Cold water turns on combi boiler

Hello everyone, I have a quick question. I've noticed recently that when I turn on cold water ie faucet, hose etc, it trips the combi boiler to either turn on or leave heating mode and go into DHW mode. I attached some pictures from the manual and a sketch of my set up. I've already tried a shark bite check valve a couple feet (maybe 3) while it's going to the HW heater but it still trips the machine. If I close the hot water valve entirely and call for cold water, it doesn't turn on, like it's supposed to. Any idea where I should place this check valve? Is there one in particular to use? Should it be mounted closer? Thanks.