Radiant heat how far can a 1/2 loop of pex be run.

Nope, basic serpentine only, in most cases. We do use PWM thermostats and reset control and/or Indoor feedback on pretty much all of our systems though, so operation is pretty consistent, if not truly constant.

Climate panel *requires* designing around a 20 degree dT, last I knew.. at least, all the 'quik trak' design info is based on a 20 degree dT.. 5/16" pipe can't handle much higher flow rates. That's what made me start wondering... if it works for that, why am I designing around 10 for everything else?

If you're getting *striping*, your water temps are too high and the system isn't running long enough on cycles. If you notice temperature changes from one end of a loop to another... not sure what to say, I've never had any feedback from anyone noticing that. Maybe if you really tried you could, but most people aren't really trying, and if they don't notice, I'm not going to bring it up

Pushing a 90 kBTU load with on 3-speed brute is pretty sweet though

George, I don't see the value of this comment. Are you suggesting that running the numbers isn't useful?

Obviously you have to calculate flow and frictional loss to size a pump. If you don't, you're guessing.

I'm not saying you're going to nail a ten or twenty degree dT in the field exactly because that's what you designed for, but it certainly tells you what kind of flow rate you need, how to balance, how long your loops can be, how big of a pump you need, how big of a pipe you need.

right? or are you just guessing at all of that too?

So it's not like design dT doesn't matter. You can ignore it and design around "rules of thumb" if you like, I suppose. Me, I'll push 75kBTU loads on a UP15-58 pump because I know how much flow the system NEEDS, I know the frictional losses in that system), and I know that the pump will do the job (because I looked at the pump curve). My system will use smaller pumps, smaller pipes, less electricity, and less loops. And the numbers will be close enough. OR, I'll know that I just can't do that on this particular job because, say, someone already installed a 350' loop in a high load area!

THAT'S the value of running the numbers, not that every factor is correct to the second decimal point in the field, but that I can approximate with great enough accuracy to make better design decisions than if I don't do the numbers.

It also tells me fun stuff, like whether this room would ever heat with a 350 foot loop or whether you'd have to eat holes in the pipe to push all the flow a room needs to keep the back end of the loop from being stone cold when you're under a load. And when you run the numbers, you realize that some rooms could handle 500 foot loops and some you really shouldn't push past 250.

I don't really care what the pump thinks, pumps are dumb... that's why I pick the pump I want and I set up the system, not the pump.

Nice Try Mike

Let's say, though, that he's got a 1000 BTU load on this room. That's only .2 GPM with a ten degree drop... very acceptable. his frictional loss would be about ONE foot of head in that case, pushing .2 GPM through 470 feet of 1/2" pex, roughly. Very doable. the water may not actually come back exactly ten degrees cooler than it goes out, but you know the pump can do this.

If he's got a 5000 BTU load, 1 GPM.. or .5, if he can handle a 20 degree drop, which might mean his supply temp has to go up a few degrees.. and at .5 GPM, that 470 foot loop is still only 5 feet of head.. a standard UP15-58 would push up to 13 GPM if that were the maximum head it were facing. Even if it's not dedicated to that loop, if he can balance, you're still not upsizing your system pump and this is totally doable.

But if he's got 20 BTUs/sq ft, for a 370 sq ft room, that's 7400 BTUs.. .75 GPM even at a 20 degree drop.. now he's up to 12 feet of head. That UP15-58 could still do that on low speed, if it's dedicated, but just barely. If he's got a system pump though, now that high speed 15-58 can only push 6 or 7 GPM through a balanced system. Probably time to upsize.

but it goes up fast after that. Since I have no idea what his load is, I would split into two to be safe if I had to do this blind.

well, no offense intended here either george, but you're designing for at least double the flow rate most loops need, even at a 10 degree drop, and no one would ever be able to tell the difference if you ran the flow slower, except during the initial cost of the system and the electrical bills.

I suppose if you're doing fixed temp, bang bang strategies this may be more of a concern. But if you're using reset, and assuring more consistent heating cycles, you really don't need that much flow the vast majority of the time. the loop will heat evenly. But then, none of us are doing bang-bang systems anymore, right?

Frankly, I've seen a lot of charts talking about laminar flow kicking it at around less than 2 FPS which is supposed to cripple our heat output, or talking about air coming out of solution at lower flow rates, and the vast majority of us out here are running around .5 GPM per loop... here, we're routinely running .2 GPM as an arbitrary minimum... with zero problems. So something's up that the charts are not reflecting... we do occasionally get a bubble in radiators or a baseboard, I think it does finally cross a line there, but in 1/2" PEX I'm just not seeing it.