteresis" where the temperature dips a degree or two below the min temperature before heating up and then carries on heating a degree of two above the max temperature. That temperature swing costs more money than maintaining an even temperature. overheating a greenhouse by a couple of degrees increases the rate at which heat is lost through the glass / polycarb, so from the money you spend on over-heating the greenhouse more of the heat is lost (whereas if you were in a well insulated house it wouldn't matter very much if you overheated a room by a couple of degrees - you'd just be "a bit warmer" for a while and it would take hours for the extra couple heat to be lost)

Next up is what capacity tube heaters do you buy? if you want to safeguard against, say, -10C outside you need some beefy heaters. When they come on, on a night where the outside temperature only dips slightly below your MIN temperature, they will generate serious amounts of heat and the temperature in the greenhouse will shoot up! but they will work fine on a really cold night ...

The other problem with tube-heaters is that the heat is only spread around the greenhouse by convection, so it can take a while for the far corners of the greenhouse to get warm.

A Fan heater, by comparison, is instant. It blows the heat all around the greenhouse, so there are no cold spots. Only downside is that the cheap ones have rubbish thermostats (they can be so bad that their swing between MIN and MAX temperature is as much as 10 degrees! at the MAX end that is money wasted, and at the MIN end that can mean dead plants )

So personally I would recommend getting a really accurate digital thermostat. That will set you back £50 ... but probably save you that in fuel in single season or two.

Personally I think electric is cheapest. Although Electricity is probably the most expensive fuel you only use the fuel you need, whereas if you have bottle gas or paraffin you will have to light the heater on any night where you think it might be cold enough to need it. Bottle gas heaters are thermostatically controlled, but their thermostat is so basic that the temperature swings are likely to be huge. Paraffin heaters (unless you get a sophisticated one) have no thermostat and will be burning all night long until you get there in the morning to turn it off.

If you want to keep your greenhouse at 10C, or warmer, all winter you need to look seriously at the best way to achieve that. Lots of insulation, ideally extend the house central heating to the greenhouse for "cheapest fuel".

But if it is only for "occasional" top-up heat on chilly nights in the Spring to keep tender seedlings growing well, then I think electricity is best & cheapest - using a fan heater, and a really accurate thermostat.

(I have logged the temperature in my greenhouse on cold nights, and my thermostat keeps the temperature within 0.5C - which is even better than I would have hoped for)

[Potstubsdustbins]

I have checked my electric convector heater thermostat with a very sophisticated professional thermometer and found it to be accurate within 1.7'C.

Maybe back in the days of yore cheapo stats weren't good enough but that has changed drastically over the last 10 years or so. If you check the packaging of the heater before purchase it will tell you how accurate the stat is.

The biggest problem with stats in appliance's is the positioning it will be near the floor so you will need a thermometer so that you can adjust it over a period of nights to get the temperature you require.

Another tip is to put your most delicate plants at the highest point where it will be warmest, I have s shelf in the apex of my roof and it is usually 3 to 4 degrees warmer up there than at ground level.

[Madasafish]

I used to heat our 6x6 glass greenhouse with paraffin. Gave up due to lack of control and condensation and cost. We have - even in this warm weather - cold nights and ground frosts so control is vital.

I insulate with bubble wrap and use a 3KW heater (bought s/hand for £20 ebay) and a proper control system (£60 ebay). and a 50 meter cable from garage. Heating is set to come on a 5C. Costs about £25 to run in March and £10 in April.

When using a blue paraffin heater I was spending £8 a week...

Far less condensation, no maintenance and total control..

[Richie88]

thanks or all that info makes very interesting reading
as i am only growing small scale toms cucumber and starting off bedding plants
gives me some thinking to do but enjoying every minute of it
cheers all
richie

[polc1410]

Are you trying to HEAT or trying to FROST PROTECT? May need different solutions.

I find a 150W tube quite effective for taking the "edge" off an out building. No leccy in greenhouse on allotment.

With two slightly differently set thermostats you can probably help balance the hysteresis Typically a relatively cheap 13amp plug one probably has a hysteresis of 1C that can't be changed, so you set it to put the heating on at 10C and it actually waits till the temp falls to 9C then switches on and stay on till it reaches 11C and switches off.

The comment with a tube being that it takes time to heat up so actually switching on at 9C the air temp might fall to 7 or 8C before starting to warm up. It then stays on till 11C but the tube remains hot so might heat it to 12C giving you a heating range of 5C or a hysteresis of 2.5C

If you put TWO thermostats on two tubes of half the power each you can probably achieve a better overall pattern. Tube 1 is set at 11C on the thermostat. Tube 2 is at 9C on the stat. (Bear in mind the precision on these is probably +/-2C so you need to accept the dial etc is not right and find the right settings yourself. To run through what happens:

Gently cold night drops to 10C and the Tube 1 kicks in, but the air temp falls to 9C before it starts heating up. Then as its not that cold tube 1 brings the temp up slowly to 12C where it switches off and continues to heat the air to 12.5C (its only half the tube remember) So the hysteresis is 1.75C which doesn't feel too bad to me.

Really cold night / sharp drop in temp at 10C Tube 1 kicks in, but the air temp continues to crash before its hot enough so Tube 2 then kicks in at 8C and it will take a bit of time to get up to speed too. But you already have half power on fighting it so probably never drop to 7C. As they warm up at 10C Tube 2 switches off leaving Tube 1 to carry on heating. If Tube 1 can't cope Tube 2 will kick back in at 8C. Assuming Tube 1 can cope, Tube 2 will help Tube 1 for a bit as it stays warm but by the time the air reaches 12C Tube 2 is probably doing nothing. Tube 1 then switches off but its only half a tube remember so 12.5C. So a hysteresis of 2.25C.

You can play and create different scenarios some of which actually make the hysteresis higher in extremes but are as a result only using lots of energy in extremes.

BUT it all depends really on how hot you want the greenhouse and how cold the outside is.

[Kristen]

I find a 150W tube quite effective for taking the "edge" off an out building.

Different heat characteristics of even a relatively thermally inefficient outbuilding, compared to a greenhouse?. Rate of increase of heat loss, as temperature falls, much greater in a rubbish-insulated! greenhouse, hence why I favour a more instant-response heat source than Tubes. I've not tried them side-by-side though (I wish a horticultural student would pitch up here asking for projects instead of turning up saying "I've designed a better spade for my dissertation can you fill in my POLL questionnaire please" ...)

For a heat source that doesn't have fan-assist I fully agree with having twice as many, half-capacity, heaters with separate thermostats so that more "umph" can be brought into service on colder nights (and avoid heat overrun on mild nights, and in the mornings as the sun rises).

I've had this argument with plumbers over the years. They always want to fit one socking great big radiator in a [large] room, whereas I would prefer two, even if side-by-side and looking like one. Whenever we have had one large radiator it has always overheated the room hugely in Autumn and Spring when the heat demand is modest. Gallons of very hot water sitting in a huge radiator gently [particularly given the small dT!] cooling down over ... hours probably

[Kristen]

P.S. On my conservatory we have radiators extended off the central heating, but I also have a large [greenhouse] fan heater with thermostat set a couple of degrees lower. If the central heating doesn't kick in for some reason, or can't maintain heat on a very cold night, then the fan heater will provide backup. I've got gas bottle heaters in the shed too - in case we get power cuts on a cold winter's nights.

[Potstubsdustbins]

Kristen, I don't know where you get your so called plumbers but sack em. It has been acknowledge by domestic heating engineers for years that any room over 12ft x 12ft requires two radiators to correctly heat the space.

[unclefudgly]

Do you think the larger the greenhouse or polytunnel the slower it is to cool down? I have no idea if this is the case but it feels like it should be true, all that extra ground mass providing residual heat etc.