50% more?

leo g_13

if i have tubing already installed, that was installed for a specific heatloss, based upon a 20* delta T, can i get 50% more heat from that tubing by going to a 30* delta T?

leo g

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Tony Conner

Say You Have...

...10 GPM circulating at 20 delta-T, that's:

10 X 500 X 20 = 100,000 BTU/hr

Take the same flow, but increase the delta-T to 30, then it becomes:

10 X 500 X 30 = 150,000 BTU/hr

R. Kalia

if...

It's 50% more IF you have the same flow rate. If you decreased the flow rate to increase the delta-T, then no.

Given that you aren't changing the installed radiation, the only way to increase the delta-T without changing the flow rate is to increase the water temperature.

Steve Eayrs

Which of course means,

> It's 50% more IF you have the same flow rate. If

> you decreased the flow rate to increase the

> delta-T, then no.

>

> Given that you aren't

> changing the installed radiation, the only way to

> increase the delta-T without changing the flow

> rate is to increase the water temperature.

Steve Eayrs

Which of course means,

you may end up with way too hot of a floor, before you even get close to 50% more btus.

Steve

leo g_13

ahhh,

so the only way to increase the DT in this case would be to add more radiation?

leo g

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hr

You could

bump 5° on the supply temperature and accept a 5° lower return to get the extra 10° delta T. A good design software will let you play with the delta t and show the output changes.

It's not uncommon to see 30° delta t used in large commercial radiant. It can help with loop lengths and tube diameters, in large spaces.

For the best comfort on residential hard surfaces I feel a 15- 20° delta t is a good guideline. Just depends on the application.

Running wide delta t's at the boiler piping and buffer tank (before the mix station) is a good way to downsize piping, extend burner cycles, and store more useable BTU's. I have a handful of systems running 40° delta T,s.

Europeans love widddde delta t designs

Robert Bean wrote an excellent Delta T article, maybe he would allow me to post it here?

hot rod

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leo g_13

that would be great

please Robert?!?!?!?!?!?!?!?!

my problem is that i am trying to help a customer who had a system installed, that is only capable of supplying about 2/3 of the necessary heat from the tubing as installed (the "letter I hate to send" thread). so i am trying to find a way of being able to push more heat from the installed tubing.

of course this could be all for not, as i am not sure how much tubing was actually installed, and whether the insulation was installed correctly under the in-joist portion of the job.

leo g

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Matt Undy

Just a thought

Perhaps runing the radiant on a frist stage and adding some baseboard or other radiation for the last 1/3 as a second stage at times of heavier load might be better than trying to push the radiant to produce enough heat? (obviously I don't know what the piping situation to new radiation is). Seems that the existing radiant would provide very even heat runing nearly constantly on less sever days.

Just a thought. Feel free to tell me my engineering or thoughts of practicality are wrong.

Matt

hr

Is this

a problem with excessivly long loops, or just not enough tubing installed? I have a simple, long loop fix method, if needed.

hot rod

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RB_2

post away

can't remember what I wrote...but post away my friend.

George_10

Leo

Our find a volume kit would help you figure the total volume and then you could deduct the boiler and back out the length of tubing in the system. It certainly would give you a usable answer.

hr

Large Delta T ( by one of the sharpest tools in the shed

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leo g_13

HR

no, the loops i doubt are overly long. this was just a job that was never, in my opinion, designed right. if the preceding company installed the tubing as per our guidelines, then the heat loss that i ran, still called for 30% of supplemental heat. this was never put into the original quote. they also used an undersized instantaneous electric water heater, and had the in joist and in floor being run at the same, lower temperature, controlled by a tekmar.

leo g

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leo g_13

Matt

we have decided that it would be more economical (as the drywall and tiling has been completed), for the sparky to run electric supplemental heat. he can run his cable on the outside of the house and penetrate the walls where need be.

thanx,

leo g

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leo g_13

yes George

but my feeling is that since the tubing no matter what is inadequate for the design day heat loss, that i am only trying to cover 70% of the load. the biggest problem with the tubing for me, is that because the loop lengths were never marked, it is going to be fun balancing them!

maybe another application for the Azzel dual temp guage?

thanx,

leo g

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Jed_2

Something's wrong

It comes up as a downloadable, executable file. Is that right for .doc file extension?

Jed_2

Leo

Is there any way you can just add branch drops to your manifold, and run tubing to one or more panel rads with TRV's. This would simply provide the supplemental you need, since you already know the 30% value. Your design heat loss should tell you the required SWT. Are you there?
30% seems like a lot to make up trying to play with Temps.

Just a thought.

Jed

hr

Try this, however no pictures

Designing with large Dt’s DRAFT v1
Copyright 2002 Robert Bean,R.E.T., revisions and changes subject to approval



Our North American twenty-degree Dt design habit dates back to the days of gravity systems. Many believe it has stuck with us in large part by the ease at which flow rates can be mentally calculated. However, there are tremendous benefits to designing with larger temperature drops in distribution systems particularly those serving low temperature heat terminal units, such as radiant floors/walls and ceilings. These systems can take advantage of the large temperature differences between plant and loads resulting in significant cost reductions and efficiency improvements.

Lets look at the impact of designing a water-based system with larger Dt’s. A 250 MBH load (ex.1a) transported on a twenty-degree Dt design has a nominal flow of 25 US gpm 1. However, by doubling the Dt to forty degrees (ex. 1b) we reduce the flow by 50% to 12.5 US gpm.

Ex.1a US gpm @ 200F Dt = Btu/hr / (60 min/hr x Df x Cf x Dt),
= 250,000 btu/hr ( 60 min/hr x 8.34 lbs/cf x 1 btu/ lb* 0F x 200F )
= 25 US gpm

Ex. 1b US gpm @ 400F Dt = Btu/hr / (60 min/hr x Df x Cf x Dt),
= 250,000 btu/hr ( 60 min/hr x 8.34 lbs/cf x 1 btu/ lb* 0F x 400F ) = 12.5 US gpm


Consider the impact of 50% flow reduction on velocities and subsequent reduction in pipe sizes. A 25 US gpm flow in a 2.0” copper pipe is 2.6 ft/sec but a 12.5 US gpm at 2.3 ft/sec is possible in a 1.5” copper pipe2. This equates to less operating differential pressure requirements and lowers the risk of water hammer effects from cycling circulators or fast acting valves. Down sizing pipe from 2” to 1.5” also translates to less fluid volumes. Reducing fluid volumes means less cost for expensive glycols and chemicals. The change from 2.0” to 1.50” has an immediate effect of lower material costs for pipe, solder, gas, control and balancing valves, circulators, insulation, and accessories such as; hangars, flanges, strainers, air separators, and expansion tanks. Greater material savings are available by downsizing to 1.25”, which equates to an acceptable 3.2 ft/sec velocity. All this shrinkage in volume reduces labor, transmission losses, and building space resulting in improvement in operating efficiencies.

With these benefits why have we not seen North American designers using these principals? A quick look across the ocean into Denmark, Germany and Sweden tells us we are far behind our European colleagues who are required by circumstance to ensure the lowest possible return temperatures from district energy sub-stations to combined heat and power plants (CHP). District energy applications are still virtually non-existent in our land yet the design principal’s can and have been successfully used in North American buildings with stand-alone heating plants. Aside from the distribution system, applications for large Dt’s include radiant systems embedded deep within a slab, piped in reverse return, having high R-value floor coverings and closer tube spacing (all of which “smooth” out the isotherms) and series piping of heat terminal units such as baseboard.

In order to take advantage of larger Dt’s at the heat terminal units we need to have a thorough understanding of heat transfer principals in light of larger temperature drops. A heating system designed for 1800F at a 200F Dt, has an operating average temperature of 1700F (fig 1a). However the same system at a 400F has a lower operating temperature of 1600F (fig. 1b). The question then becomes what impact does this lower operating temperature have on conduction, radiant and convection heat transfer.

Analysis of heat transfer at lower operating temperatures may reveal a need for larger surface areas for wall mounted panels, or duct mounted coils, or it may require more linear footage of baseboard. These possible extra capital costs must be considered in the analysis, keeping in mind the premiums are one time costs in comparison to the ongoing benefits of better plant efficiencies.



Secondly would fixing the Dt at 400F but raising the plant temperature up from 1800F to 1900F resulting in the same 1700F make sense for all or part of the system (fig 1c)? If safety and incremental operational cost at 1900F are of little concern then it is possible those HTU’s3 (heat terminal units) designed for 1800F at 200F Dt may provide acceptable performance at 1900F and 400F Dt with the advantages discussed earlier. Also, designing a primary distribution system with 400F Dt does not equate to the secondary systems if it makes more sense to use 100F Dt for the purposes of maintaining higher velocities in secondary zones with small loads.



So far we’ve considered the mechanical benefits but other advantages such as lower power consumption influence design decisions. When we double the Dt, we see a 50% reduction in flow, which can correspond to a 75% reduction in friction losses and a reduction in power consumption by 87.5%!

Now lets us consider the above in context of the plants rangeability. What does this mean? Heating plants are designed for maximum capacity, a condition that rarely occurs, making systems grossly oversized and inefficient for most of their life. However the systems minimum output can be managed effectively with larger Dt’s in combination with using weather compensation on valves, circulators and boilers to lower operating fluid temperatures. Lower fluid temperatures at low loads means control valves can remain open with better authority5 placing the system into a desirable constant flow / variable temperature mode. The cooler fluid temperatures provides better comfort, less pipe expansion, lower transmission losses and the plant remains efficient throughout its entire range from minimum to maximum output.

In summary, given a choice between dealing with this years flavor in a new generation of controls or material savings, energy reductions and efficiency improvements…most home owners would first chose better engineering and this begins with designing with larger Dt’s.

1.Based on water densities = 8.34 lbs/gal and heat capacities = 1 btu/ lb,0F
2 The velocity increase from 2.60 ft/sec in 2” copper pipe to 3.2 ft/sec in 1.25” copper pipe corresponds to an increase in differential pressure from 1.46 to 3.65 ft/100 ft. Another suitable selection would be 1.5” copper pipe at 2.26 ft/sec and a 1.6 ft/100 ft differential pressure.
3. HTU or heat terminal units is a generic term describing devices or system used to transfer heat such as fan/coils, baseboard, floor heating, wall mounted radiator etc.
4. Authority refers to the ratio of pressure across the valve to the system it is controlling. See


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leo g_13

Jed,

the biggest loss of the home, which of course needs the biggest supplement, is a sun room, 3 walls and the ceiling, which is open to the main room. what i am trying to accomplish is to get this home enough heat at the best price possible. so i have decided, after consulting all of these great replies, that the most economial way to achieve this is to have the supplemental in electric baseboard or fan unit or some such thing. the panels priced out to expensive and large, especially when i looked at the amount of drywall that would have to be replaced.

thanx!

leo g

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RB_2

try this

right mouse click on link...hit save to file....store on your desktop.


go to desktop...right mouse click on file....go to open with...select "word"

Jed_2

RB

Thank you, Robert. I sometimes get lost downloading.

Jed