Well.
My supplier is trying to charge me for an entire hardwood floor... However it was the 10th of january... during the middle of winter in alberta... the wood was delivered on day before install was done.. Install was tight. 3months down the road there is some appearent gaps between the but joints. Now is this and install issue, or humidity...

Hmmmm? must be an engineered cross-ply wood, or some early growth, harvested solid wood.

it's humidity/moisture. in the winter,a floor can shrink.It's important to humidify the house so that the floor doesn't dry out and shrink.

If you don't have cracks on the sides of the boards, but you have butt end gaps, I'd start looking at framing/sub-floor movement. Flooring is nailed to the sub-floor, sub-floor moves pulling the flooring with it.

Happens sometimes when the framers changed direction of the joists to frame out for a sun room or large L shaped homed. Things like that.

Installer issue. If you deliver wood a day before install is to take place in the dead of winter in Alberta you are asking for trouble. Wood here has to acclimate for longer, or you are going to pay for it long run...the installer should've known that, tested the wood prior and shut down the install to let it acclimate for longer

Cheers
Steve

Open end joints are almost always caused by subfloor movement, or settling of house in new construction. Humidity issues that would cause open end joints would also create huge gapping along the edges of the boards.