Steam risers heating up when heating system off

aquastatman

We have a steam heating system in a 10-unit multi-family brownstone building. Our Weil Mclain boiler produces both steam for the heating system and domestic hot water. The boiler is controlled by a heat timer that has summer and winter modes. Currently the heat timer is set to summer mode, so the boiler should not be producing any steam.

However, one of the risers that runs through the apartments in the back of the building has been heating up. The problem typically happens during the morning (c. 7-10 AM) when domestic hot water demand is at its highest, but it has also happened in the evening (c. 9-11 PM).

We had a plumber come look at the problem. He told us that the system was overheating the water in the boiler during periods of high hot water demand and this was causing steam to flow into the riser. He checked the water level and replaced the aquastat, which he set to 140 degrees. The problem went away for a few days but then recurred the other day.

The plumber is now telling us that the best short term fix is to install a shutoff valve on the header that we can close during the non-heating season. For the long term, he suggested that we install an entirely separate hot water system.

My questions are:

1. how is the boiler producing steam in the first place, when the aquastat is set to 140 degrees? The water temperature would need to rise another 81 degrees to reach boiling, so how is the system producing any steam at all?
   
2. which fix makes more sense? Install a shutoff valve or install an entirely separate hot water system?
   
3. does my plumber know what he's doing?
   

STEAM DOCTOR

New auqastat would be step number one

EBEBRATT-Ed

1.It shouldn't make steam
2.A separate hot water system would make more sense than a valve
3. No. Not saying he's incompetant. You need someone with more knowledge than he has

a.You may need heat conductive paste put on the aquastat.
b.The aquastat well may be in the wrong location or sludge may have built up around the well.
c. could be a wiring problem
d. regardless of the settings the "heat timer" may be calling the boiler on when it shouldn't be.

Not the most difficult problem to diagnose

just need someone willing to dig in and not give a quick answer

nicholas bonham-carter

Have the aqua stat installation, wiring, and the Heatimer checked.
When you have a true steam pro look at the problem, also have him look at why the errant steam fills that riser first. Typically the Heatimer salesmen promise a cure for an unbalanced system, when correcting bad piping, or a lack of venting is called for.--NBC

j a_2

Hold on, you have a tankless supplying hot water to 10 units...post some pics of this system....Ed gave some great suggestions...

gerry gill

Is it really steam heating the riser? or could it be 'gravity' hot air rising off the hot water in the boiler. I have watched that happen on steam systems without drop headers who have hot water loops off the boiler.