This question has been asked QUITE A FEW times before.

 Short answer - Ethereum is likely to go to Proof of Stake before the DAG file gets to 4GB.

HD 7870 are fairly good ZEC mining cards, 160 sol/s ballpark with reasonable overclocks.

Never had an NVidia 7xx series cards, no clue there.

Why a dehumidifier?
 No point to that I can see.

 Keep in mind that a "dehumidifier" is nothing more than an air conditioning unit with the controller set to trigger on humidity level instead of temperature and SOMETIMES better "removed water" flow control.

 I prefer the Netgear "blue box" pro-level switches to anything TP-Link - Cisco is good gear too but tends to be too expen$ive.

Dry air is NOT critical - I've yet to see any miner that wasn't specified to 80% or higher humidity for operating conditions.
Just need to keep the humidity low enough that you don't get condensation at the intake.

 Furnace filters work fine - a common 20" x 20" furnace filter is an almost PERFECT fit for most common "box" fans.

 Dripping on top of those filters = VERY BAD idea, they are PAPER filters they don't handle getting wet like that.
 Might work OK with fiberglass-based filters, but those don't FILTER very well.
 They probably would work as an evap pad, but not nearly as well as a standard Aspen type pad (lots less surface area).

 
 I concede that the MINERS don't really need the good filtering I insist on - it's more about avoiding my own problems with sinus issues, especially with the very very high smoke levels my area has suffered through a lot of this summer.

R4 is your "quiet" miner option.

 S7 and S9 need too much airflow (unless you GREATLY undervolt the S7 like the Sidehack mods did) to be able to survive with a "quiet" fan outside of the Arctic or Antarctic.

Many common 6-card GPU mining rigs eat as much power as current generation ASIC miners do.

 My *3* card GPU rigs eat almost as much power as an Avalon 741.

 However, you're not going to make ANYTHING trying to mine Bitcoin on a GPU - if you want to mine on a GPU rig, you need to be reading in the altcoin section.

Given you're using it as a heater replacement (effectively free electric), pretty much anything semi-recent should work. S7 again, S9, T9, R4, Avalon 741, etc.

 S9//T9 should be right at the same noise level of the S7 you used to have, the rest will be quieter.

 

 GPU mining WAS pretty much dead for a while except if you had VERY VERY cheap electric - and even THEN it wasn't making much.

 It is unlikely to completely die in the forseeable future, the question is "is it going to be profitable for the large majority of folks that do NOT have well under 5c/kwh power cost".

 My Asus 1070 turbo is in an open rig.
 It runs way too hot even when I had it in an unblocked end slot for a short while during testing.
 Removing the end bracket helps a LOT (at least 5 degrees C cooler when it's not there blocking airflow), but it STILL runs hotter than any other 1070 model I've worked with.

 I've never used a Bitmain PSU on any ASIC miner.
 Some of my A2s have the "factory" power supply, but 2 of them are running with Seasonic X-1250 gold units and at least one has someone else's 1300 watt Gold unit swapped into it at some point.

 I used to use a mix of X-1250 and EVGA G2 1300 power supplies on the S5 and SP20 miners I had for a while.

 Main issue for the L3+ is that it needs 9 PCI-E 6-pin connectors to power it - not many standard ATX power supplies that have that many, even when they easily have the power capacity to handle a L3+.

 They can plan all they want - but if the chipmaking capacity does not EXIST their plans aren't going to happen 'till it does - and established players are more likely to get access to that capacity first.
 I also question their stated plan to locate in Northern Europe, especially after KNC died with THEIR farm near a Northern Europe hydro plant....

 Drywall is almost always FIRE rated, it WILL handle high heat levels if it's properly installed.
 Down side is that it's not very strong structurally - it needs a solid frame and optimally something like plywood to back it for strength, or you risk poking holes in it pretty easily.

 Foam board insulation is NOT generally fire rated (I'm not sure if fire-rated foam board EXISTS), and doesn't handle high heat levels as well.

 Tarps are a bad idea, even if they are hot temp rated - they flap around too much. Tolerable for TEMPORARY situations but I would never use one in any sort of a permanent installation.

3xx series R7/R9 cards for the most part were the same GPU as the corresponding 2xx series card (the 380/380x was the exception, being based on the 285 "Tonga" GPU instead of the 280/280x "Tahiti").

 They should work the same as the 2xx series except for having slightly faster RAM and minor BIOS changes.

HD 7870s are good for about 150-160 sol/sec on ZEC - about the same as the GTX 950 depending on the relative overclocks - but soak a fair bit of power to achieve that.

 Pretty much anything from the HD 77xx series and later (aka ANY GCN-based AMD card) is useable and viable, though the lower end stuff like the 7750 isn't going to earn a LOT.

 I'd ignore the older Terrascale stuff, as a lot of miner software flat out doesn't work with it and it's very low performance when it DOES work.
 Perhaps work with it AFTER you get any GCN-based hardware you have running just to see if it CAN do anything viable.

15.12
Period.
OS does not matter, that was THE BEST driver for all AMD GCN cards prior to the Fury.

IMO it was THE LAST "good" driver version AMD ever did, though the 16.9.2 WHQL driver was fairly good and probably the best for Fury and RX 4xx series cards.

And as a result you need to stick with the various Delta "Focused Flow" fan series (FFB or derivatives) or the VERY FEW similar fans from other companies.

 Even though they have comparable CFM ratings, I would NOT put a Delta AFB series fan on any Antminer past the S5.


 Exhaust of the GE90 is a bit hot for miner usage.
 Perhaps a CFM56?

 8-P

Most areas are locked to one power provider - the infrastructure cost gets prohibitive FAST for power providers to try to compete in the same area.

Could be a loose connection from the hashboard to the controller board.

 Could be a dead hashboard.

 Hard to tell with the very minimal info you have provided.

As of right now, it would take appx. 6 Antminer S9 or T9 miners to achieve $50/day in gross income.

 NET however depends on your electric cost and how much you have to spend on cooling - if your electric cost is too high, you won't be able to earn ANY profit from ANY Bitcoin miner.

There won't be significant production capacity on 7nm (or even 10nm) that is available to a SMALL company like Caanan untill at best late 2018 - and that's assuming the node even goes into production AT ALL in 2018.

 Keep in mind that 14/16nm "production" was first announced to exist almost 2 YEARS before any major capacity showed up, due to yield issues with the new node and the amount of work to get a line up and running at ALL for those nodes, plus the HUGE equipment investment needed.
7nm/10nm node is looking to be even WORST about all of that by a lot, at least in part due to the issues with getting EUV up to the power level needed on a reliable PRODUCTION basis.

 I wouldn't bet on ANY miner manufacturer having access to more than "tape-out" level "production" before 2019, or MAYBE the very end of 2018.


 Even INTEL hasn't announced firm dates on it's introduction of 10nm process devices, and they're YEARS ahead on that node vs anyone that has announced 7nm "being worked on" (and were the first to actually start shipping 14nm production chips IIRC).