Thanks for the quick replys. Here's some more info for you.

It's a Burnham furnace, pressure dropped to 0 when we changed the
expansion tank. Current pressure is 20PSI, current temperature is a
hair under 170.

We have 5 zones with Taco Cartridge Circs Model 007-F4

The controls are Argo ARM842 (3 zone) with 2 expansion zone controls,
also Argo of course.

Sounds like the temp might be too high then? What would cause that
and is there a way for me to adjust it without calling in a pro?

Thanks.