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Just Don't Get Modcon Boiler Pump Charts!

135

Comments

  • ChrisChris Posts: 2,869Member
    Not at all

    This wouldn't be new just new to us. They have been doing

    this across the pond for years. The boilers logic wouldn't allow

    for a low flow situation.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
    · ·
  • ced48ced48 Posts: 330Member ✭✭
    Thanks, Chris-

    Would I half to be concerned about to much flow? If not, isn't this the perfect solution, while maybe a little pricey?
    · ·
  • ChrisChris Posts: 2,869Member
    edited March 2013
    Too Much Flow On Which Side

    The boiler side or the system side? Maybe we should start fresh on RISE..



    The boiler pump chosen is going to move it's flow CONSTANT. Meaning, whenever there is a call for heat and the boiler runs so will the pump. You cannot change that. It's a fixed speed pump. Look at the Alpine curves I posted above. Tom has a Grundfos 26-99 which is pushing 13.gpm across the boiler. That never changes. What changes is how many btu/hr is being carried by that 13gpm and that my friends is going to be dictated by the boiler rise or the difference between the boiler supply and return temp. Rise is not a constant and is continually moving up and down. Modulation!



    I'm not a big fan of the pumps that are sized based on the recommendation of 20 or 25 degree rise. This means you have to move a lot of water off to the system side to scrub out the boilers made btu/hr. You end up with a small rise and this is what causes SHORT CYCLING.



    If one zone is calling and you only have a flow rate of 3gpm where do you think that other 10gpm is headed? It is b-lining right back to the boiler. It has no choice. I'll say it again, What goes into a tee must leave a tee!



    You system flow rate can be designed for what ever you want it to but you really want high gpm on the system side low gpm on the boiler side. Size the boiler pump based on the flow and head of largest rise avail for that boiler and your system side for the standard 20 degree delta. This way you can take as much as the boiler gpm you can leaving little, ideally none, to b-line back to the boiler. You want your emitters to scrub off the btu/hr and send the coldest water back. You better make sure you have enough emitter and you better make sure that mixed supply water temp is going to do the job.



    The best way to read the chart is this. The pump you choose will move the stated gpm based on the HX head loss (plus fudge factor) to get the full rated output of the boiler at the given delta-t (temp rise). There it is again that Universal Hydronics Formula that our friend Mr Barba has imprinted and stamped into my head!



    gpm = btu/hr / (Delta-T x 500)



    So when we say the boiler will just mod down to low fire, we are saying the burner will, not the gpm across the HX. You still need to get rid of the btu/hr being carried. So if your moving 13 and can only take 1,2,3,4 your going to short cycle and/or stay out of condensing mode quite a bit of the time.



    You could say that it's not the boiler that is over sized it's the pump. There are plenty of condensing boilers out there way over pumped. We like big pumps, they move more water.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    A few points Ced48...

    "Can someone explain to me what the delta T rise on a modcon boiler circulator sizing chart relates to? ...  ...The water is circulating in the primary loop at a rate that hardly

    allows for any temperature drop from one end to the other, and this is

    temperature drop, not rise"



    At equilibrium, the deltaT rise in the boiler is equal to the deltaT drop across your heat emitters.  The deltaT versus flow chart is correct, but the pump recommendations to achieve those flows are not.  I think the manuals are intentionally highly conservative.  Lochinvar is probably assuming that you are assuming a very wide allowable margin of error, with the possible use of glycol in high concentration.  This seems to be consistent with the other feedback you're getting here.  Keep in mind the tables are only valid at full fire output, and the deltaTs are always proportionally less if the burner is modulating, which is like 99% of the time.



    "How do I "design" for a 35 degree delta T rise?" 



    This is a retrofit.  It has already been designed.  The fintube is already in place and you must work with what is there.  "The system works fantastic, as is..."  So it's fair to say your home heats pretty evenly and the most difficult to heat rooms are still warm enough during the coldest parts of the year?  Hypothetically, If the current delta's are around 5 to 10 degrees, I do not think it's wise to spec a pump for 35 degrees.  (Yeah I know, 20 @ 33,000btus)  It might work fine, but I think that might be a high number for a lot fintube systems.  Especially if your emitters are in series and span different rooms on the same loop.  What I'm saying is, don't go out and buy another single-speed pump specifically for that system delta.  Get something that is adjustable and capable of additional flow to match what you are delivering now.  And yes, you will have no problem moving the additional gpm required to achieve the lower system deltas in the boiler pump charts.  Even if you wanted a delta of 10 (which I recommended starting off with) you do not need a primary/secondary.





    "A delta T of only 5, or so, is not going to make for a very efficient system."



    In your simple straight through one pump system, the delta T is not the main indicator of efficiency.  That is only one parameter of many.  Remember, your new boiler will/should be heating the water to a temperature dictated by the ODR curve.  Almost all of your efficiency will be coming from the combination of ODR, modulation, and condensing technology.  Increasing the deltaT by 10 or 15 degrees is nothing compared to having the boiler reset the setpoint from 180 to 110 over the course of the season.
    · ·
  • ced48ced48 Posts: 330Member ✭✭
    Thank You, Eastman-

    Your thoughts are more or less inline with mine. I am just trying to make as much sense out of everything as I can. My understanding of how a system gives up it's BTU's puts flow rate at an almost insignificant factor in efficiency and delta T. This is why I felt, and still do, that running at 3.5 GPM constant makes sense. I will have a flow above the boiler"s minimum, yet slow enough to keep the 3/4" baseboard quiet.



    To refresh, the system is a split loop, with 2, 3/4" circuits, and a 1" common supply and return. Primary/secondary will not be used, as it is just plain unnecessary and redundant. I will be running an indirect at about 8 GPM, which should do a great job of keeping the HX scrubbed.



    Well, if I'm off track on any of my ideas, let me have it! This has all be very informaitve to me, and thanks again to all.
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    the problem is with a high deltaT

    is that your particular house may not heat evenly.  Take a look at the output curve for a fintube emitter.  The relationship between water temperature and heat output is complicated by air convection.  If you have multiple fintube units in series, and enforce a high delta on the system, the first heat emitter could easily put out double the heat that the last one on that loop is delivering.  If the loop spans multiple rooms, the room at the end of the run may never get warm enough even though the first room is overheating.



    I think 6gpm would be more typical for this type of system as that corresponds to about 10 degrees at 33k btus if I recall correctly.  And that's not a fast water velocity.  6gpm is about the *minimum* flow taco recommends in 1-inch copper to keep bubbles from accumulating.  (about 3 for 3/4 inch)
    · ·
  • ced48ced48 Posts: 330Member ✭✭
    Maximum Flow

    for 3/4" baseboard is 4 GPM. The house heats very evenly right now.
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    edited March 2013
    where did you see that?

    can you point me to that information?



    By the way, I'm not recommending running 6gpm through 3/4 inch baseboard.  You would have 3gpm in each loop, with a total of 6gpm through the 1-inch common, boiler, and system pump.
    · ·
  • ced48ced48 Posts: 330Member ✭✭
    Not-

    both circuits will have almost the same flow, not each, 50 percent less. That is why the common pipe is oversized.
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    ??

    Not sure I understand what your saying.  You wanted 3.5 gallons through the boiler right?  That would be 1.75 gallons through each loop.
    · ·
  • ced48ced48 Posts: 330Member ✭✭
    No-

    I think 3.5 gallons GPM in the 1" will give you about 3.2 in the 2, 3/4" pipes. Isn't a question of volume?
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    No

    You are making a critical mistake.  (Probably won't matter in the end but this is quite an important detail to get wrong here.)





                                                 1.75gpm

      3.5gpm                      ------------------------                 3.5gpm

    >-------------------------- Tee                        Tee------------------------------------>

                                       -------------------------

                                                 1.75gpm
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    Think of a car analogy

    Suppose you have 10 cars per minute going down a two lane one way street.  The street forks into two one lane streets.  Some cars go left, some go right, but together, there must still be 10 cars per minute.  So something like 3cpm to left and 7cpm to the right.  Or 5 and 5cpm.  But certainly you can't have 8cpm and 8cpm.  That would imply cars were magically dropped from the sky onto the road.
    · ·
  • ChrisChris Posts: 2,869Member
    Here's The Possible Problem Eastman

    First the boiler pump is going to operate at is given curve based on the head it is coming against. Second, the rise in the boiler is going to be very small and he is going to get very limited time in actual condensing mode unless he puts a ton of emitter in the spaces to scrub the btu/hr being delivered boiler. His best bet to gain the most in efficiency is to size the boiler pump based off the minimum flow and head requirement from the boiler mfg which is generally a 35 rise and pipe with a LLH and then size his system flow based on a 20. Let's leave dollars on the sideline and talk about best practice.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    I disagree

    Best practice in this case is to *not* use a primary/secondary configuration.  The deltaT is not the main driver of efficiency for this system.  He can set the flow rate to whatever he desires using any number of common products.
    · ·
  • ChrisChris Posts: 2,869Member
    edited March 2013
    Sorry But Wrong

    You want him to move 6gpm. He would never get in condensing mode until he sees a minimum of 140 degree supply water temp providing his CO2 is a constant 7. Dew point changes. If he sizes his boiler pump for a 40 Rise and his system side for a 20 he has the best chance of removing boiler btu/hr created thus giving him the lowest return water temp at a much higher system supply water temp as long as he is using a LLH. He just needs to make sure he has enough emitter for the reduced supply water temp at design temp.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
    · ·
  • tom3holertom3holer Posts: 45Member
    edited April 2013
    here are some numbers

    Hi,

    I want to thank you all for the help,I am learning a lot and really enjoying learning how to hopefully be able to set up my system to operate efficiency.



    I am currently in Paris, France, not Maine, but have been to the one in Maine also, so do not have all the numbers with me. I will be home on this comming Tuesday.



    The downstairs rooms (3) that I have been using as a sample all run off the same thermostat. The total slant/fin length is 46'. There is another, I am just guessing here,  50 or so feet of insulated piping associated with the zone. The total heatloss at design temp is 17,500. As I read the S/F baseboard chart I will need a temp of about 150 to get the needed heat. So is my reasoning right that if I need 17,500 btu's of heat for a 20* D/T that is about 1.8 GPM?

    At an oat temp of 40* the heatloss is 9700 btu. So does that mean I will need less than 1 Gpm ? That doesn't seem right although it is 2am here.

    As always any thoughts or suggstions I look forward to.
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    how about we flesh that out...

    Hypothetically, let's say the system needs 160 degree water with a 20 degree delta on the coldest days we're anticipating.  A 20 degree delta across the secondary loops implies they're returning 160 - 20 = 140 degree water.  Now if the primary loop is operating with a 40 degree delta, the boiler must be outputting 180 degree water.  So the boiler is running at 180/140, and the secondary loops are running at 160/140.  Why not just dump the primary/secondary and lower the ODR curve by 20 degrees?  How is a boiler running at 180/140 more efficient than one that is running at 160/140?
    · ·
  • SWEISWEI Posts: 4,995Member ✭✭✭✭
    dump the primary/secondary

    only works if (a) the boiler HX can handle the required water flow and (b) the emitter system can lose heat fast enough.  In the case of a water-tube mod/con HX paired with low-temp radiant floors a conflict usually arises.
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    Here's another example:

    Suppose you're canoeing down a river.  You pass a sign that says this river has a flow rate of 350,000 gpm.   You continue down river until you approach a fork.  You go to the right.  There's another sign that says this branch has a flow of 120,000gpm.  What is the flow in the other branch?  It's 350,000 - 120,000 = 230,000 gpm.  The two branches together must add up to 350,000gpm.



    You're probably thinking of velocity.  (feet per second) 
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    edited March 2013
    Yeah, but

    Ced48 is a real person with existing fintube emitters.  There is very little hydraulic resistance.  He's posted his system specs earlier a few times.  He wants to use a firetube boiler and install it in a closet.
    · ·
  • SWEISWEI Posts: 4,995Member ✭✭✭✭
    don't get me wrong

    I'm a huge fan of direct-pumped firetube mod/cons.  Just need to me absolutely certain that both source and sink are sized using one set of assumptions.
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    ?

    What assumptions are you referring to?
    · ·
  • SWEISWEI Posts: 4,995Member ✭✭✭✭
    can't reply to threads having titles that begin with punctuation marks

    Assumptions = ∆T in this case, on which both boiler and baseboard should be able to agree.  I'd probably install a Bumble Bee and see what I could get.
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    The forum doesn't allow a response to a ? mark?

    Really
    · ·
  • SWEISWEI Posts: 4,995Member ✭✭✭✭
    Can't click

    on the reply link to the right of the post if the title is ? or . or similar.  No problem replying to other posts or the thread in general.
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    Any thoughts on...

    my earlier response?  How is 180/140 more efficient than 160/140.  There's no zoning here so it's not like the total system flow is throttling up and down at all. 
    · ·
  • SWEISWEI Posts: 4,995Member ✭✭✭✭
    not much difference

    I can see -- other than a tad more pumping energy with the lower ∆T.  I aim for 30F with radiators when designing - we usually end up closer to 25F once the system is fully balanced.
    · ·
  • ChrisChris Posts: 2,869Member
    Eastman

    Haven't responded because Easter with the family. I will though. Be patient and enjoy your holiday with your family.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    this what i

    enjoy though
    · ·
  • tom3holertom3holer Posts: 45Member
    Another question for Chris

    Chris,



    I am having a hard time understanding some of what you are saying.



    "Your system flow rate can be designed for what ever you want it to but you really want high gpm on the system side low gpm on the boiler side. Size the boiler pump based on the flow and head of largest rise avail for that boiler and your system side for the standard 20 degree delta"



    If I take the largest zone I have which is 107' of baseboard using 150* water and a D/T of 20*, I come up with 4.2GPM.

    The Alpine manual calls for a range of 7.9GPM at 35* D/T to 13.8GPM at 20* D/T in the primary loop.

    What I don't get how you can get D/T's on the primary loop of 30-40* if you are returning 150* primary water and mixing it with 130* secondary water. That would mean boiler input water would have to be 120*-130*.I just don't see how you can have a higher D/T on the primary circuit then the secondary.



    I know I must be missing something here.



    Tom  
    · ·
  • bobbob Posts: 541Member ✭✭✭
    edited April 2013
    ∆T

    bob
    · ·
  • ChrisChris Posts: 2,869Member
    edited April 2013
    You Have to Grasp This

    Those are the flow rates that will get you the full output of the boiler at that given rise. Your boiler delta-t or rise is not a constant. It is a moving target. What isn't a moving target is the flow or gpm that moves across the heat exchanger.



    In the case below the boiler pump is always moving the stated flow rate based on the boiler rise you chose.



    7.9 x 35 x 500 = 138,250 Btu/hr

    13.8 x 20 x 500 = 138,000 Btu/hr



    If I run the 20 Rise flow rate (13.8) in your zone call example my boiler return would be 144 degrees. 13.8 x 6 x 500 = 41,400 Btu/hr Boiler output



    If I run the 40 Rise flow rate (7.9) in your zone example my boiler return would be 139 degrees. 7.9 x 11 x 500 = 43,450 Btu/hr Boiler output





    Both are predicated that you are running 150 Supply and 130 Return on your system side. Your piped pri/sec but you cannot by your zone need pull out the entire boiler flow. What you can't goes back to the boiler. That's why the 20 Rise in this example has a higher return water temp. You have more flow of 150 degree water going back vs the amount of flow in the 40 rise flow rate example.



    I will get to the condensing mode faster choosing the 40 Rise flow rate pump then you will with the 20 Rise flow rate pump. At a 140 supply 120 return system temp you would think you would be condensing right.



    Same zone flowing 4.2



    20 Rise = 134 Degree boiler return - Might condense

    40 Rise = 129 Degree boiler return - Will be condensing
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
    · ·
  • ChrisChris Posts: 2,869Member
    No No

    You can't change the parameters in the middle of the game. Plus who said I needed to run 180 degree water? Of course running less then 180 is more efficient not arguing that. You advise to run a 6 degree delta-t above not a 20.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    edited April 2013
    6 degrees? I feel like you are going in circles...

    Here is the text I was replying to...



    "You want him to move 6gpm. He would never get in condensing mode until

    he sees a minimum of 140 degree supply water temp providing his CO2 is a

    constant 7. Dew point changes. If he sizes his boiler pump for a 40

    Rise and his system side for a 20 he has the best chance of removing

    boiler btu/hr created thus giving him the lowest return water temp at a

    much higher system supply water temp as long as he is using a LLH. He

    just needs to make sure he has enough emitter for the reduced supply

    water temp at design temp."



    My point is there's no reason for Ced48's system to have a primary/secondary.  You seem to be advocating for one based on the grounds that it would be significantly more efficient.  Hence the following reply regarding that efficiency:



    "Hypothetically, let's say the system needs 160 degree water with a 20

    degree delta on the coldest days we're anticipating.  A 20 degree delta

    across the secondary loops implies they're returning 160 - 20 = 140

    degree water.  Now if the primary loop is operating with a 40 degree

    delta, the boiler must be outputting 180 degree water.  So the boiler is

    running at 180/140, and the secondary loops are running at 160/140. 

    Why not just dump the primary/secondary and lower the ODR curve by 20

    degrees?  How is a boiler running at 180/140 more efficient than one

    that is running at 160/140?"



    Let me clarify the above example:  The 160 system supply is the temp entering the fintube.  The 180 boiler output is the temp of the water leaving the boiler before being mixed down in the hydraulic separator.  Note that parameters are not being changed, this is simply an example of a condition where "boiler flow is larger then system flow."



    Take a look at the 40 Rise.pdf you posted earlier.  That document, once again, describes the condition when system flow in the secondary exceeds the flow through the boiler.  Notice that T3 is equal to T4, in other words, even though there is a pri/sec, the return water from the fintube is the same temperature as the water entering the boiler.



    From my perspective, the parameters are not being changed.  180 boiler output is the direct result of the other parameters. 



    My advice, in a nutshell, is to buy a pump that is at least capable of producing the target flow necessary to achieve 10 degrees.  Because that is a good starting point for a small fintube system.  If a target flow can be achieved to hit that delta, then there's no point in the pri/sec.  Just use the ODR to lower the water temps to maintain thermal efficiency throughout the seasons.



    30,000 btu/hr = 6gpm x 500 x 10 degrees delta.
    · ·
  • ced48ced48 Posts: 330Member ✭✭
    Well, Just to Get Back

    to my situation, Chris, if I install a variable speed pump, controlled by the boiler's computer, would it prevent the flow from every falling below the required minimum? And what will keep the pump from getting the flow moving to fast? Also, is the boiler's delta T pre -programed by the manufacturer, and, or , is it adjustable? (Lochinvar Knight)
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    everything is adjustable

    I wouldn't be surprised if you could even set two different deltas, one for the fintube, and one for the indirect tank.  (have to dig through the settings to confirm)



    I have to ask Ced48, what did you think of my earlier examples and analogies?  Are you still not convinced regarding the flow rates?
    · ·
  • ced48ced48 Posts: 330Member ✭✭
    I Still Think Flow Rates

    are based on the available volume in each of the two differently sized pipes. If the flow rate of the 3/4" pipes were half of the 1", what did we gain by upsizing the common pipe?
    · ·
  • EastmanEastman Posts: 618Member ✭✭
    Well...

    Let me rephrase what you wrote:



    If you have 1.75 gpm in one 3/4 inch loop.  And another 1.75gpm in the other 3/4 inch loop.  And then the two lines come together and go into a 1 inch pipe.  Then you are asking what is to be gained by having the 1 inch pipe?  What is gained is a lower water velocity in the 1 inch common.  If the common pipe was only 3/4 inch, the velocity (feet per second) would have to double in the common pipe.  By up sizing the common piping you are preventing higher velocities.



    Everyone here is implicitly assuming that when you say you want 3.5 gpm through the boiler, you are actually saying 3.5 through the boiler, hence 1.75 through each of the two even loops.  For example: here's Paul48's response to you earlier:



    "Ok.  Roughly 1.7 per split. Just wanted to make sure it wasn't an arbitrary number."



    Maybe I don't understand the layout correctly, but I assure you, the system as I understand it would function as I have described. 
    · ·
  • ced48ced48 Posts: 330Member ✭✭
    What I am Saying

    is that water traveling thru 2, 3/4" pipes will move at roughly the same rate as the water moving thru 1, 1" pipe, pushed by the same circulator, 12 percent less to be exact. maybe we are taking about two different things. I am not taking about pressure drop, or head. They will be divided in half.
    · ·
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