Excessive Steam produced when hot water loop zone calls for heat

bobtee

I have a Burnham steam boiler which supplies steam to radiators in the older portion of my home. It also has a water loop which circulates boiler water through a brazed plate heat exchanger whose secondary loop provides hot water through baseboard convectors in the newer portion of my house. The secondary loop is controlled by its own thermostat. That boiler loop water temperature is controlled by an aquastat on the boiler water loop which is set to 170 deg F. When the hot water heated space calls for heat the boiler energizes as does the boiler water circulator which pushes water through the heat exchanger. The secondary side loop circulator is also energized and pumps water heated by the brazed plate exchanger to the baseboard convectors. The problem is that when the circulator loop is energized the boiler inexplicably produces steam to the old steam radiators even though the aquastat control keeps the boiler water at or below 170 deg. Why is steam being produced at temperatures below 212 deg F when not called for by the steam side of the house? The result is that the steam side of my house gets over heated whenever the baseboard side of my house calls for heat. What can I do to keep steam from being produced and sent to the radiators when only the space heated by the hot water loop is calling for heat.

Jamie Hall

Where and how is the aquastat with a 170 high limit mounted? Unless it is in the boiler itself, in a well, or very very close to where the water for the heat exchanger comes out of the boiler, it can't sense what the boiler is doing.

Or is the aquastat set to maintain 170, rather than take 170 as a high limit? If it maintains 170, there may be enough slop in the calibration to allow the boiler to steam. It should be a high limit.

bobtee

The 170 is a high limit. The aquastat is located in the 3/4 inch boiler water circulating pipe about 8 inches from the boiler outlet and about 10 inches before the entrance to the heat exchanger.

EBEBRATT-Ed

@bobtee

It would be better if the aquastat was in the boiler water itself. Has it always acted this way or is this something new?

Try lowering the aquastat setting to 150-160 and see if that helps.

Post a few pictures of the aquastat and piping

ethicalpaul

Can you expand on that, @EBEBRATT-Ed (because I'm going to be setting up a water loop soon myself)?

In my assumption, the circulator is on any time the loop is calling for heat, so it is always pulling "fresh" boiler water across the aquastat sensor, right?

Or can it be a situation where, for example, the hot water loop might have a thermostatic mixing valve on it, and even though the loop is calling for heat, the loop itself is hot enough and stops pulling "fresh" water from the boiler for awhile?

mattmia2

Maybe the HX is plugged so it isn't circulating much water so it isn't seeing the boiler temp.

Robert_25

When the hot water heated space calls for heat the boiler energizes as does the boiler water circulator which pushes water through the heat exchanger.

If the boiler fires on a heat call from the hot water system regardless of the aquastat setting, your controls are not wired properly.

bobtee

Good comments all. I do periodically back flush the heat exchanger and that helps just a little and only for a short while. This has been a problem from day one. See attached 2 photos of aquastat location. I agree it would be better located directly in the boiler in the opening shown on the third picture but I’m afraid to attempt removal of that 3/4 inch plug. It’s probably rusted-in solid in the boiler tapping. I’ll first try to lower the aquastat temp to 150-160. It will likely help but I suspect it will take longer for the baseboard heated space to achieve temperature. I guess the answer to that would be more linear feet of baseboard convection. Do you think reducing the air vent size on all the radiators might help by requiring a bit more pressure in the boiler before the water steams? Thanks for your thoughts.

Jamie Hall

Changing the air vents on the radiators won't do a thing. Even if you managed to raise the pressure to 3 psi -- which is the upper limit of the vents (beyond that they will be ruined) you'd only raise the temperature 10 degrees. Don't do it.

mattmia2

That cap and valve is a skim port. If the brazed plate hx clogging is a recurring problem you could replace it with a shell and tube hx.

EBEBRATT-Ed

Maybe I am not seeing the picture right but Isn't that a backflow preventer mounted above the aquastat? I don't think that well is in the circulated water. Looks like its in the MU water.

@ethicalpaul

In a perfect world you want the aqustat sensing boiler water so it senses a larger volume of water, You want it to turn on the boiler for the hot water loop when it needs heat but also to keep the boiler from steaming. It's really just a low limit the same as what would be used on a boiler with a tankless heater.

Dave T_2

That 3rd picture showing a valve with cap looks like it is a skim tapping so likely too high for a good aquastat location. There no doubt is a good location in the boiler block. It is in a poor location now, not even in the flow of water, but I am assuming it worked in past years.

If by turning the aquastat down 10 or 20 degrees does not stop the steaming then either the aquastat is no good or you do not have enough flow through the boiler side of your plate exchanger (which would not surprise me).

You have a Y strainer located just before the circulator. It's there to catch debris before it gets into the exchanger, it has a screen in it. Remove the screen and check if it is dirty. If it is that likely is the problem of poor flow through the exchanger. If not ok, the next most likely obstruction would be the boiler side of the plate exchanger; remove and clean or replace if necessary. If every thing is found to be clean and you can somehow confirm there is a flow problem the circulator may be the culprit. As you can see there are many possibilities, some that have not been mentioned yet. A visit by a good tech can diagnose most of this without wasting much time and solve your problem the best, but if your willing and able - have at it!

bobtee

Great advice, thank you all. just fyi the tapping i mentioned is below the skim tapping outlet (you can see the circle cutout in the boiler casing) so it is below the water line. im just afraid to try to remove the plug. i wont try it till this heating season is over. Any experience on how difficult it will be to get the 3/4 inch plug out cleanly without stripping the tapping threads or cracking the cast iron boiler wall?

New England SteamWorks

It’s not wired or piped right. When the hot water loop calls for heat the circulator must come on. If the circulator is on, then there will also be a call to fire the boiler IF the boiler aquastat is closed (meaning the boiler water temperature is low (say below 180). Once it reaches 180 or so the boiler stops firing, but the circulator continues until call for heat is satisfied. No steam is made, assuming the steam thermostat is not calling for heat.

The high limit stops the circulator if the temperature gets above say, 200, to prevent boiling water/flashing steam in the closed system.

This means that when there is a call for steam heat the hot water loop is not going to run. This creates a problem on very cold days when the steam zone is running constantly. The Hi Limit will prevent the circulator from ever running until the weather moderates.

The solution to this problem is piping in a bypass, allowing cooler return water to be mixed with the supply water, thereby regulating the supply water temperature.

ethicalpaul

Here's where I always post this link which has lots of info about the bypass: https://heatinghelp.com/systems-help-center/how-to-run-a-hot-water-zone-off-a-steam-boiler/