What supplies the water connection? The water temperature has to drop below 110 degrees in order for the aqua-stat to shut off the fan. Have you measured the water temperature?

Yup, pipe temps at about 65 degrees and the aqua-stat set at 165 (I raised it just to make sure) that is wasn't opening the connection at a lower temp. I have a new Burnham boiler that is heating my house and that zone ,that is also the only device on that zone of heat. Given all that the fan will still run.
Boiler off, thermostat off, temp of water below 70 degrees, fan runs constantly.

I'm thinking the fan unit is some how shorting out the aqua-stat so the electrical connection to the aqua-stat just gets by passed.

Any thoughts?
Thanks

I know you said you replaced the aqua-stat, but it sounds like the aqua-stat is still the problem. It is hard to tell if your theory is plausible without knowing the insides of the aqua-stat or the connection to the fan.

Is there any other heat source nearby that might be fooling the aqua-stat?

The data sheet says the unit has a two speed motor. Does the problem occur with both speeds?

The aqua-stat is designed to open at 115 - 175 degrees as the temp rises, its adjustable. There are no insides to the aqua-stat its fully enclosed in its plastic shell. The leads on it are soldered in place to the ears and right now its not even touching the water pipe that its suppose to reading the temp from. the leads are wire nutted to the leads from the fan unit. which run up and inside somewhere. I have not taken the fan motor apart (yet). There is nothing near by that would trip the aqua-stat into thinking that it is 165 degrees, that is its current setting.

And it does occur on both speeds.

Thanks