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Tone generator with short tester

If you're replacing a boiler where there's a mono flo system, its important supply and return flow stay the same. Where in a series loop it doesn't matter if you reverse flow in most cases. I've replaced numerous boilers where's there's a mono flo system and the old had the B&G 100 on the return. New installs always pumping away. Never an issue.

And the behaviur of a monoflow system has nothing to do with the location of the pump nor even the static pressure in the system. It does, however, have a lot to do with the water velocity in the distribution pipes — so one way to mess it up which your guys may have found is to substitute a very different pump…

just to be clear the pump could be on the return and still be pumping away as long as the expansion tank is connected at or near the pumps inlet

Many mod cons have the pump on the return to help overcome pressure drop in the boiler

Here are two examples, one with pump at return, one at supply

Typical ci boilers have low pressure drop so tank at return, pump on supply is still “pumping away”

What is important is for the air purger to be at the boiler supply out, the hottest point in the system