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Carrying capacity of copper pipe

John S_2
John S_2 Member Posts: 29
If 3/4" copper only carries 40,000 BTU @ 4 gpm, how does 3/4" baseboard heat a house with a heat loss of 100,000 BTU/hr? I recently repiped a boiler (changing it to primary/secondary and adding outdoor reset control) and the question came up because another company proposed replacing all the baseboard to address insufficient heat call-backs. I said the problem was in the near-boiler piping. All the flow from two separate zones had to go through one 3/4" tee. The boiler was short-cycling on temp (it's over-sized too) because it couldn't move the Btus out into the system water. But even though I changed all the near-boiler piping it still ends up leaving the boiler room through 3/4". Did I only increase it from 20,000 btus per zone to 40,000? I don't think so because I've done this before and I've seen it improve comfort levels/heat distribution in the home. I just don't understand the engineering well enough to articulate the reasons why.
in vino veritas, in cervisia carmen, in aqua E. coli

Comments

  • icesailor
    icesailor Member Posts: 7,265
    Pipe capacity:

    The saying: "Pipes increase as do their squares". That means that it takes 4-1" pipes to carry the same volume of a 2" pipe.

    You can put the contents of a 30 gallon water heater into a 50 gallon water heater. But you can never put the contents of a 50 gallon water heater into a 30 gallon water heater. A 50 gallon water heated and full of 150 degree water has more stored energy than a 30 gallon water heater with the same temperature water. Then, as the water flows through the heat emitters, it looses heat. The amount of heat is related to how much water goes through and heat is lost. Because there is less water and less available heat to loose, the 3/4" pipe will not be able to carry the load and the water will return colder than it should. There are articles you can read. You sound like you need some reading. There are good books here that will explain it.  
  • Techman
    Techman Member Posts: 2,144
    BTU's

    What are the dimentions of the house? Tight construction ,medium , crummy?
  • John S_2
    John S_2 Member Posts: 29
    Not a heat loss problem

    It's a delivery problem. House is on a slab w/flat roof, probably 50' X 25' or so,... didn't measure it as the boiler was so over-sized already (175,000 net btu/hr) and it was more of a distribution issue (IMHO). I reversed the flow in Zone #1 (Main) because the living room was where the occupants complained of being cold (large windows and it was the last strip of baseboard in the loop) and the loop started in the kitchen (where the fridge and cooking added heat already). Now the kitchen is last and the living room is first. I changed the near-boiler piping to 2" and 1.5" from 1". The original installer had reduced from the 2" supply port right down to 1" and from the 1.5" return port (in the boiler) to 3/4". That's what I saw as the real problem. Not the old (and slightly banged up) baseboard that the other company wanted to replace (for twice what it cost for me to re-pipe and change to P/S and instal outdoor reset). Replacing the baseboard wouldn't have made much difference if the return was 3/4" @ the boiler. The house is kind of loose construction. It reminds me of a Levitt house (except for the flat roof).
    in vino veritas, in cervisia carmen, in aqua E. coli
  • John S_2
    John S_2 Member Posts: 29
    reading

    I have (and have read) most of Dan's books already. Hence the change to primary secondary and attention to near boiler piping. The question I have is whether 3/4" baseboard can put out more than 40,000 btu/hr. Is it 40,000 per 100' of baseboard? Does it put out more if you go beyond 100' or less? Doesn't it depend on the temperature drop? Greater Delta-T would seem to indicate more heat loss and therefore higher btu output. Net output. All dependent on temp of the supplied water, temp of the air in the house and flow in gpm.
    in vino veritas, in cervisia carmen, in aqua E. coli
  • SWEI
    SWEI Member Posts: 7,356
    edited December 2012
    delivery options

    I take it this is a series baseboard system and not one with diverter tees?  In either case, were the emitters sized with this in mind (lower fluid temp as the loop progresses)?



    As to the pipe capacity, the 40,000 BTU @4 GPM assumes a 20F ∆T.  Unlikely you can increase the ∆T on baseboard but (depending on the loop length) you may be able to push a bit more than 4 GPM, but that will take a bigger circulator, which will consume more power.



    Any chance you can split the loop (tee a 1" supply or return in the middle)?  That's usually the easiest fix for a bad series loop design.
  • John S_2
    John S_2 Member Posts: 29
    splitting the loop

    I thought about doing that at first. No attic or crawlspace or basement though. Probably have to tear up part of the ceiling to run the pipe. The water isn't coming back cool though. I want to wait and see how the system performs with the changes I've made. I only finished repiping it last night. Plus it had other problems that might explain the call-backs the other company was having. The 007 was seized up the first time I was there. And they had turned the feed off because the relief valve was leaking. I assumed that was why because it started leaking when I opened the valve on the feed. Turned out the expansion tank had lost it's air cushion. I fixed those three things and the heat was working well. The weather was pretty warm though so I wasn't sure it would perform under extreme winter weather. Don't want to split the loop unless I really have to. Your answer makes the most sense so far though. Thanks. 
    in vino veritas, in cervisia carmen, in aqua E. coli
  • Techman
    Techman Member Posts: 2,144
    House BTU

    The house is 1250 sq. ft. so I think 100k is kinda large for that sq.ft.
  • John S_2
    John S_2 Member Posts: 29
    BTU rating for 3/4" baseboard

    According to this chart http://www.slantfin.com/images/stories/Technical-Literature/ratings_bareelements_r.pdf 3/4" element @ 3fps flow 180 deg water will give me 118,900 btu per 100'. So again I ask, what's the max btu I can get through 3/4" pipe? If I feed the baseboard with 1.25" does it change the output of the baseboard? I suppose if I feed it with .5" it would decrease the output, right? Suppose I have 1.25" on the supply side but I return it through 3/4"? Am I stuck at 40,000 btu/hr still?
    in vino veritas, in cervisia carmen, in aqua E. coli
  • Henry
    Henry Member Posts: 998
    3/4 capacity

    Normal design range for 3/4 copper is from 1 3/4 GPM to 4.5 GPM. You can increase the GPM to about 6 but that will increase the friction loss to 5 Ft per 100 Ft. 4.5 GPM will give you a bit over 55,000 with a delta T of 25F. 6 GPM will give you nearly75,000 at 25 delta T or 90,000 at 30 delta T. Is it a two story house or single story house? A ranch style here would only have a heating load of 45,000 BTU and a cottage would have twice the load.
  • DanHolohan
    DanHolohan Member, Moderator, Administrator Posts: 16,513
    Heat transfer

    John, I talk about maximum flows through pipes of a certain size, I'm looking at charts that show the max flow that won't give such a severe pressure drop as to push me into a larger circulator and possible velocity noise when it runs.



    When I talk about the most baseboard you should put on, say, 3/4" pipe, I'm really talking about the heat loss that takes place and its affect on average water temperature. For instance, if you start with 180 degrees at the start of the baseboard, about 70 feet (of active baseboard) later, you're at 160 degrees. You're still transferring heat, of course, but the average water temperature is dropping so there are fewer Btus per linear foot coming from the baseboard. The further you go, the cooler the water gets. You'll keep transferring heat as long as the radiator is warmer than the air in the room, but the people in the room probably won't notice.



    In the house you're describing, the average water temperature might be okay all the way around, or the piping to and from the radiators might be something other than one continuous loop, and that's what's making it work.



    As for feeding it with the larger pipe, the water will slow to whatever the smaller pipe will allow. It's like traffic.



    Hope that helps.
    Retired and loving it.
  • John S_2
    John S_2 Member Posts: 29
    it's a single floor

    a ranch-style but with a flat roof, the load is no where near to the 175,000 btu (net) in the boiler. I will likely get away with the 80,000 btu that two loops of baseboard give me. The way it was piped before (with both loops returning through a single 3/4" pipe) was probably causing the complaints. That and the issues with the circulator and air-lock caused by the feed being turned off and the leaky relief valve. I'll see what happens when it starts to get really cold out, I guess. Thanks for everyone's advice and info!
    in vino veritas, in cervisia carmen, in aqua E. coli
  • CMadatMe
    CMadatMe Member Posts: 3,086
    edited December 2012
    Capacity

    The answer to the question lies in the universal hydronic formula



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



    It also lies in the universal answer, it depends. We know the max flow rate for 3/4" copper is 4gpm but gpm can give you many btu outputs.



    4gpm = 40,000 /(20x500)

    3gpm = 50,000/ (30x500)



    So your btu/hr delivery is going to be based on controlling zone delta-t which can be done with a Taco Bumble Bee for instance. I can deliver 100,000 btu/hr through 2 zones of 3/4" pipe providing I can carry those 100,000 btu/hr to the zones through the main header.



    John Barba has a nice training video on the Taco Flo Pro Team.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • John S_2
    John S_2 Member Posts: 29
    I don't know,....

    you think I have enough capacity in the header?
    in vino veritas, in cervisia carmen, in aqua E. coli
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Looks Like

    1-1/4" pipe. Is it 1-1/4" all the way to the point where the zones take off? If so, yes.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • John S_2
    John S_2 Member Posts: 29
    it's 1.5"

    But I reduce down to 1" before the zone valves. 1.5" through the second circulator then it tees off to individual zones, then it reduces to 1" right before the zone valves. So I would need 2 more pumps to control delta T?
    in vino veritas, in cervisia carmen, in aqua E. coli
  • Henry
    Henry Member Posts: 998
    Does anyone use

    It is basic engineering! Does nobody look at basic engineering tables! This is a public forum which we can link basic standards from various websites,
  • Zman
    Zman Member Posts: 7,561
    What do you have?

    I have been reading this post and am confused. How many feet of actual baseboard do you have? Is it 3/4" single pipe (one pipe running through the rad)?

    Car;
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Gordy
    Gordy Member Posts: 9,546
    Henry

    Yeah I use system syzer, engineering tool box, rad pad etc. but I' m with Zman how many feet of base board etc. need all the facts before you start throwing numbers around only knowing part of a system out there. I think you know 6 GPM through 3/4" copper pipe is exceeding velocities recommended right?
  • John S_2
    John S_2 Member Posts: 29
    Yes

    I'm not looking to exceed the recommended velocity and I'm not trying to change the boiler out. It's less than 5 years old. I just wanted to get some of the btus that were trapped in the over-sized boiler (by the under-sized near-boiler piping) out into the living space. That and I wanted to show corporate how I can save them some money by switching a significant chunk of their properties to outdoor reset. Thus the re-pipe to primary/secondary and increase in size of the delivery system. The baseboard is original to the house. I'm not an engineer (and I don't pretend to be). My background is in commercial hvac and refrigeration. I spent the first 13 years of my career in residential (10 years with a "mom & pop" heating oil company and 3 years with a major heating oil company). I'm now the "in-house" hvac person for a large non-profit with multiple properties (+200) in the Mid-Atlantic region. Many of them are residential oil-fired hydronic properties. I appreciate any and all opinions offered here. I haven't yet measured the actual baseboard in the house we're talking about. That was a mistake. I still make them pretty regular-like. Try to learn from them. Here's what I saw when I first looked at this boiler.
    in vino veritas, in cervisia carmen, in aqua E. coli
  • Zman
    Zman Member Posts: 7,561
    Ok

    Ok,

    So this is more of a theoretical conversation? If you do not know the approximate length and output of the emitters on the loop how would you know the target GPM? Maybe you are pumping to much water? I think you need to figure out what your heat loss for the rooms being heated and compare that number to the available radiation in each room. It is also important to note which emitters are on the supply side of the loop and which are on the return. This will tell you what temp and how much water you need to deliver.



    Just because some knucklehead oversized the boiler, that doesn't mean you need to overheat the house, wear out the pipes, and spend a bunch of money on circulators.



    If you needed to go to the store for a gallon of milk and the only car in the driveway was a dumptruck, would you have to fill it with dirt for the trip? No, you would just drive it and hope to find a parking space.



    Carl
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • CMadatMe
    CMadatMe Member Posts: 3,086
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
This discussion has been closed.