Low Loss Header vs Closely Spaced Tee's for efficiency

john123

For a Mod-Con boiler ----Would Closely Spaced Tee's (CST's) be more efficient than a Low Loss Header (LLH) because the return temperature would be cooler (lower)? Conversely would the LLH warm up the temperature of the return water and be less efficient as a result? I understand that in high flow, low temperature floor heating, one would probably be better to use the LLH, regardless of efficiency, but in my circumstances of using higher temperature emitters, it is a little more difficult to keep the return temperature cool (low). Would using CST's as opposed to the LLH be a good idea?

EBEBRATT-Ed

Not much difference between the two. LLH is better because it slows the flow and is much better for air removal. The mixing is based on the flow in the boiler circuit versus the flow in the heating circuit which would be the same for CST or LLH. It is just a bigger pipe basically

hot_rod

Depending on the llh you chose, you can get multiple important functions, air separation, dirt separation, magnetic separation in addition to hydraulic

The blending of the temperature, if any, is based on the difference in the two flow rates. The size of the container doesn’t change that

That temperature blend happens in either case. A sep does add a bit of buffer also. Sometimes the sep hold more water than the boiler😚

john123

How does the "blend in any case" happen with CST's , if part of the return water is siphoned off by the "first" Tee and goes directly to the heat exchanger. I understand all the advantages of the LLH---dirt and air and good separation etc. and would put in a dirt and air separator anyway (Califfi makes a really nice 1 1/4" dirt one that I intend to put in!). It's not that --cost is no object--but presumably the system is going to be there for 15 years and then maybe then even the mod com boiler is replaced. If you can get better condensation over all those years ---should there not be considerable savings in total?

john123

Thanks for your comments, much appreciated. I expect to have 1 1/4 primary loop ( with the air cushion tank) and a secondary loop to Viessmann boiler with 3/4 " in and out.

hot_rod

The "blend" happens in CST due to the flow reversal moving the mixing point. This example where the primary is flowing 10 gpm, the secondary 15 gpm. So 5 gpm is going the opposite way thru the straight section.

Use the mixed temperature formula to determine the blended temperature.

Same applies to multiple take offs on series primary loops, temperature blends based on flow rates.

john123

I agree with Rod that there would be a blend in those circumstances.

However:

Suppose the system loop is a long single loop with the pressure tank and the PONPC; is done in 1 1/4' black pipe; has a circulator pumping away at 20 gpm; and of course has all the radiators.

And the boiler loop is a single short loop connected with the CST's to the system loop in position AFTER the circulator; with 3/4" pipe of some sort; and with a small internal circulator enclosed right inside the mod/con boiler pumping at 6 gpm.

This is my approximate situation.

Would we would have 20 gpm approaching the CST's; then 6 gpm siphoned off into the first T; 14 gpm continuing through the CST's; then 6 gpm coming out of the mod/com boiler and joining the 14 gpm and making up the 20 gpm as it exits the CST's? And no blending? Is that the way it works or is there some sort of back water to make up 20 gpm going through the CST's? Or is the 14 gpm just a slower movement of the full volume in the CST's? I've been trying to figure this out on my own but am really only guessing. your help would be very much appreciated.

EBEBRATT-Ed

@john123

Your last description is correct.

Lets say you have 20 gpm of 120 degree water coming down the return.

6 gpm of 120 degree water goes in the boiler return.

14 gpm of 120 degree water crosses the "bridge" to the second tee

at the second tee the 14 gpm of 120 degree water mixes with 6 gpm of boiler water.

Lets say the boiler water is 140 degrees

The formula is as follows:

hot flow rate x hot temp + cold flow rate x cold temp= warm flow rate x warm temp

HFR 6 gpm x ht 140=840 + CFR x ct 14 x 120=1680+ 840= 2520/20gpm= supply water temp of 126 degrees,


In this case with the heating pump moving more water than the boiler pump the mixing or blending takes place in the second tee.


Now lets say using the same temps that the heating system pump is 6 gpm and the boiler pump is 20 gpm


6gpm of 120 degree water enters first tee but the boiler pump sucks in the entire 6gpm + 14gpm of boiler supply water flowing backwards across the bridge so the mixing is done in the first tee.

HFR 14X140+ CFR 6X120= 1960+720=2680/20 WFR=134 DEGREEWATER MIXING OR BLENDING IS IN FIRST TEE.

now with unknown temps


boiler output into the water is 50,000btu/hour or 833 btu/min

heating pump is 5 gpm and boiler pump is 4 gpm

assume 120 return water 5gpm into first tee return to boiler is 4gpm @120 degree

4 gpm of water 833 btu added btu added

4 gallons of water (8.4lb/gallon =33.6 lbs of water) 1 btu/lb/degree=833/33.6=24.8 degree rise in temp through the boiler.

CFR 4 x 120 + HFR 4x (120 +24.8)= wfr 5gpm @(120 +24.8)=144.8 degrees.

As @hot_rod says the radiation load will balance against the boiler output to determine the water temp.


Blending with cst or llh is the same the llh is just a bigger pipe the flow rate and water temps determine the blending

the flow rates are whatever the respective pumps are moving.

boiler puts heat into the water and the radiation take it out of the water and these two thing try and balance each other

john123

@ ebebratt-ed so would it be fair to say that when the heating circulator pump specs a higher GPM than the boiler pump, CST's provide a lower return water temperature at the boiler heat exchanger and thus create a higher condensation and subsequently a higher efficiency than a LLH would? perhaps not always but certainly in the system of mine that I described above?

Jamie Hall

In any case, you have two junction points. One where the return pipe from the heating system joins a pipe connecting to the supply junction and to the boiler inlet. Call this junction R. The other where the boiler outlet connects to the supply line to the heating system and the line from the return junction. Call this junction S.

The size of the pipe connecting the two junctions is quite irrelevant to the temperatures. What is relevant is the flows in the three connections at each junction.

If the flow in the heating loop -- controlled by the heating loop flow resistance and the pump choice -- water flow in that loop from junction S, through the loop, to junction R. Now at Junction R it has a choice. If the flow in the heating loop is greater than the flow in the boiler loop, the water will flow will split and some will flow from junction R, through the connector, and to junction S and the rest into the boiler. If the boiler loop flow is greater, however, water will flow from the heating loop to the boiler loop, which at junction S the water will flow from the boiler and split, some will go to the heating loop and some to junction R.

The temperature in the various flows will be controlled by the splitting -- or not -- at each junction.

hot_rod

Tees or seps will be one of 3 conditions

EBEBRATT-Ed

Yes. Concerning the bridge or space between the two tees...Foreward.....Backward or......no flow which isn't really a reality

john123

thanks all for the comments and 2 claps and 2 snaps to Hot Rod for the diagram on page 40 of the attachment showing that in a LLH, the return water to the Boiler is not heated and that is what they say "straight up" on page 41of the attachment --" Since no mixing occurs in the bottom portion of the separator, the water temperature returning to the boiler is the same as that returning from the distribution system..." So it is not that CST's are more efficient; CST's and LLH's act the same when the system GPM is greater than the Boiler GPM!

I now stand more informed. I guess it seemed to me (mistakenly) that there would always be at least some mixing or heat transfer in the separator body. But I am thankful for the explanation and wish to thank all for their patience.

hot_rod

With a property designed separator the velocity inside is very low, top to bottom. Thus is due to the area inside compared to the branch size.  Creating this low velocity zone is why the do a good job with air and dirt removal

TAG

hot_rod said:

With a property designed separator the velocity inside is very low, top to bottom. Thus is due to the area inside compared to the branch size.  Creating this low velocity zone is why the do a good job with air and dirt removal

Is there not a limit to the CST -- I have always used a LLH with mod con boilers for fear of exceeding the flow with CST.

on old school cast i used to pull from the primary -- pumping away