Please analyze steam production timing of my system

Motorapido

Please review and comment on the performance of my one-pipe parallel-flow steam system. I installed a 0 to 3 psi gauge on a T over the pigtail onto which the pressuretrol mounts. I timed my system from start-up and measured the following results. Venting on mains is grossly inadequate. Forum members kindly helped me calculate what I need to fix that problem. I decided to perform this test as a base test before I vent the mains, so I can measure improvements after I install my Big Mouth vents on the mains and then balance the radiators after installing new radiator vents.

House is 85-year-old brick and plaster, two-story, 2,500 square feet. Attic has decent insulation, but who knows what, if anything, is in the old walls. Windows are original, single pane, and they seal well for their age, all things considered.
Boiler is about 6 years old, Burnham, natural gas. I think it is about 100,000 to 140,000 btu, but I need to check that in the spec sheet sticker. Boiler piping seems to follow best-practices, based on my research on this forum.
I will be calculating my radiator EDR this weekend, and can't provide that info just yet.
Mains are well insulated.
Wet returns uninsulated.
Basement is concrete block walls, well sealed and bone dry but uninsulated. Basement floor is concrete. Basement windows are pretty weather tight.

Outdoor temperature 60 degrees at time of this test (temporary heat wave in central Pennsylvania), no wind.
I shut off the boiler for 90 minutes before performing this test to let the system cool down. Mains and returns felt just a bit warmer than room temperature to the touch just before the start of this test, but all radiators felt cool to the touch. Thermostat showed 66 degrees at start of this test.
Thermostat set to call for 68 degrees.
Pressuretrol set to 0.5 and 1 – as low as it will go. I am not sure how accurate the pressuretrol is, but I do know it is about 6 years old. I will soon be installing a vaporstat in series with the pressuretrol, since I like wearing suspenders and a belt.
Thermostat is a cheapo Honeywell electronic setback model, at the bottom of the model lineup. It is set for steam/water heat and set to 2 cycles per hour (lowest cph setting option).

At the beginning of this test, from the time the boiler first fired, the needle on the 0 to 3 psi gauge did not move until 4 minutes elapsed, at which point the needle still stayed on 0, but began to vibrate a little.
At 6 minutes of boiler firing time, the gauge needle moved to 2 ounces.
At 9 minutes it showed 3 ounces.
At 15 minutes it hit 4 ounces.
It took just 30 more seconds to move to 6 ounces, at 15:30.
3 more minutes to reach 7 ounces, at 18:30.
It reached 8 ounces at 19:15 but then quickly dropped back to 7 ounces.
At 21:10, it returned to 8 ounces.
For the next 40 seconds, the needle rose steadily and stopped at 12 ounces, at 21:50.
It then climbed steadily again and reached 15 ounces at 22:39.
It reached 16 ounces at 23:00.
It then quickly and steadily rose again to 18 ounces at 23:25, at which point the boiler turned off.
The thermostat showed 68 degrees when the boiler turned off, which was the temperature I had set on the thermostat before beginning the test. This test cycle increased the measured heat in the room where the thermostat is located by 2 degrees.
I cannot be sure if the boiler turned off at the moment that the thermostat was satisfied, or if it turned off on pressure as regulated by the pressuretrol. I cannot know for sure, since it is impossible to know the accuracy or lack thereof from the pressuretrol.
Within 4 minutes of the boiler shutting off, the 0 to 3 psi gauge on the boiler returned to 0. I was surprised that it dropped so quickly. I am not sure if that is normal or not. I became curious about the accuracy of the 0 psi indicator reading on the gauge, and decided to live life on the edge and open the emergency over-pressure valve, to test for steam pressure, and when I did, I felt no resistance as I opened the blow-off valve and heard no steam escape, and no water dripped out.

Please provide any insights and consultative advice based upon your analysis of this data. Many thanks in advance. I’m in awe of the knowledge of members on this forum, and I really respect the logical thinking that I see displayed in various threads. I have belonged to mechanical forums of other types in which the answer always seems to be “just get a bigger hammer.” I like the analytical thinking on this forum. The bigger hammer sometimes has its place, but more often the jeweler’s tools are the best match to the thinking mechanic’s work.

Jamie Hall

I'd say, off hand, that what you are seeing is more or less typical of a system with inadequate main venting. It will be interesting to see what it's like with the new vents. As to the rapid drop at the end of firing, that's to be expected -- the steam condenses pretty fast, and as it does the pressure is going to drop equally fast.

ChrisJ

I agree with @Hatterasguy

My system climbs up to 1/8 of an ounce with cold mains, and then constantly drops down into a vacuum as the steam condenses in them. Once the steam hits the main vents, the pressure stays positive.

If the mains are warm to hot, the pressure wills stay positive while the mains vent, but at no more than 1/4 of an ounce or so.

I realize my system isn't typical, but with regards to how it behaves as the mains fill with steam I feel most systems can behave similar.


My way of testing main venting also differs from most in that, I let the system run for a while and get everything hot and then shut it down for 5 minutes and fire it back up. I expect my mains to vent adequately under those conditions because that's what I see when we're below zero outside and I run 3 cycles per hour. Having the system off for only 5 minutes means the piping is all still very hot and the steam will go down them as fast as possible, with little condensing. The vents will have to vent far faster under these conditions than normal.

Any more back pressure than necessary means sending steam out to radiators prematurely before it gets to the end of the mains. This leads to some of the radiators heating before you may want them to.

PMJ

My first observation is that after only 90 minutes off on an insulated system the mains won't have cooled all that much. My mains would not be close to room temperature after only 90 minutes since last fire. I just have a crude preheat sensor for now which switches probably at about 150F and I doubt it would have opened after 90 minutes.

But then I don't let any room temperature air back into my mains either. Really is a big difference with that - just sayin'.

That being said, these numbers look pretty good to me with standard controls. I'm quite sure that the pressure you are building is overfilling rads and overshooting the temp some most of the time. But it isn't that much and it really isn't a problem unless you really don't like it.

Motorapido

Since outdoor temperature was high that day, it was likely off much longer. I just shut off the stat when I got home and left it off ninety minutes for good measure to make sure all steam pressure was out. That's why mains felt cool.

PMJ

Motorapido said:

Since outdoor temperature was high that day, it was likely off much longer. I just shut off the stat when I got home and left it off ninety minutes for good measure to make sure all steam pressure was out. That's why mains felt cool.

I see. In that case I am surprised by how fast you build pressure in a "cold" system. With my system cold and only one 1/2" vent on the whole thing I might get to 2 ounces after 25 minutes straight burn - and my boiler is considered oversized by most. Just too much pipe to heat up. I've never found that venting had anything to do with anything on the initial heat up. The cold pipe gobbles so much steam and the actual progress the steam makes filling them up is so slow that the displaced air can leave through a very small vent - on initial warmup only. If my one vent wasn't enough I would certainly build pressure and I don't. After initial warmup if you are going to breath in and out you need bigger vents. I don't because I go vacuum between burns.