1 GPM Thru 3/4" Baseboard - Isn't Velocity Too Slow?

JustinS

As I understand it, the ideal flow velocity range is 2 - 4 ft/s - outside this range, it is either too slow to efficient air entrainment or too fast for quiet operation... as I see it, this translates to a minimum flow rate of ~3GPM...

Assuming this is correct, why would a baseboard manufacturer even show details for 1 GPM thru 3/4" copper piping, as my manufacturer (Haydon) has shown here (http://www.2haydon.com/index.php?M=2&P=heatbase)...

It just doesn't seem like it's a valid flow rate for that size pipe...

Jamie Hall

Oh it will work... you might have problems with air binding, but it will work. Just won't get that much heat out of it!

icesailor

Well, I never bothered to compute the terminal velocity of water running through cast iron baseboard, but because it always worked at whatever flow rate was provided by a B&B Series 100 circulator, it was never something I or others bothered with.

However, the required length of 1/2" pipe or tube to feed the baseboard, and all the water stored inside, seemed to not make much difference. The water flowed at whatever speed it went through the 1/2".lowed down rapidly and gave up its heat as it slowly passed through, and picked up speed rapidly on the way out. Sort of like a 8 tube, 25" high X 25 section radiator would.

hot_rod

This journal may help clear up some of the questions about flow rates, ∆T and heat output from various types of emitters.

Graph 3-5, is based on a 12' section of typical BB supplied with 160F.
The BB manufacturers tables show 600 BTU/ft, 200F supply at 1 gpm for this BB.
In 3/4 copper tube baseboard output starts dropping sharply below 1 gpm.
Somewhere around .4 gpm the fluid turns laminar and output plunges, figure 3-5.

The takeaway is the output from a fin tube baseboard is non-linear. The output decreases slowly at higher flows, drops sharply at low flow rates.
Just beware of how varying the flow rate thru any emitter changes the heat output.


http://www.caleffi.com/sites/default/files/coll_attach_file/idronics_16_na_0.pdf

Air removal is still possible at 1gpm flow rates in 3/4 copper. If the piping has a lot of ups and downs, some high point vents may be needed to get a good clean purge.

Problematic installations might be where the installer went up and over a door with the piping creating high spots, for example, that trap air from a slow flowing stream. Same with long vertical drops, it becomes harder to push the air along with the fluid stream when velocities are low.

Higher flow rates equate to increased heat output in hydronic emitters, as long as pumping requirements don't get excessive. No need putting a 0099 just to get the highest outputs. Balance the heat output (flow rate) against pump size required to achieve it.

Most properly sized and designed residential systems, bb or radiant loops or hydro-air can generally be pumped efficiently and offer sufficient output with 80W or less, 1/2 that if you opt for ECM technology.

icesailor

HR,

That info must be in your new Idronics #16.

I wish all my old and treasured information hadn't been left in a box that wasn't meant for the dumpster. I think there is a misconception of what goes on with fin tube series baseboard and cast iron baseboard and radiators.

Cast Iron Baseboard and cast iron radiators need to be looked at like a river running into a lake and running back out after a while. Whatever the flow into the lake, it has the be the same when it goes out. If not, the lake rises If the drain is restricted. Or the level falls if it there is less restriction on the way out. In a unit of 10, the river is flowing at 10. The water slows down to 5 as it spreads out in the lake, and goes back to 10 when it leaves. As the water slows in the lake, it has the ability to give off more energy while going more slowly and more spread out. When it leaves, it collects itself and leaves with whatever energy it still has.

As far as Haydon and Slant-Fin baseboard, their literature is almost exactly the same. Especially in the information below the charts.

Or so how I learned it.

hot_rod

Interesting analogy, I think

Did you get the stack of idronics I sent to the sunshine State?

bmwpowere36m3

Jamie Hall said:

Oh it will work... you might have problems with air binding, but it will work. Just won't get that much heat out of it!

Not much difference between 1 and 4 GPM with BB… about 30 BTUs per ft. Seems it's more important to maintain velocity for air removal and keeping the flow turbulent.

JustinS

bmwpowere36m3 said:

Jamie Hall said:

Oh it will work... you might have problems with air binding, but it will work. Just won't get that much heat out of it!

Not much difference between 1 and 4 GPM with BB… about 30 BTUs per ft. Seems it's more important to maintain velocity for air removal and keeping the flow turbulent.

I'm mostly interested in how slow I can get the water to go in order to maximize delta-T... with 3GPM, I'd only get a max of 5F on my small zones of 15K BTU/hr...

Jamie Hall

The radiation from the baseboard is set by the average temperature of the water in the baseboard. Then the delta T will be simply governed by the good old hydronic formula (BTU = gpm times delta T times 50, if memory serves). Can't beat either one!

bmwpowere36m3

JustinS said:

bmwpowere36m3 said:

Jamie Hall said:

Oh it will work... you might have problems with air binding, but it will work. Just won't get that much heat out of it!

Not much difference between 1 and 4 GPM with BB… about 30 BTUs per ft. Seems it's more important to maintain velocity for air removal and keeping the flow turbulent.

I'm mostly interested in how slow I can get the water to go in order to maximize delta-T... with 3GPM, I'd only get a max of 5F on my small zones of 15K BTU/hr...

HR said it, try to keep the flow at least 1GPM. If you have a small "zone" or length of baseboard the delta t will be low… but there's not much you can do about that without increasing the length of BB or putting them in series.

I ran into the same "problem" with delta t's, but I'm planning on installing panel radiators with a design delta t of 20-30*. I'll end up using 3/8"-1/2" PEX to keep flows close to 2 fps due to small flow rates required for the large deltas and small panels (BTU load).

15k BTU with a delta t of 20* is 1.5 GPM. So within the BB itself that'll keep the flow high enough to get reasonable heat output. However within the 3/4" copper that's about 1 fps. I asked this question before and never got a firm answer whether trying to maintain at least 2 fps is critical…

What you could do is feed (supply/return) the BB with 1/2" copper or 1/2" PEX to raise the fps to aid in air removal in those lines.

bob_46

Jamie missed a zero btu=gpmx∆Tx500 . The lowest flow rate through 3/4" copper maintaining turbulant flow is 1/2 gpm which gives you a Reynolds number just a hair over 4000 .

icesailor

hot rod said:

Interesting analogy, I think

Did you get the stack of Idronics I sent to the sunshine State?

Yes, I certainly did. That's how I new about #16. I only have #1 thru #15 and that one numbers #16. I sent you a thank you. You must not have gotten it.

hot_rod

icesailor said:

hot rod said:

Interesting analogy, I think

Did you get the stack of Idronics I sent to the sunshine State?

Yes, I certainly did. That's how I new about #16. I only have #1 thru #15 and that one numbers #16. I sent you a thank you. You must not have gotten it.

If you go here and sign up,a new issue will be sent to you about every 6 months. Enjoy

http://www.caleffi.com/usa/en-us/technical-magazine

jonny88

Thanks Hot Rod for sharing,now I will be stuck here reading all day while my wife will be yelling at me to get of the computer.She already thinks I have a secret girlfriend with all the time i spend here.......

Rich_49

Tell her it's Jake from State Farm , works on my wife , she laughs and leaves me alone .