A project we have on the go currently is re-fitting a hot water plant for a residential apartment building. It’s a reasonable size job with a somewhat complicated centralised plant that services sixty apartments. To make things a little more difficult, the new system has to be built over the top of the old, in the same plant room/area, as the building remains fully inhabited and requires an ongoing hot water service.
New system schematic; yep that’s alota lines alright
Our design firm actually engineered this system and our contracting firm was fortunate enough to win the tender for the works. Possibly because everyone else had it in the too hard basket. Moving to construction, I actually picked up a couple of small errors in the above schematic; they’re only minor drafting errors so don’t affect the methodology. Being that the project was so complex, they were just overlooked. It’s funny that no matter how much you check, re-check & counter check, looking back at old drawings with fresh eyes there’s always something. I guess you get that on the big jobs; ten points if you can find them.
The wheels are now well in motion on the project. The hot, solar feed & return and building return lines all require insulation to be installed onto the pipework. The straight runs are no problem; the lengths of insulation are split with a foil lap. You just toss them on and seal with foil tape. The changes of direction are where the trick comes in. For neatness and better thermal retention, the sections have to meet neatly to minimise heat loss. Best way to quickly and consistently achieve this is using a mitre box to shape & trim the insulation. This way there’s no thinking or guessing for the guys; just cutting and installing.
This build is really a form follows function type deal; not a piece of fine furniture. I’ve got some 12mm (≈1/2”) plywood lying around that’s not really good for a lot. The board itself is fine but mould has crept into the veneer layer so it’s not particularly attractive; perfect for this application. I’ve also allowed for the uprights and base to be doubled up. A box made from a single ply thickness may hold up over the course of the works; but the guys tend to be a bit rough on the gear sometimes, so we’re going with two.
Ripped, docked & rebated; ready to join.
To increase the joinery strength between the uprights and base, 3mm (≈1/8”) rebate joints will be used as opposed to butt joints. It’s getting a little bit pedantic, but I haven’t had a chance to get out into the shed recently; because, well life really. A little bit of extra woodworking on the job is a welcome relief. The router table makes short work of this and they are arranged so the inner base is rebated and the outer walls are rebated.
Assembling the inner went smoothly. The joints are glued and fixed with a finisher nailer. I let that sit for a bit then prepped for the outer layer, which gets fully glued to the inner. One trick to help get a nice even spread of glue on larger surfaces is using an ink brayer to roll it out. Works really well. This also where things stopped going really well.
My hand design and maths were good; a pity then, that my eyes were painted on when ripping the stock down. The inner uprights were cut exactly 10mm too short. Real first year apprentice type handiwork. There’s still enough height still to just rip the excess off luckily. I had originally planned on a 25mm (≈1”) deep start for the saw, which is now trimmed down to neat 15mm (≈5/8”).
Sorry about that one boys
With the box fully formed, the two ends are docked flush and the surfaces get a light planning. This removes the saw marks for a smooth clean finish. The block plane is also used to remove the arrises; as these are rather sharp and can cause cuts while using.
I have always really enjoyed the look of tight-fitting ply joinery
Last thing to do is to add the mitre part of this mitrebox. The channel has been made to suit the insulation, now it just needs some way of making repeatable cuts. The pipe this insulation will go around gets plumbed with factory made 90° & 45° elbows to change direction. This means that we can cut preset angled slots into the box to mitre the insulation around these fittings; at 45° & 22.5° respectively. I’m also including a 90° slot to dock lengths shorter as needed. The 90° & 45° markouts are easy, as they come straight from a try square. The 22.5° one takes a little more working out to get.
I’m not sure exactly what it is lately, I still seem to have mitres on the brain.
The lines are marked out onto the box then carefully cut with a saw. The 90° & 45° ones I opted to share a slot; there should be enough material still for the stick to survive the guys. The 22.5° would have been a touch wishful here though paired with the 90°; so it has been offset some. The saw that goes with the jig is used to cut the slots; to ensure a snug fit. With repeated usage on the job, slop will invariably accumulate in these slots. Should comfortably get through the scope of works but can always cut more if need be.
So how does it work? Plonk the insultation length in, line it up with the right slot and cut away. Matching two cuts of the same angle will complete the mitre and nicely insulate the pipework.
Time to get to work fellas
What does it look like in practice? The insulation joints are measured and cut then placed over the pipe with the slit. A special foil tape is used to fix the flap down and to go around where the joints meet. Makes for a pretty tidy job.
Always a pity to have to work on such a beautiful day; especially when the jobsite comes with a view.
Kind Regards,
Walker
Sept 2021
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