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Climbing System Pressure

Canuck_3 Member Posts: 39
I had this issue years ago and completely forgot about it; now my son, who's gotten into the trade, came up to me and asked if I'd ever seen this before. Here's the scenario:

Commercial hot water heating system with a system feed valve, like a Watts U5B-LP. No bypass around the feed valve. Feed line ties in upstream of the building pump as does the expansion tank but they're not on the same connection point. They are connected on the same main line, within 10' of each other.

Here's what happens; the feed valve overfills the system to a point where the boiler SRV's start to dump. First thought is the feed valve is stuck open - but when the ball valve downstream of the feed valve is shut, the pressure gauge caught in that section of piping stays steady at the set pressure (say 15 psi). Open the ball valve and over time, the system climbs to 30 psi.

When it happened to me, years ago, my thought was that maybe there was some funky venturi effect happening at the feed connection to the heating main. As the water went rushing past the connection point, it would create a low pressure zone, which would 'trick' the feed valve into thinking the system pressure was low, causing the valve to overfill the system. Problem was, that even if I set the valve to a lower pressure, it would still overfill the system. To that I thought, there's no way (if I set the feed valve to 5 psi) - that the feed valve 'sees' less than 5 psi when the system is up to 30 psi. In the end, I never figured it out and my solution was to isolate the feed valve once the system was full.

Now that I've been asked 'what's happening', I thought I'd go to where all the bright minds collect.

Anyone have any thoughts on this?


  • Larry_52
    Larry_52 Member Posts: 182
    have to ask physically where are the expansion tank and fill in regard to pump and heater. Upstream and downstream can be sometimes confused as it is relative to source. Especially confused in a closed system. So upstream could mean in direction of counter water flow to someone (like a river) or physically ahead of some object like the pump.

    In any of the cases, suggest you buy Dan's book "Pumping Away".
  • unclejohn
    unclejohn Member Posts: 1,830
    Sounds like the feed valve is leaking. Replace it.
  • Canuck_3
    Canuck_3 Member Posts: 39
    Feed valve is not leaking. Again - if we isolate the feed on the line, we can watch the gauge stay steady at 15 psi - all day. When we open that line to the system, the system climbs beyond 15 psi.
  • Tinman
    Tinman Member Posts: 2,804
    Do I understand this correctly? You have a feed valve in the same length of pipe as a ball valve. When you close the ball valve, the pressure remains the same. When you open it, pressure rises? If that description is right, then so is Uncle John.
    Steve Minnich
  • Canuck_3
    Canuck_3 Member Posts: 39
    There are isolation valves on either side of the feed valve. When we shut the valve on the downstream side of the feed valve, pressure stays constant, indicating that there is no 'creep' on the feed valve. Once we open back to the system, the feed valve over charges.
  • Tinman
    Tinman Member Posts: 2,804
    Canuck, If you close just one of the valves, either/or, or close both valves...you are taking the feed valve out of the equation. When doing this, the problem stopped. By eliminating only the feed valve from the equation, meaning the expansion tank is still in play, what else could it be?
    Steve Minnich
  • Canuck_3
    Canuck_3 Member Posts: 39
    I'm probably not being clear enough. I'm going to sketch out a schematic so that things are more evident - just not right now because we have a ton of service calls waiting. Be back in about 12.
  • Zman
    Zman Member Posts: 7,447
    Here is what I am thinking. Because the expansion tank and fill valve are in slightly different locations, a small pressure fluctuation occurs when the circulator turns on. I am guessing that there is a problem with the fill valve that causes it to leak a little when the fluctuation occurs. Sort of an intermittent malfunction of the fill valve.

    BTW, I think you are misunderstanding the intent of the "dislike" button on this on this forum. People don't generally "dislike" posts just because they don't agree with them or understand what the person is trying to say. "Dislike" is usually reserved for posts that are nasty or in other ways not "nice". It is pretty easy to hit the button by accident, if that is the case, just hit it again and it will go away....

    Best of luck,
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • icesailor
    icesailor Member Posts: 7,265
    "Commercial" systems can be different animals. Sometimes they have pressure gauges all around for the circuit setters to set their circuits.

    As I envision the system, it would be interesting to have a pressure gauge on the discharge of the fill valve, but before it is connected to the piping. All kinds of interesting phenomena can happen in high head/high flow piping and connections too close to critical turns. Those hydraulic jumps and turbulent flows are everywhere. Same as negative pressures.

    I understand about all these PONC ships. What I don't understand is this. If you install a circulator with a pressure gauge on both sides of the circulator, and you do this "Pumping Away" thing because you are supposed to "Pump Away" from the expansion tank and TOWARD the PONC. If the system is running at 12# static, and the pump comes on, and the discharge of the pump goes to 15#, and the inlet goes to 9# (6# Delta T, if the Fill Valve is connected to the Extrol tank, the Extrol and Fill valve should be "Seeing" 9# and add water. It keeps adding water until the Relief valve blows. If the fill valve is on the return where the pump pressure is back to normal, the fill valve won't fill and cause overfilling.
    At last that's what I have seen in the past.

    Many boilers like Weil McLain Gold's came with the circulator mounted on TOP of the boiler on the supply. No matter what, unless you changed the circulator location you were always pumping away from the boiler and AT the Extrol tank and PONC.
  • Paul48
    Paul48 Member Posts: 4,469
    edited February 2015
    Out of curiousity......Is the expansion tank, first in line, upstream of the pump? Dan could probably explain this better, but I believe the expansion tank is absorbing the added pressure, and allowing the PRV to be tricked. This is probably an instantaneous thing, but happens every time the pump starts. I think it's a prime example of why you Pump Away. I don't think there's anything wrong with the PRV.
  • Mitch_4
    Mitch_4 Member Posts: 955
    is the boiler feed at the inlet to the pump? from the description you ave it seems so. remember a pump creates a differential, once the pump kicks in the inlet side pressure drops, this will cause the feed to opn
  • Mitch_4
    Mitch_4 Member Posts: 955
    ice sailor, if you are pumping away from the exp tank, you are pumping away from teh PONC. the exp tank IS the PONC,

    "... because you are supposed to "Pump Away" from the expansion tank and TOWARD the PONC."

    this cannot happen. the circ creates a differential from its inlet to outlet.if the pump is before teh PONC then the inlet pressure DROPS. if the feed is there, and set for 12 psi, and the inlet drops to 7, then the feed opens.
  • icesailor
    icesailor Member Posts: 7,265
    How can that be. I had a commercial account with gauges at every circulator. Inlet and outlet. 2-Taco 3" 1600's and 7- Taco 112's. The 1600's were on the first floor, the 7- 112's were on the second floor. With all the pumps off, the pressure on the system at the boilers and pumps was 16# PSIG. The 7 on the second floor were whatever the difference in altitude that the gauges were located on the seven circulators. If a 1600 started, and the discharge pressure went to 19#, the suction/inlet side went to 13#. For a 6# head pressure, or 13.86' of altitude head. Look on the Taco pump chart for that same pump curve, look at the head pressure and it gives you the GPM's. The ones upstairs are the same except they have all kinds of control valves with Schrader ports for a professional circuit setter/system balancer to adjust the flow/pressure to get the designed BTU's through the zone or circuit. So, if the system pressure is 16# static, and the pump starts and goes to 19#, somewhere in the system is a PONC where the pressure is 16# and starts to drop below 19# until it gets to 13# as it enters the pump. There are TWO PONC's. One in the system, and the other inside the pump. So, if one end of a running pump is 19# and the other is 13#, how can the 16# PNOC be at the center of the Extrol tank? If the pump is installed before the tank, the tank sees 19#. If mounted after the tank, the tank sees 13#. Inly if it is possible to mount the tank exactly in the center of the pump could POSSIBLY be at the PONC. And if you take the Delta T, you can even calculate how many pounds of water is going into the system and how many BTU's are being left behind. The second floor has a whole tribe if double PONC's.

    And that is the way it was shown in a Taco hydronic book that I got at a seminar at Emerson Swan.
  • RobG
    RobG Member Posts: 1,850
    Is this on a new install or a new problem on an old install?