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I have 2 pressure gauges on the system. Never read over 15psi. I have an expansion tank in place. I tested the relief valve when i flushed the boiler. I do this every fall. PRV said it is rated for 30psi. It is leaking mainly during fire up to temp (assuming as pressure builds). Never over 15 psi according to the gauges. Sounds like an end of life PRV and just replace?
I replaced the valve. Old one showed a ton of corrosion inside. easy fix.
I'm guessing you didn't fix the problem. Run the boiler up to 200 and see what the gauge says then. I'll bet your expansion tank is either full (if it's a non bladder tank) or the bladder is bad. The other thing that can cause excess pressure is a leak in the tankless coil if you have one.
Pressure gauges can become corroded too, and not indicate reality.
I used two gauges for that reason. They both read correctly. I replaced the valve and all is working just fine now.
A bad valve should have raised the pressure "constantly", not just during startup time. Your description or diagnosis may have been faulty, but as long as what you did "seems" to have fixed it don't worry about it, but do check it periodically.
I think the problem here is i am talking about a relief valve not a fill valve. Maybe my terms thru you off but this is the valve off they tank to relieve excess pressure in the system.
That would make a difference, but it would have had to be very defective to operate on the slight pressure rise that the expansion tank was compensating for.