Kettles are a little over $1,800 so you're saying fittings are another $3,600?? That's news to me and I wasn't expecting that.
The added stainless doesn't triple the cost, it is the box and the rest of the build combined.
I suppose it ultimately gets down to exactly what you want with your system. I decided to build the system as I wanted it (within reason) instead of putting something together that I would be annoyed with and start making changes that would waste a lot of money down the road. For me that meant two things, I wanted continuity and for things to be easy to take apart. That meant everything is TC. So, the water filter and Chuggers got NPT to TC fittings so I don't ever have to deal with threading connections as I switch hoses out while brewing. I have seven (IIRC) hoses that have to be built and since I hate the standard hose clamp I am using Oetiker clamps. I have bought no shortage of 90 degree and straight TC hose barbs, along with ball valves, TC clamps, hose, gaskets, and a few other things to build an inline oxygen/CO2 injection system. I went with stainless Chuggers with the 3/4" inlets, a new stainless CF from Williams (that I need to find a welder to make it TC). Than there is the cost of steel to build the brewstand, I have a mig but I needed to get shielding gas as flux core is a pain to work with for my skill level, plus all the consumables in grinding the welds and prepping the stand for paint. Further, all the nuts, bolts, washers, and hinges on the stand are stainless.
So, long story short, by the time I add the cost of everything that wasn't the kettles or the control box, I would say $1500-1800 is realistic.
As far as the control box internals, the elements, Aubur RTDs, 30 amp GE GFCI, and using automotive paint, that project has easily cost $1500...and the shipping costs of all the parts adds up. The box could be done cheaper; but, I don't like the industrial buttons and lights so I used metal DC illuminated ring lights which required me modifying the Kal design so all switches are DC...which meant a lot of SSRs. As well, auto paint is expensive...so, I figure if one went on the cheap, they could build a box for around $1k. But, if you are dropping a $1000 on a build, why make it ugly and settle for a $5 rattle can finish?
Sorry to be lengthy; but, I figure you would appreciate some some reality. The incidentals of building a brewery are huge, don't let anyone tell you differently. Figure on this costing you $5K+ by the time you brew your first batch and accept that you will no longer ever be able to tell your wife that homebrewing is actually saving you money
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