Greenhouse Heating

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Ali K

Re: Greenhouse Heating

Post by Ali K »

smoke are gushing out on every available crevice
:ahhh!: :ahhh!: :ahhh!:

Mark you really should stop trying to light your pharts :lol: :lol: :lol:
MarkD

Re: Greenhouse Heating

Post by MarkD »

Ali K wrote:Mark you really should stop trying to light your pharts :lol: :lol: :lol:
That'll be Gaz then :lol: :lol: :lol:
Don

Re: Greenhouse Heating

Post by Don »

There are now a reasonable selection of digital display electric fan heaters around which do a good accurate job. Some of them have a degree of "built in intelligence".
The power consumption decreases when the set temperature is approached and there is a "dead band" of about 0.5C which stops the thing continually switching on and off.
Even once they have attained the set temperature the fan will continue to suck and blow air through the heater and over the thermostat, for a few minutes, which means it is actually measuring the room temperature rather than the static air inside the heater (the usual source of error).
Guy

Re: Greenhouse Heating

Post by Guy »

Don, that sounds interesting. Are they expensive?
I find having a remote thermostat provides most of those advantages at low cost, but always interested in alternatives. When gardening which? looked at fan heaters last year, they found many of them were useless and couldn't keep a small greenhouse frost free. The better one was the one sold by two wests and elliot, which I use and is fine.
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Mick C
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Re: Greenhouse Heating

Post by Mick C »

I have been looking at the two wests catalogue Guy - which heater did you get if you don't mind me asking?
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Mick C
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Re: Greenhouse Heating

Post by Mick C »

Thanks Guy. I am looking to keep my greenhouse at about 5 deg over winter, which I notice is the lowest setting on this heater. Do you think it would be fairly accurate at this level?
Don

Re: Greenhouse Heating

Post by Don »

Guy, I got two of the digital heaters, one is static and the other oscillates. I can't remember the exact prices but last year they were in the region of £30 (each).
Points to remember:-
These things are not officially designed for greenhouse heating.
Some of them have a lowest setting of 10C.
I think you will find that all the professionals will back your original choice of remote thermostats which are now becoming easy and cheap to install even for the tech challenged folk.
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Dave Brown
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Re: Greenhouse Heating

Post by Dave Brown »

You have to match the heater power to the size of the greenhouse :wink: The bigger the house the bigger the surface area to lose heat. The glass or plastic will have a U-value which is the rate at which it transmits heat. The U-value is normally per sq metre/per deg C.

The below scenario is just to explain why a heater may not stop a frost. The figures used are explanitary only :wink:
Key - 1000 watts is 1Kw and are measured per hour so would be 1Kw/h

It is quite complicated to work out as heat rises so the ridge will be warmer than the lower sides, but simplistically if you have a greenhouse of 100sq mtr of glass, and simplistically the U-value meant it lost 1 watt per degree C/per sq metre you would need 100watts for every degree C temp difference inside to outside

Very,very basically.

If you wanted to maintain +5C and the outside temp was zero you would need 500w (5 X 100w) to maintain that temp, or a 1Kw heater on 50% of the time.

If the temp fell to -5C you would need 1000watts (1Kw) to maintain +5C. Your 1Kw/h heater on all the time (flat out)

If the temp continued to fall to -10C, you would require 1500watts (1.5Kw) to maintain +5C...... BUT if you only have a 1Kw heater you cannot maintain +5C as the heat loss is greater than the power of the heater, and the temp will fall. In this case with the heater on flat out would just about keep it to 0C.

For those sceintific boffs out there :lol: I know there are dozens of variables I have not taken account of but as I said at the top "very, very, basically" :roll:

Ok working out you exact heat loss is going to be difficult and beyond the average layman, but there is a practical way that you can asses this. This is very rough and ready, so if anyone has any ideas to refine it.... fire away.
icon_salut

If you set your heater up to you desired temp. Measure the temperature difference between inside and outside (that is the temp difference) so if, for instance, your setting is for +4C and the outside is 0C that is 4C difference. then measure the heater on to off time over a few minutes. If it is on 50% of the time (equal on and off) you are running at 50% of your max power. In other words to maintain a 4C difference is taking half of your heating capacity. Therefore you can expect if the temp falls -4C ( a difference of 8C) you will require 100% of your heating, and any further cooling outside will result in a cooling inside with your heater on flatout. :roll:

If we continue this further you can project that with this set up if the outside temp is -4C you can maitain your +4C inside, but if the temp outside were to fall to -8C your heating could only maintain 0C flatout icon_thumbdown

The less time your heater is switched on the more heating capacity you have left for coping with cold, so if we use the above figures of setting the thermostat to +4C, if at 0C outside and your heater is only on 33% (1/3rd) ( off twice as long as on) then the heating capacity would allow you to maintain +4C inside with a 12C difference (-8C outside)

Clear as mud :lol:

Hope I've not made any mistakes, but I'm sure if I have, you lot will tell me :lol:
Best regards
Dave
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Ali K

Re: Greenhouse Heating

Post by Ali K »

Thanks for that Dave, now I've read it through twice I understand it.

I also found this on the net earlier
Heating a greenhouse, the problem

To keep a greenhouse frost-free, the temperature must remain above 0°C even in the corners. If the current of air does not reach there, then the temperature on the heater has to be set really high.

However, then a large proportion of the warmth goes up into the gable of the greenhouse, where it is not needed, while the corners may still dip down to minus temperatures.

This is an unsatisfactory state of affairs, because heating energy is being wasted. In addition, there may be a significant temperature difference between when the heater switches on and off, especially with cheaper thermostats.

This means that if the thermostat has a switching differential of, for example, 6°C, then the greenhouse has to be heated up to 8°C if it is not to fall below the frost-free limit of +2°C before the heating switches on again automatically.

The Phoenix heater avoids the unnecessary heating up by keeping the switching differential to a minimum. This results in significant energy savings.

This is why we, as greenhouse specialists, have developed the Phoenix heater, which offers important advantages: High ventilation rate of up to 400m_/h. In normal circumstances, the current of air covers the entire greenhouse, (if the machine is correctly positioned). This means that the temperature difference inside the greenhouse remains small. Fungal attacks and mould formation are prevented.

Use of a high-quality thermostat with a control accuracy of ± 0.3 to 0.5°C. This keeps the temperature remarkably constant and avoids the expensive high temperature peaks.

Why have fans with a high ventilation rate?

The ventilation rate is set at about 3.5 times higher than with conventional fan heaters. There are three reasons for this:
1.Because the air is always circulating rapidly, the input temperature can be low, below 45°C. This means there is no drying of the air or of foliage even close to where the air is coming in.
2. Because of the high ventilation rate, all the air is circulated. An even temperature is achieved throughout the greenhouse and the dreaded high temperature differences (hot at the top and cold at the bottom) are prevented.
3.This keeps the heating within a reasonable temperature range and reduces drops in temperature due to heat loss to the outside.

The fan setting

By using the fan setting, all the air in the greenhouse can be circulated even in the summer season. With the windows open and the fan on, the greenhouse can be cooled down. Air circulation in the summer is also necessary for the pollination of many flowers.

The fan

Can be left on for long periods without using much energy. Low-maintenance ball bearings, metal blades, a built-in thermostat, extra anti-corrosion protection and an earth wire Ð these are the features that make this fan a durable, long term investment.
The Phoenix heater retails at approx. £173 so i don't think I'll be getting one :shock:
Don

Re: Greenhouse Heating

Post by Don »

Your explanation is spot on Dave but it's a bit acafingdemic as most electric fan heaters are about 2-3Kw and even two of them would blow the average ring main :shock:
Of course folk like Mark Hall don't have a problem as he just stands in his polytunnel for a few minutes each day and that's all the heat it needs. :twisted:
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helen
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Re: Greenhouse Heating

Post by helen »

Anyone know what is more 'insulating' - large bubblewrap or thin twin-wall polycarbonate? Am guessing the polycarbonate. Anyone know?
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Re: Greenhouse Heating

Post by Dave Brown »

Don't know Helen, but I would say large bubble wrap has air trapped in the bubbles so no thermals, where as the polycarbonate will need to be run horizontally to avoid the heat rising in the tubes. :wink:
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helen
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Re: Greenhouse Heating

Post by helen »

Am thinking of sectioning off part of the greenhouse (I've got a big 'un), and I'll add heat to that section, rather than the whole thing. So, I can easily create a 'tent' from bubblewrap, or build something with more expensive polycarbonate. Just not sure what yet.

Anyone done this before?
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Re: Greenhouse Heating

Post by Dave Brown »

Polycarbonate is more durable and will be reusable for many years. I have twin wall polycarbonate doors for my covered bit. Had them for about 8 maybe 9 years now. Attach them in Decmber most years and remove in Late Feb :wink:
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