Hi it doesnt sound like the boiler is at fault
more than likely the setting on the thermostat(gauge)
Is it a programmable Gauge? and may I have the full make and model # ***** the gauge?
Thank you, ***** ***** is supposed to be set as you say between the red and black. But if it isnt responding, You need to have the tech come back as that one sounds to me to be faulty for sure
it should actually turn the unit off and on
there are relays inside and one of them if seems is bad
Sometimes we all get a faulty part or device right out of the box;-(
in addition, they shouldnt charge you for the return trip as the part they installed is bad.
I hope that helps
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Thank you and have a great Sunday !