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Replacing old oversized piping

dineshj Member Posts: 17
I have a natural gas hot water boiler (not steam). The old boiler used to heat a two family home. Now, the second floor have their own forced hot air furnace. So the radiators and the pipes going up to the second floor were removed and the new boiler (5 years old) only heats the first floor. The newer (smaller) boiler runs a lot because there's a lot of mass (lot of water) that needs to be heated. I was thinking of replacing the old pipes (50+ years) and installing smaller sized pipes if it makes sense to do it.
There's mainly two sized pipes that are in the system. I measured the sizes and below are the sizes and length of the pipes.
2.39 inches Outside diameter. length 21 feet supply + 21 feet returns.
1.67 inches Outside Diameter. length 12 feet supply + 12 feet returns.

I am thinking the new pipes would be about half of the current size.
Does it make sense to replace the pipes with smaller sized pipes?


  • Jamie Hall
    Jamie Hall Member Posts: 22,952
    Not unless they are leaking. Reducing the amount of water won't have an effect of how long, in total, the boiler runs -- that's determined by the power of the boiler vs. the heat loss of the house. What reducing the amount of water might do, though, is make the boiler cycle on and off more frequently. Which is not a good thing.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • GGross
    GGross Member Posts: 974
    It does not make sense to replace the pipes with smaller pipes. What is your concern with the current systems functionality?
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,606
    edited June 2022
    There have been at least 3 boilers in that basement over the years. The first was probably gravity hot water, hand fired (coal) by the looks of the original system pipes. The next boiler had a circulator included. That is why the pipe is reduced to a smaller diameter iron pipe and fittings. the clue about the third boiler is the transition from iron pipe to copper. that is how the 3rd boiler was connected to the second boiler's smaller iron pipes.

    All this is not going to answer @dineshj's question about the pipe size. But I like to look at old buildings and imagine what the old heating plant may have been like. Sometimes you can see the old basement window where the coal was delivered, where the coal storage bin was located, and where the original boiler's fire door was located in proximity to the coal bin

    @Jamie Hall has a good point about making the pipes smaller. After you do all that repiping, you may end up with short cycling. But you can fix that by adding a buffer tank in order to increase the water volume of the system. OR... You can leave the large pipes there and use them as the needed water volume and save money on the buffer tank.

    Oh Yea... You can save money on the replacement pipes too.

    Yours truly,

    PS: I can see myself with a sore head after hitting my head on those pipes

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    EBEBRATT-Ed Member Posts: 15,346
    If they are not leaking leave, it alone. Replacing them just because will only cost you money and won't save a dime
  • dineshj
    dineshj Member Posts: 17
    Thank you all for the helpful comments. The current boiler is condensing because of the water return temperature below 140 for too long. But if it doesn’t make sense, it’s not worth it.
    @EdTheHeaterMan I do have a window and two original doors near the heater. One of the doors was closed and sealed over the years. 

    The previous boiler was a Hydrotherm boiler with circulator probably from the 60s-70s. 
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,606
    If you are experiencing Flue Gas Condensation, then a bypass might be in order. Here is a page from an old boiler IO manual (when circulators came installed on the return lines in the 1960s). I recommend figure 15 for your situation.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • dineshj
    dineshj Member Posts: 17
    Hi @EdTheHeaterMan
    I do have a full size bypass on the system near the boiler. But I think there’s condensation because the return water temp stays below 140 for a longer time. At night, I leave the thermostat at 4-5 degrees below the desired temp in the morning. And during the day, I leave the thermostat at one desired temperature throughout the day. 
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,606
    edited June 2022
    As long as the bypass is allowing as much water as it can to short circuit back to the boiler return, then there is nothing else to do. But take a close look at the piping and circulator location. Figure 14 is to keep the system temperature from getting too hot. Figure 15 is to resolve flue gas condensation. The circulator is in a different location. Just that subtle difference can make all the difference in the world.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • dineshj
    dineshj Member Posts: 17
    thank you for the information. Here’s the picture of the current setup. 
    The right side is supply and the left is return. 
  • hot_rod
    hot_rod Member Posts: 21,882
    Bypass piping or pumping is a guess at best. They work only under the one condition they are set to.
    The best way to protect a boiler is with a return protection device that can sense and respond to temperature.

    A very simple 100% efficient way is to use a thermostatic boiler return protection device, basically a high Cv three way thermostatic valve with a fixed temperature element. They are used commonly on wood or pellet fired boilers or boilers connected to buffer or large volume piping like you have.

    I did a demo of these valves for a wholesaler in NJ today and they placed an order for stock. A boiler manufacturer had been supplying a cast iron version, not quite as engineered as this brand.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • JimP
    JimP Member Posts: 86
    One way to get the impact you were looking for in smaller pipe would be to insulate those big pipes. If you don’t want the heat in the boiler area it might help you. It might be kind of expensive for the amount of gain you get. I’d only do it if you want that boiler area cooler.
  • mattmia2
    mattmia2 Member Posts: 9,469
    I see 2 valves with the handles removed that look wide open. They are probably from laarz's manual, setting them may be covered there. They can probably be used to divert more to the bypass to keep the boiler temp up. You want to keep the water in the system cooler to keep all that mass from overshooting after the thermostat ends the heat call.
  • dineshj
    dineshj Member Posts: 17
    @mattmia2 So you are saying when the boiler starts up, I should close (not all the way) the return water valve on the left pipe. So more of the heated water goes back to the boiler?
  • mattmia2
    mattmia2 Member Posts: 9,469
    edited June 2022
    I would look at the manual and see if it explains it. It looks like they are following some sort of manufacturer's diagram.(and possibly didn't understand it)

    But closing the valve on the left some should increase the re circulation and reduce the circulation to the system.
  • clammy
    clammy Member Posts: 3,094
    The wiser thought would be to have piped it primary secondary and install a thermic valve ensuring that return water is above flue gas condensation point and have a secondary pump installed on the system supply . A magnetic separator and a good air elimator would have been nice also . Just about all the mini mights and just about bout all low mass I ve every seen are always missing a by pass ,primary secondary piping and a flue liner it all seems to go hand and hand unfortunately , being a well planned out installations don’t seem to happen much when shopping the lowest pricing and looking for bargain . It s nothing new like the sun rise and set it happens daily un changed and occurring daily and never getting resolved until replacement and then once again over looked . Gotta either love the business or hate it . Lol
    Peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • heatdoc1
    heatdoc1 Member Posts: 14
    I agree with Clammy. with that boiler on a high mass system primary secondary would be the best bet to eliminate the condensing and you will get a more even heat distribution as well.
    John Ringel
    Senior Technical Support Specialist
    Energy Kinetics
  • mattmia2
    mattmia2 Member Posts: 9,469
    See page 16 and 18 for the description of the bypass. There are better options but what you have can probably be adjusted to work: