Saturday, July 2, 2016

Solar Shed: Part 8: Installing the HVAC

This is the eighth post about my solar shed, and I can say that the shed has been working very nicely for me!

After installing a bunch of plywood for your walls, one should, of course, immediately go about cutting a large hole in it!

If you have no idea why I've covered walls in plywood, or why I'm installing an air conditioner, you should start here.  This is my off grid, solar powered office!

Quick quiz: How long does it take to cut a hole in a wall and cram an air conditioner in it, when given a selection of power tools and saws?

Answer: A whole lot longer than I expected!  Walls don't give up as easily as I thought they would.

But I did succeed!

How?  And why this particular unit?  Read on!

Wall Mount Heat Pumps

When searching for an HVAC solution (not just an air conditioner - an HVAC solution!) for my office, I really, really wanted to find an air conditioner capable of operating in reverse cycle (or, to use the more common term, as a heat pump).  There are plenty of air conditioners that offer heating, but almost all of them heat with a resistive element (backup coil).  This consumes insane amounts of electricity for the heat generated.  At one level, it's "100% efficient" because 1kWh of electrical input gets turned into 1kWh of heat, but it's not a great way to heat compared to pumping heat around (a 3-5x multipler on energy to heat).

Why do I want one that works as a heat pump?  A heat pump is, quite literally, running an air conditioner backwards - instead of the inside coils being cool and the outside coils being hot, the inside coils are hot and the outside coils are cool.  The advantage here is that when the outside temperature is cool enough to require heat but not insanely cold, it uses far less power than coils.  The unit is just moving heat, not creating heat.  This makes a huge difference in power, which is really important for an off grid solar office.

That said, it's remarkably difficult to find one of these things smaller than a mini split unit.  There are very few self contained heat pumps out there.

However, I found one!

The Frigidaire FFRH0822R1 (easily found on eBay) is exactly such a unit!  This gem is a window/through-wall unit that operates as an air conditioner in the summer and a heat pump in the winter.  It includes an economy mode, in which it will turn the fan on for 15 seconds or so every 10 minutes, sense the air temperature, and only turn on if needed.  This thing is basically custom designed for off grid use, and many of the Amazon reviews mention it's getting used in exactly that type of situation.

I do understand that it will turn on the backup coils around 40F, which I'm going to keep track of and see if I can work around - I have a few ideas up my sleeve.

In any case, I need to get it through my wall.  Which, conveniently, is a whole section in the included installation direction booklet!

Frames, Instructions, and "Not Believing the Lies in the Instructions"

The instructions come explaining how to window mount it and how to through-wall mount it.  Since this is intended to be a permanent installation, and my window is going to be, you know, a window, I decided to go with the through-wall mount.  It seems nobody has actually done this with the current revision of the cabinet.

Step one is to build a frame of a given set of dimensions.  Specifically, 22 5/8" wide.  Which, I should point out, does not fit the cabinet.

Why?  There are some tabs on the side to mount the accordion window gap sealer things, and they stick out!

Here's the cabinet: 22 5/8" is a perfect width for a frame for the cabinet - if only that were the widest point.

Here are the tabs that stick out the side, and are entirely unmentioned in the instructions.  They add about 1/4" to the total width needed.  Which means a tightly fitted 22 5/8" frame does not work.  At all.

Options included "pounding these flat" or "adjusting my frame."  I decided that pounding them flat was going to be more trouble than it was worth, and might interfere with something inside.

The reason this posed a problem is that I'd already built the frame based on the instructions, assuming they were correct...

So I adjusted my frame.  Sigh.  Read the instructions, then sanity check before building something.

Cutting Holes in Walls

The next (somewhat terrifying) step is to cut a very large hole in one of my walls.  The air conditioner is mounted on the back (north) wall, centered.  This puts it near the inverter, gives it a good position to blast conditioned air all over the office, is on the shaded side of the shed, and is out of the way of other things (the center of the back wall is mostly for the mechanicals).

The hole I'm about to cut covers two studs, and is level based on my best ability to level things with a level.

Ignore the lower right hole.  I forgot you can't turn a saw blade rapidly.  I then proceeded to drill sane holes in the wall.

It takes a good bit of time with a hole saw to punch through half an inch of plywood, two inches of foam board, three and a half inches of insulation, and another half inch of siding - mostly because you have to keep stopping and clearing the hole saw out.  The plywood needs to come out, then the foam, then the insulation, and if you try to keep drilling, you make no progress until the contents of the hole saw have been removed.

About half an hour of solid work later, I had a proper hole in the wall.  I obviously fixed the lower right corner.

I expected punching a hole in the wall with a saw to be a quick, easy, painless process.  It was none of the above.  Getting through the studs took a long time, in no small part because there were screws in the studs I had to slice through.

It's not as easy as it sounds.  If you're tempted to use the phrase, "As easy as slicing a hole in a wall with a reciprocating saw" - don't.

Installing the Frame

After I finished cutting the opening to shape, I stuck the frame in.

Or, really, I should say, "I pounded the frame in with my trusty sledgehammer."  The frame fits in very tightly against the studs, and I'm totally fine with that.

I did have to do plenty of trimming as the frame went in.  The outside needed to be trimmed, and I trimmed as I went, finding the high spots and trimming them as I crammed the frame through.

Can you tell I haven't done this before?  Tolerances aren't amazing, but nothing some foam won't fix.

I also, apparently, did not cut this hole particularly straight.  Foam it is!  This is, quite literally, the first time I've ever cut a hole this large in a wall.

After getting the frame all the way through, I screwed it into the studs - not that it was going anywhere, but I may as well be sure.  Besides, the instructions said to screw it in!

Installing the Heat Pump

A very, very nice feature of this unit is that the guts can be removed from the cabinet.  This means I'm not trying to wrangle a heavy unit into the frame - just the exterior.  So, of course, I did things this way.

The cabinet mounts easily in the frame, once you've adjusted the frame to be the right size.

If it looks like it's slightly tilted down, you see properly.  The unit is slightly tilted to help drain water off and out.  It's about half a bubble down in the rear.

Once the cabinet is installed, the actual guts slide right in!  This is much easier with two people, and I actually did use a second person for this.

So, uh, I have air conditioning now!

Final Steps: Foam

The final step is to foam the gaps so air can't leak through.  The cabinet isn't the tightest fitting thing in the frame, and air (and dust) gets through easily.

So I fixed the glitch.

The foam expands a good bit while setting up (that's it's purpose in life), so there's plenty of excess after things have dried.

However, much like for my windows, it's easy to trim the excess off.

I should probably caulk the seams too, but I'll get around to that at some point.  It's now airtight around the unit, which is exactly what I wanted!

On the outside... well.  I have a bit more trimming to do.  It's kind of a mess.  I'll finish that up later.

Bonus: Wall Construction Detail

One really neat thing about cutting a hole in my wall is that I can see exactly how things worked with the wall assembly - and I'm very happy with it!  Everything is tightly secured together with screws and glue, and it's exactly what I hoped it would look like.

So, I have the ability to heat and cool my office now!  Just no ability to power it for long.

Most excellent.

Join me next week as I get the solar bits and pieces wired up!


  1. How will you get a data connection to your office? LTE? Microwave network to the house? Directional wifi antennae?

    1. I've got a few Mikrotik Lite2 (2.4ghz wifi) units doing the link between the house and office, though I may replace them with a 5ghz link at some point (when I actually get around to mounting the office unit outside instead of laying it on a shelf).

      They're not perfect right now (latency is a bit more erratic than I'd prefer), but they're good enough for now. The link is "hundreds of feet," and those radios are good for a lot further. It's not like I set the office out on a desolate mountain top (though that would be quite tempting).

      At some point, I'm going to be adding another internet connection to the property for redundancy, and that may run straight to the office (rural wifi on licensed bands).

      Basically, my noise floor on 2.4 and 5.8 is "nearly zero" (-107 on 2.4, -109 on 5.8, measured from the shed AP), so I can get away with a lot. I plan on doing a property area network at some point with good signal strength everywhere so I can toss wireless soil moisture monitors and the like in my gardens and lawn.

  2. Query: Why did you do this step now? Wouldn't it have made more sense to do it before all the insulation and wall building steps? Tighter seals and all that?

    1. Because I didn't think it would be as difficult as it was.

      I've never installed a through wall unit before.

  3. Keep it Up Russell, well done that Man.

    Hindsight is an amazing phenomenon, I have yet to master...


  4. Did the HVAC unit instructions specify angling it down for condensate drainage? You would think they would make the unit drain properly when installed level.

    1. Yes, they specified angling it down half a bubble or so.

  5. As it happens, I'm having this exact same unit delivered to my garage tomorrow. Unlike you, however, I'm not even sure I can build the square box, much less do the rest of the stuff. But is that going to stop me from trying? Nooooo. A few questions, if I might.
    My walls are 7.5" thick, so my plan is to build the box out of 2"x8"s. That sound about right to you?
    (I'm going to be expanding a smaller pre-existing opening that was used for a propane-stove vent.)
    What tools, besides the drill, did you use to cut your opening? On hand, I've got a circular saw, a recip saw and a jig saw ....
    How did you build in the angle?
    What kind/brand of foam did you use and do you just squirt it in once everything is in place?
    Got any other hints and/or tips you can offer (besides get a pro to do it, ha ha)?
    Thanks a bunch!
    Oh -- did you use a support brace outside in back?

    1. The 2x8s sound right. Be aware that the measurements in the manual do not include the tab width, so you'll need to adjust for that.

      I cut the opening with a reciprocating saw and the hole drill - that was it. Cut it small and enlarge it as you fit the square in it.

      As far as the down bubble angle? Just play with it - it's not built in. The cabinet fits slightly loose in the enclosure, so I had room to move it around a bit.

      Foam? Great Stuff from my local hardware store. And, yeah, squirt it in, let it set up, trim it.

      No support brace in the back.

      Otherwise, any tips and suggestions I might have are in the post!

  6. This comment has been removed by the author.

  7. This comment has been removed by the author.

  8. Thanks. I'm as nervous and twitchy about doing this as a cat on a hot tin roof but I'm gonna give it a go. i mean what's the worst that could happen :-)?

  9. Well, my a/c arrived ... but with some pretty smushed fins on the rear of the unit. Here's a pic: . I'm loath to send the thing back to Amazon, just because it's so damned hot out and I need a/c. Do you have a sense of how critical those fins are and how crippled my machine might be? i googled and found you can straighten em out with either a fin comb or a butter knife, etc., but most of my bent ones are almost fused to their neighbors -- ie, they're bent beyond redemption. Sigh. what do you think?

  10. here's another pic. . man, am i upset or what? i'm sure if i want to keep the thing, i can get amazon to refund me some dollars but i'll do that only if'n the thing isn't ruined.

    1. Wow, that's pretty beat up. I'd return it and ask for a replacement.

  11. I spoke to Frigidaire and was told that if 20% to 25% of the fins were damaged beyond repair, the unit would still run fine. I bought the thing from amazon warehouse for 290. it was supposed to be in 'very good' condition. I wrote amazon about its actual condition and amazon said that if i wanted to keep it, they'd refund me 35% of the purchase price (or $100). My plan today is to hook it up and see how it blows. If it still *seems* to work well, I guess i'd keep me, for a new price of 190, though that could just make me pound wise and penny foolish in the longer run. Arggg!

    1. For $190, I'd take my risks with it if they say that it's OK with damaged fins. All they affect is the heat rejection side of it, but if it's designed to work with a bunch beat up, then it should be fine. As long as it's blowing cold and hot (in cooling or heating mode), it's fine.

      Be aware that it will need to run for a few minutes before it turns the compressor on initially. Let it run for 10 minutes or so before you get concerned about a lack of compressor.

  12. Got it done two days ago and it works great, no problems at all. the only thing about the a/c i don't like is that every time you turn it on, it starts up in econ mode. does yours do the same thing? would love it to start in cool / auto. btw / thanks for your help and encouragement and for all the pix showing me what to do. knowing i had to add 1/4" to the width measurement saved me hours of sweating and cussing, i am sure. bueno!

    1. Yes, mine starts in econ mode. I don't mind, though you should run it for at least a few hours in "cool" every now and then to keep enough airflow that it doesn't get funky.

  13. Just wanted to say thanks for this series. I'm purchasing a very similar studio from Tuff Shed and will be following this tutorial pretty much exactly. I'm in Portland Oregon and will be on-grid so I'm considering just using 1" styrofoam, but I haven't settled on that one yet. The exacting pointers here were great, and it's so cool to see where you had to step back a bit as things didn't go to plan.

    Side note I liked your super-short explanation of why heat-pumps are more efficient than simple electrical resistance generators.

    1. Awesome! I'd love to see pictures of the process, and would be happy to share them if you have no objections - my contact form is over in the right column.

      If you're grid tied, it gets so much easier. You might consider getting a mini-split unit installed as well - they're a large heat pump with an outdoor unit and small indoor wall unit, but my unit works just fine, and in the winter it would be fine except for the grey days. I have a post coming up this weekend about winter heating, but it won't be particularly relevant to you, since it doesn't get nearly as cold in Portland, and you'll have grid power to brute force through.

      Going with the foam insulation and sealing up the enclosure would be a good idea - my foam shell is as much a wind block as anything else, but there's definitely no need for 2" foam if you're not going for maximum insulation.

      I definitely enjoy my plywood interior - being able to mount things anywhere is awesome.


Comments on older posts are moderated due to spam issues. If you don't see your comment immediately, and you weren't just spamming me with some irrelevant comment and a link to whatever site you're trying to SEO, your comment should show up relatively soon. If you're trying to use my blog for your SEO purposes, your comments will never show up, so don't waste your time.