Cost of heating a greenhouse overwinter

PeterS
I'm sorry I missed your question till now. I can only blame the drink. It was that time of the year.
I got mine from www.maplin.co.uk.
Stock No. L61AQ, price �£24.99.
It functions like a plug in timer. You plug it into the mains socket, then plug the appliance into it, the measuring starts automatically. The one snag that I've found is that the slightest glitch on the mains resets the readings. But it includes an hours counter, so you do know how long since the last power cut. It is also useful for spot measuring the consumption of appliances, and the current mains voltage and frequency.
Nik

 

Hi Nik, I don't know about PeterS but the gadget is exactly what I need. Thanks!

:D Is the 'hic' due to the drink? I've finished my sloe gin already... :D

 

Liz
Glad you like the gadget.
Best make twice as much sloe gin next time
"nunc hic aut numquam" translates roughly as "Its now or never", but you could be right about the hic bit.

Nik

 

Many thanks Nik - I think I will get myself one of those.

 

An update on the electricity consumption:
Up to this morning 28th Feb, apart from four days in early December when the data was lost due to a power cut, total consumption has been 601 KWh, and the min temp was 5.9 C. I've had seven lemons off the tree this winter, so that's about �£9 each and there is a bud on the Strelitzia.
I picked the last lemon today to take to Spain this afternoon. The Head Gardener does like a G&T before she can face the shops. :D
Back in a week, it'll be interesting to see what the coming chilly blast does.
I will aim at a lower min temp next year, but the thermostat will need looking at first to do that.
Nik

 

On the grounds of "Nere pull plug out till May be out", I've just disconnected the greenhouse today, after turfing out the remaining plants skulking in there. Total electricity consumption for the 05/06 winter heating was 849 Kwh, mint inside temp was 4.8 C, the cost of which has been docked from the Head Gardeners frock allowance,not that she'll take any notice.
I must make a remote thermostat and aim at 3C min for next year, as the thermostat in the heater itself seems a bit too generous.

Nik

 

Nik that is interesting, but sound a lot of money. Right at the end of the winter I got myself organised and bought a B&Q greenhouse heater, a consumption meter (thanks for the info), and an electronic plug thermostat. Because it was at the end of winter it never really got tested.

I bought the seperate thermostat because I wanted to be able to set a temperature of my choice, and also I did not trust the heater thermostat. Whilst I do not have any specific information, I am suspicious of cheap heaters that might have only a bimetalic strip. This might turn on at 5C but may only turn off when the heater itself reaches 7C or 8C. However as the heater is blowing hot air foreward, the area in front of the heater might have reached 10C or 12C, before the heater itself switches off. I put my thermostat in front of the heater so it will heat up and cool down quicker. But the system is not proven yet.

The power you use is proportional to the amount the temperature rises. So if the average night temp outside is -2C and the average inside is 0C, you are raising the temperature 2C. If the average inside in 2C, the temperature rise is doubled and so is the cost. So small differances can have a large impact on cost.

 

PeterS
I would be very interested to know where you got your electronic plug thermostat from. It would save me the hag of making one.
Keeping the thermostat away from the heater makes a lot of sense. My choice would be to have the heater low down, on the floor, and the thermostat high up behind it so that most of the green house space was being heated.

Nik

 

Hi Nik. This is what I bought. It is a thermostat capable of taking 13 amps.

http://www.tlc-direct.co.uk/Main_Index/Wiring_Accessories_Menu_Index/Therostats_All/Therm_1/index.html

You obviously plug it into an extension lead, which you can then move anywhere you like. It takes a tiny current when it has no load on it and I found this raised the reading by 0.2C - but thats irrelavent.

I have not tried my system properly yet. The biggest problem I anticipate is uneven distribution of heat. You need to borrow as many max/mix thermomiters as you can to check. There is nothing stopping you having another fan (no heat) which comes on at the same time and blows at say right angles.

 

Logically the stat wants to monitor the temperature at the plant level...no point heating the top of the greenhouse if you`ve nothing growing up there

If the heater is mounted higher, the air it used would be somewhat warmer to begin with and take less energy to heat than the cooler air at floor level.

Any building with corners will have deadspots and uneven airflow.

The only constant is that heat rises, so it may be better/cheaper to heat a sub-floor (or under bench) cavity which would create an even temperature gradient as it rises.

 

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Remember the saying, "Knowledge is Power" ?
In this case, more knowledge is less power ...electrical variety that is

you need to heat the greenhouse in winter to keep plants alive, but lack of light will prevent them from growing much.

A better plan might be to add heat and light?

Lights give off a good percentage of heat as they are relatively inefficient at generating light

If you`re paying just to electrically heat a greenhouse in winter, you might as well add supplemental lighting instead of a heater as the actual running cost won`t be much (if any) higher.

 

Hex, I did ponder about this at one time. But you have to have powerful lamps just a few inches about the plants. Ordinary lighting, say in your sitting room, is only about one thousanth (or some similar large number) of the light of an overcast day. It looked pretty expemsive. Whilst heating can just be at peak periods at night, lighting tends to be a longer term item.

But it can be done. Just ask the marijuana growers. :D

 

Hi Peter
I guess it really depends on your plants.
As they are likely to be the usual things (nothing too exotic) that can just about survive the winter in a very cool greenhouse (40F-50F?) i guess anything better than that bare minimum is a plus to them

With regard to light levels you should take into account short winter days, nasty dull weather plus loss from the glazing (probably an inch of the proverbial bubblewrap to get through too)
..so the natural light intensity is going to be pretty minimal.

Heatloss on the other hand is constant.
If you completely insulate the greenhouse for the winter months you`ll need much less energy to maintain the internal temperature, or to put another way, maintain a much higher temperature for the same energy cost.

I can`t think of anything offhand that might need 100F in the dead of winter though.