I have a whole house radiant 5 zone hot water system with hot water heater as part of the system. The hot water has dropped in pressure but the cold water pressure is okay. Is there an easy way to back flow the system to flush it out? I didn't see a valve like is on a traditional hot ware system for a flush of the system. Each zone has a flush valve, but the hot water heater side I don't see one... What am I missing...

Okay, I found the drain for the entire heating core, and flushed the core. Some rust, but basically a steady flow from the flow, esp when I over rode the pressure relief valve.

I then turned off the unit, at a shower turned on the hot and cold but used the shut off at the shower head, turned off the cold water supply to the furnace, and opened the drain again. A slow flow. I opened other faucets on hot and got a good flow. This tells me the flow after the heater in the pipes is good, both hot and cold. The flow from the core was slow.
I'm stumped, what is restricting the flow? I restored everything back to normal. Still low flow. Good pressure for about 10 second on hot when first turned on then drops to a small stream.

Any experts ?

The unit has a watts pressure regulator s 1156f and watts temp regulator 70A, could these be faulty or restricting the flow of water?

When I push on the level at the top of the pressure regulator - water flow is good out the core drain. But If I shut off the drain valve then leaks show up near the circulation pumps. Tells me too much pressure or this is the part with an issue... (this only happens when I over ride the pressure valve) Do these clog?