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Thread: Boiler Pipes hot when heat has been off for months? Boiler used for DHW.

  1. #1
    DIY Junior Member GregoryR's Avatar
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    Default Boiler Pipes hot when heat has been off for months? Boiler used for DHW.

    Hello all,

    I currently have a 25ish year old Weil McClain boiler. We have yet to use the heat up here in Maine(thankfully), so the boiler is only being used for DHW via tankless coil right now. I've noticed that the main hot water outlet coming out the top of the boiler is very hot, all the time.

    There is a section of about 8 inches where the outdoor reset has a temperature sensor, then some type of black control box(that screws into the pipe). The pipe continues and has an outlet to the expansion tank, and then to the start of the baseboard. This pipe is very hot to the touch(150+) degrees all the way to the start of the baseboard. At the baseboard, some heat can be felt.

    Any ideas on why this pipe(many meters of it) is hot when the heat has not been on for months?

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Convection...sounds like you may need a working check valve. Well, it would be good to get rid of that tankless coil as it is not very efficient and can be limited in the hot water supply from it. To be able to make hot water at any time, the boiler basically will run 24/7 between its min/max aquastat settings. Hot water rises, and if there's no check valve in the system, it can flow around the loop. Since it isn't being pumped, it doesn't flow very fast, but it is continuous, even in the summer when you probably don't want the heat. This may be one reason you haven't needed to turn up the thermostat...you're getting some heat, regardless.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    In the Trades Tom Sawyer's Avatar
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    Well, a picture or two would be nice but you have a few things happening there. If you have circulator pumps you probably have a flow check valve that is stuck open. It's probably either red or green. Try a smart rap with a hammer to the side of the valve. That usually un-sticks it. Or, you might have zone valves and one of them is not closing properly. Pretty rare but it does happen. In that case, you need to replace the zone valve head. Also, since you have a tankless coil, the control is pretty stupid and does not know that you are not using the boiler for heat so it keeps the boiler at around 160 degrees or so, so that you have hot water when you open the faucet. You should consider getting rid of the tankless coil. There are very inefficient at making hot water. An indirect water heater is the ticket here and while your wallet is open, spring for a temperature re-set control also. The upfront cost hurts a little but it all pays itself back in a couple of years.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    DIY Junior Member GregoryR's Avatar
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    Name:  111811233306.jpg
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Size:  20.5 KBName:  111811233321.jpg
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    There you are . The black box says McDonnell & Miller Fluid Control Division ITT.

    Thanks!
    Last edited by GregoryR; 11-18-2011 at 09:08 PM.

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    In the Trades Tom Sawyer's Avatar
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    The black box is a low water cutoff. Your pictures are too dark and too close. Stand back so I can see the piping coming off the top of the boiler
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  6. #6
    DIY Junior Member GregoryR's Avatar
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    The wrap insulation on the thick black pipe is covering up the temperature probe from the outdoor reset. The entire black pipe(insulated with the pre-formed white insulation) is shown in the next picture as well at the bottom left.


    To the left, you will see the same pipe in photo one, exiting the boiler, and travelling to the right. At the far right, the pipe is still VERY hot.


    Service logs


    I hope this helps, and again, thanks so much.

  7. #7
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Tank-less water heating coils are obsolete. Born in a era of cheap fuel the tank-less water heating coil is placed in a boiler that is maintained at a minimum water temperature and the water "indirectly" heated through the coil...more or less "instantly". Not be confused the a tank-less - or on demand -water heater that is only fired with when a hot water faucet is opened.

    Boilers make an excellent heat source for domestic hot water but you would be better served by an indirect water heater that would only call the boiler to duty when the water in the indirect got cool. An indirect water heater will cut the cost of heating domestic hot water in half, last about 30 years and likely never come up short on laundry day or holidays.

    By the looks of it a new boiler and indirect water heater would not be a bad plan.
    Last edited by BadgerBoilerMN; 11-20-2011 at 10:27 AM.

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    In the Trades Tom Sawyer's Avatar
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    Your second picture, the slim one, on the left side the green thing.....Smack it a couple times with a hammer. See if that helps. Or.... make sure the screw on the top of it is screwed all the way down.....righty tighty

    your last efficiency test...10% CO2 could be better. Should be 11 to 11.5 That boiler will give you 82% all day if it's tuned right.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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