Just curious, doesn't 120F EWT cause it to exceed the recommended maximum temperature for hardwood flooring?

This is a very conservative number. One of my first true staple-ups radiant floors was installed in my brother's, now 100 year old house, some 20 years ago. We stapled 3/8" barrier polybutylene under a 5" plank southern yellow pine floor and T&G sub-floor of the same material. All driven to 180°F under design conditions by a propane atmospheric boiler at a 20°F delta T. Perfect 70° in all weather conditions...oh to be young and dumb again.

It is not so much the supply temperature and certainly has little to do with the surface temerature (just consulted on a 7 year old DIY job with 93°F surface temperature. The flooring was 3/4" cherry over an insulated slab-on-grade. It was not keeping up with the load from a 16' great room with typical windows, but the floor was warm. Too warm? Let's consider that the 86°F "maximum" surface temperature does not follow and particular "law" of physics, man or God. It is simply a temperature close enough to human body surface temperature to cause some peoples feet to sweat (not mine) and therefore another one of those dreaded "rules of thumb".

The wood flooring people have jumped in, knowing little to nothing about radiant heating, as they an opportunity to sell more wood flooring than ever. The surface temperature of any radiant panel has more to do with MRT than EWT at any rate.

http://hardwoodfloorsmag.com/articles/article.aspx?articleid=378&zoneid=2

Yeah but - it would be nicer to be young again and with todays knowledge!!

Its been my experience that you can go a bit higher with solid hardwoods with out problems but the newer engineered hardwood are problematic very close to 85º. The first thing you will notice is the top edge of the groove start to rise so that when the sun hits it right you see the strips. Don't know if humidity has anything to do with it 'cause most hydronic floors tend to be fairly moisture stable. I think there might be something to do with the planks being installed to tight rather than the actual result of the heat. Another ten years and we'll have the answer but then I'll be ten years older!!

Posted By joe.ami on 08 Apr 2013 10:00 AM
....or more pipe

More pipe is a given and is the starting point for under-floor...then temp and then flow rate.

Posted By jonr on 09 Apr 2013 02:04 PM
Just curious, doesn't 120F EWT cause it to exceed the recommended maximum temperature for hardwood flooring?

The 120F might be the required supply temp to achieve a 85F floor surface temp...which is about as high as you would typically go. Having a thick hardwood floor and an under-floor hydronic installation would be challenging.