Greenhouse Heating

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This area is for Reference and discussion of plant hardiness, overwintering methods and tips, and planr protection.
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DiCasS
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Re: Greenhouse Heating

Post by DiCasS » Wed Sep 30, 2009 3:10 pm

My brother in law gave me a Hotbox when I first started in exotic gardening and I've always used it on the frost free setting. What temperature this actually is, I don't really know, but it keeps the glass free of frost (and I use bubble wrap as well) so I would imagine it's up to 5 degrees as the next setting is 'from 5 degrees'. It doesn't worry me if it has a bit of fluctuation as my plants seem to be okay on this setting.
Then two years ago I bought one of those electric Parasene fan heaters from B & Q to give my garage a little extra heat in the Winter. It cost round about £25 (no more) and to be perfectly honest, I can't see much difference in either. I've used the cheapie heater in my greenhouse and the Hotbox in the garage, and for me, both have done the job I want. If you want to be precise in temperature, then perhaps the cheapie one may not be so accurate.

I've just googled the price of these Hotboxes and was surprised how much they were, I didn't realise they were so expensive. If I was starting out again and had to buy one, then for me, I'd buy a couple of cheapies.

Di
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real_ale

Re: New Forum - Hardiness, Overwintering and Protection

Post by real_ale » Tue Oct 06, 2009 7:50 pm

here's a link about keeping the greenhouse frost free over the winter and its environmentally friendly.
http://www.reuk.co.uk/Solar-Greenhouse-Heat-Sink.htm

weve

Re: New Forum - Hardiness, Overwintering and Protection

Post by weve » Wed Oct 07, 2009 1:13 pm

I see the website mentions (in "comments") using old, water-filled plastic milk containers to keep a greenhouse frost-free. I'm trying this this winter in a plastic temporary greenhouse (which i'll also insulate with bubblewrap) erected over my most tender plants.

In theory it should work. (Hope so. I've been saving the containers for a while now!). I'm underfilling them slightly to give room for expansion should they freeze. Anyone else tried this?

weve

Ditch

Re: New Forum - Hardiness, Overwintering and Protection

Post by Ditch » Wed Oct 07, 2009 10:52 pm

I am am just a newbie, but surley water is a better conductor than air..??

So I would have thought that empty (sealed) milk containers would have better insulation properties than ones filled with water??

themes

Re: New Forum - Hardiness, Overwintering and Protection

Post by themes » Wed Oct 07, 2009 11:10 pm

Ditch wrote:I am am just a newbie, but surley water is a better conductor than air..??
I would tend to agree

dave2166

Re: New Forum - Hardiness, Overwintering and Protection

Post by dave2166 » Thu Oct 08, 2009 6:29 am

yes and no

what you are trying to achieve is a heat store with the water. even a couple of degrees stored up during the day will allow the containers to give back over a period of hours.

although the mass is greater of the water, it absorbs heat slowly during the course of the day. even better it releases this stored heat back slowly, so it helps to reduce the peaks and troughs over the course of 24 hours

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simon
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Re: New Forum - Hardiness, Overwintering and Protection

Post by simon » Thu Oct 08, 2009 12:11 pm

If you read the link again, the comment neglects to mention anything about filling the bottles with water (Weve imagined that detail) so it is hard to know what they are trying to acheive but I agree with Dave2166, the heat sink sounds a lot more plausible. Insulation wouldn't be very effective with just a few bottles stacked in the corner.

EDIT: Maybe the guy just leaves them full of milk :lol: .

weve

Re: New Forum - Hardiness, Overwintering and Protection

Post by weve » Thu Oct 08, 2009 1:38 pm

I took the implication that they were water-filled from him giving the fact that "only the outer ones freeze at night". (If they are empty and the air froze, then it really was a cold snap :) ). Anyway its quite a common method. I read and saw pictures sometime ago (It was the founder of bananas.org I thinK :?: ) where he was covering the base of a greenhouse with filled water containers to even out the temperatures. Its the same idea as planting against a south facing brick wall which radiates out the stored heat slowly.

Water retains heat relatively well to act as a heat sink, (and is not a particularly good conductor, think when you try to cool bathwater down by running the cold tap, you have to give it a swish round else it stays cold at one end). Hence also the warmer maritime climates and water being used in central heating systems and the length of time a hot water bottle stays warm compared with say one filled with warm air. Another plus about water is its latent heat (which I remember only vaguely from O-level physics) where the state change between liquid water to ice temporarily slightly warms air, (or should I say, stops it becoming cooler). I find it easier to understand the other way round, where to get ice at 0c to change to water at 0c takes a lot of heat with no temperature increase of the actual water/ice itself, so this way round it "takes out" heat.

I'm not talking just a few containers for my experiment. I have been saving them since June for this purpose and at the moment have just under forty, so we're talking 85 litres so far, that's umm around 20 gallons (?) ) with a lot of surface area to absorb heat, compared with say, one big water butt. Black plastic (diesel) cans would be better and easier to stack, but those aren't free!

Not saying it'll work but having a bash all the same. Quite looking foward to it really. (sad or what :) )

best

weve

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simon
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Re: New Forum - Hardiness, Overwintering and Protection

Post by simon » Fri Oct 09, 2009 12:36 pm

Careful Weve, You won't have any room in that greenhouse for plants.

I'm glad you mentioned about the formation of ice giving off heat. It was something I talked about last winter, that the formation of frost on leaves actually gives heat to the leaf.

weve

Re: New Forum - Hardiness, Overwintering and Protection

Post by weve » Fri Oct 09, 2009 11:22 pm

Hi Simon

You were spot on about the leaf temp. Seems bizarre, i know. but this phenomenon (latent heat) is how fridges work and how sweating cools us down, so it must work. I can get my head round the cooling bit, its the other way round, ie the warming bit, i struggle with.

I've just put up the greenhouse frame around my tenders and you are right, placing the containers in the remaining free space is going to be a squeeze. Perhaps I should just leave the plants outside to make more room for all my precious containers of water :)

Its all theoretical at the mo. but have now done some very rough maths. Now, I may be completely wrong and way off beam (being a biologist and not a physicist!-My trouble with physics is all the different units that they use. Always confuses me!) but anyway, 'calculated' (with formulae found on the web) that to turn all my water containers (if they were at 0c) into ice would release just under 1 kilowatt of energy as heat.

In practice i don't think it raises the temperature, but will stop the air temp dropping below 0c, at least until after all of it has turned to ice. Of course there is the payback next day when the reverse happens in the melting.

Forgetting the maths, which is most likely totally wrong anyway, and to put it another way, if all my 20 gallons froze it would release the same amount of heat as it would take to melt 20 gallons. which must be quite alot. So I am hoping (in the absolute worst case scenario) to achieve a minimum of 0c in the greenhouse as, according to the theory, as soon the temp starts to get lower than 0c and the water starts to freeze, the latent heat will kick in. It should work like having a thermostat set at 0c.

However. anyone who has watched the TV programme "Rough Science" will know that applying such science theories, which for all intents and purposes, should work, doesn't always actually do so in practice. I'm having fun trying it though. (Of course if Torbay gets its normal Torbay winter this time, it won't get much below 0c anyway -but at least, in any spring drought, I'll have plenty of stored water for watering :) )

I will consider it a success if I just get my replacement Echium candicans through alive. (last winter toasted the last one) A very small Ensete maurelii is also in the ground inside. If that makes it, it will be a bonus and I will be impressed!)

best

weve

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Re: New Forum - Hardiness, Overwintering and Protection

Post by Dave Brown » Sat Oct 10, 2009 6:51 am

Read through this topic very quickly, as going out to erect my polytunnel while enough daylight and not raining. The reason water remains warm longer than air is the density of it. Simon may well correct me, but the heat energy is moving molecules within the material, Gases have very few molecules per volume area compared to liquids, so water will store much more heat energy than air. :wink: Solids are even better at thermal regulation, and burying a ducting pipe 2m into the ground with a small fan may give you as much , if not more, stored energy. The temps at those levels ramians around 10C most of the year. :wink:

I have used the water containers method before but painted them matt black to absorb more heat while the sun was shining. :wink: I was heavily into heat conservation back in the '80s. With the forecast for energy prices going through the roof I'm going to revisit many of those methods.

Enough talking, going to DO..... Poly to erect...... speak later icon_thumright
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Dave
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simon
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Re: Greenhouse Heating

Post by simon » Sun Oct 11, 2009 10:35 pm

Ground heat is a good source but the problem with a ducting pipe and fan is that you may end up just blowing the cold air from the greenhouse down and back out out again with very little heat transfer. You need a large surface area to take advantage of the heat.

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Re: Greenhouse Heating

Post by Dave Brown » Tue Oct 13, 2009 10:01 pm

Good point Simon. icon_thumright

poly is now up and battened to the base....Topic to come on that.

I am also toying with the idea of underfloor soil warming cables. I have looked at a 375 watt set up to just give background heat. It won't need a thermostat as the aim is to heat all night and store as much as possible. As I have Economy 7 electricity as all electric, units are less than half price during the 7 hours between Midnight and 7am. If I can store heat in the floor overnight it will slowly give off heat during the day, with a small fan heater as backup
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Re: Greenhouse Heating

Post by Dave Brown » Wed Dec 16, 2009 4:38 pm

The under floor heating did not happen this year as the overall project finished too late so I took an old Black and Decker Greenhouse heater, must be at least 20 years old now, up the polytunnel yesterday 15th Dec, as it went down to -1.1C in there the night before :roll: It was 2kw but one of the elements expired years ago, and it was replaced as the fan was too noisy for the conservatory. The 14w extractor fan sucking ridge air down trunking blows into the back of the heater so there is contant air movement over the thermostat. The thermometer was reset when the temp stabilized at 1.5C last night, and showed this morning the temps rainged from 1.4 to 1.7C when on the frost free setting. when outside temp fell to -3.6C. That seems pretty tight control. icon_thumright
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