Vacuum and condensate return opinions

ChrisJ

I know this has been talked about from time to time, and even argued.

I'm curious why anyone thinks vacuum would stop condensate from returning? I'm sure it's been mentioned but my memory isn't what it used to be.

Jamie Hall

Depends on where the "vacuum" is. It helps — a lot — to remember that all one means by the term "vacuum" is the pressure at that location is less than the local pressure of the atmosphere.

And then… water, including condensate, will always flow from a higher pressure to a lower pressure. So there are conditions (almost always misguided piping, or traps in odd places) which can cause lower pressures out in the system somewhere than in the boiler or wherever… and then the condensate will happily flow towards the lower pressure.

ChrisJ

https://forum.heatinghelp.com/discussion/comment/1810283#Comment_1810283

Why does atmospheric pressure matter in an operating steam heat system?

Jamie Hall

Because that is the pressure seen if there are open vents. If the system is completely closed, it doesn't matter — all you are worried about is absolute pressure differences.

ChrisJ

https://forum.heatinghelp.com/discussion/comment/1810290#Comment_1810290

Is there ever a time when a reasonably well operating steam heat system has more than a very small stream of water in any of it's piping?

Jamie Hall

https://forum.heatinghelp.com/discussion/comment/1810291#Comment_1810291

shouldn't be, except the wet returns!

ChrisJ

https://forum.heatinghelp.com/discussion/comment/1810292#Comment_1810292

So theoretically speaking,

There's never anything that could cause the condensate not to leave the radiators, steam runouts, mains etc? It should all roll back via gravity just like it always does. If there's vacuum present in the radiators, it's also going to be similar at the boiler via the mains and header.

The Steam Whisperer

I don't think that is true, Chris. When a two pipe vacuum steam system shuts off, the collapsing steam in the supply side (piping and radiators) will cause quite a bit of vacuum, which is can hold water in the radiators until it subsides. On vacuum systems there is supposed to be a piping loop with a check valve under water that allows the excessive vacuum in the supply on shut down to balance back into the return so water will flow freely back to the boiler.

mattmia2

https://forum.heatinghelp.com/discussion/comment/1810293#Comment_1810293

Only if air can flow past water which may not happen at something like the seat of a steam trap.

EBEBRATT-Ed

If you study mechanical vacuum systems, the only real reasons they are used is

1. To operate the systems at lower pressures
2. to return condensate to the boiler where gravity can't be used.
3. to enable smaller piping to be used in the system

Some vacuum systems may have some return lines where the vacuum actually lifts the condensate to a higher elevation buy using piping designed with step lifts

ethicalpaul

if you’re talking one pipe residential, piped correctly, there is no condition that I’ve heard of or seen that could cause vacuum to hold condensate.

PMJ

https://forum.heatinghelp.com/discussion/comment/1810293#Comment_1810293

Correct @ChrisJ.

Speaking from experience going on 20 years now operating a completely closed 2 pipe Mouat system in as much natural vacuum as it will produce between cycles; no pipe anywhere ever has more than a trickle of water in it, the entire system is essentially at the same (below atmospheric) pressure within fractions of ounces, and I've never seen a hint of any change in the gravity induced flow of water back to the boiler from what it was open vented.

Pumpguy

https://forum.heatinghelp.com/discussion/comment/1810297#Comment_1810297

What Dave is referring to here is the equalizer line. These usually use a water sealed swing check valve, but a thermostatic radiator trap serves the purpose just as well.

In either case, when this valve opens, the pressures on the steam and return sides equalize and allow condensate to gravity flow back to the condensate or vacuum pump.

The attached file explains the details.

equalizer lines.pdf

jumper

I haven't seen anyone try this. Seal all vents, both radiator & main, on a one pipe system. Use some sort of vacuum generator in boiler room to evacuate air before firing boiler. I don't see why condensate won't return due to gravity. Using steam to push air is thermodynamically wasteful.

ethicalpaul

Sealing all vents, buying and running a vacuum pump has no cost? How far down do you want to draw it? The water will boil at room temperature and start to fill the system with room temperature steam (or something!) Does that heat the house?

It's an interesting thought experiment, but there's no free lunch. It's not that wasteful to push air IMO

But to bring it back to Chris's original question: This setup would still allow condensate to flow (I agree with you). The whole system would be the same low pressure and the liquid water would still flow downhill.

Pumpguy

"Its not that wasteful to push air IMO". Maybe, maybe not, but in the shoulder seasons when the boiler is firing just a few hours a day, the warm-up time difference with vacuum and and without vacuum can be considerable.

The attached file makes for some interesting reading on this subject.

NPGPR.pdf

ethicalpaul

I dare say if those schools required 6 to 8 psi to get the steam around, then those systems were poorly maintained and understood. The article says as much. Even after installing the vacuum they were still running too much pressure.

Regardless, a typical residence is much different than schools like that.

But I don’t know anything about such things except what I’ve read.

In the shoulder seasons, my system’s time is spent heating the mains and runouts. But it’s ok, it’s the shoulder seasons, heating is easy

mattmia2

If it is a 2 pipe system and they are using steam to lift condensate they need enough pressure differential to lift the condensate.

ethicalpaul

I admit I am ignorant of how and why condensate has to be lifted anywhere when it comes down from the steam that is already carried to the upper levels, but I keep hearing about it so I know it must be a thing.

mattmia2

https://forum.heatinghelp.com/discussion/comment/1810537#Comment_1810537

It may need to go up over an obstruction or the returns may be overhead or the emitter may be below the boiler.

ethicalpaul

Thank you. None of those seem like a good reason to run a steam boiler at 6-8psi…and if they did need it that high, I can't figure out why in the article they used vacuum to lower it. And if they could lower it, why didn't they just run the boiler less hard instead of paying for a vacuum pump? None of these questions need to be answered, you have been kind with your time already…they are just the questions that come to mind.

mattmia2

If you need to lift a column of water you need a differential greater than the weight of that column of water. if the returns are under vacuum the pressure you need at the boiler to create that differential is less so you don't have to heat the boiler as hot to get the higher pressure you would need if the returns were at atmospheric pressure or higher. Many of these systems have a condensate pump of some sort so the pressure of the returns is decoupled from the pressure in the return to the boiler.

jumper

People set high steam pressures because they think that steam "blows" air and condensate down the trap. I've heard this multiple times from guys who ought to know better.