floors, coatings and heaters

A few months ago we had our hardwood floors finished with hardwax and are quite happy with the results.
However, over winter the boards in front of the gas fireplace have caused the 110mm wide boards to shrink by maybe 2-3mm, opening up gaps. The wax coating has worked a treat, with no peeling or rippling or bonding of the boards.

My question is how does polyurethane, in any of its varieties, go under the same conditions? We've never used it but are considering it for an extension.
Does it prevent the the boards shrinking in the first place by preventing them drying out from above ie does it prevent moisture vapour escaping from the wood?
If not can it accomodate the expansion and contraction of the boards the way the wax seems to?

We're inclined to stick to what we know but if we all lived by that we'd still have leaves on the floor.

In our experience they will still shrink, the hot air from the heater tends to upset the floor. We have always kept a rug in front of the heater to protect the floor and give fido something to lie on, even then you seem to get the odd minor gap showing in the last inch or so near the hearth where the rug doesn't always cover the timber.

John

G'day.
It doesn't matter what finish you use, They will shrink and gap.
If you use 2 pack poly, you may also get edge bonding which will make the gaps look even worse.

Don't ya just love timber floors...Eh...eh...?