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Hot Water Heat - Reverse return piping questions

housefix Member Posts: 7
Looking for help with questions for a hot water heating system with reverse return piping and cast iron radiators.

1) Can the pipe size of the branches and the main supply/return pipes be the same size? e.g. 3/4" mains with 3/4" branches leading to each radiator. Or is it recommended for the branches be a smaller size?

2) Do the total length of the supply main and return main need to be approximately the same length? e.g. can I have 75' of supply piping and only 50' of return piping without causing an issue and having the system remaining as self balancing?

3) Do you typically install a shutoff valve at each radiator? Is it installed on the supply inlet side?

4) Whats the typical copper pipe size used at the inlet and outlet of new 10-20 section 4 column cast iron radiators?

Some background for context....
I'm installing a completely new hot water system for a 3 fam house (2 story, w/ 1000 sqft per floor). Each unit will have its own boiler and reverse return piping. 1st FL is one unit and will have 8 Pensotti wall panel radiators installed. Piping will be copper with 3/4" mains with 1/2' inch branches. 2nd FL will have 2 units (500 sqft per unit), each with its own boiler and pipping and standard cast iron radiators will be installed. My plumber isn't familiar with reverse return piping. He's experienced piping serial systems, mostly for baseboards and cast iron radiators but is willing to try piping reverse return. He wants to use 3/4" pipe for the branches and mains on the 2nd fl. Most examples I found have a smaller branch size vs the main -- will this be a issue? I would like to confirm pipe sizes and lengths seem generally ok.

Are there any other design issues I should consider or bring up with the plumber for a reverse return system? Like... is determining and setting the flow rate gpm the same in a serial system as in a reverse return? Water temp will likely be set at the standard 180 degrees for every boiler. Assume boiler and radiators BTU are properly sized.


  • housefix
    housefix Member Posts: 7
    No takers on the questions?... Key questions is really #1 and #2 above.
  • Paul Pollets
    Paul Pollets Member Posts: 3,419
    Supply risers are sized to the requirements of the radiator load. A 1/2" pipe can handle 12K btu's, a 3/4" pipe can handle 45K btu's. The main needs to be sized to the total btu's attached. TRV's are often installed on the supply into the radiator. It doesn't matter how long the mains are. If the mains or the radiator are undersized by the piping attached, the radiator(s) will not heat properly. It is the responsibility of the contractor to get it right. Modern design strategy often uses condensing boilers so that the radiator will heat effectively using lower boiler water temperatures. This saves energy and provides lower operational costs.
  • SuperTech
    SuperTech Member Posts: 1,670
    I did a zone of cast iron radiatiors in my basement. I used 3/4" main and 1/2" branches. B&G monoflo Venturi tees. I don't think I've ever seen a diverter tee that's 3/4"X3/4". As for the ball valves, as long as they are full port I don't see a problem.
    EBEBRATT-Ed Member Posts: 9,690
    The whole idea of reverse return is that the system is more self balancing so the supply and return pipes are basically close to the same length.

    But it's not rocket science. If the reverse return is correctly piped it will be ok.

    As far as pipe size obviously too small a pipe for the flow will result in a higher pressure drop, high velocity in the pipe, possible noisy flow and may require a larger pump.

    Oversized pipe presents no issue except for
    1.added cost over smaller pipe
    2. possible difficulty venting and removing air (not enough velocity to move the air to the air vents)

  • housefix
    housefix Member Posts: 7
    Thanks for the info.

    @Paul Pollets - Good to know. the total BTU requirement required for each 2nd FL unit is only ~19k total, so the 3/4" mains will be more then sufficient. Max BTU on a single radiator is 8k

    @EBEBRATT-Ed - Do you know if that means over-sizing the branches at 3/4" with 3/4" mains won't be a problem with a reverse return system other then the added pipe cost and possible venting issues? I was concerned about the 3/4" return not being able to handle the return volume of water from four 3/4" branches to the radiators.

    Based on the replies it sounds like I should insist on 3/4" mains and 1/2" branches for the cast iron radiators?

    My plumber wanted to put in 3/4" pipe throughout since that's what he does when he puts in a serial system and said 1/2" was too small -- which looks like its certainly true for serial. Total pipe run for supply and return mains for each 2nd FL unit is probably about 170' for one unit and 130' ft for the other -- each with its own boiler.
    EBEBRATT-Ed Member Posts: 9,690
    On the pipe sizing it's hard to say. The first thing is

    a heat loss of the space
    then size and place the radiation
    then draw the piping
    size the piping based on the flow in each section
    then size the pipe

    If you sikp a step you can't get the answer

    1/2" pipe=1.5 gpm=15000btu/hr
    3/4" pipe= 4.5 gpm=45000
    1" pipe=8 gpm-80000

    You have to figure out how much water goes through each section of pipe

    so if each rad is not over 15000 btu you could use 1/2" for the branches and 3 radiators could could be on a 3/4 main.

    1" main can handle 5 15000 btu radiators