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Pipe sizing

dante421
dante421 Member Posts: 13
Hello, 
   I bought an old farm house (1760) that had no heat installed. There is an oil furnace and a bunch of various sized cast iron radiators spaced throughout the house. I did some quick calculations and placed the radiators around the house to accommodate each room properly (or so I think) I am very new to dealing with cast iron rads or heating systems in general but I’m learning. The house is a two story maybe 1500 sq ft. I am going to do 3 zones. One for upstairs which I am just using the standard baseboard heating as I don’t have enough rads and it’s very small up there. Then I am splitting the downstairs into two separate zones due to the layout of the house and a fireplace being on one half to help heat that side. Each zone will have 4-5 radiators of various sizes. All around 2’ tall x 3’ wide x 6” deep.  I know I should provide more info as far as sizing but I am not at the place and would like to purchase all material and have in hand my next trip up. 
    So, my question is.... if I am doing a 2 pipe reverse return for each of the 2 downstairs zones, is 1/2” oxygen barrier pex suffice? Any help or advice is greatly appreciated!

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 22,941
    Ideally you would calculate the flow to each radiator -- which involves the heat you want the radiator to put out and the temperature of the water being supplied -- and figure your pipe sizes based on that.

    In my own opinion, and others may differ, you may find that to get the heat output you want in colder weather that the required temperatures will push the temperature limits of PEX -- of any size. If you install it in troughs, rather than just hang it, you can go higher, but... hmm.

    Reverse return is definitely the way to go, and I suspect -- without doing the numbers -- that the feed and return lines could probably be half inch copper, but the mains will need to be three quarters. Just a guess at this point.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,594
    edited March 2023
    Based on the year built and the 1500 sq ft. floor plan, Im thinking that you may need the near boiler piping to be at least 1" for the shared piping. Once you tee off one zone than you can reduce to 3/4".

    If however that 1500 sq ft has had insulation upgrades and window upgrades to make the home more energy efficient 1/3" might be fine.

    Here is a textbook that I used for teaching my one day seminars on Hydrionics. It will help you understand pipe size requirements after you read the first 3 pages. There is also some "Rule of Thumb" charts for selecting the proper pipe sizes.

    https://documentlibrary.xylemappliedwater.com/wp-content/blogs.dir/22/files/2020/01/FH-Z100B-BG-Zoning-Made-Easy-2.pdf
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,344
    First step is a heat loss of the house. Then compare that to the radiation you have. Then determine the water flow and water temperature you need you need to offset the heat loss.

    skip a step and it may or may not work
    mattmia2
  • dante421
    dante421 Member Posts: 13
    Thank you for the responses,  a couple of you said that the near boiler piping should be larger. Right now, out of the boiler the 1 1/4” or may be a little bigger feeds four seperate zones. One goes to the HWH and three more to the house heat. All of these get reduced to a 1” line with a valve all with separate circulators. Is that what you are referring to when you say near boiler piping? Can I reduce after the valves to 1/2” or whatever is decided?
       I did the heat loss calcs as best I could and that’s how I came of with layout of radiators and sizes. I feel like if I set it up as I have planned and it doesn’t get the house up to temp or the boiler is overworked, I can always add a radiator. I’m sure this isn’t exactly the correct way to do things but we are on a pretty tight budget and trying to use what was left laying around at the house
        Ed, thank you for the link. I will be reading it tonight. Hopefully that will help also. Maybe I should draw out a little diagram and post it on here to help understand what I have going on
        Again I appreciate all the help!
  • Ed, thank you for the link. I will be reading it tonight. Hopefully that will help also. Maybe I should draw out a little diagram and post it on here to help understand what I have going on
    Good idea! It's easier to correct things on paper before you do the work.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • hot_rod
    hot_rod Member Posts: 21,872
    Still need to know the load, or boiler size to accurately size the piping. I think 3/4 to heat zones, 1" to the indirect.
    Cast iron boiler? Are you zoning with valves? TRVs on the radiators might be nice.

    It could be this simple, delta P circ, zone valve for fin tube, 1" zone valve for indirect, TRVs on 2 radiator circuits.

    Or zone valves for the radiators on a central thermostat.

    No harm inn the piping at the boiler being 1-1/4 since we don't have a load number.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • I like the idea of manifolds to heat the radiators. One manifold on each floor with separate loops to each radiator. PEX tubing is rated to 180°F and you can certainly operate at that temperature, but in a perfect world, the radiators would be sized for 150°F. But I see your limitations. It would still be a good idea to do the heat calc's. and figure out the output of the radiators that you have. You will be able to see in advance which rooms will over or underheat.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • dante421
    dante421 Member Posts: 13
    This is a quick layout of what I am looking to do if it helps anyone. The downstairs over all sq is around 1050
  • dante421
    dante421 Member Posts: 13
    Alan, 
         So when you say sized for 150, that means radiators are better off being oversized where 150 degrees will still do the job? Also, I looked into the manifold set up. Just seemed like way mor work and material. I am on a tight budget and also trying to keep it as simple as possible. What are your thoughts on the 2 pipe reverse return?
  • hot_rod
    hot_rod Member Posts: 21,872
    I'm with @Alan (California Radiant) Forbes for a homeowner without copper or threading skills, the pex homerun is about as inexpensive and easy as it gets. Assuming you have good access from below.

    The living room could have the two radiators in series to save a homerun,
    same with the kitchen living. So maybe 5 homeruns. Down the road TRVs could be added for more room control.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • dante421
    dante421 Member Posts: 13
    Hot rod,
         I have easy access from down below via basement in most and crawl space in the rest. What are your thoughts of the 2 pipe reverse return to save time and money on doing all the extra pipe work required with your idea? 
  • mattmia2
    mattmia2 Member Posts: 9,467
    Reverse return or even direct return is fine, reverse return has little benefit with how small your zones are.

    Fitting direct or reverse return is probably more time of fitting all the pieces than just getting some multiple tube supports for pex and running a pair of lines back to the manifold for each emitter.

    Remember copper expands a fair bit with temp and pex expands a lot. You have to work such that it has someplace to go when it expands.
  • dante421
    dante421 Member Posts: 13
    Ok. Thanks for the input.... if I use The manifold set up I’m pretty sure I could definitely use half inch pex....., if I do a direct or reverse return, would 1/2” pex suffice for that also?
  • hot_rod
    hot_rod Member Posts: 21,872


    The main level is two zones?  Without a heat load, I’d use 1” Pex if your are looping 2 or 4 radiators on one circuit

    so the trade off is the cost of tube and fittings between home run and series loop

    assuming you are not paying labor cost 

    home run gives you unlimited zoning options, each radiator could be balanced or zoned later

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • dante421
    dante421 Member Posts: 13
    Yes, main level is two zones. I have a fireplace that will be getting a lot of use on one half so wanted to keep that off the zone for the kitchen and living room. I have a third zone doing the upstairs but I will be using the regular baseboard up there. Labor is all me so that’s not an issue, however, time is. Really trying to get this place up and running. So you really think 1” is necessary for just a few rads on a circuit? 
  • hot_rod
    hot_rod Member Posts: 21,872
    1" if you use pex, the ID is not much larger than 3/4 copper, and insert fittings drag down flow capacity.

    3/4 copper would probably be adequate. Have you priced copper tube?
    Press fittings if you don't solder, but those too are $$

    I think that is why we are steering you to 1/2" pex home run. Two compression adapters per tube.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream