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understanding working steam pressure / test case

4Barrel Member Posts: 125
I read the "understanding working steam pressure" thread with great interest. I have a problem that relates to this issue, but thought it warranted a new thread. here it goes:

My scenario is similar to FJL's: 4 unit building. One-pipe steam, New boiler, oversized (after considering 33% pick up), vented mains. Is the is system balanced? well, no, but after loads of experiments with different rad set-ups and rad vent settings (using Hoffman 1A's throughout), I had the system running pretty smoothly, with reasonably equivalent heating to each unit. Tenants happy.

I had my V-stat set to 12oz. cut out with an 8oz. differential, or 4oz cut in. the system rarely ran at higher than 5-6oz. The T-Stat called, the boiler ran at 5-6oz until T-stat was satisfied, and that was that. The rad vents worked fine - once they got hot, they shut. Occasionally, I could hear them open back up, "take a breath" and close again.

I installed a new radiator & run out (because although my tenants were happy, I was not). I used a thin-tube rad w/ EDR of 35. I set the Hoffman rad vent to "fast" setting, because this run out (1-1/4") is not vented, and I was relying on the rad vent to do all the work. The boiler is still undersized. after rad install the system "threw up" at first, but after a boiler blow down and refill, it settled back to normal (pressure level as described above). No other changes. It ran this way for the month of January with no issues. Now I'm happy.

I recently refinished a American Radiator "rococo" standard tube rad and installed it in the same location, replacing the thin-tube rad (the thin tube was just temporary until i finished painting the rococo). According to the books, the EDR is the same, 35. But, I recognize that it is bigger (internal volume). I installed the same rad vent, and keep it at the fast setting. Of course, the system doesn't like this at first, crazy variations boiler water level, and hitting upper pressure limit (just like when i installed the thin-tube), so I do another boiler blow down and all seems a-ok.

After a few days, the system is behaving strangely. I am getting heat to all rads (the rococo heats to all sections), but the boiler is operating at a much higher pressure - consistently at the upper limit of the V-stat setting as soon as the boiler starts to steam - I quickly achieve 12oz, and at some points, the boiler short cycles - about 90 seconds on - until it recovers and then fires again. This occurs until the t-stat is satisfied. The short cycling occurs even though I can hear air being pushed out of the rad vents - they're open!

So, why would changing out one rad cause the entire system operating pressure to rise? Nothing else has changed. This rise in observed operating pressure seems all the more strange to me because the vents are open. I can easily add more venting to the rococo - there is a second tap. Would the added volume of air that needs to be displaced in the rococo create enough back-pressure to account for the whole system pressure to double? I'm OK with raising the V-stat cut out, but want to know what's causing this first.

Thanks for reading!
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