The Orchid Forum, for the UK and Europe (previously known as The UK Orchid Forum) • View topic - Greenhouse heating (revisited)
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PostPosted: Fri Feb 17, 2017 9:33 pm 
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A few days ago i had an air source heat pump installed to the greenhouse. Early days yet but looking promising. Potentially extremely cheap to run, particulary if you are currently using leccy!


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PostPosted: Fri Feb 17, 2017 10:39 pm 
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Would be interested to know more.
I am planning on gutting the greenhouse and installing new insulation to increase cost saving.


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PostPosted: Sat Feb 18, 2017 8:18 am 
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blondie wrote:
Would be interested to know more.
,............


Me too. Cost effectiveness and the ability to maintain a given temperature in our winter would be of interest to me.

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PostPosted: Sat Feb 18, 2017 9:39 am 
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Early days yet and we havent had any very cold nights. Some very early observations though,

Bad:
Temperature control is good but currently i can only either set 10C, or 16C minimum, nothing inbetween. (Hoping to fix that with a wifi adapter and internet control).

You have to be carefull where to position it, it wouldnt be good to get the indoor unit wet, although most of it would be ok, there is some electronics inside. We put ours under a bench were a cheap fan heater used to be and covered the bench to stop any dripping water.

Expensive to buy and install!

Good:

Hopefully a 3 year payback and then very cheap heating for the next 12 or so years(15yr avg life of a unit).

7 day timer built in, can set different day/night temperature.

It easily maintained the greenhouse temperature to 14C above ambient on a few hundred watts (18x10ft, well insulated).

Its very quiet!

I used to run two 1kW heaters to keep a minimum of 12-14C, day & night. Now i can turn up day temperature to 20+C, nightime 16C and its still cheaper to run.

Theoretically they should be 4-5 times cheaper to run than a fan heater, my feeling is that does seem to work out about right, ill know better when i've got more data. The model i got should put out 3.2kW of heat for 0.580 kw of power input. (Heat output is controlled and varies to maintain the set temperature).

Overall, im rather pleased with it and am now happily turning the temperature up in the greenhouse, instead of allways wondering if i could turn it down a bit more!

What i really need is some cold frosty nights to give it a proper test.


Last edited by SilverShaded on Sat Feb 18, 2017 10:34 am, edited 2 times in total.

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PostPosted: Sat Feb 18, 2017 9:57 am 
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Data from the two heaters, unit put in on wed so the 4 is a made up number. First meter is now never being used, The second meter was swapped from the fan heater2 to the heat pump on wed (since then I increased the greenhouse temperature by several degrees).

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Image

Greenhouse Temperature
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PostPosted: Sat Feb 18, 2017 10:45 am 
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Outdoor Unit
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Indoor Unit (with emergency backup fan heater).
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PostPosted: Sat Feb 18, 2017 8:53 pm 
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Very interesting and I look forward to your results in the worst weather. Thanks for the post.

Ed


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PostPosted: Sat Feb 25, 2017 10:19 pm 
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phraggy wrote:
Very interesting and I look forward to your results in the worst weather. Thanks for the post.

Ed


Hopefully the worst weather is behind us, we've had a few cool nights and it performed with no problems.

A couple of pointers though;

In order to be able to set a temperature between 10 and 16C, had to get the WIFI adapter and use the internet app!

Temperature control is not perfect, because i placed it under a bench, it measures the floor temperature, rather than the middle height temperature, so you have to get a bit of practice to get the temperature you want!

It might not be suitable to a smallish greenhouse, heat output can be up to about 4KW, so it heats up pretty fast and then turns off. It would be more efficienct if it was just ticking over at a low duty, minimum power input is about 250W, so heat output is 5x that, hence it heats up/turns off, cools down/turns on again - not the most efficient operation.

Rather than saving as much leccy as i thought - we just turned the temperature up a lot :crazy:


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PostPosted: Mon Feb 27, 2017 8:54 pm 
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Not sure why your pump unit is outside the greenhouse - the warmer the air that goes in, the less it has to be heated it.
When we were looking for a source for our swimming pool we were recommended an air-water heat pump and told that 3kw input would produce 17kw output. That was all new to me so I researched and found how wonderful they are with many hotels sourcing all their entire hot water that way. When I got to the bottom of the articles I found those hotels were in Singapore where the input air is so warm! We were advised, too, that under 7c they are not efficient, and as the stated advantage was that they would use off-peak over night electricity it seemed to me that as we get most nights except in high summer at around 7c it was not for us - but we are trying to heat 100,000 ltrs of water. It was estimated that the big unit would take 3 weeks to heat the pool in spring - we opted for oil (no gas here) and that does the job in 36 hours.
Shall be most interested to hear as your experience develops.


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PostPosted: Mon Feb 27, 2017 10:06 pm 
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The evaporation unit is outside the greenhouse because it expels air at well below freezing on cool nights. If it was inside a building you would in essence be making a big fridge.

The unit is rated at 7C outside and 20C inside, at those conditions it has an efficiency of 550%, the efficiency will drop off, slowly, as either you raise the inside temperature or lower the outside temperature.

It will carry on working down to about -15C, with lower efficiency, (you can see working examples on u-tube) but still quite a bit more efficient than a fan heater.

So average winter temperature where i live is 5C, absolute minimum this year was -3.9C for one day. I calculated the thermodynamic efficiency at that those temperatures and its not vastly different to the 500+% ball park efficiency.

(Btw if a hotel is heating hot water to 60C, its a different ball game, theyre looking for a 30-40C temperature uplift (in singapore) in the uk the same heat pump would need a 50-60C uplift!, a UK greenhouse only needs a 10-20C uplift most of the time).



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PostPosted: Tue Feb 28, 2017 8:39 pm 
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Good point. I had forgotten it is like a fridge in reverse so the exhaust would be colder. Shall be most interested to see your updates.


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PostPosted: Sat Mar 04, 2017 10:02 am 
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I think i have enough data to say the following,

We are currently heating the greenhouse to 15C at night and 24C in the day time (regardless of weather), its costing about 70p a day on cold dark days (no heating from the sun). The last few days have been not far off winter average.

If we did that with a fan heater i'd be looking at probably 1000+£ a year or more (in the past we set the temperature to about 13-14 C, definately not 22C!


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