I think the some confusion comes from misunderstanding the relationship between heat source and load. Reynolds Number is but a small part of hydraulic and of even less significance in radiant floor heating. Where turbulence is of primary importance is in the operation of low-mass boilers in which we trade mass for efficiency and require turbulent flow for proper heat thermal efficiency of a given appliance.

Once the full output of the boiler is transferred to the water of a radiant floor panel, the sheer surface area and "intermittency" more than make up for a low Reynolds Number. This is why very low flow rates will often satisfy thermostats and creature comfort as well.

All that bubble water whips by.

Ah ha, so much velocity that the air they didn't quite get all bled/vented is being sucked along for the ride. That actually could decrease heat transfer.....

Well,
Clearly I have it all over my left foot... As I said there are many smarter than I,
Back to my original thoughts if I may..
Slow the water down, think about delta T pumps, running water at big gpm is of no gain,
I must now clean off my boot, perhaps I have that linear thing on its head,

Dan

The turbulent core rides past the the laminar wall.
All that bubble water whips by.

There is something about these words......something evocative and inspiring......reminds me of the latest Tom Cruise science fiction screenplay......  ;-)

Mission impossible IV

Posted By Blueridgecompany.com on 08 Jan 2013 07:53 PM
Well,
Clearly I have it all over my left foot... As I said there are many smarter than I,
Back to my original thoughts if I may..
Slow the water down, think about delta T pumps, running water at big gpm is of no gain,
I must now clean off my boot, perhaps I have that linear thing on its head,

Dan

It's still of some gain, but once you're in the turbulent zone it's a miniscule gain, maybe measureable in a lab, but "in the noise" from a system effectiveness point of view.

Maybe it's instructive to look a the BTU per hour per foot deltas seen in fin-tube rated at both 1gpm & 4gpm (as is commonly done.)  Increasing the flow 400% only delivers a ~6% increase in heat transfer- it's an increase that's easy to measure, but hardly enough to matter.

Beyond 4gpm it continues to rise, but it's a difference without a distinction- you're already pretty much at the practical limit- there simply isn't another 6% to be had, even at flow=infinity.  In a small system like this, just the extra power consumed by the pump (converting kilowatts into BTU/hr) going from 1-2gpm to 6-10gpm becomes greater than the difference in heat transfer performance of the system- the hot pump is delivering more heat to the room (and water) than the increased flow is buying you.

Most residential hydronic systems will do just fine with 1-2gpm flow in the radiation, and raising the water temp only two degrees makes a bigger difference in system output than any amount of over-pumping could achieve. Truly laminar flows aren't going to be a problem at 1gpm unless it's a really fat pipe (which may exist in older gravity-feed systems that have been converted to a pumped system.)

The fin-tube example is a good one. Truly laminar flow is not possible in a radiant floor panel and Reynolds numbers over the magic 2020 are not necessary for adequate heat transfer. In truth, it is much easier than that.
As you correctly point out, all energy used to pump the fluid is parasitic. All excess is waste.

The BubbleBee is an attractive unit. I wonder how it compares to the Alpha or Eco?

Precisely and very well stated Dana! Most residential systems are ½” PEX and at 1-2 GPM will be well into the turbulent flow Reynolds number range. Don’t know how many folks use slab temps in their control algorithms? However, if you plot stabilized slab temps (or just shoot stabilized surface temps) at several flow rates that transition across the laminar/turbulent Reynolds number range, you will observe a couple degree discontinuity in the curve. In other words, you will see a higher slab temp (with corresponding higher BTU heat gain) with a relatively small increase in flow rate.

If you continue increasing the flow rate, your delta T will continue to decrease. At some point the delta T will approach 0 which means the slab simply can no longer absorb the heat you are already feeding it. So there is a diminishing BTU heat gain benefit as one approaches this flow rate limit (and a hydraulic friction, pump head limit that may bite you badly too). However, since most residential systems are designed around a 10-20 degree F delta T, one should never get close to running into this limit.  And as the badger indicated, all pumping energy is really inefficiency, so one doesn't want to use more than is absolutely required to get the job done.

As you indicated, raising the supply temp has a much greater influence on BTU heat gain.

As an uninformed layman, having an unfinished 1/2" pex radiant sytem in the concrete floor of a 40' x 80' barn, I gather from this very interesting forum that I'm better to pump slowly for best effect; this is important to me, as I have no heat in the system as yet, and am looking at running the fluid through an electric water heater to start to warm things up enough to work inside, so need advice if the EWH is useless. The inside temp, at present, is about 38F, with slab insulated underneath and at edges with 3"DOW SM. Cannot hook up propane Quietside boiler until next fall. I've also been told that it is better to use water than the commercial fluid, and would like to get started while it's still above 32F. Any thoughts? Almost forgot: I have 11 zones of about 214' each.

Thanks in advance.

No. Pump slowly to save energy. not for "best effect".

heating 1200 feet of concrete with a 15,000 BTU output will take a long time.

It will take even longer, as it's 3200sf of floor area. By "best effect", I meant overall efficiency in all aspects. I haven't even checked the EWH for BTUs, but will do so today.

it's a 4500 watt element... they almost all are. Pumping slowly is great for saving energy. doesn't increase output. just making sure it's clear since we only had 3 pages of disagreement on that point leading up to now

I thought the disagreement was over heat absorption rate, and how it was affected by flow rates. Now working on page 4. ;>)) I viewed the video, but, being a layman, was just mesmerized by the flow at 21:49, in that it looked like the old movies illustrating blood flow in veins.

an electric water heater to start to warm things up

If you are going to use an electric heater, ignore the concerns about pumping efficiency - it doesn't matter.

I was thinking of heating the bulk of the fluid, then opening one zone at a time, to heat the small work area needed.

Posted By icfb on 11 Jan 2013 01:10 PM
As an uninformed layman, having an unfinished 1/2" pex radiant sytem in the concrete floor of a 40' x 80' barn, I gather from this very interesting forum that I'm better to pump slowly for best effect; this is important to me, as I have no heat in the system as yet, and am looking at running the fluid through an electric water heater to start to warm things up enough to work inside, so need advice if the EWH is useless. The inside temp, at present, is about 38F, with slab insulated underneath and at edges with 3"DOW SM. Cannot hook up propane Quietside boiler until next fall. I've also been told that it is better to use water than the commercial fluid, and would like to get started while it's still above 32F. Any thoughts? Almost forgot: I have 11 zones of about 214' each.

Thanks in advance.

ELEVEN zones in a ~3200' building?

Why? 

assuming a 4" slab, you've got about a thousand cubic feet of concrete to heat. that's about 145,000 pounds of concrete. at 0.18 BTUs per pound per degree, you're at 26,100 BTUs per degree. with zero heat loss. heat loss is subtracted from your 15k output and the leftover can be applied to heat up.... basically.

in fact, a regular electric water heater probably won't even cover half of the the full heat loss of this structure. so forget heat up time for a minute. you won't heat up until it warms up. You could probably do something to prevent freezing but I wouldn't count on even hitting 50 degrees inside. and getting there would take warm weather and a long time.

you need more oomph here for a place this size. if you need temp heat, space heaters are probably the best way to go.

Two more things to consider regarding low flow rates. 1) too much difference between supply and return will noticeably effect the evenness of heat distribution and 2) some heat sources (eg heat pumps) produce lower efficiency at higher temperatures. And you would need this higher supply temperature to get the same heat output with lower flows (higher supply/return delta T).

I was informed that each tube length should not be longer than about 215' (from hazy memory, could have been 235' or so), so the total tubing length was calculated, and it worked out to 11 runs of 1/2" PEX. The slab is 6" thick, with 6x6 wire mesh; the tubing is placed at 3" deep. The 3" SM is underneath, and also extends up the outer walls about 18", and the walls & roof are covered in R11 sprayfoam. I only need a slight bit of warm floor at the entryway, about 30' long x 40' wide. I'm thinking if I get the fluid (water, or ecologically friendly solution) up to 100-150F, I can run it reasonably quickly through one loop to start to warm that small area, then expand to a neighbouring loop, then the loop on the opposite side of loop #1, etc.
Might work, might not, but there's been no heat in the building at all as yet, and outside temp here today is about 39F, warmer tomorrow, and about the same on Sunday. I'm in Ontario Canada, so can only hope it warms up slightly, just enough I can work in street shoes. When the whole system is complete, in a year or so, it will be heated by a Quietside propane boiler, which will also supply house hot water. The house will be superinsulated, with ICF perimeter walls from footings to roofline, and, possibly, EPS or SM as ceiling insulation as well.

I hate to be the guy to burst your bubble. Your design has some flaws that need to be addressed.The building unless its subdivided into insulated zone along with the slab will rob from the zone you are attempting to heat. You have a tremendous thermal mass in the concrete.The idea is to bring the slab up to a reasonable temperature and then trickle charge energy back into it as needed to maintain a steady state. You are going to need some type of additive to your loops which will effect your pumping requirements.You should recheck your heat loss calculations and your needs for hot water in the house, your boiler selection may or may not be a good choice.