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Are those Honeywell zone valves, the 8043?

Usually ZV hammer is caused by a fast closing valve against a high velocity flow, flow above 4 FPS, perhaps. It's more about the speed, not the volume of the fluid.

However if the ∆P circ is on the correct setting it should easily address the velocity? What circ do you have and what is the gpm or W. when the valve makes the noise?

One common hack for the HW is to remove one of the two springs from the brass sector gear. This slows the close speed by 2 seconds, and that fixes many of the hammer issues.

It does change the close off pressure of the valve by a couple PSI also. So make sure that zone isn't bleeding through when it is off and other zones are flowing.

A properly adjusted pressure differential bypass on the discharge side of the pump should get rid of the water hammer.

or to put it another way — either shut it down or bail out.

If you are reading 70 ppm inside, you're not safe. Just do some good research, and do understand the dangers to anyone in that place. I would not be comfortable with a level that high indoors.

Something's wrong here. It's dangerous at 70 ppm and with those numbers you have no room for error using cheap instruments that are imprecise and a malfunctioning heating plant that may continue to degrade.

I was called into a job years ago where my brother in laws father (in his 90s) was complaining that his addition on his house was cold. He had an old gas fired steam boiler with a tankless heater that heated the HW loop for the addition. There were 2 or three CI rads and they had piped a split loop 1" supply and 2 3/4 returns. (way overkill) He already had every local plumber look at this with no luck. They had monkeyed with this for years. They had taken the CI radiators out, put baseboard in and then put the CI radiators back in. I don't think it ever worked right. I think as his father got older he couldn't tolerate the lack of heat.

It had a Taco 110 circ.

After staring at this for a while feeling the pipes and bleeding the system I stood there looking at the tankless coil with it's 1/2" connections and pictured how much 1/2" tubing it took to make the tankless coil.

I upped the pump (forget what I put in) and it was fine.

I never missed when sizing a pump for a new job but going to an existing job that sort of works it gets in your head.

You have to step back , start from scratch gather some information and figure it out.