I am getting ready to add a 150 gallon outdoor system. Plan is to close it in eventually. It will mainly consist of gravel beds and PVC runs. My question, for temp reasons, should I have continuous flow through the PVC letting it drain into gravel beds to Bell siphon out; or let the beds Bell siphon through the PVC giving the PVC an ebb and flow effect? I have heard PVC water can get hotter than beds. Love to hear from you.
Uh, I would probably recommend a layout with a sump tank to make use of NFT runs because you need filtered water to go to the NFT. And I wouldn't really recommend having the siphons dump into NFT runs because that may be like expecting your tender annuals to survive a flash flood several times an hour and if they do thrive and grow well, you may be in for overflowing NFT pipes whenever the siphons kick in.
So, I would recommend something more like this
You might also add a T and a Feed with a valve directly to the fish tank in case you need more flow through the fish tank and in case you ever want to take the NFT pipes offline.
I usually recommend at least 300 gallons for temperature stability especially in media, nft or tower systems.