Flow Rate Principles

Jamie Hall

The conversion is (roughly) 3.8 Litres is 1 gallon -- so you shouldn't have a problem with excess velocity! Your 2.5 L/min is only about 0.6 gpm. What I know about radiant floors is pretty well limited to 'they work' (steam's my thing), so I can't comment on the rest of it...

Jamie

Ben Greenfield

Hello All,

I just finished the install of my radiant heating system it is a single zone 1 large room 1,400 sqft with 9 1/2 loops. I'm waiting for the instructions for my mixing valve to show up but I'm currently running the system. My question is about flow rate. My mainfolds on the return side have adjustable flow rate valves that go up to 2.5 L/min (liters a minute?).

If a look at a table in the book Hydronic Radiant Heating it looks like I should have this set to 1.3 gallons a minute (to avoid velocity noise) I'm more concerned with efficient heating though.

If I look at formula earlier in the book it looks I should take the Heating capacity of the boiler 53,000 Net I=B=R and divide by 17,500 for a 35 degree temp drop for 3.02. This is off the chart if I try to convert this to liters I get over 11 liters a min.

So I'm a little confused right now but I'm sure someone could help me with this.

Another question I have is shouldn't I try to dump as much heat as I can into the room so that the return pipes are much cooler then the supply.

Thanks,

Ben

RB_2

Comparisons

I wouldn't concern myself with too high a velocity but too low a velocity...35 deg f dt brings you down below 2 ft/sec...just too low...if you have compelling reasons to use a 35 deg f dt then switch to 3/8 pipe...and redo your head losses.

Another issue to consider, 20 deg f could/would, depending on which model, improves the efficiency of the circulator. Ask your supplier to provide a circulator which performs in the middle third of the curve...ask them what happens if they use 20 deg f delta t...does it make for a better selection?

My guess is creating a large delta t between your s & r is probably not as important as dumping as much heat into the room to make it comfortable...yes?

The system will deliver heat as long as the floor is hotter than the room mass surrounding the floor...to make sure you don't overheat the other masses you'll need someway of controlling the fluid in the floor (weather compensator controlling the mixing valve) and additionally some type of high limit room sensor (Thermostat).

The highest average fluid temp on the floor side of the mixing valve should be determined by your supplier and is based on the load, floor covering and spacing...the variations in floor surface temperatures based on a 35 deg f delta t could be major or minor depending on your loop pattern...again if you're hell bent on 35 deg f dt use a reverse return piping method and the closer the spacing the better...another good reason to use 3/8"....other options...bury it deep and or a higher r value floor covering...both will "smooth out" any variations at the expense of higher fluid temps.

Some random thoughts on efficiency for anyone who really cares?

Efficiency is not a micro thing...condensing boilers, weather compensators, mixing valves...variable speed motor controls ... all good...all micro....Efficiency is macro...as in BIG...as in the BIG MAC [that ones for the big cheese (;@)]...anyway...efficiency is soup to nuts...taking raw goods from mother earth all the way to making us comfortable...

I would venture to guess the gas burned up mining the raw goods and converting it to parts...the fuel used to assemble it, the gas burned delivering it to some distribution center...the gas burned getting it to a wholesale branch...then the numerous trips back and forth...then add in all the fuel burned to solder up the system...and don't forget the fuel burned to heat the home whilst under construction with no windows or insulation...oh yes the fuel to obtain permits...and inspections...and collection of monies...and...and...and...get the picture...it all matters...

And if anyone tells you not to worry about this stuff ask him or her...if it weren't so important than why do the heat loss?

rb

Mike Kraft

Mr.Bean

Oncet' aGAIN he plows through a topic and hits all the light swithces (on of course) and with such a simple tact.Bean power..talk about raw goods

cheese

Gary Fereday

Mr. Bean,

? Serpentine vs. reverse return piping loops? Your Referencxe to reverse return in the above, suggests that compared to serpentine it causes a higher Delta T. Is that a correct assumption on my part? Perhaps I am all wet instead of just my head! (;) bigugh

heatboy

You're kidding, right?

"Ask your supplier to provide a circulator which performs in the middle third of the curve...ask them what happens if they use 20 deg f delta t...does it make for a better selection?". No disrespect to the distributors that hang here, but......... Maybe that's where "the deer in the headlight look" started?


Warm Regards,

heatboy

climatecadvanced.com

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RB

Design vs actual

Greetings bigugh...how's things?

The load determines the delta t...it tells us how much heat we left behind...regardless of pattern...our friend, mother nature will pull out of the house (or put in the case of cooling) whatever heat it can...construction design resists the natural physics and the mechanical systems make up for the construction short comings...at max design load we're saying our average flow temperature can keep up...again regardless of loop layout...but the greater the load along any given foot of pipe the greater the delta t...so if one were to throw all the pipe (every inch) along the exterior wall one would deliver the same output had one spread the pipe all over the floor...the difference is the floor temp in the center of the house would be cooler than the surface temp at the exterior...not a comfortable condition...so loop patterns have more to do with creating consistent or inconsistent surface temperatures...and very little to do with the actual delta t or average fluid temperature.

The cool (warm) thing about radiant systems is their redeeming qualities and the deeper the pipes in the slab and or the higher the floor covering resistance the more even the surface temperatures created...so one could royally screw up on a loop layout and not notice the difference...but apply the same technique on a thin slab application with tile serving a large high heat load area...it's not the same game...tube layout can enhance comfort conditions, in some applications.

I know a lot of great designers and the common trait amongst them is they design for failure rather than success...ie: they focus on what can go wrong and do their best to eliminate these potentials land mines...in their brains each zone/load/floor covering and type of system is a set of unique circumstances and they eliminate "failure" by selecting the best pattern, spacing, temperature for the specific application...and as Jeff said ...this is all "deer in the headlights" stuff.

I reread my earlier post and can see how one could interpret it...does this dry it up?

RB

RB

\"Venison is mine\" sayeth the Jeff.

;@)

Dan Peel

So

You deal with real wholesalers too?

Dan

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Ben Greenfield

thanks robert 2 more questions and some clarification

it turns out the flow rate is not 2.5 but 2,5 liters a minute

"My guess is creating a large delta between your s & r is probably not as important as dumping as much heat into the room to make it comfortable...yes? "

yes that is true but isn't the large delta indicative of dumping heat?
should I tune the flow rate to always maximaze the delta between s&r?

I'm writing some custom control software so I can make this the goal to always dynamically adjust the flow rate to maximize the heat dumping. When I get to that stage I will probably use a danfoss product

can I get the formulas you used to create that table?

Thanks,

Ben

RB_2

I'm going to answer in the way I understand your question...if I'm out to lunch (it wouldn't be the first time...nor the last) let me know.

Let’s presume one cranks down the flow in the loops to get a large delta t...what needs to happen in the middle of the worst cold snap to compensate for a slower flow? How about a higher fluid temperature? Ask yourself, 'is this what I want?'

Here’s another angle…Imagine this...nine loops...all the same length...all getting the same flow...or imagine totally different loop lengths but balanced out in the manifold so as to have the same flow...its no different...then in your minds eye see yourself taking one of the loops outside to heat the dog kennel...then do something insane like taking another loop and coil it around the boiler draft hood...and while we're having a silly time imagining where we can stick this stuff lets put one on the roof...south facing to make it interesting...got it? Ok...lets pretend its a nasty day in February...boilers running full bore...now lets imagine what is happening to the fluid inside the different loops...the loop around the draft hood isn't dumping heat is it? How about the loop on the roof...dumping heat? You bet! How about the dog kennel...how about the rest of the loops in the floor...remember they all have identical flow...BUT TOTALLY DIFFERENT delta t's based on their exposure...now lets pretend it's a nice hot sunny summer day...that loop on the roof still losing heat...not a chance!

The delta t is an objective design selection to establish a flow rate...pick a number any number...it is always balanced out with a design supply fluid temperature...what does this mean? It means one can pick a delta t of 50 deg f as long as the average fluid temp is sufficient to raise the average heated floor surface temp to a temperature which can deliver the required heat...in the case of a 50 deg f delta t...one would be running some hotter fluid through the pipes...in fact the mixing valve might just become nothing more than an expensive fitting...now imagine walking across a floor with a wild range of temperatures...very poor design...try picking a 10 deg f dt...what does it mean? It means a lower supply fluid temperature, greater consistency in surface temperatures but higher flows, which could mean paying for oversized equipment...again all 'deer in the headlight stuff'

The best place to start playing in the 'Paralysis through Analysis' game...is purchasing the books on this and other sites and participate in the advanced seminar circuits ...there are some very crazy but competent instructors who visit this site…maybe they'll flag your name for an upcoming cranium tapping...

…as far as your software to control individual loop flows…its admiral of you…heck it’s even impressive…but you’ll need to go places in your brain which others have gone before only to wake up a realize the need for some expensive hardware to marry to the creative software…nothing like a big invoice to make us manufactures happy at the expense of ones sanity…if ones career offers one the time, tolerance and tender to turn this size of project into a zippy Ferrari…we’d be happy to take the tender…but the time and tolerance to become part of Team Ferrari …my competitors telephone numbers are….1 800 come to momma.

In the meantime the only formula in the chart is:

US gpm = (Load in btu/hr) / ( 60 * df * cf * dt)

Density (df) for water = 8.34,
df for 50% eg = 8.76,
Heat Capacity (cf) for water = 1,
cf for 50% eg = 0.86

punch in your dt to determine the design US gpm.

…isn’t this stuff just so much fun?

Ben, all kidding aside…the stuff you are exploring is great mental gymnastics but comes up shy in practical terms for a project of this size.

If your desire is to burn up some brain cells…do some intellectual yoga so to speak...email me your telephone number and I’ll give you 15 minutes of my time.

How does this sound?

RB