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Help understanding piping; no flow to new radiator

TomTA Member Posts: 20
I hope someone here will be able to help me out. I live in an old farmhouse which was seriously "under-radiated"; 4 radiators downstairs, 4 upstairs, in a 2500 sf house with 9 foot ceilings (old Victorian cast iron radiators). I got a new Buderus panel radiator (20x24) installed in the kitchen, and it just doesn't get any flow.

I've attached a schematic (sort of) of the piping to the radiators. It's sort of a split direct-return system. The lines are coded by size; heavy lines are 1.5" copper or iron (or cast iron radiators marked A, B, C, D), medium line is 3/4" copper and fine line is 3/4" Pex. The supply from the boiler splits into an East branch (bottom of diagram) and a West branch (top of diagram). Each of those branches is then a direct return.

There's a single circulator, pumping away from the expansion tank on the supply side.

Emitters A, B, C, and D are old Victorian cast iron radiators. A, B, and C are plumbed all the way with 1.5" copper from the boiler tied into 1.5" or 2" iron to the radiators. D is tied into the 1.5" main line with 3/4" copper. E is the new Buderus panel radiator which isn't getting any flow; it ties into the 3/4 copper that goes to D.

Radiators F, G, H, and I are on the east side of the house; 1.5" steps down to 1.25" to F&G on the first floor, then steps down to 1" to H & I on the second floor.

If I close the valve all the way on "D" I do get some flow to the new Buderus panel radiator, but that sort of defeats the purpose of having radiator D. A, B, and C were probably installed 100+ years ago and their valves are long since frozen

My uninformed guess is that the supply water is taking the path of least resistance, going through A, B, C and (to a lesser extent) D, and saying to heck with E. The radiator was bled, and in fact the guys who installed it put a drain valve on the return side (just under it in the basement). After "bleeding" it through that drain valve the radiator got warm, but I imagine that just drained out the cold water, to be replaced with hot water from the supply line. At this stage, unless I shut the valve on radiator D there's no evidence of any flow at all to E (except perhaps a little gravity flow after an hour or so; the return stays ice cold).

Any help or insights would sure be appreciated.


  • delcrossv
    delcrossv Member Posts: 502
    edited November 2021

    It seems whatever flow there is is going to D. Was E installed with a diverter tee?
    Trying to squeeze the best out of a Weil-McLain JB-5 running a 1912 1 pipe system.
  • PC7060
    PC7060 Member Posts: 1,039
    edited November 2021
    Option 1: switch the 3/4” pex to copper and connect it to the main lines in the basement.  As it is the large radiator is taking all the water flow as you initially surmised. 

    Option 2 Add TRV to the large radiators to enable the flow to be modulated/stopped based on room temperature.  

    Best option: do both.  

    Does the pex have oxygen barrier?  For example PEX-AL-PEX? 
    EBEBRATT-Ed Member Posts: 13,783
    Or put in monoflow tees as @delcrossv suggested. Or put it on it's own zone
  • TomTA
    TomTA Member Posts: 20
    Thanks for all the suggestions. No, it's not a diverter tee. Given that I have to shut down the one radiator (D) completely to get even a halfway decent flow to the new one I don't think a diverter would have made any real difference. Yes, it's oxygen-barrier pex. I think the only real option is to put some sort of valving on the close/big radiators so I can try to balance the system. I was thinking that was probably going to have to happen anyway (either that or manifolds with a home run setup). It was already the case that the flow to end radiators on the east branch was marginal.

    I just wanted to be sure I wasn't making a fool of myself when the guys come back out; they're insisting "sounds like an air issue".

  • delcrossv
    delcrossv Member Posts: 502
    edited November 2021
    Dunno, monoflo will divert what 30%? I'd probably put in two. A "forward" one on the supply and a "reverse" one on the return.

    See: https://pmengineer.com/articles/85479-a-simulation-model-for-diverter-tee-systems

    YMMV, but it seems a simple fix to me.
    Trying to squeeze the best out of a Weil-McLain JB-5 running a 1912 1 pipe system.
    EBEBRATT-Ed Member Posts: 13,783

    There are several ways to fix this. First of all an air bound rad and a rad with no flow will exhibit exactly the same symptoms. The only difference is the air bound rad will have air in it and the rad not getting flow will not. So if you bleed it and don't get air stop bleeding it won't fix anything.

    The fix is several ways.

    1 Run two seperate pex lines back to the boiler and ty them in with a new circulator and thermostat and make the new rad it's own zone. 100%fix

    2 Install a balancing valve to choke off part of the system to drive some water into the new rad. Understand this might rob flow (and heat) from the other rads,

    Because when you install a valve in the supply to radiator "D" to force water through "E" you will also force more water through rads a,b,c,f,g,h,i.

    Your pump may have enough capacity to over come this and it may not

    Just know that you have upset the balance of the system by piping the new rad with undersized pipe

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,042
    edited November 2021
    So, you have an old GRAVITY design piping system where resistance to flow id almost non-existent The original boiler was hand fired and never shut off and on like your modern boiler. Then someone added a cast iron radiator (D) with 3/4" copper pipe. That radiator (D) works OK but does not get the same flow as the original radiators that are connected to the big iron piping system. Those radiators have a large water volume and will work with a flow rate of .3 to .4 gpm (gallons per minute) or less. They are slow to heat up and slow to cool down.

    Did I get the sizing correct?

    Now you add a modern european style radiator that works on a different flow rate. that radiatro wants at least a .5 gpm flow rate and you are trying to make that happen with a smaller pipe that is attached to a small (3/4") branch circuit. Your thoughts are absolutely correct. The heated water is taking the path of least resistance... All those big pipes.

    You need to add a circulator to the small pipe. or make a home run to the boiler or both. That will involve adding check valves to the existing system so that the new (small pipe) circulator does not cause unwanted flow to the other zone and the large pipe circulator does not want to take a short cut around the new zone loop. It is all explained in this booklet that I used to use for teaching a one day seminar on Hydronic heating. http://media.blueridgecompany.com/documents/ZoningMadeEasy.pdf page 4 and 5 are key to understanding how heated water flows thru different size pipes.

    The Flow-check valve thing is on page 9.


    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • TomTA
    TomTA Member Posts: 20

    Did I get the sizing correct?

    Pretty much; the common piping in the basement was replaced with 1.5" copper; that feeds into 1.5" or 2" iron to each individual radiator. Last year a leaking radiator was replaced, which is why there's 3/4" copper going to one of them. It was easier/quicker/cheaper than trying to rework the 1.5" iron to the new radiator configuration (bottom/bottom instead of top/bottom).

    Thanks for the link to that booklet. That will help me figure things out.

    I just want to be sure I've got this: tying the new radiator in to the main supply trunk upstream of the big lines, and with the return downstream from the big lines, should solve this problem since it would see a bigger pressure differential between the supply and return? It's a relatively small line, and I can throttle it back at the radiator, so there would still be plenty of flow for the rest of the system?

    I think putting in an additional circulator for this one small radiator might be overdoing it if the same thing can be accomplished with a little better planning for the distribution lines.

    Since the flow is more feeble to one side of the house (F, G, H, I at the bottom of the diagram) I was thinking about redoing the whole thing this summer with a home run set-up; I can't do it nearly as fast as the contractor with his compression fitting tool, but at least if it's screwed up I know who to blame. Would it be just as viable to set it up as a primary/secondary with two secondaries, one for each set of radiators (A-E and F-I, or even better A-D and E-I) being the two sides of the house?

    I sure do appreciate the help.

    EBEBRATT-Ed Member Posts: 13,783
    What you need to do is forget the new radiator for the moment. How does the rest of the house heat? If it's ok then just solve the new radiator problem.

    If the rest of the house is not ok then plan to rip and replace the whole system.

    Putting the new rad on it's own circulator may seem like overkill but you can't mix two different designs and expect them to work. Anything you do "Might work" only a separate zone"will work