Cons of operating system in vacuum?

Fizz

We know what are the benefits of operating steam in a vacuum; but what are the potential drawbacks?

EBEBRATT-Ed

Air leaking in through leaks or bad traps can cause problems with the vac pump if the system is not maintained. Basically vacuum systems suffer more from lack of maintenance than other steam systems.

Cost of operating vac pump and maintenance may be offset by increased comfort and system efficiency....generating steam at a lower cost due to reduced operating pressure

Fizz

So not much. My system is Richardson, and with no vac pump. All I did was replace 2 air vents with vacuum vents. This will be 3rd year. Run system at 3cph(hot water setting) and heat is steady. vacuum remains at between 2-4hg.

EBEBRATT-Ed

Thought vacuum vents were no longer made?? No drawback then

Fizz

Actually Mepco quick vent QV-1 is a vacuum vent, that can also be ordered without the check ball featured in vacuum product.

The Steam Whisperer

I suspect one may be that there could be more oxygen introduced into the system if it is leaking. This could lead to increased corrosion. As for vacuum vents, we make our own using Apollo number 61-104-A1 check valve with no spring along with a standard vent. Another advantage to vacuum systems ( if it is an induced vacuum system) is that you shouldn't need nearly as much venting... saving costs on vent installation. Our induced vacuum systems tend to run as deep as 8 to 10 inches right after a firing cycle and then drop to 4 or 5, IIRC, at the beginning of the next cycle.

Fizz

That's what I get when running steam cycle(1cph), 8-10, then drop to 2-4.

MilanD

So, in theory, one could adapt this on 1-pipe using reducer from the rad vent and putting 1/4 version of this Apollo check valve between the rad and the vent, then also one on the main vent? Literature say cracking pressure on the check valve A1 is 0psi? Would this be the way to turn all 1 pipers (w/o condensate receiver) into a vacuum system??

Fizz

I'll leave that to the pros on this site.

The Steam Whisperer

Probably so, you just need something to allow the air to vent at low pressure and then not let it back in.

MilanD

> @The Steam Whisperer said:
> Probably so, you just need something to allow the air to vent at low pressure and then not let it back in.

Wouldn't this check valve not allow that if cracking psi is 0? I'm thinking scenario is:

1. Cold start, air in pipes.
2. Steam pushes air out of check valve/vent combo on the main and rads.
3. Steam hits check/vent - vent closes.
4. Steam condenses, vent opens creates pull on the check valve - check valve closes.
5. Steam condenses more - creates vacuum.
6. System goes into vacuum- water boils at lower temps.

I can't be the only one thinking of this. @IgorZhadanovsky @AMservices @izhadano does this make sense on 1 pipe system, without doing what you're doing? What am I missing?

PMJ

Fizz said:

So not much. My system is Richardson, and with no vac pump. All I did was replace 2 air vents with vacuum vents. This will be 3rd year. Run system at 3cph(hot water setting) and heat is steady. vacuum remains at between 2-4hg.

Right. No negatives in 2 pipe for sure. Only big positives. Systems running induced vacuum have dramatically less air in them overall and therefore less corrosion. Are there any positives to backfilling the entire system with room temperature air every time the burner goes off? I can't think of one.

The Steam Whisperer

So, in theory, one could adapt this on 1-pipe using reducer from the rad vent and putting 1/4 version of this Apollo check valve between the rad and the vent, then also one on the main vent? Literature say cracking pressure on the check valve A1 is 0psi? Would this be the way to turn all 1 pipers (w/o condensate receiver) into a vacuum system??

Actually I was agreeding with you, the above should work just fine.

AMservices

I've been thinking about air vents with check valves and the reasons why they stopped working with modern steam boilers.
What I've read is people experienced unbalanced heating, after converting from wood or coal to oil.
The boiler comes on, starts making steam and the air is pushed out through the vacuum vent ( some of the air).
Steam hits the air vent and it closes trapping air in the radiator. The boiler continues to fill the system until it trips the pressure troll and shuts the burner.
Now that the Boiler is not filling the system, The pressure drops away and a vacuum starts to rapidly form in the radiator. But steam is not the only gas that's expanding in the vacuum. The air will also expand.
When the pressure troll closes again, There can be a difference in pressure between the boiler and the radiators. The vacuum forming in the radiators is keeping the check valve closed At the same time the boiler is trying to cram more steam into the piping.
In my mind I'm seeing a tug-of-war of pressure differences throughout the system with air caught in the middle.
When enough pressure finally forms to push the check valve open, The steam could have been beating the air To the vent and rapidly closing.
Everything happens for a reason and there must have been a good reason for something is wonderful as a vacuum vent to disappear from the market.
It was different with the coal fired boiler because the heat source was always burning. So even when steam hit the vent causing it to close the vacuum forming would still be pulling steam out of the boiler at lower temperatures, Steadily compressing the air Against The vent, Pushing the check valve open and allowing more air to escape.
So I don't mess around, I like taking air vents out and controlling the lowest point of pressure with a mechanical vacuum pump. I balance the air leaving the radiators And steadily filled the system on the 1st cycle.

The only way I see a vacuum vent Being reborn is when steam boilers become fully modulator, With an even more accurate pressure control than a vapor stat.

All this only applies with single pipe steam systems.
2 pipe steam systems can still work beautifully with a single vacuum vent at the very end of the line, Homemade vent or not.

MilanD

@AMservices - Thank you for that great explanation!

AMservices

> @MilanD said:
> @AMservices - Thank you for that great explanation!

My pleasure

MilanD

> @AMservices said:
> > @MilanD said:
> > @AMservices - Thank you for that great explanation!
>
> My pleasure

I knew I was sure missing something otherwise vacuum vents would still be around, as you said. Thanks again!

PMJ

> @AMservices said:
> I've been thinking about air vents with check valves and the reasons why they stopped working with modern steam boilers.
> What I've read is people experienced unbalanced heating, after converting from wood or coal to oil.
> The boiler comes on, starts making steam and the air is pushed out through the vacuum vent ( some of the air).
> Steam hits the air vent and it closes trapping air in the radiator. The boiler continues to fill the system until it trips the pressure troll and shuts the burner.
> Now that the Boiler is not filling the system, The pressure drops away and a vacuum starts to rapidly form in the radiator. But steam is not the only gas that's expanding in the vacuum. The air will also expand.
> When the pressure troll closes again, There can be a difference in pressure between the boiler and the radiators. The vacuum forming in the radiators is keeping the check valve closed At the same time the boiler is trying to cram more steam into the piping.
> In my mind I'm seeing a tug-of-war of pressure differences throughout the system with air caught in the middle.
> When enough pressure finally forms to push the check valve open, The steam could have been beating the air To the vent and rapidly closing.
> Everything happens for a reason and there must have been a good reason for something is wonderful as a vacuum vent to disappear from the market.
> It was different with the coal fired boiler because the heat source was always burning. So even when steam hit the vent causing it to close the vacuum forming would still be pulling steam out of the boiler at lower temperatures, Steadily compressing the air Against The vent, Pushing the check valve open and allowing more air to escape.
> So I don't mess around, I like taking air vents out and controlling the lowest point of pressure with a mechanical vacuum pump. I balance the air leaving the radiators And steadily filled the system on the 1st cycle.
>
> The only way I see a vacuum vent Being reborn is when steam boilers become fully modulator, With an even more accurate pressure control than a vapor stat.
>
> All this only applies with single pipe steam systems.
> 2 pipe steam systems can still work beautifully with a single vacuum vent at the very end of the line,

I'm not so sure about this analysis. Someone needs to try it starting one rad at a time.

If you want to run natural vacuum I would never fill to a pressure stop. Rads are much fuller than they ever need to be at a pressure stop. Never got anywhere near that high(or that full) in the original design. When the burner goes off the whole system will be sub atmospheric with only tiny pressure differences in the rads. Steam from the mains will continue to flow into the rads. When the burner comes back on all the vacuum will be gone in just a few minutes. The entire system then changes gradually to very low pressure and vents will open again. I just don't see air being trapped. I agree that vent lines to a pump is preferable but I think this will still work without.

Someone with a one pipe please vacuum vent just one rad. I think it will grab a bigger share of the total steam than it was open vented. Doing more of them will do the same thing.

One thing to consider might be to put the vacuum vent near the bottom. That is where the air goes out on two pipe.

AMservices

@PMJ, there has to be a reason why vacuum vents went obsolete around the same time the fuel became automatic.
Really don't think it's as easy as testing 1 radiator.
Any vacuum forming will just pull air in from a non-vacuum vent.
Air Vents need to be a little ways up the side of the last section on the radiator because if it was too close to the bottom the condensation that collects at the bottom of the radiator and the steam passing over it could create a wave that would hit the air vent and break it.

PMJ

> @AMservices said:
> @PMJ, there has to be a reason why vacuum vents went obsolete around the same time the fuel became automatic.
> Really don't think it's as easy as testing 1 radiator.
> Any vacuum forming will just pull air in from a non-vacuum vent.
> Air Vents need to be a little ways up the side of the last section on the radiator because if it was too close to the bottom the condensation that collects at the bottom of the radiator and the steam passing over it could create a wave that would hit the air vent and break it.

Why not test one radiator? And if you like what happens try another. No harm is there?

With the dramatic improvement in my system with natural vacuum I can't explain why it went away. The explanation now is always that air gets trapped and does weird stuff. I admit I only know 2 pipe. There is no weird stuff there. It is definitely more difficult to get a system going in vacuum than vented. Maybe customers just wouldn't pay for it.

These systems were designed with mains always hot and always full of steam. That is how they are when you cycle in natural vacuum. I ask again - what advantages come from filling the rads and mains with room temperature air between every burn?

AMservices

These systems were designed with mains always hot and always full of steam. That is how they are when you cycle in natural vacuum. I ask again - what advantages come from filling the rads and mains with room temperature air between every burn?

There's a give and take with everything mechanical.
If people don't want to shovel coal, they can't use vacuum vents on there single pipe steam systems.
First generation central steam heating was designed with the intention of using vacuums to their advantage. When the fuel changed, so did the boilers controls.
What are people supposed to do?

PMJ

What should people do? Don't run to a pressure stop, deliberately cycle instead. Everything changes then and the system can be run much more like originally designed and it is much easier to take advantage of natural vacuum. I am doing it and so are a few others. The improvement is startling.

I can't know that it will work in one pipe without an air line. Perhaps not. All I can say based on what I have seen is that if my system were one pipe I would be trying it.

AMservices

Depending on the size of the pipes used by the engineer, In order for the boiler to heat the radiator at the end of the line it needs to be able to build a little pressure to overcome the system's pressure drop.
Usually no more than 2PSI.

PMJ

AMservices said:

Depending on the size of the pipes used by the engineer, In order for the boiler to heat the radiator at the end of the line it needs to be able to build a little pressure to overcome the system's pressure drop.
Usually no more than 2PSI.

I don't find this to be true. If enough steam to heat could travel to the rads in these systems with a coal fire without such pressures then any other fire can do it too - run the right way. The coal systems had sophisticated pressure monitoring devices that automatically damped the fire. I think we all know they were not set to anything like 2psi or everyone would have been roasted. So my conclusion is that nothing like 2psi is required - and others have confirmed this too. Enough steam will travel to the rads at very low header pressures.

PMJ

One other comment about pressure drop:

Pressure drop is related to flow rate. Coal fired systems delivered steam very slowly all the time. The flow rate and pressure drop to the last rad were both much lower because there was much more time to get the job done. The vented, hurry up and get there on/off approach changed all that and in my opinion not for the better.

I have been suggesting for some time now that horsing air in and out every cycle really degrades the performance of these systems a lot. When you vent rads and mains to the room you start sucking the steam back to the boiler the instant the burner goes off. The room air cools down your delivery system - the opposite of what you want. I am saying that running with even controlled cycles and natural vacuum in between them results in forward motion of the steam at all times and no unwanted cooling of the delivery system. The result is much more like the original design. The heat is more even by a lot and also more efficient.

AMservices

@PMJ
I've never worked on a coal fired steam system, so I guess I really can't say what pressure the dampers closed at.
All I can say, from what I've read is 2 psi was the standard.
I agree that steam should always be running at as low of a pressure as possible.
Most steam systems today can't support a vacuum very long in between cycles (due too a lot of small leaks) without major restorations.
I'm always up for the challenge, but l don't think your average home owner is.

One more comment about vacuums.
Mechanical induced vacuum systems are better then naturally induced vacuum systems

PMJ

@AMservices ,

It would seem that any homeowner that ends up on this site is definitely not average and therefore unusually interested in knowing something more about his system. It is those people - the unusually interested, that I think might be looking at this.

I expect also that contractors won't be particularly interested. I can't imagine homeowners paying them for the time required to fiddle with vacuum - even though it isn't all that much. There are no inexpensive off the shelf controls for them to use - though @Fizz seems to be doing well with a standard control.

I don't know what 2PSI is a standard for exactly, I just know that no where near that is required to heat. Many on this site have demonstrated this in all kinds of systems one, two pipe, vented and vacuum. Quite a few have reported that they operate at low single digit ounces.

What I am writing for is just to make those unusually interested aware that even low levels of vacuum achieve a lot of improvement and that low levels are easy to achieve even on leaky systems. Mine is leaky. But if you run 3 CPH the vacuum only needs to last 10-15 minutes between cycles which is almost nothing. I and several others like @Fizz in this thread do it. We have done very little work to achieve significant vacuum in old systems.

I am not attempting in any way to suggest that natural vacuum is better than mechanical. I am saying that natural vacuum is so easy and inexpensive in 2 pipe and the results so significant that I encourage readers here to try it. I am also interested in what would happen in one pipe and hope someone will try some things and tell us about it.

jumper

For a house the following may be too complicated. Plug the air vents;evacuate the system mechanically. Control with timed firing. Determine how often you have to re-evacuate.

Some buildings need most of its heat at only certain times of day. For example,ski lodges need heat early in the morning and then again late afternoon.

An advantage to hard vacuum heating on single pipes is that you can get away with smaller radiators. We always talk about square feet of radiation. But does a normal single pipe ever get the whole radiator hot?

jumper

PMJ said:

> @AMservices said:

>
One thing to consider might be to put the vacuum vent near the bottom. That is where the air goes out on two pipe.

>>An advantage to hard vacuum heating on single pipes is that you can get away with smaller radiators. We always talk about square feet of radiation. But does a normal single pipe ever get the whole radiator hot?<<

Koan

@Fizz - Im looking for more info on the QV-1, can you please give me a hand?

Fizz

I'll try. What do you want to know?