Best Of

Thank you JH for the clarification and addt'l info!
Thank you NY_Rob for your personal example and vital info!
And thanks to hydro_newbie and JUGHNE as well for your input! I like the idea of installing a sacrificial copper pipe so you can easily tell if the neutralizer is working.
You guys are GREAT!
The way you answer questions and provide detailed and concise answers, backed by experience and tremendous knowledge is so incredibly valuable!
I feel like I learn so much with each post.
Wishing you all the BEST!

They look strangely familiar.

Yeah, they're in the meme thread.
But they're just as good the second time around.

Officially that is a straight threats and should be sealed with a gasket fitting. Actually a 1/2 npt threads into it, use Loctite or teflon tape and it seals fine. Just tried all these 1/2 npt options on the bench.

How about that vintage Heatway gauge!

We did come up with this NA10425 gauge well for a Viessmann boiler customer that wanted to use one or our I-Solar control sensors into the boiler. It allows a 6 mm sensor to slide in and capture with the rubber "keeper" It is a straight thread, gasket fitting.

6 mm is about 1/4" and is a typical sensor size on most mod cons, or any control of European origin.

Fed through an SCR instead of a relay? Those will float up to supply voltage without a load on 'em.

A relay is an ON/OFF device, there's a physical gap that interrupts current flow. SCRs are more like low-resistance/high-resistance. There's no physical gap to make it really hard for current to flow, μA leakage will slowly, over the course of nanoseconds, raise the output terminal to the same potential as the input without some way of draining off the electrons. Imagine a leaky valve connecting two tanks. Eventually, the leak will bring the second tank up to the level of the first one.

Check with your local code official if this is something that will be inspected. Some jurisdictions require minimum 1/2" OD. As Steve said it can be harder to get all the air out, especially if the line goes overhead.

I'd only go bigger if it's recommended by the pump manufacturer for proper sizing. Bigger can make more it harder to eliminate air, especially if it's not a gravity job.