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Finding Vacuum Leaks

Hi folks

I have an old vacuum system that I've been trying to restore for the past few years. With the help of some more-than-competent plumbers--some who post here regularly (thanks Dave and Charles!)--we have made vast improvements to the system. We are heating evenly at low pressure and the vacuum system is back on line after many years of being out of service.

That said I have been chasing down vacuum leaks for the past couple of heating seasons, and though I have made significant improvement (it could hold a 2"hg vacuum for about 20 sec and now go for about five minutes) it is still quite leaky. The way I have been operating it is that I manually turn on the vacuum on the coldest days during the shoulder months and let it pump away continually for the really cold spells. I'm not sure if it can be wired so that the vacuum pump comes on only during a call for heat but the pumps are still actuated by the floats in the receiver at all times. It takes about 15 min. to evacuate the system to 2" hg, but maybe it is enough that it is pulling air only at start up?

Here's my main question, though: does anyone have a good method to quickly identify and seal vacuum leaks? I have been using a mechanics stethoscope, which has not provided sure-fire results, and it turns out than many of the leaks we've had are from less-than-perfect-seals at the thermostatic trap covers (we did complete gut replacements a couple years back) As well, I think I that one or more of the boiler sections is leaking air into the combustion chamber (I can detect a ssssssss through the rear of the boiler vent only when pulling a vacuum), yet I have not seen steam coming out the chimney in any significant way. The boiler has been abused and may not have many years left in it, but I'd like to keep it going for as long as I can. Can anyone recommend a cleaner/stop-leak product combo that I might have decent results with at this point? I've been looking at the Hercules products.

I figured now that spring is here I would have a chance to completely fill the boiler and bring it up to temp with the proper stop-leak product, then flush and fill, cycle it a couple of times and let it rest. That might give me an opportunity to chase down more leaks in the summer with only the pump(s). I know nothing about these products, however, and do not want to do more harm than good.




  • Charlie from wmass
    Charlie from wmass Member Posts: 4,166
    Hello Jim

    The best way to find a leak is to close off the boiler supply and return valves, remove the side jackets and bring it up to 5 to 10 pounds pressure. A leak is very straight forward to find under pressure. The stop leak often fails to work if the leak is above the water line and causes issues with flow pipes if used in excess. You can always call my cell # to talk in detail.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • Jim_from_Worcester
    Jim_from_Worcester Member Posts: 25
    Hi Charles,

    Thanks for the tip. I'll call you next week.

  • Pumpguy
    Pumpguy Member Posts: 445
    I assume

    you have a 2 pipe system with vacuum condensate return pumps.  If so, you may be dealing with 3 separate problems; valve leakage in the vacuum pump, air leakage in the return lines,  and also leakage on the steam side.  

    As a first step, I would check for valve leakage on the vacuum pump. You may be loosing vacuum here, rather than in the system.  Different vacuum condensate return pumps have different valve arrangements.   If I knew what vacuum pump you had, I could offer specific guidance. 

    As for finding vacuum leaks in the return lines, the peppermint vapor test is probably the best.  You don't need a high volume compressor for this.  Using the air discharge from a shop vac should do the trick.  Valve off the vacuum pump's receiver tank, and then blow peppermint vapors into the return lines.

    I can't tell you where leaks usually occur, but I would suspect around valve stem packing or maybe corroded threaded joints. 

    Ideally, your vacuum pumps should be controlled by vacuum switch(es).  The usual settings are on at 3" Hg, off at 8" Hg., maintaining an average of 5.5" Hg. vacuum throughout the heating season. 

    What steam pressure are you heating with?  You really don't need more than 2 psi.

    Please let us know how this turns out.
    Dennis Pataki. Former Service Manager and Heating Pump Product Manager for Nash Engineering Company. Phone: 1-888 853 9963

    The first step in solving any problem is TO IDENTIFY THE PROBLEM.
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,166
    He is firing low pressure

    with a two stage Powerflame dual fuel firing on natural gas. How much peppermint would you need to use for a 7 story building?
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • Pumpguy
    Pumpguy Member Posts: 445
    I really can't say

    how much would be needed.  On a vacuum heating pump of the type I believe this is, the usual arrangement is to add the peppermint oil to the receiver to mix with the condensate.  Valve off the receiver from the system, and open it to allow air from atmosphere to enter.  Now, make a temporary connection from the vacuum pump's air discharge pipe  to the return line, upstream of the closed off valve, blow the mint vapors into the system, and follow your nose to the leaks. 

    I suppose a shop vac with some water and mint oil in it's tank and it's air discharge piped into the return line could also be used to blow in the vapors. 

    I was advised to use "Creme de Minthe water soluble oil flavoring".  My records show this is available from Bickford Flavors, Cleveland, Oh.  Phone 1-800 283 8322 or 1-216 531 6006.  My notes from 2007 show a price of 6.95 for 4 oz. or 24.59 for 12 oz.  I'm told they take Visa and MasterCard. 

    I hope this is of some help.  I would really like to know if this test is actually performed and what the results were.
    Dennis Pataki. Former Service Manager and Heating Pump Product Manager for Nash Engineering Company. Phone: 1-888 853 9963

    The first step in solving any problem is TO IDENTIFY THE PROBLEM.
  • LarryC
    LarryC Member Posts: 331
    How about a sniffer?


    I am under the impression that HVAC techs have a device used for determining locations of refrigerant leaks.  Could something like that be used to pinpoint leaks?  It might be more sensitive in finding the leaks without smelling up the building.  ( I personally do not like the smell of peppermint.)

    I am also curious what you would use to seal up any pinholes or leaking joints you would find.

    Do you have the ability to isolate a single floor at a time when searching for leaks?
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