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Flow problem in secondary heating loop

samwelch11 Member Posts: 3
edited October 2022 in THE MAIN WALL
Hi all,

We live in an old home and have a hydronic heating system with cast-iron radiators. (The system is set up with two pipes, one send, one return, in a large U-shape, with the cast-iron radiators forming bridges between the send and return loops.) We installed a new propane boiler two years ago. The system worked well until I fired it up for the first time this year (after a summer of inaction) last night. It worked fine at first--radiators heated up per usual, it seemed, for maybe about an hour or two before we fell asleep--but then when I woke up this morning and walked into the kitchen, the radiator was cool despite the thermostat calling for heat. Here is what I have determined since then:
  • The boiler seems to be working fine. No errors, heating to appropriate temps, etc.
  • The system has a primary and secondary loop. The primary loop is very short (a few feet) and the secondary loop contains the rest of the radiators in the house. The primary loop is powered by the circulator pump in the boiler; the secondary loop is powered by a Taco ECM0018e. The Taco is on and--at least according to the bluetooth data I can access from the app--working normally. (See attached photo; these numbers are roughly what it was running at last year, if I remember correctly).
  • The piping in the primary loop is hot (quite hot, hot as it should be, and hot as the secondary loop normally is when the system is working). The secondary loop is cool to the touch.
  • If I turn off the Taco pump for a minute or two, the circulator pump in the boiler pushes hot water a little bit of the ways into the secondary loop but not very far. If you look at the attached diagram: Point A is always hot, whether Taco pump is on or off. Point B (the second of the closely spaced tees) is cold when the Taco pump is on but becomes quite hot after a few seconds if I turn the Taco pump off. With the Taco pump off, the heat makes it about as far as Point C but never as far as Point D. Point E even warms up a little if the Taco pump is off. As soon as I turn on the Taco pump, everything gets cold except for Point A.
  • If I turn off the Taco pump, the boiler heats the water in the primary loop to its proper temperature and then the water slowly (very slowly, over like ten minutes) cools off and eventually the boiler kicks back on for a minute or two. If I turn the Taco pump on, the water in the boiler immediately starts to cool off, the boiler kicks on, and seems to run more or less normally (i.e., its semi-constantly heating what seems to be cold water coming into the radiator). And yet the radiators in the house never get hot.
  • We did some work on the pipes this summer (replaced some bad valves) and had to drain the system, so we did just put some new water in (but I treated the water as we always do).

At this point, I am at a loss. It seems like the Taco pump might have crapped out? If it was turned off (i.e., not receiving electricity), would it spin freely? Thereby allowing the circulator pump on the boiler to push heat a little bit into the secondary loop but not that far? But if it crapped out, why is it telling me via Bluetooth that it's pumping 6 gpm? And why does it appear to be working otherwise? (I.e., the lights come on, it makes the same very slightly vibration it normally does...) Did something become dislodged during our work this summer and plug the secondary loop? If so, why did it work at first? Is this what vapor lock looks like? I've read about it but never had this problem before (and I filled the system like I normally do, by bleeding each radiator, one at a time).

Any thoughts appreciated. To reiterate: The system is unchanged from when it worked last year, save the replacement of two valves in the secondary loop.

Thank you and let me know if you guys want more info!!


  • Jamie Hall
    Jamie Hall Member Posts: 20,804
    Is there anything controlling the flow in the little short pipe between point B and point E? And what happens to the return pipe -- between point E and the boiler -- when the Taco is on or off?

    While you're playing with that, check the pressure in the system. Also you may need to bleed more air out of the radiators.

    In answer to one of your questions -- yes the water will flow right through a pump which is not running, but the amount will be determined by the relative resistance of the loop with the pump in it vs. the loop or loops without the pump.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 18,808
    Could be an air lock in the boiler if that circulator is not moving heat across the tees. Assuming the circ inside the boiler is spinning?

    Is it a fire tube style boiler?  They need to be well purged after a drain down   A pic of the boiler would help. There may be a vent up high on the boiler or a relief valve to burp air from

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • samwelch11
    samwelch11 Member Posts: 3
    Y'all are amazing. Purged the boiler--I had purged it very briefly when I started it back up to make sure the valve was working, but I let it run for about ten seconds--and it immediately started heating the secondary loop. I didn't notice much air come out but it really seems to have solved the problem. For now. If it stops running again and I can't get it to work by purging more, I'll return. But for now, THANK YOU THANK YOU THANK YOU!!!
  • hot_rod
    hot_rod Member Posts: 18,808
    Hopefully you have an air purger that can take over removing any last air, now that you have  flow moving
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream