Excessive(?) boiler make up water

[bmehank]

Our condo building is heated by a Weil-MacLain LGB-11 steam boiler (31 HP, 1050MBH) supplying about 250 radiators in a two-pipe system with thermostatic traps. Steam pressure is less than 1psi at radiator inlets. The building is heated adequately and silently. But I am surprised that our seasonal (7,000HDD) make up water volume is about 130 cubic meters. A few liters are blown down weekly. Residents have not reported escaping water or steam nor are any losses seen in the boiler room. The condensate tank is vented outdoors and on very cold days wisps of condensate may be visible. The main condensate returns are hidden in concrete trenches. Our contractors are not surprised by the make up water volume. Is this volume typical? Can anyone suggest where to look for the losses?

 

[TBP]

I'd suspect the hidden condensate lines are leaking, or there's a water leak into the firebox of the boiler, and the resulting steam is going up th stack.

The make-up required for these systems should be VERY low. With the volume you describe, you're no doubt getting oxygen corrosion & scaling in your boiler, plus paying a for a lot of extra fuel, as you're heating dead-cold make-up water to steam temperature, instead of the nice running-start you get with hot condensate.

 

[bmehank]

Thanks for your ideas. The gas flame is below the cast iron water sections so potentially leaked water could be steamed off and sent up the chimney. A small wet patch did appear beneath the sheet metal flame pan last spring and two cast iron sections were replaced but this fall make up water volume continued as before. We added automatic water treatment a few years ago and I believe it deals with the chemistry problems you raised. We have the significant cost of treatment chemicals plus water/sewerage fees plus energy to warm 45F city water to "a running start" of about 85F. There is no easy way to examine the main condensate lines but it sounds like there's nowhere else to look for our 130 cubic meters of make up water.

 

[TBP]

Your condensate should be getting back to the boiler at at least 160 deg or so. Most of these old low-pressure steam heating systems did not require condensate tanks.

The fact that your contractor thinks your systems's make-up water volume is "normal" suggests to me that you have some guys who aren't all that familiar with steam.

You may wish to get hold of a copy of "The Lost Art Of Steam Heating" by Dan Holohan. It covers the whole range of low-pressure steam systems, and yours will be in there somewhere. His website is heatinghelp.com, and there a lots of very knowledgeable contractors who post on that site. There's very likely somebody local who can help you.

 

[ApC2Kp]

bmehank,

There is some loss of steam through the thermostatic air vents at the radiators. Assuming that there are air vents on the radiators, consider the following.
"Steam pressure is less than 1 psi at radiator inlets." would result in partial vacuum in radiator during cycle 'off' when thermostats shut flow of steam. After the vacuum draws air into radiator, the air needs to be expelled at start of steam 'on' cycle. The thermostatic air vent at each radiator lets out the air with some steam, until steam temperature closes the air vent.
You might estimate the steam loss thru the air vents by a cycle rate of say 2 per hour, x 1 ft3 steam per cycle, x 24 hr/day x number of heating days x 250 condo units. If the estimated steam loss volume is 2,400,000 ft3, convert the vapor to liquid by dividing by 100. The 24,000 ft3 of water is almost 5x the 130 m3 of makeup water.

 

[TBP]

Shouldn't need vents on the radiators in a two-pipe system. The traps will get rid of that air. These systems will have main vents on dry returns, the base of the wet return risers, top of the steam risers - depends on the type of system.

One-pipe steam systems, however, MUST have air vents on the individual radiators, as they do not use steam traps.

 

[bmehank]

Thanks for helping me work through this problem. The only vent I've seen in this system (other than one leading from the condensate tank) is one at the most distant end of the condensate return, on the lowest floor served. An odd-looking gadget with a bell-shaped cover. I'll get a copy of Holohan's
book to see what I'm missing.

Condensate leaves our radiator traps at a peak 170F but it reaches the condensate tank at about 85F. We time-modulate the main steam feed according to ambient and exterior and anticipated exterior temperatures and that is reflected in the average temperature being less than 170F. Also, the two main condensate returns are below grade for 100ft and 200ft and a lot of cooling must take place during that journey.

 

[GenB]

I had to study and imagine the start up and condensation of steam. I think many boilers systems heating buildings experience the same problem. I installed a boiler and had to correct its problem and learned the lesson.
everytime you startup (daily I suppose in most cases)
when you start up you evaporate into an empty space and depends on the volume of the system, the boiler will call for water... but what do you do with the water sent to the system... it will comeback to the return cond. tank eventually overflowing to the sewer.
there is your answer.

I manage to set the water level in the boiler so it will be a little high at start and lower when running, you can only do this if you are experience with controls. and on some boilers will be difficult to do so, in my case the boiler uses probes/sensors and easy to manage levels.
I built the boiler so was authorized to play with it.
genblr

 

[TBP]

GenB- These old low pressure steam heating systems were originally designed with no condensate tanks, or pumps in the system. It wasn't - and shouldn't be - necessary. They were made to run a 2 PSIG maximum, so that the condensate could gravity drain back into the boiler with a 5 foot fill-head.

These old systems are beautiful in their simplicity.

 

[GenB]

TPB, I know both systems very well.
Condensate as he puts it is the same if a simple one pipe or 2pipe system. the start up will always have lots of return, the thing is: what to do with it, return it to the boiler if enough storage capacity or dump it to the sewer, in this case they are overflowing it to the sewer.

What is your advise for the post?
beautiful will not fix the posted problem.

 

[TBP]

GenB - Where does bmehank state that his condensate tank is overflowing?

I see where YOU say the condensate tank is overflowing...

 

[bmehank]

Thanks to everyone for sharing experiences and opinions. In light of some comments I should provide background to help explain our situation. Until about 2003 the system was fed by two Weil-McLain LGB-11 boilers that provided steam at about 5psi. The building was historically overheated and residents had become used to regulating room temperatures by opening windows. This became a big problem for residents on upper floors because the building became the dominant radiator and open windows provided no relief. We were unable to convince residents that windows could not substitute for rad valves and in any case we could not prevent overheating because of inaccessible uninsulated steam lines and open-web floor construction. Finally energy cost and discomfort forced a single boiler, time-modulated steam feed solution. Since 2003 our energy usage has been halved and residents are comfortable as long as they leave rad valves open and windows closed. A lot of condensate is generated each time the main feed valve is opened and that is returned to the condensate tank as is all condensate. There is no sewer connection.

 

[TBP]

Ah - opening the window to deal with overheating...

Holohan describes windows used this way as "double-hung zone valves".

 

[GenB]

If you do not understand the system, iot would be better that you consult an expert.
also
Please try the steamforum, a lot of low pressure posts out there. Murphy is a expert...

 

[bmehank]

I received a copy of Holohans "The Lost Art of Steam Heating" recommended by TBP. This book delivers a how and why for components of our 1929 system that have puzzled me, and contractors, for years. The writing is crisp and clear and full of interesting asides and anecdotes. Almost a page-turner.

Our problem is definitely leaking condensate returns that are virtually inaccessible. Holohan has a solution for that.

Thanks to all who shared their experience and offered ideas.