Why glycol instead of water?
Unless the top floor baseboard is more than 65 vertical feet above the boiler, there is no reason to run it at 30psi, and running it there would usually require a different pressure relief valve than the one shipped with it. Most cast iron boiler can be operated safely at 50psi, but are shipped with 30psi relief valves. Most 2- story houses with the boiler in the basement do fine with the system pressure of 12-15psi at idle. Water pressure increases 0.433 psi for every vertical foot of height, so at 12 psi the piping at the top of the system 27-28' above the point where the pressure is measured (say, the gauge on the boiler) would be neutral relative to atmospheric pressure. The rule of thumb for setting the system pressure on taller houses is to measure the vertical distance in feet, multiply by 0.433, then add 3 psi to guarantee that the top of the system is still always at positive pressure even while pumping. The lower limit is about 8-10 psi at the boiler at which point flash boiling becomes a problem.
It's likely that your system is currently at 30psi because that's the pressure at which the relief valve opens up. Any time the boiler fires it increases the volume of the water, increasing the pressure, and the valve spits water.
So...
With power to to the system turned off and the boiler at idle, turn off the valves at the glycol tank or any potable supply to the heating system water, then bleed system water until the pressure reads 12-15 psi or so, and carefully note the reading. Turn the power on, but keep the isolating valves turned off. When the boiler fires on a water heating call it will probably come up a few psi, but it should fall back to the original pressure setting after an hour or three. If it spikes to 20+ psi in only a few minutes into the burn it's likely that the pre-charge on the expansion tank is way wrong, or the expansion tank is shot. (Tap the air-valve end of the expansion tank with something hard- it should ring rather than thud.)
If it seems to behave fine, no pressure spikes, no hammering banging noises from flash boil, keep observing it over several days to see if the pressure is creeping up. If it is increasing, it means water (or glycol) is slowly entering the system over time, and the possibilities are pretty much limited to seepage at the valves isolating the system from the glycol or potable fill points, or a leak in the heat exchanger inside the indirect.
With 146,000 BTU/hr of DOE output the CGa-6 is ridiculously oversized (for most) homes in the US with less than 8000 square feet of conditioned space. It's even more ridiculously oversized for the radiation on individual zone calls for a house with 4 heating zones + 1 hot water zone. To analyze what makes sense for any adjustments or boiler swaps do this bit of napkin math:
1: Measure the amount of baseboard on each zone by feet. The multiply the feet by 500 BTU/hr (which is roughly that amount of baseboard will emit with an entering water temp of 180F.) Add up the total feet of baseboard, and the total BTU.
Then repeat, using 200BTU/hr per foot, which is roughly the output at an EWT of 125F (the temperature it takes to hit the mid-90s for efficiency with a condensing boiler.)
2: Run a fuel use heat load calculation at your
99% outside design temperature. (For SLC that's +11F.) The short explanation of that is:
Take some mid to late winter gas bills (no shoulder season bills) noting the exact meter reading dates, and the amount of fuel use, and convert the fuel use into BTU, whether it's expressed in therms, CCF, or decatherms. Multiply the BTU by 0.83 (the steady state efficiency of a CGa-6), which is the net amount of heat that was delivered to the heating system (the rest went up the flue.)
Then find a local weatherstation on degreedays.net close to your house, and download a daily spreadsheet of base 65F degree-days that covers the days between meter readings, and add them up, first including the beginning meter reading date but not the last, then conversely. They should be pretty close, but average those two numbers, since you don't know exactly what time of day the meters are read.
Divide the net-BTU by the total degree-days, and divide that number by 24 to get a degree-hour constant.
The presumptive heating/cooling balance point is the heating degree-day base, which in this example is 65F. The 99% outside design temperature is +11F, so you have 65F-11F= 54F heating degrees. So, multiply your degree-hour constant by 54F, and that is your implied heat load. Since it doesn't separate out hot water use it's actually an upper bound, but in sunny SLC solar gains offset that error a bit.
The optimally sized cast iron boiler would have a DOE output no more than 1.4x that derived number, and even a 1.25x oversizing factor will cover your load at the temperature extremes well below the 99th percentile temperature bin. Do NOT upsize the boiler for domestic hot water, since you have an indirect tank, zoned priority.
A more detailed
explanation of fuel use load calculations lives here.
If installing a modulating condensing boiler it's more complicated, but the smallest zone's baseboard will ideally be able to fully emit minimum modulated output of the boiler at condensing temperatures, which almost always rules out a 199K combi-boiler.
The napkin math on that lives here.
