I replaced the loud stock fan by two 120mm fans I had in spare. The fans are in push-pull configuration and powered directly from the PSU (constant speed).
It makes my S5 really quiet, about the same as my desktop computer. But I had to downclock to 250MHz (~800GH/s) to keep the temp below 60°C.

So at this point I loose 30% of hashing power to reach an acceptable sound level..
Next step I'll try more powerful fans with the "fan shroud" trick mentioned by dog1965 (https://bt.irlbtc.com/view/908494.msg10250546#msg10250546

I also noticed a big spread between the 2 temp sensors... Any idea what it could be??

https://i.imgur.com/yoUwCpd.jpg
 

Just run PSU A to S5 X and one blade of S5 Y, then run the other blade of S5 Y and S5 Z off of PSU B.

Perfect thanks - we ended up giving it a shot that way and it's perfect.  Thanks for the help!

Strato

hopefully this clarifies it a bit so people dont plug both PSUs into the sockets on a single blade

You could still stack. You would just need to block and vent 100% of the exhaust from each level.

But the unit also takes up 50% of its normal footprint when vertical.

So that considering as well.

Strato

I had an interesting thing happen with one of my S5's

I had it running for about a week, then one of the boards stopped working - all x

I rebooted it and it started back up for about 15 minutes and died again, after another reboot it just showed all -

So I figured the one board dies and I would let it keep going with the other board, but as it ran the hash rate would drop for the next 20 minutes down to 0. Reboots would bring it back then about 20 minutes later it would be 0 again.

Frustrated, I shut it off and left it for two days.

This morning I decided to take a look at it. I pulled the cover on both sides and did not see any signs of anything burning/frying etc. I unplugged the power to each board one at a time and powering on to determine the bad one. Then I decided to try changing the plug on the controller board, unplugged it from the outer socket and plugged into the inner one. Upon powering back on both boards started hashing and have been happily humming along for the last 6 hours.

Can you PM me or post back here in a few days how you get on with that, its an.... interesting result which may be useful. Is that an RM1000 I see?

Yeah I thought it was interesting too, that is why I posted it.

If something happens I'll let you know, but still going great right now.

RM750

I use all Corsair for my miners.
RM750 for each S5
RM850 for each set of two S3's
RM1000 for each set of two S3+'s

As others have said, nothing wrong with the temp delta between the two boards, its quite common and a product of a not very accurate measurement system. It does what it needs to though. What power supply are you using, the controller rebooting rather than blades dropping or X'ing is rather uncommon.