Question regarding pressure at various points in residential system

ethicalpaul

I'd appreciate any and all (positive and negative) input on my drawing here which I tried to make as accurate as possible. Does anyone see anything wrong with the pressures I have here?

Given: a typical residential one pipe system currently running at a pressure of 5psi as measured by the accurate gauge. Allow very small tolerance on these "5psi" values, well under 1" of water column, due to very tiny localized pressure differences due to boiling action, etc.

The picture is trying to illustrate that in a call for heat, the radiators are condensing steam which causes a low pressure zone which allows the steam to flow from the steam chamber, up the main, to the radiators, with generally lower pressure seen as you get closer to the radiators.

As a result, there is also going to be a lower-pressure area at the water level in the drop to the wet return, and this causes the the well-known raised water level at that point.

Thanks for all your input, I'm making sure I'm thinking correctly about this!

Jamie Hall

Nothiing wrong with the pressure relationgships -- but the overall pressure is MUCH too high. You shoould never run more than 1.5 to 2 psi. Adjust your pressuretrol.

If nothing else, most vents misbehave -- if not fail -- if subjected to more than 3 psi (their rated working pressure).

ChrisJ

That header needs pitch and the boiler water looks dirty.

ethicalpaul

Jamie Hall said:

Nothiing wrong with the pressure relationgships -- but the overall pressure is MUCH too high. You shoould never run more than 1.5 to 2 psi. Adjust your pressuretrol.

If nothing else, most vents misbehave -- if not fail -- if subjected to more than 3 psi (their rated working pressure).

It was just a number for ease of discussion...you must know that I know appropriate for pressures after hanging out around here for 3 years (or is it 5??). Or you are just funnin' me?

Jamie Hall

Soirry, @ethicalpaul -- I didn't read who had posted. My bad.

Long Beach Ed

Looks great. You may note somewhere that the steam main's friction is what is reducing the pressure as the steam finds its way to the return. Otherwise it may be somewhat a mystery.

bburd

The condensate must also overcome friction in the return pipe to return to the boiler. Given the very low flow rate though, this is probably a very small effect.

ethicalpaul

Thanks all above! Those issues Ed and bburd raised won't affect what I'm thinking about, they are very very tiny I think. Thanks again and more comments are welcome! And thanks Jamie, I appreciate your comprehensiveness here and always!

ChrisJ

There's energy lost from the radiator legs dragging on the floor as the radiators heat and cool too.......
And the piping rubbing on hangers as it expands and contracts....

EBEBRATT-Ed

If the steam in the boiler is at 5 psi the return water entering the Hartford loop must be over 5 psi so the condensate will return to the boiler. Where does this pressure come from? The left over steam pressure in the main is helping + the height of the return water backing up in the returns so the blue portion of the Hartford would be above 5 psi, but the difference is small and hard to measure.

ethicalpaul

Thanks Ed. We know that condensate does return to a boiler that is holding a given pressure, so as long as my values are pretty close I think I'm OK. I think the height of the return drop water column as you said keeps us at a consistent water level. Stay tuned!

Tim_D

If you are looking to illustrate the effects of operating pressure on the water level in the return this would be a good example and if were teaching with this I would note that a 5psi operating pressure would cause the return water to stack to 11.55 feet above the water line.

ethicalpaul

Thanks Tim! I agree but in operation it won't do that because the air/steam pressure at the end of the main is surely above atmosphere and probably pretty close to 5psi. Sound right?

Jamie Hall

ethicalpaul said:

Thanks Tim! I agree but in operation it won't do that because the air/steam pressure at the end of the main is surely above atmosphere and probably pretty close to 5psi. Sound right?

In a one pipe system such as you have diagramed, yes -- there will normally be very little pressure diffference, and that due to friction in the main.

Two pipe dry returns... different story.

ethicalpaul

I know so little about two pipe—would the pressure be a lot more different at the boiler compared to the end?

109A_5

Tim_D said:

If you are looking to illustrate the effects of operating pressure on the water level in the return this would be a good example and if were teaching with this I would note that a 5psi operating pressure would cause the return water to stack to 11.55 feet above the water line.

Except for the function of the equalizer pipe, which is in the illustration above.

ethicalpaul

Interesting you said that @109A_5

what is the function that you see it performing?

Jamie Hall

ethicalpaul said:

I know so little about two pipe—would the pressure be a lot more different at the boiler compared to the end?

No. What is different is that the pressure in the dry return is -- or at least should be! -- very close to atmospheric. Pressure in the wet returns is as you have shown it.

ethicalpaul

OK thanks Jamie! I would add that is also the case for correctly-sized one-pipe systems. Mine (still slightly oversized) is usually 1" of water column at the boiler and I think it must be near atmosphere at the return...I'll install a gauge there to see!

If a one-pipe is running at 5psi I think we'd expect the entire system to be pretty close to 5psi with the lowest pressure in the radiators, but not too much lower because, due to the obvious over-sized boiler, steam is always getting pushed everywhere it can with significant force, so localized areas of low pressure don't last long. Sound reasonable?

ch4man

hmmm, I like to think in terms of absolute pressure. +pressure at the steam chest happens due to the expansion of the steaming process. now in the radiator steam collapses and can in fact lower to the point of vacuum on the gauge scale. that difference in pressure between boiler and radiator is the driving force to move steam through the system. now condensate being way more dense than steam "falls to the wet return and stack higher than the NOWL that also must induce a delta P to drive condensate into the boiler. so sorry Paul there are at least two different points in the system that have a pressure difference. (sorry about the one big paragraph, my laptop tab function is eluding me.

ethicalpaul

Thanks @ch4man but I don't think we are disagreeing. I see different pressures at each radiator, at each point in the main (especially just past each radiator runout), and at the drop to the wet return.

Everywhere where I indicated "< 5psi" I can't know the exact pressure of course but I think that saying it's less than 5psi is accurate. And each of those is likely different if that isn't clear.

I do think the pressure at the far end of the wet return is the same as the steam chamber when measured at the boiler water line in the drawing (with occasional variation that levels out over time).

All my pressures indicated are actually PSIG, sorry about that

ChrisJ

Wouldn't the equalizer push against the wet return?

ethicalpaul

Yes it would push against the wet return with a pressure that already exists at the wet return and the boiler, resulting in no effect.

The belief seems to be that it provides pressure that keeps the water in the boiler, but it looks to me like it pushes equally toward the boiler and away from the boiler (again, with the same pressure that is already there, so no effect anyway)