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Will removing large cast iron pipes increase system efficiency?

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DJDrew
DJDrew Member Posts: 89
edited December 2019 in THE MAIN WALL
Hello,

My wife and I were talking about getting some headroom back in the basement by removing the old 2" cast iron piping and re-piping/rerouting with smaller diameter copper or switching to pex-al-pex home runs in the basement. I have seen threads on this site that talk about how people have successfully implemented such changes, but my wife and I are wondering, other than headroom and comfort via better possible balancing with individual pex runs, would it lower our gas bill?

Doing some math, we have 122 feet of cast iron pipe running like a water freeway around the basement. Using online calculators, that is almost 20 gallons of water in just the basement pipes before the volume for the boiler and radiators is added up.
Switching to 1-1/4" Copper would reduce that down to ~8 gallons.
Switching to 1/2" pex-al-pex home runs would increase piping to ~549', but take the water volume down to ~5.5 gallons.

As we look to justify the cost of all this work, would having to heat less distribution water make the system more efficient and thus save us gas? I am torn if the water heated in the large pipes right now is really "wasted" - as I'm sure there is natural ghost flow and the warm water is naturally rising & circulating after the heating cycle ends.

My wife and I like the idea of more head room and that the home runs will really help balance the system... but is there likely to be a payback on the gas bill? Or will we still use roughly the same btu's worth of gas and this is just a project that we have to determine if the cost is worth the value in improved personal comfort and headroom?

Any thoughts are greatly appreciated!
Leon82

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 23,675
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    From the standpoint of heat loss from the pipes, you will make a small gain in efficiency if the pipes aren't insulated. The gain will be small, however, as there just isn't much surface area there to begin with. There is no gain to be had by changing the total volume of the system. Most of that heated water isn't just sitting there, after all, when the system is running -- it's all circulating. Even if the total heat in the system were lost every time it stopped circulating, which it isn't -- it goes into the building envelope where it is useful -- how much heat, really, is there? If you switched to PEX, you would save 15 gallons, which is roughly 100 pounds of water. If that water started at the end of a heat cycle at let's say 160 F and dropped at the beginning of the next to let's say 100 F, that's a delta T of 60 degrees in 100 pounds of water -- 6,000 BTU. At current prices for oil somewhere around 10 cents worth of oil... it would be similar for gas.

    Bottom line. No economic case can be made.

    On the other hand, there is something to be said for gaining headroom...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    DJDrew
  • JUGHNE
    JUGHNE Member Posts: 11,111
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    Once that water is heated by your boiler and in your pipes, regardless of size, it is within your building envelope and heating your basement or floor. If you have a cast iron boiler without an automatic flue damper some heat would be lost up the chimney if the water circulates by gravity thru the boiler. But we are talking a small amount.

    As Jamie said the head room and system balancing are a positive item. But how will you heat your new usable space in the basement after you get rid of those large pipes?
    DJDrew
  • SuperJ
    SuperJ Member Posts: 609
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    If you have some rooms that currently overheat, you will be able balance things out better with home runs and thus save a bit by not overheating.

    Also if you go primary, secondary with your home runs, you could run constant circulation and possible add TRVs for even more comfort and room by room control. If the heating is pretty even, TRVs may not be worth considering.

    The boiler would control off a stat placed in one of the cooler rooms and inject heat into the constant circulation loop. The TRVs would keep the the warmer zones from overheating.
    DJDrew
  • DJDrew
    DJDrew Member Posts: 89
    edited December 2019
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    Thank you, all this is very helpful as we weigh the decisions and set expectations should we make any modifications.

    @Jamie Hall you were very mathematical in your assessment, thank you!
    @JUGHNE Right now, even though the heat from the big pipes is probably minimal, the current arrangement keeps the temps fairly comfortable, especially once a carpet pad and carpet is added. We recently insulated the basement block walls too, which helped. There is an old trash-burning fireplace from the rural days that we have considered just putting in a gas insert in to bump up the temp when needed.
    @SuperJ If you ever find yourself in Pittsburgh, we'll have you design and modify. I like your thinking.

    Cheers, all.
  • retiredguy
    retiredguy Member Posts: 931
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    I guess that there are not many contractors or others that use "mono-flo" tees any more. (go steelers)
  • GW
    GW Member Posts: 4,743
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    Retired guy, why use that method? That ship left the port a long time ago

    Anyway, head room is a perceived and actual value, chop the old pipes if you want real living space. Sorry for my math less answer 😄
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    gary@wilsonph.com
    SuperJIronmanDJDrew
  • ch4man
    ch4man Member Posts: 296
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    all you'll gain is some headroom IF it's done right. if not done right there will be no gain and maybe a loss on the important stuff. you know like heating well, air control, flow issues and the like.
    i have seen this done before and the house was butchered. and ive seen it done right. have a care, the entire system was designed to work as it sits. sometimes its better to either leave things alone or a total upgrade when all components work with each other.
    if you proceed. do it right. it can be done
    good luck.
  • mattmia2
    mattmia2 Member Posts: 9,981
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    Keep in mind you will reduce the total system volume so you will shorten the cycles somewhat, heating that water and taking the heat out of that water helps buffer the on/off nature of the burner and helps even out the heat in the system.
    IronmanDJDrew
  • GW
    GW Member Posts: 4,743
    edited December 2019
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    yes indeed
    Gary Wilson
    Wilson Services, Inc
    Northampton, MA
    gary@wilsonph.com
    DJDrew
  • DJDrew
    DJDrew Member Posts: 89
    edited December 2019
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    mattmia2 said:

    Keep in mind you will reduce the total system volume so you will shorten the cycles somewhat, heating that water and taking the heat out of that water helps buffer the on/off nature of the burner and helps even out the heat in the system.

    Interesting. So right now our cycles are 35-75 minutes in length based on outdoor temperature. Usually about 4-7 cycles per day.

    If by reducing the water volume our overall cycle run-time will shorten shorten, but then I'd assume that the amount of heating cycles would then also increase? As the same BTU requirement will still exist...?

    I did an experiment recently by adjusting the differential setting on the thermostat from 2 degrees down to 1 degree. The overall cycle lengths were shortened by about 30-40% since water wasn't allowed to cool down as much. However the number of overall cycles increased to ~16. Gas usage seems almost equal. I ended up switching the differential back to 2 degrees as it seemed like double the wear-n-tear with the flue damper and igniter when the differential was lower.