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FHW plumbing issue

RecruiterRecruiter Member Posts: 20
I have a 110 year old multi family house that has asbestos covered FHW supply and return lines feeding radiators on the first floor. (BTW: the asbestos is already encapsulated). It appears to be a two-pipe direct return sytem. The previous owner uncovered the 1 1/4" pipe, to a fitting, then connected copper reducers down to 3/4" in order to install a Slantfin 4 ft. baseboard radiator in the bathroom. Due to major renovations that are needed in the bathroom, I will be replacing the baseboard heater, since it was damaged during the renovations and the copper connections now leak at the fittings.

I am looking to replace the copper with Pex.

1. Is the original 1 1/4" piping black pipe? 2. If so, will I be able to use a black pipe 1 1/4" to 3/4" reducer directly to a Pex brass connector without an issue of corrosion or electrolysis? 3. What would I use on the threads? Is tape OK? 4. Do I need to use a balancing valve, and if so what type and in what configuration.

We live in RI, USA.
We purchased this house in 1999, complete with all the former owner's mess-ups, which we have been trying to fix, one project at a time.
This baseboard unit is the last unit on the loop. I am not able to tell if there was originally a cast iron radiator like the rest of the apartment, replaced by this baseboard heater, or if he simply decided to tap into the line and add a new baseboard unit to it. More than likely, it was the first situation. Now I'm stuck trying to figure out how to work around it.


  • BrewbeerBrewbeer Member Posts: 611
    Can you post some photos of what you are describing?
    Hydronics inspired homeowner with self-designed high efficiency low temperature baseboard system and professionally installed mod-con boiler with indirect DHW. My system design thread:
    System Photo:
  • SteamheadSteamhead Member Posts: 13,998
    This sounds like an old gravity system that now has a pump at the boiler. If I'm right, there was a radiator there originally, and the baseboard never worked well because of the reduced pipe size.

    You can still get radiators. That's how I'd handle this, complete with proper pipe sizes. PEX might not be a good choice, since the fittings reduce the flow thru the pipes. In this type if system, any extra resistance in the pipes can completely kill the flow to that rad, because all the other pipes would then offer much less resistance and the water would go that way.
    All Steamed Up, Inc.
    "Reducing our country's energy consumption, one system at a time"
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  • RecruiterRecruiter Member Posts: 20
    I checked this out, again. Steamhead, you are correct, it is an old gravity feed system. The boiler was replaced several years ago, and it now has circulator pumps installed.

    I actually found the 2 original fittings that were removed to install the baseboard heater. Curiously, it is a 1 1/4" Tee (capped at the end) with a 3/4" leg. When I checked out all the other radiators in the apartment, they are all running on 3/4" pipe.

    At first I thought there may be a 1 1/4" loop somewhere, but it's not the case. There are 2, 1 1/4
  • IronmanIronman Member Posts: 5,815
    "Recruiter"? I've been looking for you for over forty years!
    Bob Boan

    You can choose to do what you want, but you cannot choose the consequences.
  • RecruiterRecruiter Member Posts: 20
    Sorry, I didn't realize my last post didn't go up completely.

    My system has 2, 1 1/4" distribution lines. Each radiator has a 3/4" supply and return pipe. One to each large pipe.

    If as had been suggested, I would be choking the heating system by stepping down the pipe size of the Slantfin down to 3/4", what is the difference with what had been installed over 100 years ago, with what is being done now?

  • Jamie HallJamie Hall Member Posts: 13,439
    The biggest problem with converting old gravity systems to pumped is in getting an even flow distribution. Odd pipe sizes don't help -- keeping in mind that the head loss is inversely related to the square of the pipe size (that is a 1" pipe as 4 times the head loss of a 2" pipe).

    If that is kept in mind, and some means of balancing the flows is provided on all the lines -- such as ball or globe valves (NOT gate valves!) -- you can make it work.

    Or you can follow @Steamhead 's suggestion!
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • Eric_32Eric_32 Member Posts: 264
    I agree with Steamhead. The biggest issues with that IMO.. are the much smaller pipe tapping to the baseboard vs the cast iron radiators.. and 2nd... the mixed type of heat emitters on the same zone..

    Mixing cast iron and copper fin tube baseboard on the same zone will never work right. They heat at different rates. The cast takes a long time to get warm and they stay warm for a much longer period of time.

    You would be better off with either making a separate zone with it's own thermostat, to keep the fin baseboard. Or go back to cast iron in that room.
  • RecruiterRecruiter Member Posts: 20
    Thanks Eric
    Now it all comes together. Its not so much the mixing of cast vs. copper vs. Pex. Or even so much as the sizing, as long as it is consistant throughout the system. It is more a case of how fast the two systems would heat up and cool down that is more likely to mess up the system.
  • Eric_32Eric_32 Member Posts: 264
    Yes the room with the copper fin will always feel colder being on the same zone with cast iron radiators or baseboard....Even if the pipe sizing was not an issue. You can mix them on the same system (as in your whole heating system) but not the same zone.

    I have zones of cast iron baseboard heat in part of my house, zones of fin baseboard heat in others, and radiant under floor heat in rooms with tile.. my kitchen and baths... they all work fine but run off separate thermostats.
  • RecruiterRecruiter Member Posts: 20
    Gotcha! Thanks Eric. Guess its time for me to check in on some radiators.
    Now I just need to find out how many BTU's I need in an 8' x 8' x 8' bathroom w/ a 2x5 window (w/storm but single pane), one outside (uninsulated) wall and heated above and below.
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