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Electrical usage in your observatory

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#1 Travellingbears

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Posted 04 July 2020 - 08:27 PM

My question to others is how did you implement your electrical plan in observatory? Interested if you’re primarily relying on utility power (eg 120V), and if observatory near/convenient to main house or easy access. How many Amps did you feel were required? How many dedicated circuits? Out of curiosity wondering how many folk did new sub-panel vs adding circuits to existing panel (answer might depend on distance from main house or local code)?

 

In July I’ll be coming to end of ‘primary’ build stage of my 8x8 backyard observatory which is positioned about 15-ft from main house. I had relied on a convenient house 120V outlet at back porch and 25-ft GFCI extension cord to serve my power needs for portable viewing. I can initially continue to use this scenario at pier in observatory. In near months I’m considering adding a small ductless AC/heat pump (220V system) and a minimum of two 120V dedicated circuits in the observatory for astro equipment and a third 120V for general use. I’ve got plan to run single imaging session from pier and perhaps also do portable (tripod) getting power from observatory while setting up in yard if I have guests.
 

I have two 200A panels with adequate space to add additional circuit load. In my case I could add up to double-pole 50A breaker in one of my main panels and run 6-3 w/G UF-B to a new sub-panel in observatory and have branch 120V and 220V (hvac) breaker circuits there. The other choice is to add the required new breakers in one of existing panels (skipping sub-panel method) and directly pull multiple runs of 12-2 w/G UF-B in same conduit to observatory for 120V/A and the 220V hvac. Probably a difference in cost but the sub-panel at observatory would likely be method recommended. I’ve still got to get bid from local electrician. 

 

Thanks

 

Dave



#2 outofdark

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Posted 04 July 2020 - 11:58 PM

I personaly like having a sub panel, it adds flexability in the observatory.  Plus you can kill circuits without having to run over to the house.   

 

YMMV

 

len


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#3 Travellingbears

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Posted 05 July 2020 - 06:48 AM

I might add that pier setup would be plugged-in Paramount (120V/48V system using SB power supply), plugged-in PowerWerx 120V/12V units for distribution thru Rigrunner fused systems to cover primary and secondary cameras, Moonlite focuser, StarTech powered usb hub, Kendrick dew heaters (depending on scope used), plugged-in 120V/19V Win10 NUC (runs imaging session). When using my PW14 I’d use the supplied 120V block adapters (plug-in) as well.

 

On portable (secondary for visual) I’d be plugged-in Paramount (120V/48V using SB power supply), plugged-in PowerWerx (120V/12V distribution) to Kendricks dew heater (eyepieces/scope), and plugged-in Mac laptop (SkyX to mount). That power support uses outlet and my 25-ft GFCI with 3-outlets. I’d have electrician put weather-resistant box outlet on outside wall of observatory.

 

The roof has Aleko AR920 setup which is minimal drain on 120V. With split roof I’ll have one for each side once I have it operational. I have 24V battery and charger/solar panels for backup but I’ve only tested from 120V.

 

I’m thinking minimal lighting (white/red selectable) inside observatory. I’ve got WiFi from house but am considering ‘hard line’ solution option for data traffic since I’ve got to add conduit anyway. So might small network switch and do fiber run vs CAT6. I have visual security on premises (covers yard/observatory well) and have extra camera (WiFi) which will go inside observatory so I can see how scope is moving etc.
 

Does anyone have any suggestions on requirements that I might be missing/overlooked? Generally we tend to underestimate capacity and wish that we had more outlets, etc. Since most astro stuff is running at 12V (after 120V conversion) each 120V/20A circuit gives substantial available amperage and should be no issue with available outlets since normal practice to have several outlets on each 20A circuit. 

 

Dave
 



#4 kathyastro

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Posted 05 July 2020 - 07:05 AM

I ran a two-pole 15A service to my observatory, off the main panel at the house.  Getting rid of the previous owners' hot tub freed up a panel slot that I am now using.  This gives me two independent 15A circuits at the observatory.  Because the cable run is 250 feet, I upgraded two sizes, to 10 gauge cable, to minimize voltage loss. 

 

I didn't install a sub-panel at the observatory.  Instead, I installed a kill switch.  It means that, if I need to turn off the power, I have to kill both circuits, but that is a relatively minor inconvenience.

 

I have several 12v power supplies, ensuring that motors and relays will not affect imaging electronics.  My current project is to build a remote power switch that will allow the power to be switched off automatically at the end of a session.  It will incorporate battery backup, using the marine battery that I have for field trips.


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#5 Travellingbears

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Posted 05 July 2020 - 07:29 AM

Within the observatory itself, how many chose to left walls unfinished (not drywalled) so electrical wiring is exposed on framing? If exposed style as final state of interior walls did your electrician encase wiring runs between boxes (eg, flexible armor conduit or pvc). I often see Romex (indoor rated NM-B) run between drilled 2x4 and stapled in sheds. Wasn’t sure if folk got inspections (or not) and were ‘dinged’ on having ‘unprotected’ romex in exposed walls. I’m in Maryland and less certain how ‘inspections’ might be handled. Anyone in my area have an observatory which underwent ‘scrutiny’?

 

With roof opening was electrical design/materials for observatory setup as if it was an outside wet space. So was it all done in UF-B wire or conduit in interior walls, water-resistant electrical boxes w/covers, used outdoor/waterproof lighting etc. How far did you go (if at all) if concerns that it was potentially a ‘wet space’. 

 

Also were you specifically instructed/advised that detached observatory was to have GFCI protected? If GFCI you do implement as GFCI breaker or just GFCI at first outlet/load off a normal breaker?

 

Dave



#6 kathyastro

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Posted 05 July 2020 - 07:38 AM

I finished my walls, so there is no exposed wiring.  It is a dome, so the only wiring potentially exposed to the elements in the event of a sudden downpour is at the pier.  The pier wiring is enclosed in conduit.  And I have an automatic-close system that will button up the dome in the event of rain.  Maximum elapsed time from first raindrop to weathertight is 90 seconds.

 

I used GFCIs for my outdoor outlets, but not for the indoor ones.

 

I didn't get it inspected, but I am pretty sure it would pass an inspection.



#7 macdonjh

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Posted 05 July 2020 - 09:10 AM

My observatory is at my club's dark site.  Six years ago (or so) the club I belong to decided to open twelve small plots at the dark site to members for construction of private observatories.  The club provides 10A of 120VAC for each observatory site.  I ran a buried conduit from the "drop" provided by the club to my building.  I ran the conduit through the slab so it enters the wall with no exterior fittings.  

 

Like others, I installed a small panel on the exterior.  If I remember correctly my panel has four slots, I use three.  The external panel is intended to allow a club member to turn power off if something goes wrong.  Luckily it's only been needed once, luckily again, not for a fire but just because my interior lights came on when I wasn't there.  They were able to cut power and turn the lights off.

 

One circuit provides 120VAC to my observatory.  I have a thermostatically controlled exhaust fan to keep my building vented.  I also have power outlets on the west, south and east walls inside.  I also have power on the west and north walls outside.  Finally, there is an outlet at the base of my pier.  I keep a clock and use a hair dryer (dew control) there.

 

One circuit provides power for a 30A(?) 12VDC power supply which goes to my pier and a set of 12V connections on the outside of my west wall.  Currently only my mount and telescope fans draw power from that power supply.  I have an 8'x8' slab outside my west wall and I've had guests connect to the outside power sometimes.

 

Finally, the third circuit provides for a second 30A(?) 12VDC power supply for everything else.  I have 12V LED lighting and eventually will install an old car stereo head unit and speakers.  That last is a project I've been meaning to get to since I installed my scope five years ago...

 

As to the question, "how much do you need?": if you think about it, not much.  The only big load you're likely to have is a small air conditioner.  Just check at Home Depot to see what the current draw is for what you're planning to install, if you're planning to install one.  After that, your astronomy gear will draw hardly anything, at least when measured on the 120V side of the circuit.  Even if you count up all the amps your mount, fans, camera, filter wheel, focuser, guider, etc will draw at 12V, you get to divide that by 10 when figuring out how much 120V power you need to supply to your power supply.  OK, for your Paramount you only get to divide by 2.5...

 

I think your current plan (in your original post) is way more than you'll need.  Perhaps you'll need a dedicated circuit for your heat pump (again, find out what the current draw is for the unit you want to install).  Having a separate circuit for it may be a good idea anyway, it may generate a bunch of electrical noise you'd rather not have on your mount and camera circuit.  After that, I'd bet a single 15A or 20A circuit will provide enough 48VDC and 12VDC power for everything else.

 

As to the question about running directly off your house's existing panel or installing a subpanel in your house or at your observatory, that's up to you.  The most cost effective thing is to install a couple new breakers in your house's panel and then run wire directly to your observatory site.  I don't see how having a subpanel in your house gains you anything.  A subpanel in your observatory may get you a small amount of convenience since you wouldn't have to walk from your observatory to your house to cut power.  I have never used the breaker panel I installed in my observatory (but I needed it because I share my circuit with another observatory and had to protect my wiring).  


Edited by macdonjh, 05 July 2020 - 09:14 AM.


#8 Travellingbears

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Posted 05 July 2020 - 09:38 AM

Kathy

 

Thanks for the feedback. So two 120V/15A have handled your astro needs with everything running on 12V systems. Makes me wonder if I could realistically drop sizing from current 100A sub-panel being considered with three single pole 120V/20A (astro) plus one double pole 220A/15A (hvac). I would have like to avoid expense of buying larger 12-slot 100A sub-panel and get down to 70A/4-slot variety (size of larger outdoor spa panel) or just pull lines from added breakers in panel at house.

 

Fortunately I have only about 15-ft to dig/trench within backyard and then conduit can emerge and be strapped against side of house under our deck/walkway to mid-point along side of house where wiring can pass thru an outer wall to reach panel in basement. So my overall distance from pier to house panel is probably less than 60-ft with the turns.

 

Dave



#9 kathyastro

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Posted 05 July 2020 - 11:06 AM

I would think that 100A is overkill.  I was being extravagant with two 15A circuits.  I only did it because I figured that, If I was going to pull three conductors, it wasn't a big upgrade to pull four.  My biggest single load is a 1500 Watt portable heater, that I only use when doing maintenance.


Edited by kathyastro, 05 July 2020 - 11:08 AM.


#10 HunterofPhotons

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Posted 05 July 2020 - 11:08 AM

Dave,

In circumstances that you have described I usually run a single 220V feeder to a subpanel.  Make sure that someone who knows how to balance circuits wires the circuits.

I also like to do separated common and ground bars in the subpanel with the ultimate combination in the main house panel.  There is a school of thought that doesn't do the separated bars.  Opinions and local regulations vary.

Exposed romex isn't the safest way to go.  Conduit isn't that expensive.

As noted, power requirements shouldn't be made just on 'how much power do I need now' calculations, but add in a generous amount for future, undetermined uses.

 

dan k.



#11 Travellingbears

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Posted 05 July 2020 - 11:14 AM

Macdonjh

 

I’ll revise to two dedicated 120V circuits for astro stuff. Never will exceed capacity of one 120V/20A but with two circuits can split/isolate items which might be prone to ‘electrical noise’ like exhaust fans, etc from imaging.

 

You mentioned the exhaust fan to help reduce inside temps. I’m in Chesapeake Beach MD (so not struggling with Texas level of heat) but interested in good airflow for humidity reduction on equipment sitting in observatory. We regularly experience 80-90 F with 70-80% humidity in summer which persists into evening hours. 

 

In my small 8x8 I didn’t have space to partition off a ‘warm room’ (or A/C cooled area for summer) so installing a ductless A/C-heat pump is a moderately expensive purchase (but with low Amp running costs) for general temp regulation and humidity control. The A/C-heat pump would have capacity to keep observatory at 60-70 F at 35-40% humidity thru the year when roof closed, so might help preserve equipment condition. Of course I have option to start off with a Home Depot portable AC/heat units (<$600) which sits on floor (but those operate at high Amp). The split heat pump would be $1800-2000+ installed but do better job and save on electric bill. 
 

Dave



#12 Travellingbears

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Posted 05 July 2020 - 11:46 AM

Dan

 

Thanks for feedback. Within observatory not sure yet that I’ll finish interior 2x4 walls. If I add XPS rigid foam between joists there is a requirement (fire code) for it to be covered (can’t be left exposed). On sub-panel, I know that have to unbond (remove screw) on neutral and then set ground rod if panel is in separate building. Would also have separate ground bar in sub-panel. I’m not considering the ‘newer’ plug-on-neutral panels which were convenience for newer style arc-fault/ground fault breakers (w/o pigtail). I looked initially at Eaton brand outdoor rated (NEMA 3) sub-panel using CH breakers (CH12L125R). Not needing capacity or spaces but didn’t see smaller unit readily available. Lot more offering for BR style breakers in smaller Amp panels.

 

Dave



#13 Travellingbears

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Posted 05 July 2020 - 12:28 PM

Kathy

 

Leaning towards ‘conservative approach’ with implementation via new breakers at existing house panel for simplicity. I’d pull two 12-2 w/G for 120V/20A for astro (for certain) and the third wire set (12-3 w/G) for future 220V/15A hvac when dig trench and set wires runs in conduit this summer.

 

Might try seeing how observatory does with thermostat regulated exhaust fan vs expending the high $$ for ductless hvac install this first year. I’ve got a rough opening (30x24) in South wall between the two 24x24 awning windows which I’d planned to put framed 24x24 removable insert. I could buy exhaust fan and stick in the ‘window’ sized slot. I didn’t want to use a window A/C unit in that location. If needed I can always try out interim solution like portable dehumidifier too.
 

Dave



#14 mark77

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Posted 05 July 2020 - 01:14 PM

My observatory is not exactly typical, but I have a 100 amp 220v feed that goes into the largest breaker box Home Depot sells, and it is FULL.

 

See link below for details.



#15 Travellingbears

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Posted 05 July 2020 - 01:57 PM

Mark77

 

You’ve made good use of power with that substantial investment in infrastructure! I wasn’t coming close to capacity on the 12-space Eaton panel that I was evaluating. 
 

Dave



#16 MJB87

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Posted 05 July 2020 - 02:36 PM

I had a subpanel installed with four breakers. I seem to recall it is 80A but I'd have to look it up.

 

One circuit is dedicated to the telescope pier and runs all of the sensitive electronics associated with the telescope. I leave that breaker off (disconnected) unless I am actively using the telescope.

 

One runs a set of outlets primarily used by the small air conditioner.  It also runs my computer and monitors when I'm in the observatory.

 

One runs another series of outlets primarily used by the roof motor. It also powers my Sonos One speaker,  my light box, and some white and red light bulbs.

 

The final one runs a few outlets that run 24/7: a POE switch that connects with my security cameras, local WiFi access point and local weather station console. It also powers an iPhone charger.  (I took an old iPhone and installed Sky Safari Pro on it -- that is my hand controller.)

 

I found it very helpful to segment the usage this way: one breaker normally off, one always one, the other two either way. The panel also has a surge protector on it.



#17 Travellingbears

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Posted 05 July 2020 - 04:31 PM

Marty (MJB87)

 

Sounds like a logical/functional partitioning with multiple breakers. I like the inclusion of surge protection at panel. I could easily implement similar plan for power management at session startup to bring devices online. I’d rather not be concerned about loss of equipment via lightning strike. In past I was only running portable astro sessions off GFCI extension cord for power. With permanent setup best plan for safety of equipment (no risk) is when I can leave ‘optional’ breakers and ‘pier/imaging’ breaker in ‘off’ position. 
 

I had been focused on load (which is not a lot) and option for some segregation to keep fans, AC, roof motor etc on different path from sensitive equipment. For my needs I might setup sub-panel with 3 x 15A breakers (rather than running 2 x 20A) with on-panel surge protector (utilizes two slots) in the observatory. You mentioned 80A panel which seems more than reasonable. 

 

I doubt that I’ll pull 30A (load) even if I include the DP 220V/15A hvac within the Astro sub-panel. But the setup would have me with double 50A breaker at main panel where I require 6-3 w/G wire for the 60+ feet run. If I were to separate off the hvac load to its own circuit, it would be reasonable to put lower double 25A or 30A breaker at house main panel and I can run less expensive 10-3 w/G to sub-panel and set 3 x 15A breakers for astro. Based on astro loads I’m not expecting to get much over 12-15A total load on 120V side across all three circuits on regular use and never above 20A max across all 3 if I had guest on their portable outside too. I’d put mini-split under its own breaker (double 15A) at main panel and probably run as 12-3 w/G  (vs 14 because of distance) to the AC Disconnect box on outside of observatory. I could find smaller outdoor Eaton brand panel 100/125A max and utilize 5 spaces (3 breakers + surge)

 

Dave
 


Edited by Travellingbears, 05 July 2020 - 04:32 PM.


#18 macdonjh

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Posted 05 July 2020 - 07:34 PM

Macdonjh

 

I’ll revise to two dedicated 120V circuits for astro stuff. Never will exceed capacity of one 120V/20A but with two circuits can split/isolate items which might be prone to ‘electrical noise’ like exhaust fans, etc from imaging.

 

You mentioned the exhaust fan to help reduce inside temps. I’m in Chesapeake Beach MD (so not struggling with Texas level of heat) but interested in good airflow for humidity reduction on equipment sitting in observatory. We regularly experience 80-90 F with 70-80% humidity in summer which persists into evening hours. 

 

In my small 8x8 I didn’t have space to partition off a ‘warm room’ (or A/C cooled area for summer) so installing a ductless A/C-heat pump is a moderately expensive purchase (but with low Amp running costs) for general temp regulation and humidity control. The A/C-heat pump would have capacity to keep observatory at 60-70 F at 35-40% humidity thru the year when roof closed, so might help preserve equipment condition. Of course I have option to start off with a Home Depot portable AC/heat units (<$600) which sits on floor (but those operate at high Amp). The split heat pump would be $1800-2000+ installed but do better job and save on electric bill. 
 

Dave

I only suggested Home Depot to get an idea for how many amps it takes to make a ton of A/C.  It sounds like you've done your research, so please ignore me. smile.gif

 

By the way, I live on the Third Coast, so I deal with Texas heat.  The combination of radiant barrier and my exhaust fan does a good job of keeping the inside of my building cooler than outside ambient.  My building is nowhere near sealed up (I have pretty big gaps between my roof and fixed walls), so condensation has not been a problem.  Unless I forget to close the roof when I'm finished on a dewy night.  I've had puddles on my floor when I forget.

 

Good luck with your observatory.  You're going to love having one.



#19 MJB87

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Posted 05 July 2020 - 07:44 PM

 I’m in Chesapeake Beach MD 

Nice spot. You are across the Bay from me.



#20 Travellingbears

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Posted 05 July 2020 - 09:47 PM

Marty 

 

We are right at boardwalk in Chesapeake Beach about 1/4 mile below main marina. The photo below was taken from our boat in summer 2019 as we cruised by. We are the gray house with white trim and red roof adjacent to the stairwell going up from the boardwalk. I was doing a lot of viewing from deck then and had my Tak TSA120 under the silver Telegizmo 365 on the deck. The observatory ended up being built this year in backyard rather than front hill/slope (which had better views) since less controversy from neighbors and better privacy/security inside fenced yard.

 

Dave
 

View from water


#21 Travellingbears

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Posted 05 July 2020 - 10:14 PM

Macdonjh

 

It sounds like you are doing just fine managing more severe environmental conditions than I might experience with just radiant barrier and ventilation from exhaust fan. In MD we can wake up with dew on outside deck/furniture and windows.

 

I too will have some gaps in roofline connection under soffits to walls etc. I will likely add some type of ‘brush sweep’ in this 1” space to minimize access of flying insects. I might try an exhaust fan first along with planned use of reflectix type material stapled on roof joists (leaving air space for ventilation from soffits) and try the XPS rigid foam on inside walls. Might not find that I need to spend the bigger $$ for mini split system. I’ll still pull the wire from house panel to support future hvac (in case I need to implement). I should hear back on Monday-Tuesday timeframe from my nephew (who’s in plumbing/hvac) as to what install costs would be. His business is aligned with supplying Mitsubishi Slim ductless systems which tend to be premium $. If he can’t pull a good discount for me I’ll pass on it. 
 

Dave



#22 Travellingbears

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Posted 06 July 2020 - 03:00 PM

I mentioned solution of temp triggered exhaust fan to provide improved airflow in observatory to my nephew (plumber/hvac contractor) and he thought it was likely a better use of my $ than letting him install min-split AC-heat pump. So I’m scratching off the 220v hvac from the electrical plan. I can find a decent sized 120V exhaust fan which draws only 0.5 amp.  
 

Without the dedicated 220V hvac circuit I could reduce to just adding double-pole 30A breaker at main panel and only need to pull single wire run of 10-3 w/G UF-B. I might use 3/4” conduit for convenience (easier than 1/2”) and terminate in small load sub-panel where could partition into four 120V/15A GFCI protected circuits. Functionally I’ll partition breakers along lines of ‘always on’ general items group within observatory (exhaust fan, WiFi security camera, lights, etc), and two breaker subsets for devices associated with pier/imaging primary and secondary group which can be turned off between sessions for protection against lightning strikes. Probably leave fourth breaker dedicated for outside wall circuit for guests setup in yard (normally circuit left off). I have surge protection associated on each of my two main panels so need to verify if downstream sub-panel from a protected main still needs plug-in surge breaker. I know that sub-panel in separate building should have a ground rod and the ‘neutral’ bond screw is removed at the sub-panel. For wiring within observatory itself will proceed with thought that walls might stay exposed so wiring goes in conduit between electrical boxes. Hopefully I’ll get the wiring done in July so won’t be dependent on my 25-ft GFCI extension cord when mount and scope get fitted in observatory later this month.

 

Dave


Edited by Travellingbears, 06 July 2020 - 03:03 PM.


#23 macdonjh

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Posted 07 July 2020 - 07:46 AM

Macdonjh

 

It sounds like you are doing just fine managing more severe environmental conditions than I might experience with just radiant barrier and ventilation from exhaust fan. In MD we can wake up with dew on outside deck/furniture and windows.

 

I too will have some gaps in roofline connection under soffits to walls etc. I will likely add some type of ‘brush sweep’ in this 1” space to minimize access of flying insects. I might try an exhaust fan first along with planned use of reflectix type material stapled on roof joists (leaving air space for ventilation from soffits) and try the XPS rigid foam on inside walls. Might not find that I need to spend the bigger $$ for mini split system. I’ll still pull the wire from house panel to support future hvac (in case I need to implement). I should hear back on Monday-Tuesday timeframe from my nephew (who’s in plumbing/hvac) as to what install costs would be. His business is aligned with supplying Mitsubishi Slim ductless systems which tend to be premium $. If he can’t pull a good discount for me I’ll pass on it. 
 

Dave

I guess I should add I've had no trouble storing my gear in unheated buildings these past five years (observatory) and eight years (my garage).  The glass and aluminum doesn't care about the ambient conditions, and what electronics I have (just my mount controllers, I don't use a computer at the scope) have survived well.  Whenever somebody asks about gear storage, I remind them the professionals at the top of Mauna Kea and in the Atacama desert store their gear in unheated  buildings.

 

Also, my comment about not having condensation and big gaps was meant to convey I have good ventilation in my observatory, so moisture doesn't get trapped inside.

 

As for planning for the future: if you think you might want an A/C or heat pump in the future, I like your idea of pulling the wire now while your trench is open.  Another alternative is to consider running 1" conduit, pulling the wire you need today but also leaving a pull string in the conduit.  That way if you decide to pull new wire in the future you'll be ready.



#24 Travellingbears

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Posted 07 July 2020 - 11:37 AM

Macdonjh 

 

I’m going to see how my results are relying on good airflow from an installed exhaust fan. My interior air space would be under 500 cubic-feet (8x8 on ground with 6-ft walls and 7.5-ft at top of A-frame roof). For moisture control I’m not certain that we’d apply same guidance that they use when sizing an attic fan.

 

What size is your observatory and what capacity (CFM) fan did you install? For temp setting, what did you use? If I understood you had installed one fan which ran on thermostat versus having one which ran 24/7 at a pre-determined speed and second to kick-in (backup) if temp got above 80F (for example).

 

My walk-in basement has served as ‘astronomy man cave’ for a while but I’m planning to create shelving with storage boxes to allow me to leave items in observatory. I’ll be moving primary mount (ME II/OAE) onto pier and have some ‘rotation’ between PW14 and my Tak scopes.

 

Good suggestions with larger conduit and leaving pull-cord. I recall being thankful when adding more electrical or communications cables. 
 

Dave


Edited by Travellingbears, 07 July 2020 - 11:42 AM.


#25 kathyastro

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Posted 07 July 2020 - 11:52 AM

For temp setting, what did you use?

If I was going to install a thermostatically-controlled fan, I think I'd look for a differential thermostat, with one sensor indoors and one outdoors.  I would set it to come on when the indoor temperature is one degree or more higher than the outdoor temperature. 

 

If you have an ordinary indoor thermostat set to 80, you could have a situation where the indoor temperature is, say, 85, and the outdoor temperature is 90.  You would be blowing out that nice cool 85 degree air and replacing it with 90 degree air.


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