The points were in the bag within five minutes of the start of the second half after Danny Graham had ended his goal drought to follow up strike partner Heidar Helguson's quick-fire brace.
But the opening period was all about Helguson, who boosted an already impressive scoring record since returning to Vicarage Road.
The Icelandic international, who will always be regarded as a hero at Vicarage Road for his scoring exploits in his first spell for the club, went into the game with three goals from as many appearances after returning on loan from Queens Park Rangers.
Within the space of three first-half minutes, that record read five in four.
The former Fulham and Bolton Wanderers striker might have already had an assist to his name - Graham racing on to his flick on to find the back of the net inside the opening minute, only for it to be disallowed by an assistant's flag - when he made the breakthrough in the 19th minute.
Henri Lansbury fed Tom Cleverley on the left side of the Scunthorpe area and he turned before clipping a fine lofted ball into the heart of the area where Helguson got goal-side of his man to head firmly down and past Josh Lillis to make it 1-0.
Within three minutes he had doubled his tally, but this time the quality of the ball was arguably even better as Graham dug out a stunning cross from tight to the right touchline.
Scunthorpe showed exactly why they had shipped nine goals in their previous three league, leaving Helguson totally unmarked to head home again.
The Iron's best chance of getting back into the game came shortly before the break when England Under-21 keeper Scott Loach made a fine save to deny Sam Togwell, but within four minutes of the restart it was game over.
Iron boss Nigel Adkins made a double change at the interval, but one of his substitutes, Bondz N'Gala, was involved in an awful mix-up that allowed Lansbury to latch on to the ball on the left side of the area and square for Graham, who side-footed home from the edge of the area to score his first goal in eight games.