Pressure test with air?

Use water.

Seriously. If you use air, at any significant pressure, and something lets go, you are going to have pieces flying all over, and I don't like shrapnel. Scares me. If you use water, all that will happen is you will get wet. Second reason: air pressure is very temperature sensitive, A rising temperature will hide a small leak. Dropping temperature will look like one.

Use water.

" I plan on useing a bicycle pump to bring the pressure up to the manufactures recommended max psi of 55 "

Wow, ever fill up a flat (empty) truck tire with a bicycle pump ? You may be there a while, pumping and pumping and pumping…….. I'd at least fill the boiler with water. The more water in the system the less air needed to build the pressure up.

As @Jamie Hall stated Hydrostatic testing is safer.

you can get a test gauge that will screw on to hose thread that you can screw on to a boiler drain. you can also drill a hole in a plug and solder a schrader in to it, possibly one that you salvaged from an old innertube and stripped the rubber off of.

I really don't care whether you use water or air or LP or whatever. It's your bomb.

But whatever you do DO NOT EXCEED THE RATED PRESSURE OF THE LOWEST RATED ITEM IN THE SYSTEM.

If the boiler is rated at 30 psig, that's your maximum test pressure. Period.

I’d also isolate the expansion tank

This is another common test block, just add a 3/4 female 3/4 hose female adapter

@EdTheHeaterMan — that rule of three is occasionally used, to my knowledge, for strict testing under very controlled conditions, and it is really only during testing to determine ultimate strengths and the validity of design safety factors. It is ASSUJMED that something will break; the question is what and when…

I could be wrong…

Ah yes! Theatre. And overhead rigging. That's a different matter! The rule of three there is an absolute minimum, or at least it was in the hemp houses I have encountered. Some other types of overhead work require a safety factor of 5.

Not, if you believe some of the YouTube clips I've seen, that people always pay attention to it…

Lighting bars can be insanely heavy…

Off topic? Who us?

Speaking as the colleague in question 👆🏻 ( Hi Alan!)

Here’s a pressure-testing puzzle that stumped quite a few engineers, contractors, and other problem solvers I’ve shared it with.

Back in the early 2000s, California inspectors I worked with would accept a 100 psi air test held for at least 30 minutes on hydronic PEX tubing before concealment—whether in a slab or behind drywall.

I’d connect my compressor, bring the tubing up to 100 psi, and isolate it. If there were no leaks, the gauge would stay at 100.

Later, the inspectors began requiring a hydrostatic test. That’s when things got interesting.

I’d fill the tubing with water, pressurize it to 100 psi, and isolate it. An hour later, the gauge would read around 50 psi. I’d bring it back to 100, then return an hour later to find it around 75. After another top-up to 100, it might drop to 85.

That pattern would continue, with progressively smaller drops, until it finally held at 100 psi. Assuming there were no leaks, that’s where it would stay.

Same tubing. Same test pressure. Different behavior with air versus water.

I posed this question to plenty of people over the years without getting a satisfactory explanation, until I eventually figured it out.

What do you think was happening?

Temperature? I usually spend 5 minutes or so topping off the N₂ when I pressure test my linesets. I'm pretty sure it's just the gas settling in to the temp of the copper.

Temperature and trapped air.

We installed a couple of chillers (maybe 2 40 ton chillers ID remember) with a 500 gallon buffer tank in a 30% glycol system. Brand new system. The floor mounted expansion tank was piped to the top of the vertical 500 gallon tank with an isolation valve at the expansion tank. There was no water MU because it was glycol.

It ran a month or so and a couple guys went over and the pressure in the system was always too high or too low. So they would fill or drain (screwing up the glycol mix of course). This went on a few times.

Finally they sent me there. The chillers were shut down (this was a process job) so the water was room temp the pressure was high. They had a hose connected to the drain on the buffer tank.

I put a gauge on the system by the drain and it was reading 40psi. I opened the hose and took 1 coffee cup full of fluid out and the system pressure dropped to 0. My first thought was "no expansion tank"

I climbed up on top of the buffer tank and found a 3/4 ball valve in the expansion tank line that had never been opened.

If you don't have air in the system or a connected expansion tank even a tiny loss or gain of water will change the pressure radically.

If the pressure drops and you can't find a leak there is probably some air in the system.

This is what baffled me: if 100 psi pushes outward against the inside of PEX tubing just as hard whether it’s air or water, why does the gauge behave so differently?

I kept getting stuck on that point: same pressure, same tubing—why the different result?

My understanding now is that there are two possible contributors. If air remains trapped when the tubing is filled with water, some of that air—not just the oxygen—can gradually dissolve into the water under pressure. That can contribute to a pressure drop without a leak. Separately, the PEX can slowly stretch under pressure, increasing its internal volume.

What finally helped me understand the air-versus-water difference was this example: isolate a water-filled pipe under pressure and crack a valve. A small squirt of water can cause a large drop on the gauge. Crack the drain on an air compressor tank, and air keeps hissing out while the pressure gradually falls.

It’s easy to imagine that instead of releasing a little water, the tubing stretches enough to create that same amount of extra room inside. Nothing has leaked, but the pressure still drops.

Compressed air can expand into that additional space with relatively little pressure loss. Water has very little ability to expand, so a small increase in tubing volume can cause a much larger pressure drop.

The distinction I was missing is that the tubing can stretch with either test medium. The same amount of stretching simply produces a very different response at the gauge.

This is the best I could do with a GED and a life in the trades🤷‍♂️

Talk to an old refrigeration guy. Vacuum testing. Pull a vacuum and guesstimate tightness from how rapidly system loses vacuum. Seconds mean bad; hours mean….

I have seen 100 year old HHW systems develop leaks.

the air, the water, the tubing all expand and contract at different rates as the temp changes. here the municipal water is close to saturated with as much air as it can hold at the supply temp and pressure. it may not be the case where you are getting your water from. as the pressure decreases and the temp increases it can hold less air. drastic temp changes in a lot of water with little air on top of it may cause it to absorb or evolve air in a measurable amount.

also many hydronic components are only rated for 40 psig or so.