Snowmelt - Pressure jumps when on.

Hi there. My snowmelt system is set to 12psi (50% water/ 50% glycol).

I notice when the system goes on the PSI jumps very high - well over 30psi (triggering the relief valve) if the boiler is set anything above about 135. At 135 it takes forever to melt snow.

I'm wondering if I reduce pressure in the system to about 5psi to give more buffer if this will cause any problems? Right now there is a 2.1 gallon expansion tank connected.

Thanks

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