Heating Your Greenhouse in Europe This Winter

[Bern]

CG100 wrote on October 6th:

"As for heating the plants - I am not heating the greenhouse this year - I have constructed large bubble-wrap "cloches" over the benches and I am only heating those, not the entire greenhouse."

This is an excellent idea of a way to save expenses on utilities while keeping your plants healthy and alive in your greenhouse this winter.  How do you heat the area under the bubble wrap and control the temperature?  Do you use a large seedling heat mat with a thermostat and heat probe?  Do you have a small radiant heater(s)?  Do you use LED or fluorescent grow lights during the winter as well?  Do you ever have trouble with excess humidity inside these enclosures during the winter?

Do you have photos of your bubble-wrap enclosures that you would like to upload and share?  I'd be interested in seeing them.

I've been thinking of setting up a bench in my garage for some winter growing South African bulbs.  I'd have to use grow lights and a bench sized seedling heat mat for sure, but it had never occurred to me to enclose the entire bench with bubble wrap or a thick plastic wrap. I'm interested in hearing about your experience with humidity inside the enclosures during the winter.  Perhaps a warm, well lit, and more humid enclosure in my garage might be suitable for some of my more temperate and even tropical plants that I could keep growing during the winter instead of an enforced dormancy inside the house. 

Thanks for responding.  Great idea!

[CG100]

Hi Bern
You have asked an awful lot   

My benches all have solid (as opposed to slatted) tops to them, so the base of the cloche is effectively sealed. As the plants are very largely dry habitat species, they are watered rather little - anything that needs more moisture is bagged, though not neccessarily sealed. I have used the system previously when I had a large Sansevieria collection and had no problems with moisture/RH.

The frame of the cloche is made from what is known as roofing batten in the UK - timber around 30mm x 25mm - there are different sizes. The height is fixed by the height between bench-top and the eaves of the greenhouse - so around 60cm. Bubble-wrap is just stapled to that.

Heating - I have always used domestic fan heaters - the common, cheap, simple, ones in the UK have two settings - 1kW and 2kW (I have used more than one in any greenhouse, always set at 1kW, to get better and more even temperature control). In anything but a huge cloche, even 1kW may be too much in most weather here. Years ago, I actually built a simple lower power fan heater.....................

The home-made is a length of galvanised pipe with a mains V computer/electronics-cooling fan attached at one end. The heaters are industrial cartridge heaters and/or ceramic heaters which are secured to the pipe, through the wall. This uses a very crude capillary thermostat between the fan and the heaters - the strong air-flow speeds up the reaction time (reduces hysteresis). The fan runs all the time, only the heaters are on the capillary thermostat. The fan could/would run literally all the time but the whole fan heater is on a simple central heating thermostat, within the greenhouse, set at around 8-10C (a couple of degrees higher than the heater capilaary thermostat).

In a cloche in particular, air/temperature distribution is difficult, so I always butt a plastic pipe against the intake (rain-water down pipe or whatever), so the fan drags air mostly form one end of the cloche, and pushes it out at the other. No need for any join as such, just put the pipe end against the fan, (When heating a greenhouse, the cheap fan heaters have an intake on the top, so I fitted a "chimney" over the intake grill so that air was mostly drawn from near the roof.)

Thermostats - I built electronic thermostats around 20 years ago - in an 8 x 12 foot greenhouse, two, 1kW fan heaters one at each end, with temperatures 5-10C different inside-outside, a glass thermometer showed almost no change as the heat switched on and off.
I will see how the capillary etc. works first.......................... I can always easily cahnge to the electronic control.

From what little I have seen of commercial electronics.............................. they leave much to be desired.

Lighting. I do not need extra lighting in the greenhouse, but did grow LOTS of seedlings in the past, using ONLY artificial lighting. Back then it was SON/HPS/Lucalox (all the same lamp), plus a high CCT (high blue) fluorescent strip. Today, anyone would use LEDs.

I am no fan of heater mats unless they are buried under a few inches of sand or the pots are on a mesh shelf /support half an inch or more above them. The temperature produced differs massively between under and between pots and is VERY difficult to control adequately. Buried in sand, it would be a major achievement to get even temperature distribution. I see lots of problems and no advantages over heating the air in some way.

If you can't find things that I mention, or it isn't clear, let me know and I will try to explain better, or post links to any products.

[CG100]

Some typical components -

see below

[CG100]

Fan heater

Capillary thermostat

Cartridge heater

Ceramic heater

[David Pilling]

In a cloche in particular, air/temperature distribution is difficult

I bow to your expertise, thanks for writing all that down.

It has always fascinated me how my greenhouse is (too) hot at the top in daytime and (too) cold at the top at night.

I never got around to setting up a solar powered fan to shift air from top to bottom.

[CG100]

Quote from: CG100 on 09/10/2022, 09:08:02In a cloche in particular, air/temperature distribution is difficult

I bow to your expertise, thanks for writing all that down. It has always fascinated me how my greenhouse is (too) hot at the top in daytime and (too) cold at the top at night.I never got around to setting up a solar powered fan to shift air from top to bottom.

Presumably you do not have the convenience of mains electricity in the greenhouse?

I have had a heated greenhouse for something like 30 years and never considered anything but electricity, and fan heaters, from day 1 and have laid underground SWA cables to do that. Far too easy, clean, near maintenance-free and easily controlled to go any other way, for me anyway. But then, half a lifetime ago, I spent several years working in industrial air-conditioning/ventilation........

[Bern]

Many thanks to CG100 and his generously detailed response to heating the bench in his greenhouse as opposed to heating the entire greenhouse.  This is the first comprehensive response to my original post about heating greenhouses during a time of potential energy scarcity.  I will probably copy CG100's write-up and send it along to the President of the International Euphorbia Society who expressed concerns in his letter in the April 2022 journal of the IES about energy costs and heating greenhouses.

So many kudos and thanks to CG100.  Good going and good luck with your project and your plants!

[CG100]

You are very generous Bern.

I should say that the idea is no more than an extension to what people called a "hot box" when I was active in the cacti and succulent hobby - an isolated area to keep spp. such as Melocactus, Discocactus etc..

The other advantage is that on especially cold nights, maybe even every night, is that it is VERY easy to throw extra bubblewrap over a cloche (and to remove it when not needed) - not so to add multiple layers of insulation to an entire greenhouse.

Major limitation - plant size.

[David Pilling]

Presumably you do not have the convenience of mains electricity in the greenhouse?

Correct. I have no mains electricity or gas. For my plants all that matters is temps staying above 0 degrees C. I live very close to the sea. It rarely goes below 0 C outside, I can keep my greenhouse warm enough with a small paraffin lamp - in the last few years I've burned it only a handful of times, and could have not bothered (forecast says frost, but frost does not arrive).

[Bern]

I've been in touch with my contact at the International Euphorbia Society in the Netherlands.  Here's a snippet from his email to me. I've added a few corrections to help out Google Translator.

"Yes, heating costs are keeping us busy. I isolated (insulated) my greenhouse last week. I hope it will work and safe (save) the usage of gas. The other week I spoke to an elder couple with three greenhouse. They used 9000 m3 gas last year. This costs now about € 30.000 a year. I am not sure what they will do this winter."

So, now there is another idea about how to save energy for your greenhouse this winter.  It is possible to insulate your entire greenhouse, perhaps with bubble wrap along the walls and the roof.  I remember seeing photos of a greenhouse where someone had done this many years ago.  It was the interior of the greenhouse that was lined with bubble wrap, not the exterior. 

And the other solution was by CG 100 who is making a cloche inside his greenhouse.  Effectively, he's making a smaller greenhouse within his larger greenhouse.

It's good to see that people are using their creativity and ingenuity to find ways to keep their plant collections snug and warm through the coming winter.

Good luck to the couple with three greenhouses.  I wonder what the price of 9000 m3 of natural gas cost last year in the Netherlands?  30K Euros this year would be a a lot of money for heating. 

[David Pilling]

9,000 m3 of gas... that's about 6 times what I used to heat my house and cook last year.

To get from cubic metres to Kilo Watt hours, there is a formula:

(volume) × 1.02264 × 39.9 ÷ 3.6

9,000 m3 is 102,000 KWH.

Call it £0.10 per KWH and it comes to £10,000 (latest UK prices).

[CG100]

I am unsure how many people in the UK use gas, mains or bottled, to heat domestic greenhouses, but I suspect that the number is rather small.
I was suspicious at a simple comparison, as burning any fuel is not/ cannot be 100% efficient, which electrical heating is, but was surprised that domestic gas and liquid fuel burners are from 80-95% efficient. I could not find a figure for horticultural heating of any size at all, not even for commercial greenhouses.

So far as insulation is concerned, there must be few people indeed within the UK who do not fully line their heated greenhouses with bubblewrap, even going back 20 years or so.

I have not searched for it, but maybe 40-50 years ago, Kew did a very extensive study on greenhouse heating and all associated with it across the whole UK, comparing different locations, types of greenhouse construction and much besides. It was extremely detailed and comprehensive and provided some simple maths to calculate an average energy usage for anyone's greenhouse.

[David Pilling]

"Greenhouse gas: how soaring energy bills are squeezing Dutch fruit-growers
Rising prices have forced many greenhouse owners to stop production or skip a season"

https://www.thenationalnews.com/weekend/2022/10/07/greenhouse-gas-how-soaring-energy-bills-are-squeezing-dutch-fruit-growers/

The gas crisis becomes a food crisis...

[CG100]

The gas crisis becomes a food crisis...

It has been for a long while this year - NL banned export of cucumbers a few months back and there was recently an early morning report on the farming programme that large parts of the Lea valley horticultural area have been sold off for housing development as a consequence of growing becoming uneconomic.

[Martin Bohnet]

I was suspicious at a simple comparison, as burning any fuel is not/ cannot be 100% efficient, which electrical heating is, but was surprised that domestic gas and liquid fuel burners are from 80-95% efficient. I could not find a figure for horticultural heating of any size at all, not even for commercial greenhouses.

Actually, Methane gas heatings may have up to theoretically 111% efficiency - which is of course a mathematical trick (or a clever choosing of boundary conditions as we engineers say) calculating in the condensation enthalpy of the water formed in the reaction. In the greenhouse I'm not sure if condensate would be part of the solution or part of the problem, though...