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We bought a new house in Colorado Springs last July. The gaps between the hardwood slats began appearing in late summer and have continued to worsen over time. The homebuilder and reps from the company who installed the floors have looked at them. Both have mentioned filler and talked about increases/decreases in humidity causing the separation.

They said that some gaps are normal, but recognize that several of the ones in our floor need attention. They are sending out representatives from the manufacturer late this week or early next and I want to be armed with as much information as possible. Does the flooring need to be redone?

Your probably wasting your time and effort with the manufacture. I can tell you that from experience. You lowered the r/h in your home when you turned on the heat last fall and your floor shrank as a result. When wood dries out it shrinks, when it gains moisture it swells. That looks like a 5" site finished floor. 5" will show this faster and more extreme because the wide widths are prone to this if not properly handled (less nails, less separations to disperse the movement also.) Id guess the r/h in your home got down in the teens this winter and thats way way to dry for proper wood floor performance. Get the r/h back up and a lot of that will fix itself. Those extra large ones wont but all those smaller one should improve significantly. Cracks are normal in solid wide plank someone should have told you that. Do you know the moisture content of the floor and what is your average indoor r/h? Colorado has a desert like climate for the most part, id look at all the solid wood products in your home and see if they have shrank as well, if your trim is separating at the joints and your furniture is pulling itself apart then its not the floor it's the condition of the site.

The boards are 4". We have a whole house humidifier that I'm told is working, although I don't notice it at all. All of the other furniture & molding in the house looks fine.

Kevin is right no 2 ways about it.s were installed at a humid time of the year and then you fired up the A/C which made it shrink then it was winter time when the air is very dry. Your whole house humidifier won't bring it back to the original level.

Those one attached to the hvac only runs when the hvac runs and that wont get you where you want to be. You will need a stand alone unit that you can set the desired r/h and let it work. A 4" is still considered a wider "plank" floor and seasonal gaps are expected in most installations especially when you let the r/h dip real low. We had the most extreme winter here last year we have seen in the last 50 years. Ive looked at several and talked to even more about the same thing over and over again and no one is happy about it. Let me say it again, when wood looses moisture it shrinks, when it gains moisture it swells. You floor obviously has changed the moisture content since the time of install/finishing. Its not the wood's responsibility to keep it temperate in the home, it's the owners. You ever wonder why grandma kept a pot of water on top of her wood stove? .

And I heard people had pots of water under piano's, especially grands so the top wont split.
In your pic's you can see where the wood has separated from the filler.