Vacuum system with zone valves - help needed

MarcyMgmt

Hi guys,
I have a client with a problematic heating system.
It's two apartment buildings served by a single steam boiler and a single vacuum pump.
Attached is a quick sketch of the piping for reference.

Basic system overview:

• 2-pipe steam, vacuum return
  
• Has boiler feed tank
  
• Scotch Marine - 250 BHP
  
• (2) Zones, 1 for each building, each controlled by its own Heat Timer
  
• No condensate pumps/lift units in building (there is one in garage but they're not currently heating the garage and it's valved off)
  
• System piping is a problematic because it's mostly inaccessible. Have found multiple major condensate leaks, rotted-out pipes, clogged strainers and traps.
  
• Radiation - all convectors, some steel, some CI. All radiator traps were changed in the last few years.
  

Primary Issues

• Uneven heat in buildings
  
• Overheating/underheating
  
• Inconsistent return line temperatures at Heat Timer system sensor.
  

We drastically improved the system (more consistent heat and eliminated banging) by locating leaks, getting the vacuum pump working, and adjusting the control settings including moving the system sensor. But it's still not quite where I'd like it to be. Still have up to 10deg apartment temperature differentials. I.e. one is 72 and another is 82.

Primary Questions

• Should there be an equalizer line between the vacuum pump and header? I know all vac pump schematics show it, but I haven't found a schematic that also includes motorized zone valves.
  
• Should there be that vacuum breaker on the boiler?
  
• Looks like when there's no call, vacuum pulls to the mot. valves. When valve opens, water in sight glass disappears and calls for water.
  
• Vacuum is inconsistent- Between 2-9". It's not bouncing around, but goes up and down over the course of a cycle. Seems to be related to when traps start closing. Any thoughts on this? Is it a problem or indicative of a problem?
  

Appreciate any help!

- Jake

Pumpguy

Equalizer lines are recommended downstream of each steam zone valve.

The equalizer lines should be fitted with either a thermostatic radiator trap, or dropped swing check as shown in the attached file.

Depending on weather the condensate pump that discharges to the vented boiler feed tank is discharging from a vacuum, the piping diagram as shown could cause the condensate transfer pump(s) to be come air bound and just spin but not discharge any condensate.

If you ever experience this problem, answer back and I'll go into detail about why this happens, and cures.

I don't recommend a vacuum relief valve on the boiler, but do recommend motorized valves on the discharge of the boiler feed pumps.

IMO, the problems you are describing are consistent with induced vacuum in the steam lines that are not equalized with return line vacuum.

MarcyMgmt

@Pumpguy Thanks!!
Equalizer lines downstream of each zone valve makes sense. Another thing I didn't mention before was that they have the burner cycling off at setpoint and back on at 0, rather than modulating at setpt.
So, maybe steam is condensing and forming a vacuum mid-cycle?

Without an equalizer or vacuum breaker on the boiler side of the motorized valves, couldn't we end up with excessive vacuum in the boiler?
I understand that adding solenoid valves on the feed pumps will prevent feed water from being pulled into the boiler in the off cycle, but could there be any other issues with having excessive vacuum in the boiler? Just trying to wrap my head around it...

Interesting re condensate pumps being airbound... I have not experienced that, but it doesn't mean it's not happening. All of the vents dump into a covered sump pit, so no one would notice if anything is overflowing. The water meter shows 20-50 gals of make-up every day recently.
Would you mind elaborating on that issue?

Thanks for your help!!

-Jake

Pumpguy

MarcyMgmt said:

@Pumpguy Thanks!!
Equalizer lines downstream of each zone valve makes sense. Another thing I didn't mention before was that they have the burner cycling off at setpoint and back on at 0, rather than modulating at setpt.
So, maybe steam is condensing and forming a vacuum mid-cycle?

Without an equalizer or vacuum breaker on the boiler side of the motorized valves, couldn't we end up with excessive vacuum in the boiler?
I understand that adding solenoid valves on the feed pumps will prevent feed water from being pulled into the boiler in the off cycle, but could there be any other issues with having excessive vacuum in the boiler? Just trying to wrap my head around it...

Interesting re condensate pumps being airbound... I have not experienced that, but it doesn't mean it's not happening. All of the vents dump into a covered sump pit, so no one would notice if anything is overflowing. The water meter shows 20-50 gals of make-up every day recently.
Would you mind elaborating on that issue?

Thanks for your help!!

-Jake

As my handle suggests, I'm a pump guy, not a boiler, burner, or steam system expert. I am sure others here can comment with more authority.

That said, I don't see what problem there would be with vacuum on the boiler. As we both agree, vacuum on the boiler would allow atmospheric pressure to push feed water from the vented feed tank into the boiler unless there were positive closing valves on the discharge of the boiler feed pumps that only opened when the boiler calls for water.

With vacuum on the boiler, we would continue to be making steam, but just at a lower temperature. This is especially useful in mild weather.

Without functioning equalizer lines, especially downstream of the zone valve(s), condensing steam will create a vacuum which can hold back returning condensate. This could cause a low water condition at the boiler feed tank to take on make up water.

Now when the zone valve opens, the zone pressure increases and that held back condensate now flows back to the vacuum pump and then gets pumped back to the BF tank which is now overfilled and gets dumped down the drain. IE, a starve then flood situation.

Equalizer lines do just that; equalize the pressure between the steam side and return side. Equalized pressure allows the condensate to gravity flow down to the lowest point in the system, the vacuum pump.

If your condensate pump(s) ever did get air bound, the symptom would be the vacuum pumps would be throwing water out their air discharge; condensate going down the drain. This air binding condition is caused by the vacuum pulling fluid from the boiler feed tank through the casing of the condensate pump. The first fluid would be condensate in the transfer piping. If the condensate pump did not come on before all the condensate in the transfer pipe drained out, the next fluid would be air, which pushes the prime water out of the case of the condensate pump. When that happens and the float switch turns on the condensate pump, there's no prime water in it's case, so its impeller just spins in an air bubble and doesn't pump any condensate.

The primary cause of this air binding problem is a failed discharge check valve on the condensate pump. A secondary cause is boiler feet tank inlet piping that didn't take this possibility into account. An immediate solution to this problem is to replace the discharge check valve with one that is air tight. A permanent solution is to revise the transfer piping to the boiler feed tank inlet to have a permanent water seal. This can be done by filling the boiler feed tank from the bottom, adding an internal submerged outlet fill pipe, or a tie in from the boiler feed tank's botom drain connection.