DIY HVAC For Control Room

[mtl777]

I'm thinking of doing a cheapo DIY HVAC for my control room (see picture). Basically, I'm going to use a wall A/C unit with its front portion enclosed in a box to enable connection of flex duct from the A/C to the control room. This will take care of blowing cold fresh air into the room. Then a centrifugal blower connected with flex duct to the room's exhaust vent will suck the stale air out (see arrows indicating direction of air flow). These will all be placed on the attic space above a room that is adjacent to the control room.

First of all please let me know if you think this plan is workable or not. One problem I see is how to remotely control the A/C. An ordinary remote control would clearly not work because in this setup it would not have an unobstructed path to the A/C unit. Can you suggest a workaround or maybe some units that have separate, wire-connected controls?

Your help is greatly appreciated. Thanks in advance!

Mitch

[Sword9]

radio shack and other such places sell IR repeaters that will relay the IR signal from a remote to another room or area that's not directly in line of sight. I can answer that part for you, but don't know about if your idea is a good one or not.

[knightfly]

Mitch, you first need to follow the forum guidelines

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=3231

including the part about updating your profile to include a location;

Also, have you seen this thread?

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=644

I'm anything BUT a HVAC guru, Rod Gervais is the man for that (whenever he shows up)... Steve

[mtl777]

Sam:

That Radio Shack repeater is a nice idea! Solves one problem. I'll definitely give it a try. Thanks!

Steve:

I've seen the thread you mentioned. However, my design is different, that's why I started a new thread. I think it is cheaper and simpler. I hope it will work. Thanks!

Mitch

[Dan Fitzpatrick]

Mitch,

I'm not an expert but I've been thinking/planning about ventilation a bit (see my comments here http://www.johnlsayers.com/phpBB2/viewt ... 0147#30147 ), so here are some comments for what they are worth:

1) is your attic finished/insulated etc? if your attic is anything like mine (unfiinished), this is dirty, dusty nasty air up there, which may have fiberglass or even asbestos, etc. you don't want to breathe that air. when i even go IN my attic i wear a mask. so hopefully your attic is an actual room of some kind. anyway your AC unit will not like it either.

but even if it IS a room, is this going to be a real source of fresh air? don't you need to get air from OUTDOORS in??

more importantly, if your AC unit is in an enclosed room, after you suck all the air out of that space, you will have a vacuum and there will be no more air coming into your system. it won't work and maybe will damage the AC unit too.

2) where i come from attic air gets HOT, like maybe 130 degrees? or more? not the ideal starting temperature for a cooling system.

3) flex duct for the exhaust. i worry that if you try to SUCK air through flex duct it may collapse ... obviously this flex duct will have to be pretty rigid, otherwise if it collapses (and may do so over time, when you can't see it) your system will fail or not work very well. so maybe don't use flex duct on the outtake leg. or make sure it is rigid enough.

otherwise, this plan looks reasonable. but i haven't done this myself, so again take that for what it's worth.

also i don't think it is ideal from a sound isolation point of view, but you didn't ask about that

dan

[sharward]

I agree with all of Dan's comments. There's no way that would meet code (and I'm by no means a code expert), for good reason.

There's also no way a window air conditioner could cool 130+ degree Fahrenheit air to a comfortable level inside the room. You'd be darned lucky to get a 40 degree reduction.

[knightfly]

I don't necessarily agree that this can't be done; need to know more about your construction first.

For example, if construction is such that insulation is rigid foam board ABOVE the attic space, then the attic won't be that hot and the window unit may be able to keep up.

Also, anyone who subscribes to Recording Mag, check out the September issue; there are a couple articles (kind of vague though) on using just such an approach. I did notice though that Chet McCracken (Doobies, along with Keith, RIP ) had used a mini-split in his space, no mention whatever of fresh air Steve

Oh, the REALLY cheap way on this would be to use one of the more basic units with REAL switches, (Friedrich comes to mind) then just leave it turned to MAX and connect the power through a dedicated 20 amp breaker, and just turn it on/off with the breaker...

[mtl777]

Guys, thanks for pointing out those factors! My attic has wooden slats on the side walls for ventilation but it's only for a small portion... probably not big enough for a good, strong inflow of fresh air. Also, the insulation is below the attic, not above it, so it would be pretty hot especially in the summer. And the attic is dusty from years of neglect as nobody ever cleans it.

I guess the option then is to cut a hole like a window on the side wall of the attic and hang the A/C there so it gets fresh air from outdoors. The only thing is, this location is like 17 ft. away from the power outlet and the power cord won't reach. Is it safe to use a heavy duty extension cord with 12-gauge wires?

Thanks,

Mitch

[sharward]

I'm not a fan of the plan at all, but at the very least, do not use an extension cord on the unit. Typically the operating manual advises against this. There is risk of electrical fire. You'd be better extending a circuit to place another outlet in that area, but be sure not to overload the circuit. Even better, run a new circuit.

[mtl777]

Alright, I'll give this project a try. It's the only way to find out for sure if it will work. Now for the details:

1. How much BTU rating for the A/C? Is 5400 BTU enough for my small room area/volume of 124 sq.ft./1027 cu.ft.?

2. Recommended diameter of the flex ducting?

3. Recommended type and CFM rating of exhaust fan - Is centrifugal the best type for this application? Is the exhuast fan even necessary at all? (I'm beginning to wonder because Steve's design in the other HVAC thread does not use an exhaust fan.)

BTW, I got the idea of this design from my PC case, seeing that it has a front intake fan and a rear exhaust fan.

Thanks,

Mitch

[sharward]

1. How much BTU rating for the A/C? Is 5400 BTU enough for my small room area/volume of 124 sq.ft./1027 cu.ft.?

If the window air conditioner were located in the actual room, you could use the sizing chart on the California Energy Commission's Web site. However...

Recommended diameter of the flex ducting?

When you use ducts, static pressure will reduce the flow. Also, window air conditioners were not designed for ducting to be added to them. (If you or anyone find one that does, post a link because I'd be extremely interested!) Determining how to fasten the duct to the unit will be a challenge in and of itself, let alone the loss of pressure you'll encounter. I can't advise you on the size of the ducting here, mostly because (as I've already gone on record to say, sorry to be a buzz kill) I don't think it's a good way to go at all.

Recommended type and CFM rating of exhaust fan - Is centrifugal the best type for this application? Is the exhuast fan even necessary at all? (I'm beginning to wonder because Steve's design in the other HVAC thread does not use an exhaust fan.)

See my post about California Building Code requirements for habitable space without a window. You'll need to determine for yourself exactly what model codes your municipality uses.

[Dan Fitzpatrick]

mtl,

if i were you i'd multiply the height and width of your AC unit's output grille and use a duct that gives you AT LEAST that same square inch cross section.

to figure square inch cross section of a round duct, simply multiply 1/2 the diameter times 1/2 the diameter times 3.14 (pi). so for a 8" duct that would be roughly a bit more than 48 square inches (4x4x3).

so an 8" round duct would be a good match for a grille that is 6 inches by 8 inches (also 48 square inches ... 6x8=48 )

you could try to do it without the exhaust fan. if it doesnt work, then add the exhaust fan later

however it might be a good idea just to do the exhaust fan from the get-go, because there may be a risk of overheating the AC unit's fan (and ruining it ) if your design results in too much backpressure (static pressure). it will certainly make it work better.

certain fan designs do work better than others. your best bet is to buy a fan that tells you how much CFM is lost at a given static pressure. then you will know what you are getting.

remember, i haven't done this before, i'm not an expert etc.

dan

[mtl777]

Dan:

I haven't bought the A/C yet, but the one I have in my bedroom has a 16"W x 2"H = 32 sq.in. area where I can feel most of the air flow. I guess that's what you call the "output grille", correct me if I'm wrong. Based on that, an 8" round duct should satisfy and even exceed the minimum cross sectional area requirement. Is larger better? What if I use 10" or even 12" duct? What is the point of diminishing returns of increasing the duct size?

Thanks,

Mitch

[Dan Fitzpatrick]

i don't know. but i imagine that if you go TOO big then you won't get enough airflow speed, which might mean that the air warms up by the time it reaches its destination. my thought was to make it just a little oversized. so 8 inch would be in that range i'm guessing.

imagine the duct as a small room, and the room you want to cool is the one next to that room. the bigger the first room, the less cool your second room is going to be, right?

at some point the sheer voume of the air in the oversized duct probably absorbs all the coolness your ac puts out.

but as i said, i'm no expert. i'm just making comments until someone who knows what he's talking about comes along ...

[mtl777]

From what I've read, the recommended room ventilation in the U.S. is 15 CFM per person and in Canada it is 20 CFM per person. Most of the time I am the only one inside the control room, so I would like to design the ventilation for only one person. Between the U.S. and Canada standards, I think I'll choose the Canada one (20 CFM) just to be on the safe side. Do you think 20 CFM is fine for one person in a small 1027 cu. ft. room?

Assuming that I aim for 20 CFM air flow inside the room, does it mean then that the window A/C supplying the cold air must be able to deliver that amount of air flow also, 20 CFM, after all the static pressure along its duct path have been considered?

And does it also mean that the exhaust fan for the return must be spec'd for 20 CFM at whatever the static pressure is in its duct path? Thus, exhaust CFM must equal intake CFM to maintain the desired air flow?

Now what if I want slightly positive pressure in the room. Can I spec the intake for 22 CFM and the exhaust for 18 CFM to maintain the desired 20 CFM (since the average of 22 and 18 is 20 CFM)?

I'm asking these questions to verify if my understanding of what I've read is right. Please do not hesitate to correct me of any misconception I may have.

Thanks!

Mitch