Hydrotherm boiler pressure steadily rises

I have a Hydrotherm two stage Boiler, with a 15psi Watts feeder to the boiler and an Everhot indirect water tank feeding domestic water via coil. There is a steel expansion tank, probably a 30 gallon tank.
The issue I have had is that the pressure rises 5 psi or so every two to three weeks to the 20 psi level and continues to rise after that point. I have replaced the Watts Feeder, turned the valve off before it and the problem still exists. I have also replaced the Expansion Tank drain valve and the problem still exists. I have repeatedly isolated the Expansion Tank via a shutoff valve, turned the boiler off (gas burners) and drained it completely, takes about an hour, until blackish water trickles out of the tank and the problem still exists.
The Watts feeder stops feeding at 15 psi and I turned the valve before the Feeder off and the pressure still rises 5 psi every two weeks.
If the Everhot Tanks coil has a leak in it wouldn't my domestic hot water be blackish and wouldn't the pressure in the boiler rise faster than 5 psi every two weeks?
Could the expansion tank have a leak in it? I don't see a puddle, could it be a pinhole leak? How do you detect that?
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wrote:

    Not if the leak is in the other direction. from potable storage ( 60 PSI ) to boiler ( 15 PSI ). Which it is, it has to be.

    That's not your problem, even if it did.
    Drain the boiler, leave the fill to the potable open, and see if your boiler fills up. Remember, water will only move from high presure to lower presure. Your potable is always ~ 60 PSI.
Or conversely, turn the potable fill off and watch what happens. If you want a few weeks without water :-). Caution - this method can result in boiler water ( nasty ) in your potable tank. So don't do it.
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