heating zone heats up when it should not

[notaplumber]

HI sorry for the cross post, but maybe i put this in the wrong section the first time that i posted it.

To start with let me say that I am not a plumber. My father is a contractor and lives out of state. He helped me remodel our bathroom. Almost all of the DHW plumbing was replaced during the process. This should not be related to my problem since the DHW is not connected to the heating zones but i am mentioning this incase it it does matter.

Part of the remodel included moving the heating for the bathroom. The bathroom was a 1' baseboard unit with a fan on it (i think that it is called kick space). This was replaced with a standard 2' baseboard unit a few feet away.

We removed the old plumbing (the kick space was being fed through flexable tubes from and replaced it with copper pipe. We tested the unit by turning the zone heat on and letting it heat up. It works fine when the zone calls for heat, all of the elements attached to that zone heat up. I should mention that all other elements on this particular heating zone are radiators. I thought that you were not supposed to mix baseboard with radiators, but this is the way the house was plumbed.

Now a few days later, i notced this problem. Since the weather is still warm out, i noticed that the radiators were hot even though the thermostat is shut off. I verified that the heating zone was off by turning off the power to the circulators. What i noticed was that when the DHW turns on the pipes to that heating zone heat up. The return pipes are auctualy hotter than the supply pipes. As a temproray fix for this problem, I shut off the valve on the return side of that heating zone and the pipes are no longer are heating up.

A general over view of my heating system: I have a boiler with three heating zones. All three heating zones retun to the boiler at the same point and each have shut off swithces.
zone 1 is to heat up the water heater.
zone 2 (zone with new baseboard) has 2 radiators and one baseboard The supply and return pipes are 1 1/2" cast iron.
zone 3 has several baseboards and does not heat up. this zone is all 3/4" copper pipe.

Does anyone know what my problem is and how to fix it? Can this somehow be caused by air being trapped in the system?

I am having trouble visualizing what is going on in the system and do not know where to begin.

[havasu]

Not being familiar with zoned baseboard heating, I can only give you my take on the description you wrote. If the return lines were getting hotter than the supply lines, You have a serious malfunction. Are there check (one way) valves to prevent the water from circulating backwards, and if so, this is where I'd start looking. Also, I understand different mediums and thicknesses of pipe will alter the amount of heat in any one zone, so how do you control this difference? Lastly, air trapped in the system can cause havoc on your lines and should be bled out.