Too much pump?????

hr

you have the additional drag of spinning the rotor in the fluid. I think the B&G PL series makes a bit more sense from and energy efficiency stand point, but they are so darned noisey compared to a wet rotor design.

hot rod

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Wayco Wayne_2

I was

talking to an engineer last night. A real smart fellow, who was telling us about a job he straightened out. One of the problems, he said was too much flow through the radiant floor. He changed out the circs to get a wider delta tee and the capacity of the floor increased. I'd never heard of such a thing. What happens? The water train is going so fast that the btu passengers can't jump off? I always thought you could increase your capacity by increasing the flow. Can someone explain this to me in laymans terms so I can get a grasp. Also what delta T do you design for in a radiant floor?? WW

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ALH_4

It sounds

to me like he was confused. You are correct. With the same supply temperature, you will increase the output by increasing the flow rate and decreasing the dT.

-Andrew

Matt_21

you can increase btu by

increasing flow but you get to a point where too much gpm will cause the water to flow too fast and the heat will not be transferred. most radiant floor systems are designed for a 20* delta t.

Wayco Wayne_2

At what point

is too fast?? I've been designing for 10 degree delta tee on radiant floors. 20 on other heat emmitters. Am I over designng? WW

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Brad White_9

Got Pump?

Flow is a funny beast in heat emitters.. changing the flow by a factor up or down of 50 percent or more yields very little difference in output for all of that effort.

Check any fin-tube radiation table- See the output at 1 GPM (500 PPH) versus 4 GPM (2000 PPH)? 520 versus 560 or so by one selection. Radiant floors are no different.

And think about the pressure drop with associated energy! Wow. Pressure goes up by the square of flow, HP goes up by the cube.

Four times the flow in a given system, 16 times the pressure drop. 64 times the Watts. America is over-pumping.

The only reason you get more capacity is that you change your average water temperature and hence the surface temperature of your emitter. If your starting range is 20 degrees (at whatever temperature) and you double or halve the flow, you can only change your average water temperature 5 degrees (half on either side of the median ten degrees).

That is the only margin you have to work with until you start to modulate your water temperature (using a control valve in parallel to your circuit as an injection device for example, as opposed to in series as a throttling device).


BTW: I normally design for a 20 to 30 degree Delta-T. Depends on a lot of factors and what I start out with in the way of water temperature. If I have fan coils requiring a warmer temperature which governs boiler output, I can take higher delta-T's because I am starting at a higher point. If all low-temperature, I weigh that against what emitter surface I have available and what it can reasonably do. Many variables, but 20 is not a bad place to start. I am sure your 10 degrees has some basis and would be five degrees different in water temperature than my 20 degree equivalent system.

ALH_4

Heat Delivery

Delta T is dependent on a lot of factors. The performance of your heat delivery system is important. Suspended tube will have a much lower dT than extruded plates at the same flow rate and under the same conditions. Increasing the flow rate and therefore decreasing the dT will increase the uniformity in the floor surface temperature. This will increase output somewhat, however it is marginal compared to increasing the overall temperature of the system as Brad mentioned.

-Andrew

hr

The best explanation I've found

http://www.pmmag.com/CDA/Archives/de452c49e3fc7010VgnVCM100000f932a8c0____

The answer is in the numbers. The higher the temperature of the emitter, the greater the output. Ask them to show you a thermodynamic engineering formula that proves otherwise

If that link does not get you to the article click on the archive button then the June 1997 issue for the article "The Water's Moving Too Fast!"

hot rod

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Mike T., Swampeast MO

It defies most explanation but similar [seemed] to happen to me.

I simply tapped in some small radiant loops to my constantly circulating mains on a gravity conversion system with TRVs. Mainly for bleeding purposes I installed ball valves at supply and return connections. Driven by B&G 100 circulator. With the valves wide open there was exceptionally little delta-t between supply/return of the radiant loops and the floor barely heated. Throttled the flow WAY down on both supply and return (think a bit over ¾ closed ball valves) to get a 20° delta-t and the floors got warmer!

When I replaced the boiler and had the benefit of a variable-speed circulator of MUCH lower capacity I had to open the ball valves significantly (think a bit under ¼ closed) to achieve similar delta-t across the radiant loops.

Yes, I know that this is not supposed to happen and presume that there could well be some other factor involved, but with flow rates WAY above the norm, strange things might happen.

Bob Sweet

Equations are not my strong suit

but I do remember an equation that I read here, that kind of put it in perspective for me. 500 x gpm x delta T = btu output. B&G system syzer works well or Siggys HDS puts all doubt aside.

Wayco Wayne_2

Thanks for the replys guys

I read Siggy's article and the conclusion at the end was that increasing flow does increase capacity, but...there is a point of diminishing returns the higher you go. This still does not explain why this engineer got better performance by increasing the delta T in his radiant floor by downsizing the circ. Or for Mike's experience to the same. I guess I put this one on the shelf with the other mysterys I havent figured out yet. (The shelf is sagging from the weight. Yikes!) I will also investigate the delta T question for radiant floor loops. Someone once told me use 10 degrees to decrease the temp difference so it warm and cold spots would be less likely. That coupled with the fact that my Wirsbo software defaults to that Delt made me assume it was true. WW

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Plumb Bob

> I read Siggy's article and the conclusion at the

> end was that increasing flow does increase

> capacity, but...there is a point of diminishing

> returns the higher you go. This still does not

> explain why this engineer got better performance

> by increasing the delta T in his radiant floor by

> downsizing the circ. Or for Mike's experience to

> the same. I guess I put this one on the shelf

> with the other mysterys I havent figured out yet.

> (The shelf is sagging from the weight. Yikes!) I

> will also investigate the delta T question for

> radiant floor loops. Someone once told me use 10

> degrees to decrease the temp difference so it

> warm and cold spots would be less likely. That

> coupled with the fact that my Wirsbo software

> defaults to that Delt made me assume it was true.

> WW

>

> _A

> HREF="http://www.heatinghelp.com/getListed.cfm?id=

> 255&Step=30"_To Learn More About This

> Professional, Click Here to Visit Their Ad in

> "Find A Professional"_/A_

Steamhead (in transit)

Time is a factor too

If the water is moving too fast, it will not have enough time to pick up heat in the boiler or shed it in the radiation.

In extreme cases, the boiler will run up to limit and the water will come back at about the same temperature at which it left the boiler. And you won't get much heat in the rooms.

For an extremely extreme case, go here:

http://www.heatinghelp.com/newsletter.cfm?Id=119

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KBP&H

too much water

sometimes--- if the boiler cant handle the amount of return water the floor is producing, the boiler even w/ prim sec loop, can start to condensate and will run @ 140,150,or whatever,, until the floor starts to do it's thing,, w/ any floor-- slab or floor sensors w/ a wall tsat ( tekmar 509) are the only way to go, if your are not doing injection pumping.

bob_50

Boy am I gettin confused

so-- if I'm reading this right I'll get maximum heat delivery with the pump off?

EBEBRATT-Ed

I have posted a similar question before. Mathmatically as gpm goes up output should go up. Smaller TD with increased gpm, larger TD with less gpm. Ive herd the "over pumping" issue before and don't quit get it.



ED

Steamhead (in transit)

Not quite

but you have to find the "sweet spot" where the water moves at just the right speed.

"Steamhead"

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[Deleted User]

What bob sez....

You guys are ALL wet... It's physics. No one's even mentioned the Reynolds numbers yet:-) Think they'll go down or negative if we slow the water down enough? Maybe it makes water thermogenic....Yeah, that's the ticket. Thermogenic hydronics!

Does anyone remember the Russian "Heat Source" that was like a 5 HP pump that whipped water in a circle, creating heat from water friction, mechanical input of the impellar, as well as the motor being wrapped in soft copper for recovering that heat. Guess electricity was cheap back in them days...:-)

Then there was the condensing boiler from Russia that looked like an egg. What an innovative design. Couldn't get the UL seal of approval on that puppy...

It was MADE to be WORKED on....

I think by slowing down the flow, you are front loading the thermal emmitter on the cold side, hence it feels like its working better.

Or, maybe it's one of them things that makes you stop and go "huh..."

Which moves more energy, 1 gallon per minute at a 100 degree rise, or 100 gallons per minute at a 1 degree rise?

Beats me!!

ME (Formerly wellhead)

ALH_4

Balance

I look at it as a balancing act.

The customer wants:
1) Price
2) Comfortable, evenly warm floors
3) Efficiency
4) The least equipment required to accomplish this

So the balancing act is:

1) Widest dT possible that keeps the floor an "even" temperature - cool return temps help condense
2) Coolest supply temperatures possible to maximize the efficiency of the boiler - this can conflict with wide dT's
3) The smallest pipes and pumps possible to accomplish this
4) Use a minimum of equipment to achieve this

So where is the balance? Say your design temperature is 120F. Assume a 20F dT. What is the floor temperature at 120F and 100F? Let's say it's around 6F difference. If the floor surface starts out at the beginning of the loop at 80 and ends at 74. Is that noticeable? How wide can it be before it is noticeable? That's pretty subjective. Some people just want the room to feel comfortable. At the other end of the spectrum, some people want the floor warm at all times and open the windows to control the temperature.

Tighten the dT's and the pipes and pumps get larger. More electricity is consumed, and the installation is more difficult and expensive. A 20F dT seems reasonable to me for radiant floors. With radiators and TRV's, this can be wider without changing the comfort because the occupants are not in contact with them.

There is a boundary layer in every pipe with fluid flowing inside it. The higher the velocity of the flow, the thinner the boundary layer. Use the smallest tube size that is practical to help increase velocity and turbulence.

It's no coincidence that radiant floor systems are designed the way they are. It's evolution.

-Andrew

Steamhead (in transit)

It's simple (with two more-recent examples)

picture a large walk-in freezer with a door at each end. You're wearing a t-shirt and shorts. You open the door at one end and run thru the freezer and out the other end. OK, you're a bit chilly.

Now you stop for a few minutes to get warm, then re-enter the freezer...... but this time you're walking very slowly. When you finally reach the other end you're, well, freezing!

So it is with water in a hydronic system. It needs a certain amount of time to pick up and shed heat.

I've written about the over-pumping phenomenon on old gravity systems, but recently ran into this a couple times on relatively new baseboard installations:

The first one was three zones of 3/4" baseboard, piped in the usual manner with 3/4" copper. Each zone had a Taco 571 zone valve, and the circ was the usual Taco 007. Delta-T was about 5 degrees even with all zones calling. The boiler would run up to limit but the baseboards gave off little heat. We switched to a Grundfos UP-15-42F, whose lower flow rate got the delta-T to 20 degrees and the baseboards finally got hot.

The second was four zones of 3/4" baseboard, each with a 571 ZV. The main difference here was they ran 5/8" PEX instead of 3/4" copper. The fittings used with this size PEX have an internal diameter of a bit less than 1/2". Originally it had a Taco 007 which couldn't push much water thru these constricted fittings, so the original installer installed a Taco 0011. Big mistake- that high-head circ made the water literally scream thru the system with almost no delta-T- at least when it wasn't cavitating. A Taco 0010, with its flatter curve, was the right choice here, along with high-capacity baseboard in an under-radiated room (original installer never did a heat loss). Now the delta-T ranges from 20 to 30 degrees depending on how many zones call, and the rooms warm up nicely.

You can see this for yourself if you invest in an inexpensive infrared thermometer. Measure the delta-T of the next system you work on. What you find may surprise you.

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Gordy

analogy

An excellent analogy Steamhead, similar to passing hand over a flame. Sometimes the theoretical does not always reflect the reality of what is actually happening.

Gordy

hr

So if I remove the fan belt on my truck, Steamhead

it will cool better? The air won't be rushing across that hot fluid allowing more linger time

Or if I downsize the circs on all my snowmelt jobs I would increase output?

I still think in the case of converted gravity systems it has more to do with the fluid flow pattern through the radiator. With a pumped flow across the bottom connections there is no way to get a pumped flow in the upper portion.

I'll dig for some pics of a demo I did that showed that concept when I get back from work.

hot rod

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[Deleted User]

So...

What you're saying, although it goes against all principles of heat transfer, is that there IS a use for variable speed main circulators operating on delta T as opposed to delta P, no?

Field experience trumps book learnin' to a point. Then you have to explain why it is that the books are wrong.

Got any good reasons why slowing down the velocity of the water delivers more heat Frank?

Always yearnin' for a learnin', with an open mind...

ME

Steamhead (in transit)

The flow pattern is one factor, HR

because if the water is moving too fast, it will simply take the shortest route between the inlet and outlet points- which is a straight line. It won't diffuse thru the rad. That was one of the problems on the old Spencer job I wrote about, and I've seen it elsewhere too. It can also happen in a radiator connected top and bottom on the same end- the water just flows thru the first couple of sections.

You could do the same thing by orificing all the rad inlets. But why? A smaller circ pumping against less head is less expensive to buy, install and operate.

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Steamhead (in transit)

The only thing I can figure

is that you're giving it more time to do its work.

Whatever assumptions we make in designing, the proof is in the actual performance. When you have a reasonable delta-T you know beyond any doubt that you are successfully adding heat to the water at the boiler and shedding it in the radiation.

I have a feeling that the pipe design techniques we've been using have enough conservatism built into them, that the actual head pressure is less than we think. But I haven't done any head measurements to support this.

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Mike T., Swampeast MO

Not Frank and while I [think] I observed this happen, I can't be sure and it certainly does seem to defy most explanation.

Two possibilities:

1) High velocity combines with friction to set up a thin, slow-moving, cool layer against the tube walls that effectively insulates the tube from the hot, fast moving "core" flow.

2) If heat energy (like light) does indeed have characteristics of both massless waves and particles with mass then perhaps an inertia-like force applies to the heat as well. Move the BTUs too fast and they tend to stay on the train instead of jumping off...

Gordy

Marble

Thinking about the marble in the pipe, time,and space. The length of the LOOP giving up the heat is a key factor in this. Fire two marbles at the same temperature, but at different rates of speed through the same length of pipe. The faster traveling marble will give up less heat in the same time, and space than the slower traveling marble.

Now change the pipe length so that the marbles exit at the same temperature even though they are traveling at different speeds. The faster traveling marble needs more pipe to give up its heat. Now your gonna want to prove it.

All I can say is in the world of mechanical advantage, aerodynamics, thermal dynamics, gravity, there is always diminishing returns from what theoretically should work,and what will work when you go to the extreme end of the theory

Gordy

Mike T., Swampeast MO

Interesting Gordy--particularly considering the loops in question are only 40' of 3/8" copper in Thermofin™.

Larry Weingarten

I see Andrew's thoughts...

... as a good reason to consider radiant wall heating where possible. A wide delta T in walls would not affect comfort, unless your client walks on those walls ;~)

Yours, Larry

hr

I wonder in the case of the fin tube

example Steamhead used... At some point it seems you would have to increase the convection air flow to pull more heat from a fin tube element.

I know a kick space heater, in a wall cabinet with open air flow, gives off a lot more heat than the equal amount of fin tube without the forced convection. Easily proven by turning the fan off on.

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Steamhead (in transit)

That's what convectors did

the chimney effect of the enclosure boosted the air flow thru the unit. With baseboard there is almost no chimney effect.

There has to be a measurable point at which increased water velocity thru a baseboard element reduces its heat output. To my knowledge no one has tried to measure this.

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Brad White_61

Convectors and Velocity...

Steamhead is exactly right on chimney effect. With commercial BB where a variety of enclosures are available using the same element, the tables tell you that height gives capacity (more airflow) with nothing else changing.

As an aside, I designed a heating system for a house with practically full-height book cases all around most rooms. Punch windows and clerestory glazing were the rule. The three inches behind the casework was the "chimney" and the baseboard was near the bottom, all air drawn in via the toe-space. Being the conservative type, I used the base "Bare Element" rating. Based on experiential reset supply temperature, I would have to say we got a 30 percent boost. Plus, once that channel space is warm, there really is no transmission heat loss as far as the space is concerned.

As for velocity increase hitting a "zero output increase" point: Interesting concept, but I think you would never get there. Go from a 20 degree temperature drop (say 180 in to 160 out with a 170 AWT) to a 0.5 degree drop (180 in, 179.5 out) you still have an element with a 179.75 degree average water temperature. May be hard to measure the water delta-T but the output is there.

Of course you are pumping 20 times the flow more or less so the erosion leading to tube wear leads to leaks... so you are right, eventually you will get zero output!

Steamhead (in transit)

I think

you might be surprised if you measured output at such a low delta-T. I'm sure it would drop considerably. It has to- I've gotten the same results on two different systems, so it's definitely repeatable.

In both cases I cite, the heat coming out of the baseboard increased when we slowed the flow rate with a smaller circ. The greater delta-T corresponded with the higher output.

I think these measurements will be as earth-shaking as Gerry Gill & Steve Pajek's steam air vent and trap measurement project.

What kind of equipment would be needed to measure this?

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Brad White_9

Agreed,

it is a conundrum, Frank. Less delta-T generally means less heat transfer, but the flow rate has to be considered. It all works together. But I can see what you say happening, at the extreme end of things, so much flow moving so quickly. But your emitter temperature is that much higher. Diminishing returns..

To set up such an experiment you would need an accurate flow meter (low flow range such as Blue-White flow or dosing type as you might use in medical or compressed air measurement, except for water), and a very accurate (10k Ohm or higher) thermistor sensors, direct immersion not in wells.

Oh, and a circulator on a VFD. Better yet, three circulators on VFD's, three different sizes to get the full (20:1?) turndown range at all associated pressures.

Could be fun!

[Deleted User]

I have...

pretty much all that you describe in my house heating system, plus. I have a Letro flow meter on the main of my one pipe system, thermistors on the supply and return (from the Munchkin T-50) as well as the OSA. I also have a pulse output gas meter (American AL50) that gives me a pulse closure for each cubic foot of gas that goes through it.

I have experimented with higher and lower flow rate through the system at design condition. It made no difference in either comfort or btuH delivery. I varied the flow from between 1 GPM and 4 GPM in 1 GPM increments.

The only thing close to this co-relationship theory that I did see was an increase in RFH output from my kitchen floor, and I attribute that to the fact that I choked the living crap out of the mains flow, diverting a GOOD portion of the flow through this 300' below the floor, in heat transmission plate zone.

I hate to bust up your theory, but what I found takes me back to the same thing I've always said, that being a btu is a btu, and although changing flow changes delta T, it is all related to the panel load. Speaking of which, I can remember one (1) job where I actually witnessed a 20 degree delta T. It was a brand new hot water baseboard job that we started at design condition. Other than that one time, I usually see significantly less differential (more like 5 to 7 degrees F) and the occupants are perfectly comfortable.

Now, in the case of a gravity conversion, all bets are off. I agree with Frank that the water can WHIP through the bottom tappings of a radiator, like an arrow, not allowing ANY side traffic up the tubes, thereby significantly affecting the panels output. That is the reason I generally recommend using NETRV's for individual emmiter control. It slows the water down to whatever is the most ideal flow for the room/emmiter load.

I did not save any of the data from my testing because there really wasn't any significant findings, and with spring on the cusp, it will be a while before my home is again exposed to design conditions.

Here's some info on my physical plant.

ME

Ron Schroeder

"It defies most explanation but similar [seemed] to happen to me.

I simply tapped in some small radiant loops to my constantly circulating mains on a gravity conversion system with TRVs. Mainly for bleeding purposes I installed ball valves at supply and return connections. Driven by B&G 100 circulator. With the valves wide open there was exceptionally little delta-t between supply/return of the radiant loops and the floor barely heated. Throttled the flow WAY down on both supply and return (think a bit over ¾ closed ball valves) to get a 20° delta-t and the floors got warmer!"

Hi Mike,

I think that that is like when my wife is waiting for a traffic light to change and she creeps up closer to the intersection and the light changes. She is convinced that the moving forward triggered the lighting control when it was just TIME doing it's thing.

Remember, only Lawyers and Marketing (and ocasionally Management) can violate laws of physics. ;-) Engineers aren't allowed to.

BTW, sometimes a change in flow can change the heat distribution becoming advantagious or disadvantagious to where the temperature control sensor is located in THAT PARTICULAR installation.

Darin Cook_2

Interesting Facts

The average wet rotor circulator operates at about a 23% efficiency. That circulator is sized ( providing it is sized correctly to start with ) for design day and so is oversized the vast majority of the year. You will see delta P circulators in the near future. There is much energy (watts) to be saved by using them. They have been on the European market for about 10 years already. There was a study done in the EU if every circulator was changed to a delta P pump something in the range of a trillion gigawatts could be saved. It was said to be able to power about 700,000 homes for a year over there. Impressive to say the least. There are many oppurtunities to save energy with technology that is already existing here in the US. When will every one get on board?

Darin

Ron Schroeder

Assuming there is no air or cavitation in a system, the only way that heat output will actually decrease with increase in flow at a constant supply temperature is if the PATH of flow changes. That won't happen in a small tube but can easily happen in radiators.

Flow PATH can also change as seen as a balance change between zones if gravity flow is contributing to some zones more than others. The percentage of gravity flow vs. circ. flow will change differently in different zones as circ. flow changes, especially at low circ. flows.

Ron Schroeder

Hi Daren,

I once figured that if I changed all of my low wattage zone valves to conventional circs, they would put out enough heat that I wouldn't need the boiler anymore.

Ron

Steamhead (in transit)

Then

how do you guys explain the results I'm getting?

"Steamhead"

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Brad White_9

New Definition of

> Hi Daren,

>

> I once figured that if I changed all

> of my low wattage zone valves to conventional

> circs, they would put out enough heat that I

> wouldn't need the boiler anymore.

>

> Ron