Hydronic System Cleaning and Flushing

NOTE This standard governs the one-time preparation of a hydronic system's wetted interior between the piping pressure test and the start of the ongoing water treatment program: the fill, the removal of construction debris and residue by chemical cleaning and high-velocity flushing, the acceptance of the flush by measurement, the passivation of the cleaned metal, the installation of the corrosion coupon rack, the initial glycol charge where the system carries glycol, and the disposal of the water the work produces. (1.1)

NOTE The systems in scope are closed hydronic loops of every service, including heating hot water, chilled water, dual-temperature, water-source and ground-source heat pump loops, closed-circuit condenser and fluid cooler loops, inhibited-glycol loops, snow melt loops, and runaround heat-recovery loops, together with the field piping of open condenser water systems between the tower and the plant. (1.2)

NOTE The standard applies to new systems, to new piping that extends an existing system, and to any pipe material and system size. (1.3)

NOTE A new hydronic system contains mill scale, weld spatter, cutting oil, thread-cutting lubricant, jointing compound, flux, sand, and whatever fell into an open pipe end during construction. Left in the water, that material lodges in the first coil, control valve seat, strainer, or heat exchanger passage it reaches, and the loss of flow it causes is discovered as a comfort complaint months later rather than as a construction defect. Cleaning and flushing exist to remove the material while it is still cheap to remove, and passivation exists because freshly cleaned steel begins to rust again within hours in untreated water. (1.4)

NOTE The work of this standard ends with the delivery of the signed cleaning, flushing, and passivation report for each system. The ongoing program under HVAC Water TreatmentHVAC Water TreatmentResolves to the current adopted revision.sync/hvac-water-treatment begins at that hand-off. (1.5)

NOTE The following are outside this scope and are governed by the companion standards named. (1.6)

NOTE The sequence runs in one direction. The pressure test proves the pipe before the water is prepared, the flush removes the debris before the balancing agency sets flows that a fouled screen would later change, and the passivation charge is in the water before any plant equipment runs on it. Each later stage assumes the earlier one has already happened, which is why each is a stated prerequisite rather than a coordination item. (1.12)

NOTE BSRIA BG 29 is a United Kingdom industry guide cited for its treatment of flush velocity selection, method statement content, and sample-based acceptance. It is a reference for method, not an adopted code, and the values in this standard's datasheets govern where the two differ. (2.3)

NOTE The velocity calculation is the submittal that decides whether the flush will work. A system pump sized for design flow against design head rarely moves water fast enough through the mains to lift settled debris with every valve open, and the calculation is where that shortfall is found while a temporary pump can still be arranged. A method statement without a section-by-section calculation is a plan to flush the parts of the system that happen to be easy. (3.1.3)

NOTE The closeout package is the only evidence that the cleaning happened. A coil that fouls in the second year is either a construction defect or a treatment failure, and the flush results, the passivation residuals, and the coupon baseline are what let the Engineer of Record tell which. Without them the argument is settled by whoever is more persuasive. (3.2.2)

NOTE The installing mechanical contractor installed the pipe, owns the temporary connections, and runs the pumps, which is why that contractor performs the flush on most projects and is the default. A specialty cleaning contractor brings its own flush pumps, filtration, and sampling equipment and is engaged where the system is large, the schedule is short, or the installing contractor has no flushing experience. The water treatment service provider performs the whole operation where the Owner wants one party accountable for the water from the first fill onward, at the cost of a contractor on site who does not control the piping work. The chemistry is supplied by the service provider on most projects because the passivation chemistry has to match the ongoing program, and a cleaner the provider did not select is one the provider will later blame. The installing contractor's own supplier is chosen where no ongoing program has been let at the time of cleaning. (4.1.3)

NOTE No witness is the norm across projects. The commissioning authority witnesses where the project is commissioned, because the flush report is a prerequisite the authority has to accept anyway. The Engineer of Record witnesses where there is no commissioning authority and the Engineer wrote the acceptance values. The water treatment service provider witnesses where the provider will inherit the system and wants its own sample at hand-off, and is a poor choice as the sole witness where the provider also performed the work. The Owner's representative witnesses where the Owner operates its own plant and has staff able to read the results. (4.1.6)

NOTE No extent is the norm, because the answer follows the condition of the existing water. Cleaning the new piping in isolation keeps the existing treated water in service and keeps new debris out of existing coils, and it requires a temporary circulation loop and a fill point on the new section. Cleaning the combined system removes the existing deposits at the same time and resets the whole loop to one baseline, at the cost of draining treated water, disposing of it, re-passivating existing steel, and an outage the Owner has to accept. (5.1.4)

NOTE The building make-up water is already connected, already protected by the backflow assembly on the fill connection, and is the water the ongoing program was designed around, which is why it is the default. Softened water is chosen where the supply is hard enough that a phosphate cleaner or an inhibited glycol would precipitate calcium in the system, and it costs a softener and its regeneration. A temporary supply is chosen where the building service is not yet live or where the fill assembly cannot deliver flush volumes in a useful time, and it brings water of a quality nobody has characterized unless it is sampled. (5.2.2)

NOTE Every flush acceptance comparison in this standard is made against the fill water baseline, so a baseline drawn from a different source than the flush water, or not drawn at all, leaves the acceptance values with nothing to be compared to. (5.2.4)

NOTE Aluminum is the material that most often turns a routine clean into a damage claim. An alkaline cleaner at the concentration that lifts cutting oil from steel sits above the pH at which aluminum corrodes, and an aluminum boiler heat exchanger or an aluminum-cored coil connected during the clean is etched before the flush ends. Galvanized components lose zinc to the same chemistry. A solvent degreaser attacks EPDM gaskets and seals that a water-based alkaline cleaner leaves alone. The compatibility statement is where each of these is caught on paper. (5.3.4)

NOTE A water flush alone removes loose debris and is chosen on a small system of copper or press-joined pipe assembled without cutting oil, where there is nothing for a cleaner to dissolve and the discharge needs no authority coordination. An alkaline detergent with a dispersant lifts cutting oil, thread lubricant, and jointing compound from carbon steel and holds the fines in suspension so the flush carries them out, which is the condition most new systems with steel mains are in and is why it is the default. Trisodium phosphate is a commodity alkaline cleaner that does the same job at lower material cost and adds a phosphate load that many receiving authorities limit. An inhibited acid or chelant descaler dissolves mill scale and rust rather than lifting oil, and is chosen where the system stood wet through a long construction period, was used for temporary heating before cleaning, or carries heavy mill scale. It requires an inhibited formulation, continuous monitoring, neutralization, and passivation on the same day, because the metal it leaves behind is bare. (6.1.2)

NOTE An alkaline cleaner works faster warm, and a 24 hour circulation at ambient temperature is the duration that lets it work at the temperature most systems are cleaned at, which is why the two defaults go together. Heated circulation shortens the clean and improves oil removal on a heating system, and it needs a heat source: the system boiler cannot be fired on cleaner-bearing water without its manufacturer's written acceptance, and BoilersBoilersResolves to the current adopted revision.sync/boilers requires the boiler to be placed on treated water, so heated cleaning usually means a temporary heater. A chilled water system is cleaned at ambient because nothing on it heats water. (6.2.6)

NOTE A cleaner that dries on the pipe leaves a film that the flush water does not dissolve and that the passivation inhibitor has to form its film through. Draining and refilling in one operation is what keeps the wall wet between the clean and the flush. (6.2.10)

NOTE No flush velocity is the norm across projects, because the value trades debris removal against the pump needed to reach it. Settled sand, scale, and weld slag begin to move in a horizontal pipe at roughly 3 ft/s, and are carried reliably through fittings and risers at 5 ft/s and above, which is the band most method statements aim for. A velocity above the design velocity of the section is only reachable with a temporary pump or by flushing the section alone, and the calculation shows which sections that applies to. A low value makes the flush achievable with the system pumps on a small system and leaves the heavier debris where it lies. (7.1.2)

NOTE A dead leg is a settling basin. Debris that the main flush lifts drops out where the water stops moving, and it migrates back into the cleaned system the first time the isolation valve on that leg is opened for service. Every leg either gets flow during the flush or gets looked at. (7.1.7)

NOTE No pump source is the norm across projects. The system pumps are already installed and wired, and on a small system with all valves open they reach the flush velocity in the mains, at the cost of passing construction debris through the permanent pump's impeller and seal faces. A temporary external pump is sized for the flush rather than for the design point, keeps debris out of the permanent pumps, and costs a rental, its connections, and a temporary power supply. The split arrangement uses the system pumps where they are enough and brings a temporary pump to the branches and risers the calculation shows they cannot reach. (7.2.2)

NOTE Once-through flushing carries every pass of debris and cleaner out of the building and consumes a system volume of water per pass, which on a large loop is a discharge the receiving authority has to accept and a water bill the Owner pays. Recirculating through a temporary bag or cartridge filter removes the suspended debris while keeping the water, and leaves dissolved cleaner and iron in the loop until the final rinse removes them. The recirculated arrangement is chosen where water or discharge capacity is limited, and it needs the final rinse to reach the pH and conductivity values that a filter cannot deliver. (7.3.3)

NOTE The photograph log and the screen record are the running evidence of the flush. The trend from brown to clear in the sample jars and from heavy to empty in the screens is what a witness reads to judge that the next pass is the last one, and what the Engineer of Record reads later when a strainer loads in the first season. (7.3.6)

NOTE Debris moving at flush velocity scores a control valve seat, packs the narrow channels of a plate heat exchanger, and lodges in the return bends of a coil where no later flush reaches it. Bypassing the equipment keeps the main flush out of it entirely and then flushes each item at a flow its own connections carry, which is the most protective arrangement and the default. Flushing through behind a temporary fine screen avoids the jumpers and lets the flush reach the coil connections, at the cost of a screen that loads quickly and throttles the branch until it is cleaned. Flushing through the permanent screen relies on a 20 mesh screen that passes much of what a flush mobilizes, and is chosen on a small system of unit heaters or fan coils where the devices protected are not fine enough to be damaged by what it passes. (8.1.2)

NOTE A finer screen catches more of what a flush mobilizes and loads sooner, so the mesh count follows the passage size of the device behind it rather than a single value: a plate heat exchanger or a pressure-independent valve cartridge needs a finer screen than a coil with a large tube diameter, and every screen is cleaned between passes regardless of mesh. (8.1.5)

NOTE The signed report under this standard is the release that BoilersBoilersResolves to the current adopted revision.sync/boilers and ChillersChillersResolves to the current adopted revision.sync/chillers require in their pre-startup verification before the equipment is operated on the loop, which is why the equipment is kept out of the flush and brought in only once the water is treated. (8.2.3)

NOTE A jumper left in place is a permanent short circuit that starves the equipment it was protecting, and a flushing screen left in place loads steadily until the branch it serves stops. The record of installation against removal is what makes the count come out even. The cleaning of the permanent strainer screens and the removal of the startup screens after flushing are recorded separately under Hydronic SpecialtiesHydronic SpecialtiesResolves to the current adopted revision.sync/hydronic-specialties. (8.3.6)

NOTE A sample drawn at the plant on a system whose far ends were never sampled is a sample of the section nearest the pump. The low point is where the heaviest material settles and the far end of each main is where the velocity was lowest, which is why both are sampled every pass. (9.1.3)

NOTE Each parameter answers a different question. Clarity and turbidity say whether suspended debris is still being carried. pH and conductivity returning to the fill water baseline say whether the cleaner has been rinsed out, because the cleaner is what raised them. Total iron says whether the steel is still shedding scale or has begun to corrode, and total suspended solids put a number on what clarity shows. The default set is what a field kit and a photometer can measure the same day; the laboratory parameters are added where the Owner wants a laboratory record or where a descaler was used. (9.2.2)

NOTE Iron that falls with every pass is scale being carried out. Iron that holds steady pass after pass is steel dissolving in oxygenated water, and more flushing adds oxygen rather than removing iron. The distinction decides whether the next step is another pass or an immediate passivation, and it is the Engineer's call because the two look the same in the sample jar. (9.2.10)

NOTE No discharge route is the norm, because the answer follows the chemistry and the receiving authority. Plain flush water and a rinsed alkaline cleaner at flush dilution are accepted to the sanitary sewer by most authorities on notice, and the authority's acceptance is the only paperwork. Spent alkaline cleaner, phosphate, and any descaler sit outside the pH and metals limits that EPA 40 CFR Part 403 lets a local authority impose, and are neutralized and tested before release where the authority permits that, or hauled where it does not. Hauling costs the most per gallon and is the only route open where the authority declines the discharge or where molybdate or dissolved metals exceed its limits. (10.1.2)

NOTE Freshly cleaned carbon steel in oxygenated water begins to rust within hours. The rust that forms is loose and porous, and the inhibitor film that passivation builds has to form through it rather than on clean metal. The passivation charge goes in the same working day the flush is accepted for that reason, and a system that has to wait is kept circulating so the surface stays uniform until it does. (11.1.1)

NOTE The passive film is built by one chemistry and maintained by the same one. A system passivated with nitrite and then placed on a molybdate program spends its first months with a film that is neither, and the corrosion rate during that transition is what the first coupon reads. Taking the chemistry from the ongoing program is what makes the passivation and the program one continuous treatment. (11.2.2)

NOTE A passive film forms faster and denser at operating temperature on steel, which is why a heating system is passivated warm where its boiler is ready to run, and why the boiler is allowed on the loop at this stage and not before: the water is now treated. A chilled water or condenser water system is passivated at ambient temperature with the pumps running, and needs the longer end of the duration band to reach the same film. (11.3.5)

NOTE The field piping of an open condenser water system is cleaned and flushed under this standard, and its passivation is the first weeks of its treatment program, during which the galvanized surfaces are passivated by pH control and the system receives its start-up disinfection under HVAC Water TreatmentHVAC Water TreatmentResolves to the current adopted revision.sync/hvac-water-treatment. The cooling tower basin and fill are cleaned under Cooling TowersCooling TowersResolves to the current adopted revision.sync/cooling-towers. (11.4.1)

NOTE The coupons installed at passivation stay in place through the first read interval, and their reading, replacement, and acceptance criteria are governed under HVAC Water TreatmentHVAC Water TreatmentResolves to the current adopted revision.sync/hvac-water-treatment. Installing the rack at passivation rather than at hand-off is what gives the program a baseline that includes the passivation period itself, which is the period in which a poorly passivated system shows its highest corrosion rate. (12.1.8)

NOTE Glycol charged into a dirty system binds the construction debris into a fluid that cannot be flushed out without dumping the whole charge, and a cleaner circulated in glycol consumes the fluid's inhibitor package and sends the spent glycol down the drain as regulated waste. The sequence is water first, glycol last. (13.1.1)

NOTE No charging method is the norm across projects. A premixed solution arrives at a known concentration in dilution water the fluid supplier controlled, which is the simplest to verify and the most expensive to buy and ship. Field blending in a tank uses concentrate and site water and reaches the design concentration before the fluid enters the pipe, at the cost of a tank, a transfer pump, and a dilution water analysis. Blending in the system saves the tank and leaves the concentration unknown until circulation has mixed it, with the residual water in the low points diluting the result and the first refractometer reading taken at the plant telling nothing about the far end. (13.2.2)

NOTE The inhibitor package in an inhibited glycol reacts with the calcium and magnesium in hard water to form a precipitate that settles in the coils and consumes the inhibitor, which is why the fluid supplier publishes a dilution water limit and why hard-water sites blend with softened or deionized water. (13.2.4)

NOTE The report is the single hand-off document. The balancing agency reads it as the release to begin, the boiler and chiller startup technicians read it as the release to run on the loop, and the water treatment service provider reads it as the baseline the program starts from. One document with one set of signatures is what keeps four parties from each asking for a different one. (14.1.3)

NOTE The unassigned cost in a cleaning warranty is never the flush itself. It is the coil that has to be pulled, the glycol that has to be drained to reach it, and the balance that has to be redone afterward. Naming those costs here, and naming who decides whether the debris came from construction, is what makes the warranty enforceable without an argument. (16.5)