Rainwater Harvesting in a Greenhouse during Winter

I am building a new greenhouse and I would like to harvest the rainwater that falls on the greenhouse and use the water in the greenhouse.  

I live in zone 6b and we currently have freezing winter weather.  The greenhouse (still under construction) has a climate battery underneath it which should protect it from freezing during the winter.  The greenhouse frame is from a greenhouse kit for gutter connected greenhouses.  We only have a single bay, but it looks like the gutters that will run along the north and south sides would easily allow for the capture of the roof rain water inside the greenhouse.  The greenhouse walls are 12' high and I am considering getting a 750 gallon storage tank that is 8' 10" tall.  In addition to the climate battery and backup heat, we are insulating the north wall, north roof, and the north half of the east and west end walls to help hold in the heat during the winter and installing an energy curtain that can be closed at night over the south half of the roof which is glazed.

There are 2 issues I am pondering regarding the rain water capture:

1) I would like to put a leaf/debris filter screen and a first flush diverter where the water enters the greenhouse from the gutter.  However, I am concerned that a 3" pvc downspout with an open air leaf/debris filter might let a lot of cold air in and hot air out of the greenhouse during the winter.  I am wondering about creating a p-trap in the pvc tubing about a foot below the gutter which would hold water in the pipe and then feeding the leaf/debris screen with water coming out of that p-trap.  I think this would prevent air moving through the downspout, but I am not sure if the water in the p-trap might freeze or cause other issues.

2) I would like to sit the water storage tank against the north wall of the greenhouse where it can hold the water and serve as extra thermal mass in the greenhouse.  Running water into the storage tank from the north gutter will be easy.  I am wondering how to best get the water from the south gutter to the storage tank.  I know one option is to put the water in a 50 gallon barrel with a sump pump to move the water using electricity and a hose.  However, I saw an interesting video that Brad Lancaster made in Arizona where he was harvesting rain water from the roof of one building and putting it into a large storage tank next to a different building.  He used a 3" or 4" pvc pipe to run from the top of the downspout on the first building, down into the ground, then over to the other building, and then back up to where it could empty into the top of the storage tank.  The pvc tubing made a large U shape which allowed the water to run out almost at the same height he was capturing it at on the first building.  The pvc pipe stayed full of water all the time, which worked for Brad Lancaster at that site where it never freezes.  With the climate battery and insulation, my greenhouse shouldn't be freezing inside.  I am wondering about using this same method of running a pvc pipe down, underground from the south wall to the north wall, and then back up on the north side of the greenhouse to empty into the the top of the rain water storage tank.

I would like to know if others have tried anything similar to this.  I would greatly appreciate any additional input and thoughts you might have that could help me figure out the best way to capture the rain water.

Go to the topic listed in my signature it deals extensively with rainfall collection.
Answers to your questions
1/
- use standard 90mm pipe, most fittings suit it and its less likely to block
- fit a back flap to the pipe before the flow hits the first flush unit, it will stop air flow, but allow water flow.
2/
- why not run the water from the south side across the greenhouse to the discharge point outside.
- would a larger tank be useful, the larger the cleaner the water

I am currently assembling a 30x60 semi gable roof greenhouse. On the south side I plan on using a 6 foot wide raised bed with a self watering resevoir. 3 foot of the bed within the greenhouse and 3 foot outside. A central barrier that will accept the roll up side. On the north side a fair weather gutter system removed when snow season arrives connected to an IBC tote. Raised beds on the north side would be contained within the structure. I've built a previous greenhouse with interior perimeter raised beds, essentially a ground level semi wallipini. The raised beds working as a solar battery. In winter just seeing all the snow from the roof seemed like such a waste. Hinged panels on the south side exterior bed will be closed to create a cold frame component, but open for times of precipitation.

Thanks for the suggestions John

- use standard 90mm pipe, most fittings suit it and its less likely to block

In the USA where I am located pvc pipe is typically sold in 3" or 4" diameters.   It looks like 90mm is just over 3.5".   I can go with the a 4" pipe if the larger size is better to avoid blockages.

- fit a back flap to the pipe before the flow hits the first flush unit, it will stop air flow, but allow water flow.

A back flap would likely work better than a water P-trap.  I hadn't thought about that possibility. I will investigate and see if I can find some type of flap that will open with water pressure but not air/wind pressure.

- why not run the water from the south side across the greenhouse to the discharge point outside.

I am not sure what you mean by outside.  Nighttime temperatures here this week are at 5°F/-15°C and the winter temperatures here sometimes go down around -10°F/-23°C.  Pipes with water outside the greenhouse can freeze and burst.

- would a larger tank be useful, the larger the cleaner the water

Yes a larger tank would be nice to have.  I need to fit the tank inside my greenhouse to keep it from freezing and I would like to minimize how much planting space is lost to the tank.  The 750 gallon/2839 liter tank I have been looking at has a 4 foot/1.2 meter diameter and a 8.8 feet/2.7 meters height which is low enough that I can drain water from a 12 foot/3.7 meter gutter into the tank.  I will look around some more to see if there are any larger tanks available with either a 5 foot/1.5 meter or 6 foot/1.8 meter diameter which are still short enough to drain into from my gutters.  The available choices for height and diameter combinations of the tanks I have found are somewhat limiting.

100mm pipe will be great
Here are some images of bits I spoke iof, I think they are sold in North America.
Can you send some images when the place is starting tolook good?
Flap valves
first-flush-diverters/

Thanks John!

I found that Home Depot sells a 4" flap valve that should work to keep the warm air in the greenhouse and the cold air out.  Using two 90° elbows I could mount this flap valve horizontally in a section of pipe directly below the rain gutter outlet and then run the water from this into the leaf/debris filter.   The rain gutter outlet will open directly into the greenhouse, so all of these pipes will be inside the greenhouse where it won't freeze.
https://www.homedepot.com/p/Oatey-4-in-PVC-DWV-Backwater-Valve-439042/100144734

I am looking at a first flush diverter similar to the one in the link you posted above.
https://www.rainharvest.com/rain-harvesting-pty-4-inch-first-flush.asp
https://www.rainharvest.com/rain-harvesting-pty-leaf-eater-ultra-downspout-filter.asp

Looking for larger tanks I found this one which holds almost twice as much water and which should fit inside the greenhouse ok.

1300 gallon/4921 liter tank with 5 feet/1.5 meter diameter and 9.5 feet/2.9 meter height
https://www.rainharvest.com/snyder-1300-gallon-vertical-water-storage-tank.asp

The climate battery air tubing will be installed in the ground below where this tank would sit.   Specifically a 24 inch/609mm air inlet manifold tube runs horizontally underground along the north side of the greenhouse.   I will investigate whether I need to add a concrete pier or 2 to help hold the weight of the tank.  The water in the tank will weigh 10829 lbs./4910 kilograms if the tank is full.

I dont think you will need a slab.
If the ground is covered with fine stone or sand, so nothing sharp can puncture the base.