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max btu's that copper can carry

allen Member Posts: 12
If a taco 007 pump is running a 3/4 inch loop what is the max btus that one can get?

What I have is a 3/4 copper line from cellar to attic and the previous owner made a series - no monoflows - loop around the attic. Then he dropped down to 1/2 inch copper and hit kick heaters (10 feet each way in series). The kick heaters are rated for 10k btus at 180 degree water but the last kick heater in the series loop is getting much colder water thus less btu output.

I am weighing options as to go with a common header that hits separate zone valves to each room (4 bedrooms and a bathroom); or install a loop with monoflow T's.

Does anyone know the formula of how to figure out the temp drop of 3/4 inch copper at 180 degrees with a taco 007 where it takes out 10k btus? I am guessing that the drop is 5 or more degrees as when it gets to the end of this loop... there is little heat coming out. And that last heater in the line is the master bedroom / biggest room / my room. I am even thinking of reversing the flow!




  • JimGPE_3
    JimGPE_3 Member Posts: 240

    Temperature difference = Btu/hr divided by (GPM times 500)

    dT= Btuh/(gpm x 500)

    3/4" copper = 3.5 GPM
    1/2" copper = 1.3 GPM

    These flows will keep the piping quiet.
  • allen
    allen Member Posts: 12

    So if water leaves a 100,000 btu boiler "output" travels to a heater that assuming its 180 degrees and loses 10k btu's then what is the output temperature? This is 3/4 inch copper. And what is the formula (to figure next and next and next kick heater)?

  • Frank_3
    Frank_3 Member Posts: 112
    What is the actual flow?

    If you assume 3.5 gpm, 10k btu/h loss then the formula says delta T = 10k/(3.5 * 500) = 5.71 degrees, if my math is right.

    If you started with 180 degrees, and lost 5.71, then the next unit will see 174.29 degrees at its supply.

    Those are big assumptions about flow rate and water temperature, though. If you're sending hot water up from the basement to the attic then a) you'll lose some heat in the pipe leading up to the attic especially if it's hidden in an outside wall, and b) the 007 may not be moving the water at 3.5 gpm -- it could be going slower or faster depending on the total head in that loop.

    Consider also what the design flow rate is for the radiator you're using. I think I remember seeing somewhere that a manufacturer rated their baseboard at 680 btu/h/foot at a flow rate of 1gpm, with a supply temperature of 180 degrees. So, if you're not providing the radiator with that temperature water, at that flow rate, then you're not going to get the rated heat output.

    Which will throw all your numbers out the window. :-)

  • hr
    hr Member Posts: 6,106
    Not sure

    exactly what you are asking, 3/4 copper can easily move 40,000 BTU at a 20 degree delta t. The amount of btus it loses as it mover through baseboard or fan coils depends on the flow rate and also the air temperature entering the bb or coil. Of course the colder the air temperature the greater the delta T between the supply water temperature. So the more BTUs will be given off per foot of element.

    Of course the last convectors on line will see the cooler water temperatures and their output will be related to this.

    Also as others mentioned un-insulated pipe runs from the basement to the attic will lose a bit also. Quite a bit if they travel through cold wall cavities un insulated, once again the temperature lose will be related to the temperature of the space they travel through.

    It can all be calculated if you know all the variables.

    hot rod

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  • allen
    allen Member Posts: 12

    You have made it clearer. Basically a 3/4 line will push 40k btus. If I have 5 kick heaters that can all produce 10k each then the last one isn't going to do as much heat especially due to the fact that they are all in series...

    If I am losing around 6 degrees per unit and they are all in series... Then of course the last heater - call it number 5 is about 35 degrees cooler with input hot water. And doing the calculation at 180 degrees this would put out 10,500 btus thus at 155 its around 8000 btus. The kick heater specs are here: http://www.vrvproducts.com/html/products.html

    But the person who did this started with 3/4 in cellar, ran to ceiling above second floor, then dropped down to 1/2 inch ran 10 feet to a kick heater then back up with 1/2 inch to the 3/4 pipe "big loop". He then ran the 3/4 about 20 feet and dropped to 1/2 inch again with a 10 foot run to the second kick heater and back up in half inch to the 3/4 loop. Every time he connected to a kick heater he dropped down to half and made them all in series. Thus the last heater is not producing as much And this last heater is in the largest room. So the thermistat for the entire floor is in this last room which never gets satisfied so it runs all night long. I have adj the rocker switches in first rooms in lowest position to not blast them out so they open a window! AND we are not even calculating what is lost with 1/2 inch.

    This house that we bought has a new second floor and was completed about 18 months ago. I know that they must have frozen in the master bedroom last year. AND they didn't have insulation either in the attic above. So those pipes are basically in the breeze as there is a full ridge vent. Yet another loss of heat. I discovered all this while installing R 30 insulation.

    My solution is the following... I will make a header in the attic and run a separate line to each heater. Thus I can install zone valves off a thermistat in each room. And this way I will have a full port ball valve on each line to throttle up or down so all heaters get the same temperature. Tonight its supposed to get down to around 10 degrees - should be really interesting!

    Plus for the half inch pipe this further reduces the btu output. Too Bad that half inch is buried in the finished walls. But even if I have a separate line to the heaters from a common header - that's an improvement.

    I do remember 680 btu for baseboard per linear foot at 180 degrees. And I think that was off an 007 circulator pump.

    Thanks for the assistance here in Massachusetts!


  • hr
    hr Member Posts: 6,106

    The total length of the 3/4" loop and the number of fittings would play into the calcs. Could be on a cold day the last couple fan coils will be seeing a low enough supply temperature to make them useless.

    Most have small klikson t-stats strapped to the coil the will not allow the blower to start until the coil sees that temperature. The temperature rating is stamped on them usually. Could be the last ones never even rev up, depending on the load the first ones are handleing :)

    I'd look at running some new PAP supplies to each unit if possible, home run them to a piping arrangement at the boiler that would assure the same supply temperature to all the units.

    Back up even further, however, and do a heatloss calc. for the job. Make sure you have the load covered and the boiler and components sized right to do the job. There is a very nice heatloss calculator a click away at the left of the home page here. Try it you'll like it.

    hot rod

    hot rod

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  • Gary Fereday
    Gary Fereday Member Posts: 427
    Dear Sir,

    Know this series loops add the pressure drop linerly, and parallel connected ad pressure drop as the sum of recriprocals. So if you ad parallel units the pressure drops will become totally lower than the smallest drop. That given the heat becomes easier to the larger units and the smaller units have less water to gather heat from. Balance here becomes a real science. and expensive. for if you change the flow in one they all will change. You can chase your tail forever if you would like to. Your series loop now is doing only what 1/2 " will do at the pumps ability. The 3/4 is only handling the water the pump will push through a 1/2" pipe.If your are lucky that could be more than 1.5 gpm. anyway bigugh
  • Steve Ebels
    Steve Ebels Member Posts: 904
    That 1/2\" drop

    To each unit isn't going to hurt you if you have the same water temp available for each heater. 1/2" will easily carry the 10K btu's. That's only 1 GPM at a 20 degree drop. If it were mine I would cut your loop and make a header at the end of the supply coming from the basement. Run from this header with 1/2" Pex or PAP to the existing 1/2" copper. On the return, just make another header to dump all your loops back down to the boiler. Control would be your choice. Let 'em all run off one t-stat if the heatloss of each room is close to the same. If it's not you may have to do the zone valve thing. The point is to get each heater seeing the same water temp. I would try that 1st if it were mine rather than the added expense of the zone valves. At least to see how it worked. Working off a manifold like that you could even throttle back the flow a little on zones that are running to hot. You may have to experiment with pump size. Try the UP15-58 from Grundfos instead of a 007. It has 3 speeds and will push a little higher head than the Taco. If high is too much you can select the next lower speed. Just some thoughts for you to consider.

    Happy thanksgiving and let us know how it turns out.
  • Mike Kraft
    Mike Kraft Member Posts: 406
    One more wrench for the pile!

    Great advice so far.I had a job that presented me with a problem that was not the same but similar.I spoke with a man who has an uncanny knack for seeing the forest despite the trees.This man would be Dan.His suggestion and one that would make your system work smooth would be to make your loop in the attic a primary loop.Then isolate your 1/2" branches as individual zones with zone valves from the attic.If your 1/2" feeds and returns are accessible from the attic this should be an easy solution.Then all you need to do is add a relay and some thermostas and wiring.

    I would dump the fan coils in the beds and consider a panel rad or rads instead.That fan would keep me up!!

  • Mark R.
    Mark R. Member Posts: 40

  • Mark R.
    Mark R. Member Posts: 40

  • Mark R.
    Mark R. Member Posts: 40

  • Mark R.
    Mark R. Member Posts: 40
    Good Question..Great Responses

    Allen, I'm glad you asked, because the responses from these guys are really loaded with information. I recently had a question about pipe noise in a single loop system and Mike (bigugh) helped me there, but Jim's response made me realize that my noise has increased due to the new circulator moving the hot water faster, and the temp rises quicker and pipes expand faster. Hats off to the experts on this forum that help people like me!!! This is like going to Hydronic school. Mark R.
  • Mark Eatherton1
    Mark Eatherton1 Member Posts: 2,542
    Heads up....

    According to the maker, each unit has a pressure drop of almost 3 foot of head per coil, times 5 coils equals 15 feet of head, PLUS S&R piping.

    I concur with what everyone here has said, too much pressure drop equates to too little flow which equates to a MEGA delta tee which means that the last FCU is starved for heat. Parallel the buggers and your distribution problems should go away. Then as Cheese said, there's the noise issue... HMMMMmmmmmmmmmm



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