?? for the steam experts...Mod burner + one pipeTRV's'...BP

ttekushan_3

for another job, pondering a few clues revealed from a school steam heating system that was fully restored and updated complete with new Kewanee boilers and controls in 1999. Turndown ratio of 5:1 with 20% being the lowest rate. Instead of TRV's, pneumatic thermostats and actuators were used.

[I'll delete the explanation about boiler sequencing and co-modulation unless this is what you will also need to do. They had each boiler sized to do the entire heating job under average conditions, and it was vacuum return so there was no problem running one boiler. If you have questions on the total operational configuration, just ask. It was quite interesting.]

If I recall, it was a set of Cleveland Controls that operated the firing rate, and in this arrangement as demand decreased the firing rate would follow. Surprisingly, it STARTED at low fire, and increased firing in predetermined length steps until it reached the minimum firing level that would still achieve the target pressure. [That system never made a peep ever, even on extremely cold startups.] The firing rate would then drop according to demand. The system would shut off based on the boiler's duration at lowest fire. After a specified period of continued low fire, it would shut off.

This would get you the "reset" period you need to address your concerns. A simple time delay relay would do the trick, as I think they must have done.

I don't like the idea of using a control with a fixed cycle length under these circumstances. It would work against your efforts to get long, low burns based on the system's response to actual conditions as communicated back to the boiler in the form of a changing and dynamic pressure gradient. Ideally I'd want a control that starts the process over based on outdoor temperature and the length of the last cycle.

Do you suppose anyone makes an off the shelf hydronic control that could be used in this application?

By doing what you are attempting to do, you are effectively matching firing rate to pressure gradient which, in turn, is responding to the aggregated actual room heat losses/heat demands. Steam offers us the ability to monitor the system at the boiler's pressure gage. You're on the way to startlingly efficient operation.


P.S. If you're wondering how that school's system restarted, it didn't. Once off, the boilers stayed off. Each cycle was initiated by the push of a button by custodial staff early in the morning. Since the school was occupied during the typical hours, usually one cycle per day was adequate. It could continue darn near into the afternoon if conditions were extreme, or quite short in spring and fall. Talk about setback! Occasionally fast and violently cold weather fronts would catch the system off guard, but staff would notice and give that button another push. Really, that old building had its plumbing at the core, and classrooms would have doors closed at night. The heat would be off all night and all weekend. The Christmas break would probably entail a few "runs" to prevent pipe freezing, but no big deal since custodial was present to clear sidewalks of snow anyway. Today, we would use a snap disk freeze protect temp sensor to do the same.

The old solid masonry buildings (with original windows and all), steam heated with modulated, sequenced dual boiler set ups like that one for FAST recovery were quite energy efficient, actually. In talking informally to district on-the-ground staff, the gas and electric usage wasn't bad at all, and many of the new replacement schools have wound up using more energy per square foot that the old ones. Maybe there's something to be said for thick walls, high ceilings, natural light, natural ventilation and deep setbacks.

The Steam Whisperer (Formerly Boilerpro)

Questions

I have been fortunate to be working with an individual that is looking to resurrect a one pipe steam system in a 3 story building that been out of use since the 1970's. He is also looking to see the building LEED certified. What we are looking at is using the Power Flame X4M modulating gas burner in a Slantfin Intrepid Steamer. My concern is that if we use TRV's on the radiators, they need some time where the pressure is off to allow valves that are satisfied to breathe in so on the next steam cycle they won't get any steam because the radiator is filled with air. If we use a modulating burner, we may see the boiler fire at low input for many hours with no period of time to allow the off valves to breathe in air, thus rendering the thermostatic nature of the valves useless. If the air vent TRV is off and the radiators are still full of steam, then the radiator will contirnue to heat becaue of the vacumn created by condensing steam.

Any thoughts or experinces to share?

I won't be around for the next day, maybe two to answer any questions.

Boilerpro

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Brad White_203

Vacuum Breakers

Would vacuum breakers on your valves mitigate that issue?

Even with modulating fire, you will make pressure at some point- the modulation is not "that" low, would it be?

And suppose that low fire was a constant as would be steaming at a low level- would that not mean that your load was still present and that all radiators (and TRV's) would be in a position to still be venting and then receive steam?

bob_46

cycle

use a time-clock

nicholas bonham-carter

leed 1-pipe

possibly the dead men will have sized everything in the system so perfectly, that you won't need trv's, just an outdoor reset.
and maybe this IS the perfect place to go "sub-atmospheric". last time i looked the check valves at "the big box" were amazingly inexpensive.--nbc

The Steam Whisperer (Formerly Boilerpro)

Ah, but the TRV's...

can shut off with steam in the radiators but won't stop the steam from continuing to come in and continuing to heat them. At this point the vacumn breaker won't see vacumn because steam is still coming in. I was thinking along the lines of some other responders, a timeclock cycling the boiler off like once an hour so the rads with closed TRV's will pull air in through the vb and then on the next boiler cycle the air locked in the rads will prevent steam from entering.

BP

Barbbarossa

TRV one pipe

You are right about the condensing steam pulling in more steam on a one pipe system and the TRV’s do have a vacuum breaker to let air in but the vacuum produced by the steam condensing will be met by the supply side piping and the will be no appreciable air flow in the radiator by the TRV no matter what they say. I had to fix this issue on a school and what we did is use an interval timer that if the system is calling for heat that out of a 20 minuets total cycle we drop out 1-1/2 minuets and are on for 18-1/2 the vacuum forms quickly with a pre and post purge 5 seconds is enough with the work of forming a vacuum being done by the purges. We lock it out on recovery from set back then we go time based. I concur with the other reply; that if at all possible try to balance the system. Ours was a 1911 Parochial School that had been ineptly cared for and after balancing and venting we still had to do the timer, for there was no money for the restoration required to make the piping right at that time. Your question is will 20% in put form a vacuum in the radaitors that need them. I would hazzard that is site specific and long odds.
Good luck and let us know.