Cost of heating a greenhouse overwinter

Hi Hex - there is a US forum "growing under lights", which I have found interesting.

http://forums.gardenweb.com/forums/lights/

 

Hi Peter
I took a quick look and it seems to discuss mainly flourescent fittings.

Ideal for providing supplementary lighting with low running costs but not a great deal of heat generated unless you had a lot of them

To get an idea of the benefits (if any) you`d have calculate how much the heating bill would be to keep a given greenhouse at eg: a constant 40F when the external temperature is a constant 30F over the entire winter (worst case scenario)

That way, you can directly compare the calculated running costs of a conventional "frost protection" heating only system to almost any alternative plan you might have for the same greenhouse.

 

Hex, my apologies to you - I am suffering from the drought and have gone a bit droopy. I have only just realised what you are saying (I think).

I think you are saying that if you are going to be heating a greenhouse by electricity in the winter, why not supply the heat via light bulbs. That way you will get light as well at very little extra cost. I seem to remember the average light bulb gives 90% of its energy as heat and 10% as light.

In fact such a system could be even more efficient, because you could put the bulbs inside a large propagator (or inner greenhouse), with a suitable controller. And consequently only keep a smaller volume at a high temperature. The heat lost from this inner greenhouse (for your exotics) could well keep the frost off the rest of the greenhouse, where you keep the less exotic items.

 

Hi Peter
The main question is, would it be more economical to completely insulate the greenhouse for the winter to minimise heatloss.
The only reason for not doing that.. would be the fact you`d lose all natural light.
here`s the trade-off:
An average glass greenhouse 8x6 has about 225 sqft of surface area.
Uninsulated with temps of 40F inside and 30F ouside it will need ~660w per hour to maintain 40F. (with bubblewrap this would drop to perhaps 550w per hour with some reduction in light levels)
Heavily insulated with 3" thick rigid foam for the same temperature difference (10F)it would need
~55w per hour to maintain 40F and of course.. artificial light.
Assuming a light-to-heat efficacy of 30% a 100w discharge type light should provide the 55w of heat.

A larger lamp would be needed to produce sufficient light though, a 400w version would maintain an internal temp of 70F (30F outside) in the same insulated gh and still be 30% cheaper to run than just the heater.

[ 22. July 2006, 12:53 PM: Message edited by: Hex ]

 

You raise an interesting point Hex. 660 watts per hour sounds pretty horrendous - I calculate that at �£140 for three months continuous, and only �£12 for the 55 watt case. But then many people will pay �£140 pounds for a hobby for three months - especially if the hobby is the pub! But I think you would need quite a lot more light than provided by 100 watts of incandescant bulbs, if there were no natural light. One snag is that you want the light during the day, but the heat at night.

Should you be heating a greenhouse. I don't have a greenhouse. I have a summerhouse with 5 wooden sides, but the front 6' x 10' is totally glass, with wooden glazing bars, and faces south. It is not much good as a greenhouse in the summer - but do you need one in summer. In winter, it must be warmer than a greenhouse, and has just about enough light to store plants. Perhaps you should be putting up your insulated panels on the north side of your greenhouse, but not on the south.

 

Hi Peter
Incandescent lighting is quite useless for plants ..you`d need metal halide or high pressure sodium.
If the "greenhouse" is in total darkness you can make the "daytime" any time of the day..even at night

Your summerhouse is ideal but remember that "any" glazed area will lose heat a lot more rapidly than an insulated area
In the last example if 25% of the gh were glazed with 75% heavily insulated, for same 10F inside/outside temp difference, it would require 180w per hour instead of 55w

You really don`t need 360 degree glazing as the sun only travels about 135 degrees (summer solstice) and a meagre 90 degrees (winter solstice).

The angle of the glazing is important to make the most of the available light and heat, the sun is higher in summer and much lower in winter (60deg vs 14deg at noon)

Quite a few variables in what are classed as fairly simple buildings :D

 

Hex, I have seen the system that you want. I went into a hydroponics warehouse in Leeds a year ago, and looked round, trying to make sense of what they were selling. One item was a cylinder about the size of a large refrigerator. It cost �£2000 and was apparently a growing chamber. It had heat, light and circulating hydroponic liquid.

I had my doubts about overwintering my Pelargoniums in it, but all became clear when I saw a poster in the office with a large picture of a marijuana leaf on it.

 

Hi Peter
As you said it`s likely geared for illicit plants,rather than usual stuff you`d find growing in a greenhouse over winter..besides that it isn`t a greenhouse.
In fact, it`s probably be more like an oven :D

For �£2000, a good 1200 sqft energy efficient greenhouse should be do-able.
I know which i`d prefer but i just haven`t enough space

 

Lighting is often overlooked when heating a greenhouse in winter. At the winter solstice the South of England gets about 8 hours between sunrise and sunset whereas here it's a meagre 7 hours. Couple that with cloud cover and you can be saddled with only 5 hours of effective daylight so would need to supplement 7 hours of artificial daylight (not ordinary lighting) on top of any heating requirements to give a tropical 12 hour day.

Unless you are into exotics or have a very deep pocket then I just don't think its worth it. Use a propagator to give seedlings an early start. The rest are better grown a bit hardy but kept out of those biting early spring winds that do the real damage.

 

Hi frogesque,

Heating a greenhouse in winter is pretty much an expensive exercise in heating the street, especially with the ever increasing cost of gas/electricity

I`m sure some avid gardeners (with deep pockets) spend a lot of money each winter keeping the greenhouse just a few degrees above freezing.

The purpose of the post is to explore alternative ways to heat it and perhaps keep the greenhouse productive 12 months instead of 6-8 months a year

I think 12 hours lighting at night in a well insulated structure would be one way to go.

You don`t rely on unreliable winter light levels /duration and should keep the temperature reasonable 24/7 without breaking the bank.

There will be a point where the lighting provides all the heat required.
The insulation keeps it from escaping too quickly during the day, when the lights are off.. the plants will assume it`s night time

For similar or less energy cost it should be possible to keep things actively growing instead of just keeping things alive

 

Do the continuous hours of light affect plant growth adversely?
I found last year with a small heater that the temperature was erratic and patchy as I had bad insulation and no fan on the heater.
I was considering for this winter a larger propagator with a light for trying to maintain growth in a smaller area, and trying again for frost free in most of the green house. I noted that one member in yorkshire had a picture of his greenhouse covered in snow in feb/march, but was growing his tomatos inside it with a lighted propagator!

 

Hi Liz
I don`t think you`ll want 24/7 light, although some plants may tolerate it.
Most plants should be ok with a 12 hour day length as it`s close to the equinox.
It will depend to some extent on the type of plants though, as daylength is a trigger for flowering etc in some plants.

Tomatoes would be an expensive proposition if you want to get fruit in the dead of winter as they have a high light requirement with 18 hours a day, not forgetting they are pretty large too so you`d need a powerful light to get the coverage

They`d be expensive tomatoes

 

I meant this was when he was starting them off, as opposed to me half way through april!

 

Hi Liz
As seedlings it wouldn`t be a problem but i think you`d have to get the timing right, otherwise they`d soon outgrow the propagator with nowhere to go
While on the subject of propagators..
If you consider a greenhouse to be a super large propagator.. the best way to heat it is to use bottom heat underneath the beds rather than a fan type heater.
The air temperature can be considerably cooler than usual (5-10F) if the roots are kept warm.
With the air being cooler (less temperature difference) you`d also lose less heat through the glazing.

As Tesco would say..every little helps

 

You mean something like soil cables in trays?