Milivolt is a very low voltage, so any increased resistance in any circuit powered by a milivolt source can become problematic quickly. Glad you found the problem.
You mentioned experience with high pressure steam. I'm an ex-Navy snipe; when I heard civilian boilers operating at around 100 psig referred to as high pressure boilers, I laughed. I was used to 1,200 psig, superheated to over 600 degrees F. But then I realized that 1200 psig high pressure was required for propulsion, not heating.
I started / owned a company with a partner which manufactured personal care products...shampoos, lotions...where we used process steam. In order to operate a high pressure steam boiler I had to pass NJ Boiler Operators exam. The pressures were a very low (60 psi) as compared with what you worked with. As you may be aware anything over 15 psi is considered high pressure. The exam focused on operation and of course safety and not repair of high pressure steam boilers.
My partner who was licensed for many years installed the 10 HP Fulton boiler system with me acting as assistant. As we grew, we moved up to 1 40 HP Fulton.
Thanks for your feedback