Greenhouse heater sizing calculation problems!?!?!???

Hi,

I'm totally new at this, so please excuse this maybe stupid question.
While learning about sizing a heater for a greenhouse I encountered some issues:

I first found this site http://www.littlegreenhouse.com/heat-calc.shtml, so I used their calculator using imperial measurements, although I'm European and more used to metric.

The formula states BTU=A*DeltaT*U-Value which weirdly is the same to W=A*DeltaT*U-Value I got from a German site. Anyhow my greenhouse has a total exposed surface area of 2887 sq.ft/ 268.2m2 My lowest are 25°F/-4°C and I would like to keep it around 65°F/18°C the twin wall pc has a U-value of .58.
The result on the littlegreenhouse site was 66978 BTU which I then converted to 19.63 kWh. Going at it with the formula I found on a German website calculating in metric it comes to 3.42kW.

Where is the problem?

Wow that took me a while to find, you've made a mistake on the German site, you used the 0.58 U-Value figure which is in imperial you need to convert that to metric the formula works for both metric and imperial but you cannot mix them. the 19kWh figure is correct.


Edit this post has been massacred by the "you cannot use abbreviation" software very hard to write formulas without them
for the bold imperial read British thermal unit per hour foot squared F and for the bold metric read Watts per meter squared Kelvin

Skandi Rogers wrote:Wow that took me a while to find, you've made a mistake on the German site, you used the 0.58 U-Value figure which is in imperial you need to convert that to metric the formula works for both metric and imperial but you cannot mix them. the 19kWh figure is correct.


Edit this post has been massacred by the "you cannot use abbreviation" software very hard to write formulas without them
for the bold imperial read British thermal unit per hour foot squared F and for the bold metric read Watts per meter squared Kelvin

Thanks for taking the time!!! I really was hoping for the German result to be right... That would have been much cheaper to heat.

John Steadfast wrote:

Skandi Rogers wrote:Wow that took me a while to find, you've made a mistake on the German site, you used the 0.58 U-Value figure which is in imperial you need to convert that to metric the formula works for both metric and imperial but you cannot mix them. the 19kWh figure is correct.


Edit this post has been massacred by the "you cannot use abbreviation" software very hard to write formulas without them
for the bold imperial read British thermal unit per hour foot squared F and for the bold metric read Watts per meter squared Kelvin

Thanks for taking the time!!! I really was hoping for the German result to be right... That would have been much cheaper to heat.

I've looked at heating a greenhouse for seedlings for my market garden before, I ended up using lights in the house as light worked out much cheaper than heat.

I'm thinking of something like this...

https://www.klimaworld.com/pelletkessel-atmos-p25-24-kw-inkl-a25-schnecke-silo-bafa-forderfahig.html?gclid=CjwKCAiAkan9BRAqEiwAP9X6UXt3VaGbw_jHZVPO4xSL4cnLdWW9zoHHQLfoWRGVeUfZ1WJ5DUo3ChoCufMQAvD_BwE

Still looking at all the options for producing and distributing the electricity.

I'm looking into buying a house in France. Since it will be a watermill, I might have too high of a water table to put a climate battery under the greenhouse.
The greenhouse will have lighting, but they're LEDs, so they produce less heat and while France is sunny and shouldn't be an issue in the daytime most of the time, the night will be a problem.
I will not be using lights at night.

Maybe the water heating pellet stove and fin radiators might solve the problem. The Germans like to use what they call BHKW Blockheizkraftwerk, which in English are called Cogenerators, I believe. But those are around €30k. I can see it as an option in the future...

John Steadfast wrote:

Maybe the water heating pellet stove and fin radiators might solve the problem. The Germans like to use what they call BHKW Blockheizkraftwerk, which in English are called Cogenerators, I believe. But those are around €30k. I can see it as an option in the future...

Not heard of them on a small scale before, plenty up here on a municipal scale running the "fjernvarme" (communal heating/water) but they are phasing them out as they burn stuff and that is BAD.

Depending on the cost of pellets yes an old pellet furnace should work, the one we have for the house is a 30kwh model though it's generally running at around 30%. a second hand furnace and some old radiators would certainly be cheap enough to set up. The cheapest way to do it here would be to combine your house and greenhouse heating system and use straw for both. Of course that requires a tractor with forks.

John, I had no idea those formulas existed. I reviewed with my hubby. We are very impressed. Nice work. You get an apple.

Thanks to you and Skandi for taking this to a whole new place. I really enjoy being a part of this community.
Y'all rock. ⛰️

Sena Kassim wrote:John, I had no idea those formulas existed. I reviewed with my hubby. We are very impressed. Nice work. You get an apple.

Thanks to you and Skandi for taking this to a whole new place. I really enjoy being a part of this community.
Y'all rock. ⛰️

Hi Sena, you are most welcome. Thanks for the apple!

http://www.ibpsa.org/proceedings/BS1997/BS97_P008.pdf Here is an in depth paper on the expert calculation tool. You can get the tool at the UNiversity Siegen website.

IF you prefer a simpler tool, you can use the climate battery calculator. http://www.ecosystems-design.com/climate-battery-calculator.html

So you're all talking about some very large greenhouses as compared to mine. I have a 14 ft X 14 ft X 8 ft peaked hoop style greenhouse.

How do I heat that? The new material I purchased to cover it is 6 mil thick white plastic obtained from the Greenhouse SuperStore. I'll keep the 1' square openings netting that is presently between old torn plastic covers will be on top as a help against high winds.

I've only used this in the Spring to get plants going after starting them on my front porch. With heat I could do ALL of that work in the greenhouse - plus have cold weather crops growing in the winter at lower temperatures.

Yes, I'm planning a medium sized one 6x20m or approx 20x65ft. Will be using twin wall polycarboante. Would like to go acrylic, but that's a whole other can of worms with contraction and expansion of the material and more expensive. Maybe one day...

We’ve been thinking of warming strategies for a planned greenhouse. My sense is with any greenhouse a lot of the heat collects at the top and is wasted. I’ve seen some Chinese designs where there is a secondary layer of plastic deployed above the plants to keep warmth closer to the ground. One strategy is to install low tunnels inside of high tunnels (examples can be seen in this PDF). Warming mats under trays of plants seems a direct, less wasteful method than heating the entire space. Heated water in tubes would be another strategy.

Skandi Rogers wrote:I've looked at heating a greenhouse for seedlings for my market garden before, I ended up using lights in the house as light worked out much cheaper than heat.

We do the same here, we have shelves with lights for starting seedlings in our boiler room in the house. The waste heat from the boiler and the lights is plenty, and especially for the very early plantings there would be need to be supplemental light in the greenhouse anyways.
We try to put off heating the greenhouse (17'x34') as long as we can in the spring... April? May? And then we keep the heat low, and use heating mats or electric blankets on the benches under the plants, as well as covering them if it will be very cold.
Our move to the greenhouse is driven by space for new seedlings indoors, arrival of bought seedlings, and longer days.

James Whitelaw wrote:We’ve been thinking of warming strategies for a planned greenhouse. My sense is with any greenhouse a lot of the heat collects at the top and is wasted. I’ve seen some Chinese designs where there is a secondary layer of plastic deployed above the plants to keep warmth closer to the ground. One strategy is to install low tunnels inside of high tunnels (examples can be seen in this PDF). Warming mats under trays of plants seems a direct, less wasteful method than heating the entire space. Heated water in tubes would be another strategy.

Sadly the double plastic will not work for me. I would be going with Lexan Thermoclear twin wall polycarbonate panels.