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Large residential heating loop, questions and theroy

aircooled81
aircooled81 Member Posts: 205
Intermitent pop-off of both boilers pressure releifs, sometimes at start-up of pump, sometimes while pump is running. What could the cause of this be?
What are the advantages/pro's of converting this system to a primary secondary loop?
Details for the heating loop:
2 boilers, 1 pump (1hp b&G series 60), 14 fan coils (qty11 2-way valves, qty3 3way valve), pump is located upstream of both boilers (pumping into boilers), both boilers are piped in parallel with circuit setters upstream, make-up water pressure set to 20 psig located at return, expansion tank set to 20 psig located further upstream on return, pump suction 20 psig / discharge 42 psig, pressure releif valves are 50 psig, pump cycles on based on call from any one fan coil for heat.
I am convinced the pressure drop through the boilers and the entire loop is so substantial, it causes the suction side of the pump to briefly pull down the return side and causes make-up water to feed briefly before the discharge side of the pump has an oportunty to equalize the suction side of the pump to 20 psig. This and air in a loop, that can get down to 110*F, allows the pump to briefly spike the discharge pressure above the 50 psig releif valve setting. That inturn cause the releif valve to discharge, pump suction to crash, make-up water to fill again, and this repeated symptom makes for a symphony of pressure releif valves hammering away in the boiler room.
Is this diagnosis correct?
I am certain in this instance the pump needs to be moved to the leaving side of the boilers, as the low/suction side of the pump should be the pressure the boilers are exposed to.
I am contemplating the advantage of 2 smaller pumps, and creating a primary secondary loop.
My concern with the significant pressure drop, when I move the pump to the leaving side of the boilers, I will still have a brief drop in suction pressure that will be equalized by the make-up water feeding. Is this correct or possible. I have no clue if the expansion tank is supposed to be able to react quick enough to conteract this issue?

Comments

  • RobG
    RobG Member Posts: 1,850
    Can you draw a diagram of the piping and components or take some photos of the boilers and associated piping? Is there an indirect water heater piped into the system? Is this a new problem or has it always happened?
  • Ironman
    Ironman Member Posts: 7,366
    Like Rob said, pics or a diagram would be very helpful.
    You're probably on the right track and moving the circ to the supply side is a major step in the right direction.
    From your discription, I see other issues:
    1. All of the fan coils should have either a 3way zone valve or there should be a pressure bypass relief on the loop. An ecm delta P circulator such as the Wilo Stratos or the Grundfos Magna would eliminate the need for either of these and would reduce the circ's energy consumption by over 80%.
    2. The circ should not only be on the supply side, but is should be immediately downstream of the point where the expansion tank connects to the system. This is known as the Point of No Pressure Change (PONPC). The expansion tank does not push water back into the system, that's why it's called the PONPC. Sometimes, that's not easy to grasp at first, but it's true. With the circ pumping away from the PONPC, the pressure differential that it creates is ADDED to the STATIC fill pressure. When it pumps towards the PONPC, the pressure differential that it creates is DEDUCTED from the STATIC fill pressure. This is NOT where we want the circ. By moving it to pump away from the PONPC, you can very easily gain 10 psi at the top of the system when the circ is running and eliminate air issues. You may also be able to lower the static fill pressure by half of that which would also help to keep the boilers from over a pressurizing.
    3. P/S piping is a good idea, but may not be necessary in your case and in and of itself may not remedy your problems. I've always considered the primary loop to be the one with the PONPC, others may differ. With p/s, the secondary loop sees the primary as its PONPC and therefore the primary would have to have the same static fill pressure as it now does to overcome the elevation of the secondary (building) loop.

    I would highly recommend that you get Dan's book " Pumping Away" from the online store on this site.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
    RobGTinmanZman
  • aircooled81
    aircooled81 Member Posts: 205
    Rob, no indirect water heater piped in. This loop is for reheat/fan coils only.
    pipe diagram attached
  • aircooled81
    aircooled81 Member Posts: 205
    Ironman, I love Dan's books. I'm still finishing a steam book, but will pick up pumping away next.
    I attached a drawing, hope it shows up easy for everyone to see. Its a bit crude, i did it with microsoft paint.
    I like the pressure bypass idea, as replacing zone valves is not a viable option. An idea to install a frequency drive came-up but is not viable either. My expeirience with bell and gossets pressure bypass has been poor. They are small, and in an instance such as the size of this loop, I would require more than one i think.
    I think 2 smaller pumps would be quieter, that is a bonus considering this 1 hp pump conducts quite a bit of noise do to its inherent size and head.
    Can you speak a little further regarding why my suction pressure drops when the pump starts? My experience has been the suction pressure should not drop at start-up, as it does with this system.
    Thank you for the replies folks.
  • aircooled81
    aircooled81 Member Posts: 205
    Rob, I forgot to reply to your other question.
    Yes, this has aparently been a problem since conception. This original boiler loop was configured to run off of the cast iron boiler being suplemental to a heat pump and geothermal loop outside. This original idea failed, so the second boiler was installed, and the bypass/mixing valve had been left in place (noted in my drawing, set to full bypass).
    When I first took on this account, we noticed an obseen amount of chem (tyon b20) needing to be made up annually. This problem of the dumping is more predominant in the summer/spring, as is the t&p dripping issues.
    My customer agrees this is a very problematic issue, and has tasked me with finding a solution.
    I reason the issue is more predomenant in the warmer months, as this is when the pump is more likely to cycle off due to no call for heat. I think the pump cycling on, and the current boiler configuration exaserbates the problem.
    The piping is very convaluted, and my customer would like to see it redone, with a much more mechanical and straight forward apperance.
    We will remove the unneeded 3-way valve, relocate the air seperator currently ontop of boiler 1 to the return loop. relocate the pump to leaving side of the boiler. pipe the expansion tank and make-up water into the air seperator. relocate some poorly placed sensors for the BMS system.
    I am toying with the idea of creating a primary and secondary loop. and also deciding if I should make boiler 2 (the condensing boiler) the primary boiler. maybe even isolating the boiler 1 in some way, so not to loose heat through this one while the conensing boiler is running. Posiibly get rid of the circuit setters by using two seperate pumps. Only use the 2nd boiler when the load exceeeds the condensing boilers capability.
    what do you think?
  • Zman
    Zman Member Posts: 7,561
    You have been handed a mess there.

    Aside from the piping which is obviously a mess:

    The circulator is way too big. Who knows why it was installed but the size is not even close. You are trying to move 200,000 btu so you only need a max of 20 gpm. Unless the whole place is piped 1/2", you do not need the 50 feet of head you now have.

    I don't think either of those boilers are rated for the 50 psi relief valves.

    I think the valves are popping when one small air handler is calling and the circ is pushed to the top of it's curve.

    Your idea of piping this primary/secondary with the small boiler as lead would work just fine. A tekmar controller would take care of the controls.

    As long as you are going to all this trouble, what condition are the boilers in?
    One properly sized mod/con would be more efficient and less work.

    Carl

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 22,018
    A system like that cries out for a hydraulic separator. The boilers certainly do not need a 1 hp pump! And maybe not the system?

    With a sep, both boilers get the correct pump and so does the distribution. An ECM delta P would be ideal for the system pump.

    http://www.caleffi.com/sites/default/files/file/idronics_15_na.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jamie Hall
    Jamie Hall Member Posts: 23,159
    And furthermore... with the setup you have, when the pump turns on you are going to get a wicked water hammer in there -- well over 50 psi. You may, even with a smaller pump. On a system that size, you may find that you need an additional expansion tank just after the pump, to absorb that water hammer (as has been noted, the main expansion tank must be before -- just before -- the pump; that one is for thermal expansion. The one after the pump is a water hammer arrestor.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • I'm thinkin, since the boilers are different manufacturers it might be an ideas to install a boiler dedicated circ on each boiler. Either manual variable speed or Delta T (set at 20 def F).

    Then install a small system circ (be careful not to oversize - it only needs to overcome the head of the system - not the boilers and interconnected piping). If you want to maximize system efficiency this circ could be delta T or delta P).