Hydronic CSI: Full flow boiler piping


Full Flow Piping Hydronic CSI copy


Mechanical Hub and Lochinvar are proud to team up to bring you the second feature in our Hydronics CSI series. Hub friend Paul Rohrs, design and application specialist for Lochinvar, has produced a hydronics design with intentional flaws in it. There are multiple mistakes with this diagram and it is up to you to use your hydronics knowledge to figure out what they are.


This was to be a Full Flow/Variable Primary System. Problem with current boiler piping: New install with six boilers, three boilers on one housekeeping pad, three boilers on another housekeeping pad that is across from it.   Find all flow related and other potential issues.

The contestants who come closest to the answers will be put into a drawing to win a jacket from Lochinvar.

Good luck!

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10 thoughts on “Hydronic CSI: Full flow boiler piping

  1. No balancing between boilers to accomplish equal flow. Reverse Return would work well. No balancinng between the two banks of boilers. A larger common header and reverse return the whole thing or some type of verifiable flow balancing would also work. Verify that the system pumps do not exceed the maximum flow of the boilers. Air Separation is only on one bank of boilers, should be in common piping. No check valves depicted on BMP pumps. If one is off flow may short circuit.

  2. Hydronic puzzle

    – Air eliminator and thermal expansion tank; wrong place. Should be on the supply side where air bubbles are easier to catch. It should also be installed on a piece of pipe that has all of the system water running through it. (Instead of only three out of the six boilers).

    – Also, circulators should be pumping away from expansion tank.

    – No make up water.

    – Piping should be in a primary and secondary style per battery of boilers. Also, boilers should be connected reverse return to the primary loop.

    – Missing a circulator for each boiler

    – System circulators should be taken off secondary loop of piping.

    – If we are looking at the obvious things as well, we are missing relief valves, isolation valves, temperature and pressure gauges, hose bibbs,

  3. The primary/secondary setup needs to be done properly.
    Also, I would put check valves on each boiler to ensure flow over the system and not over another boiler in case not all of them are running at the same time.

  4. There is a lack of flow control, the point of no pressure change is wrong, the boiler’s should be piped in reverse return…lacking circulators assuming each primary and secondary piping required. Also lacking true hydraulic separation.

  5. 1 the air separator should be on the main return line before branching off to all six boilers, 2 each boiler should have a pump to keep a constant flow when called for and 3 both supply and return lines should be looped together .

  6. 1. Air separator should be in piping common to all boilers.
    2. Air separator should be in the hot side piping, not the cold side.
    3. Expansion tank should be located in common piping just before the pump inlets.
    4. Use closely spaced tees and generously sized manifold piping on the return piping (similar to what is shown on the supply side).
    5. Reverse-return piping for the boilers would help ensure balanced flow.
    6. Provide flow limiter for each boiler.
    7. Provide on-off control valves for each boiler, valve to close when boiler not operating.
    8. Stay within minimum / maximum flow rates for pumps and boilers.
    9. Ensure system piping allows for minimum flow rate by providing adequate amount of three-way valves at end use equipment or a bypass valve.

  7. Morning Mechanical Hub!

    These boilers are not set up as a Primary/Secondary system OR as a reverse return flow through system.

    Judging by the title of the article, we are using the system pumps to Flow through the boilers.
    This system should be piped as a big reverse return header through the boilers.
    One supply Starting at B4-B5-B6, then swinging down to B3-B2-B1.
    And one Return Staring at B1-B2-B3 and swinging up to B6-B5-B4.

    The air separator should be moved to the supply side of the system were the water is hottest, air is less soluble at high temperatures and will therefore be removed more easily.

    The expansion tank needs to be moved to the supply side as well so we are pumping away from the point of no pressure change.

    There needs to be check valves on the outlet of the pumps so they can not backtrack through each other.

    There is no system fill.

    Nothing has isolation valves.

    And that is a Sweet Jacket.

    Happy Tuesday Mechanical Hub!

    Nolan Stanley
    Ferguson Enterprises

  8. I already have a nice Lochy jacket, but need to take a shot at this anyway just to keep my installer/designer brain working 🙂

    The pumps do not have individual check valves on their discharges. This will create flow bypasses when pumps are in the off position, turbining the OFF pumps due to backwards flow through the OFF pumps.

    Flow through the boiler array is parallel direct return. This will cause greater flow in the first boilers, and less flow in the last boilers, causing them to bounce off their high limits, thereby short cycling. Should either have balancing valves on each boiler to spread the flow out, or (recommended) the boilers should be piped parallel reverse return.

    The air separator is on the system return. Not an ideal location. Should be located where the mechanically induced pressure change is the least, and the water is the hottest. It is in the lowest mechanically induced pressure point, but only because the main pumps are not flowing away from the POLMIPC (Point of lowest mechanically induced pressure change, formerly known as PONPC). )I just invented the term POLMIPC yesterday during a code discussion. Pronounced poll my PC) 🙂

    The expansion tanks connection in regards to the main pumps locations is incorrect. As designed, the pumps have no option but to present their whole pressure differential as a negative pressure change. This lowering of operating pressure can cause the boilers to flash to steam under certain conditions. Not only will it make your sphincter do the momba, and your feet want to run away with your body attached, it can cause cavitation at the pumps inlet, destroying the impeller, and creating noise that sounds like firecrackers going off inside the pipe. I’d move the expansion tank and MBR to the common header for the main pumps suction connection. I’d also add an isolation ball valve to the EXP tank, along with a drain cock between the tank and the ball valve to allow the system pressure at the tank to be lowered to zero, so that diaphragm pressure can be tested and properly adjusted.

    Although not required, if it were my installation, there would be two way motorized ball valves on each boiler (pick a port) and when the boiler is in the OFF position, the valve would be closed, thereby lowering radiant energy losses from the equipment, reducing potential stack losses from the appliance. If flow control valves are added, a pressure activated bypass will need to be installed to avoid dead heading the pumps as currently piped. This current method of piping is not conducive to the application of a VFD on the pumps, because when all boilers are in the off position (assuming 2 way valves) system flow would drop to zero.

    Ideally, boiler array should be set up as a secondary, with either one variable speed pump serving all boilers, based on delta P, or individual boiler pump$, based on staged boiler operation and then the distribution pumps could be controlled based on either delta P (assuming 2 way flow control valves at terminal units) or delta T (assuming the use of 3 way bypass valves at the terminal units). THIS would be the ultimate system with the greatest energy savings. Ex$pen$ive, yes, but efficiency comes at a cost, but will pay for itself over time. How’d I do boys? Thanks for your efforts in supporting our industry. ME

  9. Expansion Tank doesn’t serve the full system
    Relief valve doesn’t serve the full system
    No check valves are installed at the pumps
    No strainers or air dirt separators
    No dp sensors for VFD operation
    no 2 way valves for VFD operation
    No reverse return piping

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