Whole house distributed low-voltage power

[hult]

So, I'm very curious about this distributed 28vdc you have in your house. Care to spill the beans? What guage wire did you distribute it over? Whats the source? What do you use to step down the voltage (say, for example, to 12v)?

Well, none of the following is a recommendation. I'm a geologist, not a licensed electrician. If you take your electrical advice from someone whose profession is to break rocks and splash around in dirty water you may get what you deserve ;-)


"LOW VOLTAGE" IS NOT SYNONYMOUS WITH "SAFE".

LOW VOLTAGE WIRING CAN BE AS DANGEROUS AS HOUSEHOLD AC.

IN PARTICULAR, HIGH-CURRENT SOURCES LIKE BATTERIES CAN AND HAVE CAUSED FIRES AND SERIOUS BURNS AND WORSE.

ALL LOW VOLTAGE SOURCES NEED TO BE CURRENT-LIMITED.

That said:

The wiring varies, but where I could, I pulled up from the basement 4-conductor w/ ground 10 AWG NM ('romex') that I special-ordered through my local Home Depot. The electrical inspector --- apparently not a fan of low-voltage lighting -- begrudgingly remarked "At least you didn't use undersized wire" when approving it. He remarked that he's seen a lot of places where folks incorrectly thought that 'low voltage' meant 'low current' too ....

Unlike AC wiring where two 'hot' conductors on different phases can share a single neutral conductor, each DC 'hot' requires its own neutral if the full 30A rating of the 10AWG wiring is needed. Hence the atypical four-conductor cable. With nominal 24vdc, the maximum rated power over this cable is 2 X 30 Amps x ~27 volts ~= 1600 watts - slightly less than a single 15-amp (14 AWG) 110VAC circuit. Obviously power-handling is not low-voltage wiring's strong point ;-) This cable is fully National Electrical Code (NEC) and UL-compliant for up to 220 VAC 30 amp and so could be re-purposed if needed for retrofits in the future. I pulled a pair of these cables to the kitchen ceiling, piano room ceiling, and to the second floor when I installed a 220VAC load center, ethernet fiber, CAT5e, shielded twisted pairs for audio, pipes for solar/hot-water heating, and replacements for the aluminum wiring of the 2nd-floor HVAC and laundry room.

Elsewhere I have used other smaller (14AWG and 12 AWG) NM ('romex') and 2-conductor, stranded 14 AWG code-compliant CL2 cable commonly used for in-wall installation of speaker wires. The latter is plenty good enough for most all non-lighting, electronic needs. Locally there is multi-conductor wiring from a security system that was installed in the early 1980's that I have repurposed in a few places. There is also direct-burial-rated 10-conductor 18 gauge wire in places too.

Dimming of conventional, commercial halogen 12vdc track (ceiling) lighting is through surplus 24-->0-12vdc variable output DC-DC converters from (and in) a DMX-512 lighting controller custom-made by www.dfd.com that was originally used to light a floating Disney display/exhibit/thing. I was originally thinking of doing homebrew bare-wire 'artsy' 12vdc halogen installations but for now have opted for boring commercial tracks. In the kitchen the 12vdc is accent and task lighting to supplement the existing ceiling cans which become CFLs. In the piano room, they are primarily to illuminate the artwork.

Rapid advances in LED technology may cause our conversion from halogen 12vdc to LEDs to be sooner rather than later which would likely obsolete the voltage-control dimmers.

Where isolated grounds are needed for electronic gear, I use isolated DC-DC converters at point of use. EBay typically has a variety of converters with 18-36vdc ("24vdc") inputs. The distributed PC's use isolating nominal 24->12vdc converters that in turn power picoPSU-120 ATX and similar power supplies http://www.logicsupply.com/products/picopsu_120 . This allows for _local_ 24vdc PC backup on top of the intrinsic DC backup with the simple addition of two sealed 12vdc lead batteries in series, a trickle charger and diodes. (The picopsu PC power supplies are simply astounding IMO ;-)

The main DC power sources are located in the basement and are from a pair of UL-listed Inteli Power 9100-series converters that are designed to both/simultaneously charge their associated big golf-cart 12vdc deep-cycle battery and provide power. An 80-amp PD9180 powers the 0-12vdc (the 'lower bank') and a 60-amp PD9160 powers the nominal 12-24vdc bank. So an additional 20 amps of 12vdc is available compared to 24vdc.

Plans are to add two 7AHr 12vdc sealed lead batteries and associated small supply/chargers to provide 48vdc output for Power Over Ethernet (POE). The nominal 48vdc output will need to be voltage regulated because the combined battery voltage is actually ~ 4x13.6 =~ 54.4 vdc which exceeds the National Electrical Code limit of 50 volts for some classes of 'low-voltage' wiring. This will be done through multiple monolithic voltage regulator IC's that also provide current limiting. Last I knew, the IEEE-802 POE limit was ~13 watts, with 26 and 52 watt standards being worked on.

Important parts of the infrastructure remain to be finally installed. I purchased, but haven't yet decided to use, a 110 VAC circuit breaker panel with built-in low-voltage DC fuse panel that is marked as UL-514 listed. The AC circuit breakers would provde local control of the PD9100 power supplies and other HA-related uses. The DC fuses would provide DC power limiting. It looked great in the pictures but I discovered it was Chinese-made when I received it. I'm trying to determine whether there is an American-made equivalent.

Wherever practical, the plan is to have both auto-recovery, current-limiting devices as well as fuses and(or) circuit-breakers on each circuit so that the system can recover gracefully form a momentary condition without creating the need for a manual re-set that might not be possible in un-attended operation. Current-limiting and ground isolation are the _technically_ challenging parts. At least so far ;-) Installing the cabling involved much physical labor in a house with mostly solid brick exterior and interior walls including the hard, dirty work of chiseling, and drilling and uncomfortable crawling on my belly through places with 185 years of accumulated dirt.

Most of the DC wiring infrastructure is in place, and parts purchased, but with some final design decisions and installations pending. I'll post some pictures when it looks more presentable. The likelihood is that some parts will always be a work in progress adapting to the tasks and purposes to which it is put.

To re-iterate, I am a hydrologist, not a licensed electrician. So the preceding is a description of what _I_ am doing and is not advice to others. You do what you do at your risk, not mine. That should be clear enough, right?

Remember: "LOW VOLTAGE" IS NOT SYNONYMOUS WITH "SAFE".

... Marc

 

[politics123]

Marc --

Thank you for the detail, that's a fantastic post, and I think deserves a new thread! I'm no electrician, but I know my way around a wall-switch. Here's what I think you just said:

You have:
1) high efficiency AC--> DC conversion. Essentially, no wall-warts that suck power when not in use, and I bet the gold cart chargers do a reasonably efficient job of conversion. (generally, there are increasing efficiencies to scale)
2) Eliminated the need for localPC power supplies (eg: fans). This means that your PCs are probably silent, produce less heat, and are less likely to get fried by (bad power supply being a major cause of PC and/disk failure)
3) Have central battery backups for your house (instead of many distributed UPS), that control a bunch of stuff
4) Are all set for any future PV Solar install --> you might be able to get by without purchasing an inverter!
5) Seems like you're energy efficient, especially on lighting.

I'm curious on costs. The wiring, golf-cart chargers, and batteries don't seem very expensive. What about your controller? (I found a 10 channel for $155CAN @ http://www.escience.ca/hobby/RENDER/0001/2...035/12075.html) Does your controller integrate with an automation system?

 

[jasv]

Sounds like the general area of interest is something like "House-wide, distributed low-voltage power".

I would love to learn more.

 

[electron]

As requested, I created a new topic with the original posts. This really is a very interesting topic!

 

[politics123]

I asked Electron, and he gave us a new topic to discuss! Thanks Dan!!

Hult --
I think I understand why you needed 4 conductor cable. Technically, you could run distributed DC over 2 conductor, except that you could only run at ~13amps, right?

You basically provided an equipment list in your first post. The batteries, charger, and electrical cables are seemingly inexpensive. Do you have an idea of how much more efficient the electronics in your house are than using wal-warts?

To power a set of low-voltage lights (eg: LEDs), you only need thin cable, right? I'm wondering how great this would be as a retrofit standard (instead of pulling thick cables all over the place, just run a few micro-cables around to LED spots)

Can you walk us non-electricians through how you rewired your ELO? I'd rather not have to bury a 120VAC outlet in my wall, and if I could send it DC power, that would rock)

 

[carry15+1]

Marc,

Impressive installation. What is the end goal of this installation? The 4/c #10 must have been plenty expensive. Any plans to recoup this cost in some sort of savings - like solar? Surely the low voltage isn't to save energy as a watt is a watt no matter what voltage you're using. I'll be interested to read more as you progress.

John - SW Missouri

 

[jasv]

I think some energy is saved from not having the wall warts dissipate heat but what I am most interested in is

2) Eliminated the need for localPC power supplies (eg: fans). This means that your PCs are probably silent, produce less heat, and are less likely to get fried by (bad power supply being a major cause of PC and/disk failure)

Are the power supply fans not needed for overall cooling of the pc? I think we would all love pictures.

 

[CollinR]

This could be pretty dangerous, how many total watts are you actually using?

It is very interesting.

I am about to start doing some of this for my MVP clients, as when enough try to connect at one time (after power failure) the swamp the DHCP server or the SageTV miniclient server and don't get connected. This results in manually having to tell them to reconnect, kind of a PITA. Luckily it's only v6 at 1amp so not much current.

 

[dwayne]

WOW! Very interesting. Give Marc a Cocoon shirt!

 

[hult]

Marc,

Impressive installation. What is the end goal of this installation? The 4/c #10 must have been plenty expensive. Any plans to recoup this cost in some sort of savings - like solar? Surely the low voltage isn't to save energy as a watt is a watt no matter what voltage you're using. I'll be interested to read more as you progress.

John - SW Missouri

John's asking all the hard questions as he some times does in comp.home.automation ;-)

There are some specific technically-related objectives, some of which have already been teased out by others in this thread.

But there are other goals that are unabashedly idiosyncratic and personal. Many of us participate in HA partly as a hobby that compared to many others is not all that expensive. I prolly spent less on HA that many a person does on, say, what some of us dub 'recreational lawn mowing'. I also enjoy putting to use skills I've acquired in other contexts including scientific instrumentation and environmental monitoring.

John's questions also raise other broader topics. Are all watts really the same, as asserted ? The watts I reap from the solar panel on the roof are far more precious to me than the ones that come through the electric meter for a variety of reasons. In that context, in which the savings comes from constraints (that some folks would consider arbitrary, even silly) a "watt is not a watt" -- the physics of the matter notwithstanding.

This relates to a broad objective that revolves around the concept of adding an infrastructure _parallel_ to the existing one. That's an important component of the thinking behind my/a distributed DC system. Designing an electrical system that can be handled by a backup emergency generator is part way down the trajectory of designing an electric infrastructure based on local energy sources. Both force examination of priorities and -- dare I say it? -- energy conservation. I'm "starting from the top" with a home-brew solar-powered weather system on the widows walk above the third floor and designing expansions and improvements with the major constraint being available energy.

Another set of goals involve primarily aesthetics. Our house is a National Historic Landmark. From the street, except for the number of stars and stripes on the flag, one would be hard-pressed to identify the century. There are no visible utility wires, or other tip-offs to modernity (although green-thumbs would recognize some modern ornamental plants). Our whole house isn't that way, but white X-10 wall-warts and gizmos are arguably the antithesis of that aesthetic and, for lack of a less pretentious term, I'll call 'historic preservation'.

The X-10/wall-wart aesthetic also grates because it repesents in my mind (fairly or not) a highly visible reminder of the contemporary short-term, throw-way mindset which the term "HomeToy" captures well with respect to HA. Hence the preference for the built-in, long term, intrinsically valuable infrastructure that could be re-purposed in the future.

The cost of the wire itself was/is a relatively inexpensive part of a complete kitchen rebuild and other needed retrofitting. A principal expense is time. The context is the overall value of the house (US median ~ $210,000)

To minimize time spent, I tend to do infrastructure installations like wiring in advance of the need and coordinated with something else that _is_ needed, like installing new heating system. In those contexts, another few rolls of wire seem minor. Our house is 185 years old, the the list of what is needed seems longer than the list of what isn't ;-).

So some of the 10 AWG was pulled at the same time as I was replacing the aluminum wiring installed in the 1980's which was a major rewire of the knob-and-tube electric that coexisted with and later supplanted the gas lighting, which was itself a new-fangled and modern introduction 50 years after the house was built. When I have need for the Next New Thing --- say, a way to get DC from a roof-mounted solar panel to the Electric VW Bug -- it's "in the wall" and available, as are four high temperature plastic water lines and fiber optic

More later ... Marc

 

[hult]

This could be pretty dangerous, how many total watts are you actually using?

It is very interesting.

I am about to start doing some of this for my MVP clients, as when enough try to connect at one time (after power failure) the swamp the DHCP server or the SageTV miniclient server and don't get connected. This results in manually having to tell them to reconnect, kind of a PITA. Luckily it's only v6 at 1amp so not much current.

Low voltage is intrinsically less dangerous than high voltage owing to the reduction in risk of electrocution. But low voltage still needs to be correctly designed and implemented and appropriate cautions taken to be safe with respect to fires and burns. So your point is well taken. "LOW VOLTAGE" IS NOT SYNONYMOUS WITH "SAFE".

The first rule in the Three R's of Conservation is "Reduce". So, for example by redesigning the kitchen lighting -- which was 9 x 90 watt cans = 810 watts !! when we bought the house a decade ago -- with a "parallel" low wattage spot and flood task lighting which is also low voltage, we can save energy.

Migrating to LEDs in the same system could/will save even more. Of course the ceiling cans are still there and will be converted to CFL to provide uniform (i.e. boring but utilitarian) lighting when that is needed. Ultimately the savings will depend on use patterns. When all is said and done, if we don't modify our behavior, there will be net energy increase.

Adding monitoring systems (esp. illumination levels and energy consumption) that are intelligently cooperative with control systems (i.e., "Home Automation") would allow choice of lighting by alternative paradigms such as illumination level (letting the system decide whether existing daylight suffices and/or how to supplement it) or energy objectives (like the power modes on a laptop). Humans are pretty good at doing this manually when motivated, so doing it much better through HA in a way that isn't distracting or unwelcome or stupid is non-trivial IME.

For computers, a/the technology to watch is Power Over Ethernet which at the current ~13 watts suffices for the very smallest (first ) VIA mini-ITX computer and some laptops. The next level up at 26 watts will open up many more possibilities. And at 52 watts, we should be able to "rule the world' (which some of us thought we were doing in the 1980's with an AMPRO Litttle board 8088 clone;-)

... Marc

www.NeuralHome.com
www.NeuralHome.org
www.NeuralHome.net
www.NeuarlHome.info

 

[hult]

I think some energy is saved from not having the wall warts dissipate heat but what I am most interested in is

2) Eliminated the need for localPC power supplies (eg: fans). This means that your PCs are probably silent, produce less heat, and are less likely to get fried by (bad power supply being a major cause of PC and/disk failure)

Are the power supply fans not needed for overall cooling of the pc? I think we would all love pictures.

Google: mini-ITX fanless

A small PC that doesn't need a fan for the CPU does not need a fan case in a large enclosure that can dissipate heat.

I use 24”x18”x6” electrical enclosures (Hoffman J&P Box 431A catalog A-SE 24x18x6) for HA and instrumentation "playpens". See www.econtrol.org/ssr_panel.htm

... Marc

 

[carry15+1]

This is an interesting thread Marc.

I can see many benefits to a parallel energy system, not the least of which is no stumbling around in the dark during a power outage. And for the sensitive electronics elimination of transits and brownouts would be a big benefit - assuming you suffer such to begin with.

You know (properly fused) knob & tube was a pretty efficient wiring method, and safe in comparison to the non-metallic (extension cords) they install all over the country today (I'm a EMT man myself).

Which brings me to the question of aluminum. Was your wiring copper clad aluminum (CC)? I've run into CC aluminum used in 15 amp residential circuits, but the only pure aluminum wiring I've run into was for the service from the meter to the panel, or as branch feeders in commercial installs. CC was notorious for breaking off if it was knicked during stripping. I know it was responsible for many fires back in the 70's in the midwest.

My extent of energy conservation is CF lamps and some home automation. For backup I did buy a Honda 6500 watt generator and split out critical circuits to an emergency panel and made a 16' SO cord for connecting them together so the wife can do it without too much trouble. We've experienced a few long term power outages over the past few years so this was more a necessity than preparation for something that may never happen.

I would expect LED lighting to come out with some great products soon. I have a 5 watt LED flashlight that is superb to say the least.

And, "a watts a watt", even if you place more value on one ;-) My statement was more a fact of energy usage than anything else. Some people think it's cheaper to use low voltage lighting.

I do some of that environmental monitoring. I monitor activity of our shared well, monitor the temperature of our two freezers and refrigerator as well as the usual weather stuff. I cheat on the weather and use a Davis and just DL the data once a month to my laptop.

 

[hult]

I asked Electron, and he gave us a new topic to discuss! Thanks Dan!!

Hult --
I think I understand why you needed 4 conductor cable. Technically, you could run distributed DC over 2 conductor, except that you could only run at ~13amps, right?

In the US and Canada, the National Electrical Codes permit 15 amps on 14 AWG NM cable, 20 amps on 12 AWG NM and 30 amps on 10 AWG NM. This applies generally to AC and DC and low and line voltage. So I don't know where you derived the 13 amps from.

With 4 x 10AWG one can run two 30 amp circuits regardless of voltage. For 24vdc this is 2 x 24 x 30 = 1440 VA (watts). That's 720 watts per circuit.

My arrangement is a hardwired lighting system with a 'Centralite'-type centralized topology except that it is low voltage DC, not 110 VAC.

I ran two pair in part because at present the DC dimmers are in the basement where the AC dimmer panel is and I wanted two different banks of low voltage lights: one "mood" and one "task". In the future, if needed, I could move the dimmer controls to the point of use in the ceiling.

This is almost surely what I would/will do when/if we migrate to LEDs or other new technology.

The 12vdc MR-16 halogens are just as inefficacious as 110vac halogens. But the 12vdc lighting has the advantage of built-in UPS by virtue of the batteries, and the potential for smaller lamps in narrow spot (10°), spot (20°) and wide flood (38°) Super Flood (60° ) And these are available in 20, 35 and 50 watts. Especially in a kitchen this flexibility is very useful in lighting design.

Moreover, there are already LED lamps in MR-16 bases that run rings around halogens with respect to efficacy, are moderately priced, and a few mouse clicks away.

You basically provided an equipment list in your first post. The batteries, charger, and electrical cables are seemingly inexpensive. Do you have an idea of how much more efficient the electronics in your house are than using wal-warts?

Not sure what you mean here. A recent wall-wart that meets EU efficiency requirements is about as efficent as one will typically find. Older transformer-based wall warts and AC-DC converters are significantly less efficient. Course folks using powerline controls like X-10 and INSTEON have learned to cross their fingers when they plug in a switching poweer supply, and wall acne is not very aesthetic. And who has not has the blessed things fall out or otherwise become ineffective at the wrong time?

To power a set of low-voltage lights (eg: LEDs), you only need thin cable, right? I'm wondering how great this would be as a retrofit standard (instead of pulling thick cables all over the place, just run a few micro-cables around to LED spots)

Right. If it is less than 50 volts, you can pull "thin cable" and comply with the NEC.

If it is 110VAC, it has to be a minimum of 14 AWG regardless of how much curent (power) it needs or draws. 1 watt or 1500 watts -- both require at least 14 AWG conductor. (This doesn't apply _inside_ a fixture, just to distribution wiring.)

Can you walk us non-electricians through how you rewired your ELO? I'd rather not have to bury a 120VAC outlet in my wall, and if I could send it DC power, that would rock)

I'll be off HA tasks for the next few weeks, but will work it up when I get back. The 2nd and 3rd touchscreens arrived today so I can do them all at once. It will involve using DC-DC converters that I already have. In one case, the DC-DCs will be in with the computer in a 24X18X6" enclosure under the lowest shelf in a kitchen counter (IOW, on the floor). Two others screens are mounted on solid brick walls. In those, I will likely put the DC-DC converters in the touchscreen case.

HTH ... Marc

 

[hult]

This is an interesting thread Marc.

I can see many benefits to a parallel energy system, not the least of which is no stumbling around in the dark during a power outage. And for the sensitive electronics elimination of transits and brownouts would be a big benefit - assuming you suffer such to begin with.

My principal problem is with lightning/surges. I've lost much equipment even after having the power company install whole- house surge protection at the meter.

I have built air-gap filtered disconnects to help with that. See www.econtrol.org\power_conditioning.htm . The device shown there was recently modified to add X-10/INSTEON filters and current monitoring. I described that mod recently in comp.home.automation but haven't updated the web site (for that or anything else in years ..)

My server and HA PC are on a galvanically-isolated rubber-wheeled rack with audio and ethernet over non-conductive fiber (not copper). Connections to RS-232 devices are over TCP/IP via the fiber using ethernet-> RS-xxx converters on the 'house' side.

The HA PC runs off a built-in 24vdc-input ATX power supply (not the typical 110/220VAC). The 24vdc is provided by a pair of batteries and transformer-based chargers. So in this application, DC is involved in providing galvanic isolation via a transformer. This time I'm ready for a direct strike ;-)

You know (properly fused) knob & tube was a pretty efficient wiring method, and safe in comparison to the non-metallic (extension cords) they install all over the country today (I'm a EMT man myself).

When we moved here, I was flabbergasted to see that NM is commonly used on outside walls from the power lines to the meter and between the meter and the entrance panel. In our 1906 house in St. Paul Minnesota, all of the original wiring inside the house was at least 12 AWG and in threaded steel black conduit with walls as thick as standard gas and water pipe so knob and tube was not universal.

Which brings me to the question of aluminum. Was your wiring copper clad aluminum (CC)? I've run into CC aluminum used in 15 amp residential circuits, but the only pure aluminum wiring I've run into was for the service from the meter to the panel, or as branch feeders in commercial installs. CC was notorious for breaking off if it was knicked during stripping. I know it was responsible for many fires back in the 70's in the midwest.

Thirty years ago, about 20 miles from where we now live, one of the country's worst supper club fires (165 deaths) made national headlines and Stan Chesley's name and fortune in establishing what became class action suits. It was caused by faulty aluminum wiring.

Six years later, ~ 1983, the folks that rewired this house used solid aluminum (not copper clad) for both of the AC compressors, both electric stoves, and the electric dryer. The same electricians cut slots from the bottom to the middle of the floor joist in the mid-point of the joist span in one of our first floor rooms in order to install wires. The joists were then 161 years old. Before I scabbed it up, the floor was like a trampolin. And there were enough bronze placques heralding the historical significance of the house plastered on the outside to _make_ the needed copper wire. And yes, it apparently passed local inspection -- sticker and all . ( I still shake my head ...)

When we moved in, the lights on one of the two side-by side electric stoves was intermittent. When found that I could 'fix' it by wedging a fork between the two stoves, it was time to rewire. The aluminum neutral on the one stoves had broken and the fork let one stove 'borrow' the neutral of the other through their chassis ( yes, these were 3-wire, not 4-wire also.) The heating elements still worked because they are 220vac and don't use the neutral

More recently, the 2nd floor AC compressor stopped working. Turns out the aluminum contacts in the circuit breaker in the entrance panel were melted and the plastic of the CB cooked .... That's the last of the inside aluminum except for the service entrance which really should have aluminum contact 'grease' added (no aluminum connection has/had it) and be retorqued.

My extent of energy conservation is CF lamps and some home automation. For backup I did buy a Honda 6500 watt generator and split out critical circuits to an emergency panel and made a 16' SO cord for connecting them together so the wife can do it without too much trouble. We've experienced a few long term power outages over the past few years so this was more a necessity than preparation for something that may never happen.

For reasons I cannot now explain, I bought an inexpensive, natural-gas-powered _24vdc_ generator. It isn't installed yet. I should have bought an AC one because the DC one can't handle the refrigerator and freezer. If it were AC, I could simply connect the fridge, and the DC chargers, and one or two undedicated outlets to it and the DC system would have been backed up for even long outages with no switch-over needed for all the devices on the DC system owing to thge intrinsic battery backup. I may sell it and buy an AC one.

I would expect LED lighting to come out with some great products soon. I have a 5 watt LED flashlight that is superb to say the least.

There's all manner of stuff showing up now even on eBAy.

And, "a watts a watt", even if you place more value on one ;-) My statement was more a fact of energy usage than anything else. Some people think it's cheaper to use low voltage lighting.

"What's a watt?" I thought you knew ... ;-)

If you substitute a 20 watt halogen MR-16 for a 90 watt can, you save energy regardless of whether the 20 watter is AC or DC ;-)

I do some of that environmental monitoring. I monitor activity of our shared well, monitor the temperature of our two freezers and refrigerator as well as the usual weather stuff. I cheat on the weather and use a Davis and just DL the data once a month to my laptop.

This is an area that I've spent a lot of time on and am pretty close to finalizing after many different approaches. I'm 90% certain that the system will use National Instruments Data Acquisition (DAC) hardware, and about 60% confident that I will end up springing for their Visual Studio-based programming tools (not Labview).

I've designed the circuits and circuit boards for most of the instruments and sensors and accumulated most of the pcb components. There's is a fateful interval between when "it's all done except for the doing" and actually pulling the trigger on the "doing". Every room is planned for multiple occupancy, temperature and light measurements as well as humidity. And most circuits and branches will be monitored for power. So there's a big pile of parts involved and lotsa soldering and such. Even with the effciency of making many of the pcbs at once, iit will still be herky-jerky progress because of installation, programming and still-needed wiring.

... Marc