What would stop or slow flow in pipes?

DJDrew · January 2020

Hello,
Everyone on this site has been helpful as we worked through other issues with our heating system. I am hoping to get some thoughts on another issue we are trying to solve. We have an over radiated house with lots of cast iron, cast iron radiators, cast iron pipes and a cast iron boiler. From what we can tell, it was an old gravity coal system originally, two large pipes run around the perimeter, one for the feed and one for the return - each radiator has it's own tapping from those two pipes.

We have always had inconsistent heating of the radiators, so after some searching on this site, we suspected the pump was not sized correct, so we just installed a Taco 0010 pump based on the recommendation of this article matching up close to the EDR of the system. After the pump was installed, I instantly noticed most of the radiators in the house heating up much more evenly from room-to-room.

However, on half the house, two radiators were still significantly - nearly solid cold - compared to all the other radiators on the same circuit. These two radiators just happened to be toward the end of the same line. In feeling the pipes, I can literally feel the main feed pipe go from hot to luke-warm and then cool. I have included a picture with labels to show what is happening.

1) Does anyone have any thoughts as to what would be causing this issue to occur?
2) Based off the picture, is there an easy or feasible way to unscrew those unions and clean/check for issues?
3) Before the Taco 0010, we had a Grundfos 15-58F. If this was put on medium or low speed, those two end radiators would eventually get warm, but then all radiators in the house were heating up unevenly. So the new pumped fixed some issues, but exacerbated the issues on these two radiators.

Any thoughts would be greatly appreciated!

Jamie Hall · January 2020

I doubt that there is actually a restriction in the piping... other than the piping itself. Water is extremely lazy, and will take the path of least resistance every time. As I look at your piping, I see nice hot water coming into a T in a larger pipe, and exiting the T in two smaller pipes -- one to a first floor radiator and another one to another T, again coming in in a medium pipe and going out in two small pipes -- one to a second floor radiator and one, finally, to another first floor radiator.

Now, if I'm water, I'm going to take that first right turn to the nice nearby first floor radiator in preference to heading on to the other two -- and at the other two, I'll split more or less evenly, but possibly with a preference for the first floor radiator -- unless that pipe is long.

So... is there a valve on that initial first floor radiator that you can close a bit? You might try that, and see if you can persuade the water to head out to the other two.

JUGHNE · January 2020

Changing the pump may have required the system to be drained.
This would require air bleeding of all radiators. Air could be trapped in the colder rads. Have you tried to bleed air from these.
With hot flow only so far and little flow in the reducing tee to me it indicates lack of flow that could be caused by air in the rads.

Big Ed_4 · January 2020

It sure looks like an old gravity system . The good gravity installs used reducer and 45's . They used smaller pipes for the taller risers like on the second floor .The plumbing controlled the rising warm water .I would love to see their instruction sheet .Sometimes they added orifice in the radiator supply valves ,maybe to fine tune . I always liked a gravity system for it's simplicity once installed ..Just add fire and no electric needed if burning coal .. With the mass of the water in the system the boiler temperature only ran up to what is needed to heat the home at the given time.. Simple outdoor reset .
The circulators and flow check were added to gain control of heating the building once they used the boiler to heat up the hot water or added zones . Adding a circulator changed the animal , changed it to a forced water parallel system . The circulator will create pressure drops along the system to move the water up into the radiation .. I feel the best way to equalize the pressure drops along the system now would be to run either the supply or return to the far end of the line and cap the connection near the boiler .. I would also use a variables speed pump to fine tune the flow and avoid any unwanted venturi across the tees .

What would stop or slow flow in pipes?

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