He said "never seen........" because the reading looked good and he has not seen gaps at the tops only sides

So no pin meter, just a non-invasive meter. Those are great for finding wet spots. But you need to check quite a few other areas for an average. I'm sure it's dry by now with the heat on. But was it on before and or during the installation? I do not remember you saying one or the other.

It was on low during installation and brought up to temp per the information that they gave me. Like I mentioned, its only happening in one section, around 10 boards. Everything else is tight.

Very intriguing, I wish I could come look at it. I find it highly improbable that the wood in only one box was wet like that. Has anyone measured these planks and are they random planks or semi-random, the later meaning different sized planks but only 3 or 4 sizes with the same dimensions?

The planks have not been measured. The company that manufactures the wood is sending someone out to do this and to make sure staples were put in every 6 inches. The installer says he did it every three inches in spots. The wood is very random, but all have the same width by what I can see. I had a builder over last night. It took him about three minutes of asking a few questions and he thinks he figured it out.

The room below the area that has the gaps is where the hot water heater and radiant heat starts out, meaning all the tubes leave this area bunched togather and then span out around the rest of the house. This room generates alot of heat and thats why I am seeing these gaps only in this spot. He said he thinks come spring they will tighting back up considerabley and I shouldn't worry. Thoughts? and why wouldn't a professional wood guy bring this up, especially since they looked at the entire house?

Thats a possibility, I'd wait and see. Also, hardwood guys are not radiant heat experts. I suppose those tubes have to come out some where and I am not sure that is a proper installation either. You should get an opinion on the way the tubes are placed, other than the original company that installed it..

I was on a job with radiant heat in a concrete slab and got my laser themperature thermometer out. What I saw was amazing. The tubes coming directly out of the floor were 10 to 15 degrees hotter at the point where they came out than the outer regions. Obviously an installer cannot be responsible for issues caused by that.

BTW, kudos to that builder. He sounds very smart.

Yeah if that is indeed the problem I would agree. Just nice that somebody shed some light on what MIGHT be going on. I have a radiant guy coming over to check it out. Some say, just leave the door open to let the heat out and that should help.. Do you think I would see the gaps tighting up once spring comes around? if not would filler be a good idea for this situation?

and, should I wait until spring before I fill the gaps?

A "paper underlayment" is not used with a "combo" installation,( glue& nail).

When did you start seeing the end gaps?

about 3 weeks after installation. Really only in one area, a few other spots but minor.

They used glue and staples. Not sure if that means anything

How about venting out the room below? If thats what you mean. I'd wait to fill.

Good idea