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One Knock only in my Heating system

h12721aa Member Posts: 17
edited February 2015 in THE MAIN WALL
Some time ago I posted here about a “One Knack” in my heating system. I found the problem and it was eliminated.
. I hooked up a pressure Gage to the system. Boiler is off and no call for heat.
Hot side of the boiler = 25# and on the return side it showed a vacuum. I open Zone Valve manually and heard the knack and Vacuum disappeared. The pressure went back to 25# on the return side. I then closed the zone valve and it went back to a vacuum on the return.
Could some one confirm that when a pipe cools from 170deg. to 140 or less, a vacuum would develop? Or I am on the wrong Track?
I am to post a PDF of the hook up,. Don't know if the Site takes it. The PDF shows a By Pass between the system supply and the return. It was omitted as the print shows “ If required “. I did add the By Pass after the last post on here. With the By Pass, the vacuum disappeared, and all is OK now. No more Knocking


  • Zman
    Zman Member Posts: 7,447
    It sounds like you had a piping arrangement that was isolating some of your zones from the expansion tank. This was causing a pressure imbalance that was relieved when the zone valve opened the next time, creating the "knock" you heard.

    The bipass valve on the drawing is to prevent boiler condensation caused by low return water temps. It was not intended to solve the problem you where experiencing.

    My guess is that your system had a check valve on one end of the zone and a zone valve on the other. This was trapping the cooling water.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • Jamie Hall
    Jamie Hall Member Posts: 21,111
    And yes indeed -- if you ave some sort of arrangement where a section of piping is isolated, it will develop a vacuum as it cools.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • icesailor
    icesailor Member Posts: 7,265
    I promise you, the PDF drawing of how your boiler is supposed to be piped, is NOT the way it is piped.

    Most Zone Valves are Motorized Check Valves and close tightly. If you have standard mechanical flow check valves on a zone valve system, you can easily develop vacuums between a zone valve and a check valve. If the boiler feed/Pressure reducing valve is installed between the two, and the vacuum occurs, it will overfill the boiler and cause the pressure relief valve to open and leak.

    IMO, you don't need flow checks or check valves on zone valved zones. The zone valve is the check valve.
  • h12721aa
    h12721aa Member Posts: 17
    edited February 2015

    Thank you to all of You.
    I went down in to the cellar and checked again. The system is piped according to the Burnham Manual. The manual shows.
    Burnham MPO-IQ 84 oil burner, two HW baseboard Zones and Amtrol 41gal IDW Heater set for priority. One circulator and two zone valves for heat and one circulator and no zone valve for the DHW. This is the system I have. I will ad the whole print.
    I sure hope that there is no hidden check valve installed in between the joists or on the base board element.

  • icesailor
    icesailor Member Posts: 7,265

    SHut off that foolish by-pass you rigged up, and if those new circulators have IFC's, take them out. The zone valves do the same thing.

    IMO, you're creating reduced pressure zones between the IFC's and the zone valves. For my minority way of thinking, the circulator is supposed to be pumping into the Extrol tank. Not SUCKING away from it. If the circulator was on the return, it would have the whole boiler to diffuse the pressure and be pumping into the Extrol Tank.

    Another crash/lockup at Heatinghelp.com

    Especially if there isn't a lot of resistance in the system. Your knock may simply be the two pressures balancing themselves out.

    Did it do that before you replaced the Taco 007 circulators with those scientifically smart circulators? If not, what was changed?
  • h12721aa
    h12721aa Member Posts: 17
    edited February 2015
    In August 2014 in a different post see my earlier Question, Title : zone valves and Circulator.
    In there I was told:” Circ Placement
    "The proper place for the circ is on the supply line, after the air separator IF the expansion tank connects to the air separator. The place where the expansion tank connects to the system (Point of No Pressure Change) is what determines where the circ should be placed. The circ should be place immediately down stream of the PONPC pumping away from it, not towards it.”
    The circulators are Taco 007 not IFC’s and yes right from the start the knock was there but only if we have a large temp difference, like now “ baby it is cold out side”. The by- pass was installed just lately not at the beginning. That was the only way I could think of to get the thing in between the two valves. Now the knock is gone. But I also have to replace the pressure reducing valve. It is not function correctly, I can not get the pressure below 25# It should be around 12#. The adjustment is not responding, it is all screwed out.
    On one zone the base board runs true the garage and this is the coldest spot, between 45°-50°. IT IS THE END OF THE ZONE. May be that is where the problem comes from. The Old boiler was piped as you mentions. This one I did as the Mfg. suggested. It is also a smaller unit. 84000BTU. Old was 120000BTU. All piping has to wait till Summer time Ha Ha.
    PS: I should have turned the Pix around, sorry.
  • icesailor
    icesailor Member Posts: 7,265
    edited February 2015
    Why did you change the green Taco 007 circulators to the new black ones? Are you sure that they aren't IFC's?

    You have a 1 1/4" Well Poppit type check valve on the return, just below the return control valve. Do you have zone valves?

    Someone that really knows what they are looking at needs to cast their eyeball upon that.
  • h12721aa
    h12721aa Member Posts: 17
    I needed two circulators one for the DHW one for the system heat.
    How would one see from the outside if they are IFC?
    I did not see any thing special on those from looking at them when I had them out.

    The green one came with the Boiler supplied.
    The black one I got for free. Is new, not used.
    The check valve is as the print shows.
    Yes I have two zone valves.
    The whole system is as the print shows.

    I reversed the print and pix's

    I don’t know about the eyeballs!!! But I need all the help I can get.
  • icesailor
    icesailor Member Posts: 7,265
    edited February 2015
    The system has green Taco flanges. If you have zone valves, you do NOT need that well check ale where it is. In fact, IMO, you don't need it at all. You might need one on the Indirect if there is no zone valve, and if there isn't, you check valve But that is probably source of your problems. If the numbers on the back of the circulators have any "IFC" in the string, they came with checks. If they were added later, you can't tell.

    So many think they are the greatest thing since bread slicers that Counter guys or Internet Salesmen automatically give them to you. Then when problems occur (like yours) you don't have that thought as a cause.
    A tight closing zone valve can become a pressure isolator if it isolates the Expansion Device when the valves are both closed. If the ZV is on the system side of the Extrol tank, and the Check Valve is on the return at the boiler (like yours) the piping from the ZV to the Check Valve isn't under the influence of the Extrol tan. Until the valve opens. Sometimes, with a bang.

    Or so I have personally seen. Often.

    And under the proper conditions, especially cold start boilers, it can cause serious damage to the boiler. Like make it leak.
  • h12721aa
    h12721aa Member Posts: 17
    By having the by–pass in the system and “OPEN” would this not put the Extrol tank back in operation??
    Thank’s for all the suggestion, but for now I will use the BY-Pass, as it is to cold for shutting the heat off and be working on the system.
    Thank,s again
  • icesailor
    icesailor Member Posts: 7,265
    I wish I knew then, what you know now. I put hundreds of heating systems like yours over the years. Not a one with any kind of Bypass like that. And they all worked.

    Maybe they stopped working now.