Bill,
It sounds like your HH is operating normally if there is not a high heating demand. The HydroHot unit has a much smaller boiler fluid reservoir than the larger AquaHot unit, and therefore, at low demand it will reach operating temperature quickly causing shorter burner intervals. However, you should mention this the next time that the unit is serviced to be sure that your aquastat is operating correctly to turn the burner on and off at the correct temperature.
Gerald