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Thread: Boiler Radiator Cold

  1. #1
    DIY Senior Member Kiko's Avatar
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    Default Boiler Radiator Cold

    How is it possible that a 3 ft section of one of my radiators is cold (on the end opposite the inlet). If there was a plug in it, wouldn't that prevent the water from circulating through the rest of the system?

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    In the trades Dana's Avatar
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    Maybe it's a steam system, not pumped hot water?

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    In the Trades Tom Sawyer's Avatar
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    It's frozen
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    In the trades Dana's Avatar
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    Tell us more about the system.

  5. #5
    DIY Senior Member Kiko's Avatar
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    This boiler system pumps and recirculates hot water. The temp. is set at 180 degrees, and the pressure is 20 psi. The radiator in question is an 8 ft baseboard-style rad, which is warm at one end and cold at the other end, with the middle being neither warm nor cold. All the other baseboard radiators are working properly, with the entire radiator being warm. This room is not as warm as other rooms in my house, which is why I want to know how to address this problem. I doubt the radiator is frozen, because it is not on an exterior wall.

    I was under the impression that water must circulate through the entire radiator in order for it to return to the boiler. Am I wrong?

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    DIY Senior Member Kiko's Avatar
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    No boiler experts on this site?

  7. #7
    In the trades Dana's Avatar
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    If the other baseboards & radiators on the string are all still putting out heat, this system is probably plumbed with "monoflow Tees" bypassing each radiator, and an air-lock/bubble has developed in the baseboard section. To know for sure you'd have to air-purge that loop.



    If there is a valve on the main loop pipe in the middle of the bypassing element to the radiator that can be closed, forcing the water to flow through the section of baseboard that alone may be enough to stop the symptom, but if there's that much air in the system it may recur (the same place or elsewhere). Most hydronic systems have an "air scoop" and vent valve to get rid of residual bubbles after air-purging the system by opening it up (best near the high point of the system) and letting massive amounts of water flow for awhile.

    A system pressure of 20lbs is a bit high for a simple 1-2 story (12-13 psi is enough). If the pressure is too low it can let corrosive amounts of oxygen into some systems (8psi is an absolute minimum), but there's no advantage to going higher pressure than necessary. On 3-4 story buildings 20psi may be necessary to keep the pressure high enough at the high point of the system, but many boilers can't handle more than 30psi. Some ultra-modern low-mass boilers need 20psi for optimal performance, but I'm assuming you have a traditional cast iron unit. The system pressure can often be set by tweaking a screw on the autofill valve, and system pressures can rise if that valve is leaking &/or the expansion tank has failed (or needs to be air-charged). Read up on it before making any adjustments (or hire a pro.)

    The middle of winter isn't the ideal time for a DIYer to learn the joys of air-purging hydronic systems, but you know better than me what your capabilities are. No rocket scientists are out there wasting their careers purging heating systems, but some forethought and patience is required (and sometimes some plumbing skills.) If you're lucky there will be purge valve (or multiples) on the system, preferably near the highest point on the system.

  8. #8
    DIY Senior Member Kiko's Avatar
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    Thanks for the info.
    I wouldn't mess with it in the winter, but I might in the spring once I can cool it down.

    I have a couple more questions:

    1. How do you remove the outer frame that covers the heating element and fins, in order to find the purge valve?
    2. Can you attach some kind of a hose to the purge valve to drain the water outside?

    Mine looks like the one in this photo, only longer and older looking:


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  9. #9
    Retired Defense Industry Engineer jadnashua's Avatar
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    The thing may not have a purge valve at all. Usually, the end plates pop off, then you can unscrew the cover in the middle. If you're lucky, they put a valve on one side of the thing, but it may just be solidly connected.
    Jim DeBruycker
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  10. #10
    In the trades Dana's Avatar
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    It's worth poking around underneath to see what ball-valves etc. might be available for adjusting flows. There may be hose-type valves plumbed in near the boiler for bleeding down the system to lower the pressure or for help in purging too. Valves specifically for air purging are a hit-or-miss thing, and I've never seen one installed under the tin of a fin-tube baseboard (but I suppose it's possible.)

    Snoop around, try to figure out how the loops are plumbed and diagram it for yourself. There's no guarantee that it's set up with diverter Tees as in the picture- that is a configuration that could explain the symptom, and I'm not coming up with an obvious alternative. But any loops that are plumbed in parallel could have an air lock on one leg keeping the bulk of the flow going down the other.

  11. #11
    DIY Senior Member Kiko's Avatar
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    There's only two zone valves and two loops-- one for the main floor and one for the upper floor. Each radiator has a brass fitting sticking out of the pipe at one end, with a slotted screw in the middle of it.

    Is that a purge valve or something else?

  12. #12
    In the trades Dana's Avatar
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    It's likely that it's a bleeder valve, which would be good.

    To tell if it's a simple air bleed valve, turning it about a quarter to a half turn (with the boiler & circulator off) would result in either hissing of air or a small stream of (potentially hot, feel the pipe first) water. If it hisses, wait until the water appears and is a non-sputtering stream. Some are designed with a collar around the part with the slot, design to be used with a "bleed-key" tool which can keep you from getting splashed by hot water, but if the system is (relatively) cool a screwdriver can handle it. Others are more open, designed for screwdriver operation.

    Give it a shot- any air you can remove from the system is worthwhile, and there's a good chance that if it hisses air when you first turn it, the symptom may soon disappear.

    You may want to put a shallow pan or towel underneath it to make less of a mess, and use gloves if any of the baseboard pipes on that loop are hot when you start.

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