I have seen most people/sites recommend to keep the lenght of a single loop under 300ft for 1/2" I even understood the reasoning behind it. However, 2 days ago I read that this 300ft thing is a leftover from the old days when the boilers came basically with the cheapest aka weakest pumps the mfr could get away with. By using a pump with a bigger head you could use longer lenghts, up to 500ft Thoughts?

not true,
Manufactures these days are doing there best to bring quality products to you.
it is about the resistance of water in that peice of pipe.
If you have 1000 feet of 1/2 inch pipe in a straight line next to 4 250 foot pieces of pipe on a manifold it will take a much larger size pump to push the water through the 1000 foot pipe than the 4 250 foot lines. More friction/head.
so 300 foot seems to be one of the magic numbers, 250-330 is where we design to.
Dan

maximum length is determined by flow rate and pump sizing.

it is never advantageous to upsize a pump if you don't have to. it's a waste of energy, forever; it's another maintenance item; the extra cost of installation offsets a lot of cost of loops.

We always design to the lowest amount of pump energy we reasonably can. Since it's also extremely important to isolate rooms on their own loops, this isn't very often an issue.

however, that said in a basement or other low load situation we will routinely run 500 foot loops. Without upsizing the pump. In higher load areas where more flow is required, you might only be able to run a 250' loop.

300' is a rule of thumb to keep people who are not running the numbers from having totally non-functional systems. More than one person has put in a 1000 foot loop because "that's the size the coil came in".

Anyone suggesting upsizing a pump is without consequence is not engaging in "Green" design by any measure.

Well below is where I read it. Has the explanation and a graph. http://radiantdesigninstitute.com/page8.html It just made sense what is written. Don't get me wrong, I do understand what you guys are saying but I would really like to avoid any connection in the house itself and have all manifolds and zone headers in my mechanical room. Any fitting, no matter how well made up, has a bigger potential for leaks than just hose. Ben

and it has nearly no potential for leaks at all if it doesn't start out with any.

you are making a bad choice if you upsize your pump just to run longer loops. the "radiantdesigninstitute" is way out in left field on that one. consider that upsizing your pump can cost $5 to $10/month additional in the heating season... per 0.10/kwh... in additional running costs. and around here, we're staring down the barrel of 0.20/kwh.

oh boy.......
The Radiant design institute.....
I should start an institute........
Lots of good info there but way out dated on pumping, loop length, open loop, Takagie flash units,contact to surface area, pipe spacing,
We sell to a lot of there clients, but I am a skeptic.
I can't get behind a 500 foot standard loop.

Think supply return from boiler to sub manifolds, Average 300' foot loop, avoid staple up, surface mount with the evil/affordable thin heat transfer plates and a RHT floor panel system or better a topping pour, 1 1/2 light weight.
If you are concerned about pex unions place your joints where you can notice a leek, Alternately use copper supply returns.

Good Luck,
Dan