Hydronic make-up water and glycol treatment field guide for HVAC

What keeps a hydronic loop healthy

A hydronic loop almost never fails because the pipe wore out. It fails from what the water does to it. Air gets in and the system goes noisy and air-bound and the steel starts to rust. The water expands when it heats and has nowhere to go, so the relief valve weeps and drips fresh water back in. Freeze splits a coil over a long weekend. Dirt and magnetite build up and choke the flow. Every one of those is a water-side problem, and every one is preventable with parts that cost a fraction of the equipment they protect.

Five jobs hold the loop together. Feed it the right amount of make-up water and no more. Get the air out and keep it out. Give the water room to expand. Protect it from freeze where the cold can reach it. And treat the water so it does not corrode the metal it touches. Skip any one and the system runs wrong quietly for years before it fails loudly.

This guide is the water side. Setting the flow at each coil is the companion balancing guide, and picking and piping the pump is the companion pump guide. The three overlap at the same loop, because a system that is balanced and pumped right still dies young if the water is full of air, low on glycol, and eating the steel from the inside.

The make-up water assembly

The make-up water assembly is the small package of valves that connects city or well water to the loop and feeds it just enough to hold pressure. Three parts do the work in a line. A pressure reducing valve, called the fill valve or PRV, drops the building water pressure down to the loop fill pressure and holds it there, commonly around 12 psi on a small residential boiler and set to suit the system height on a taller building. A backflow preventer sits ahead of it so loop water, often full of glycol and inhibitor, can never run back into the drinking water supply. And a water meter records every gallon the loop swallows.

On small systems these come pre-assembled in one body, like the common combination fill valve and backflow preventer units. On a commercial plant they are separate devices piped in series, often with isolation valves so the fill can be shut and serviced without draining the loop. The fill valve has a fast-fill lever for charging the system and a normal mode that trickles in make-up only when the loop pressure drops below its setpoint.

The PRV setpoint is not a default. It has to push water to the highest point of the system plus a few psi of margin so the top of the loop never goes to vacuum. The rule of thumb is the static height in feet divided by 2.31 to get psi, plus about 4 psi, then that becomes both the fill setpoint and the expansion tank precharge. Set the fill too low and the top floor air-binds. Set it too high and the relief valve lifts.

Fill valve / PRV

The pressure reducing valve that drops building water to loop fill pressure and feeds make-up only when the loop drops below its setpoint

Backflow preventer

The device that stops loop water, glycol, and inhibitor from flowing back into the potable supply

Make-up water meter

The meter on the feed that records every gallon added, so a rising count flags a leak

Why is make-up water the source of corrosion?

Make-up water is the source of corrosion because every gallon of fresh water carries the two things a closed loop should never get more of: dissolved oxygen and minerals. A closed hydronic system is supposed to fill once, consume the small amount of oxygen the first charge brought in, and then sit oxygen-starved for the rest of its life. Starved of oxygen, the steel stops rusting because there is nothing left to feed the reaction. The trouble starts when the loop keeps drinking fresh water.

Oxygen entering through make-up water or a leak reacts with the steel and makes rust, which becomes the magnetite sludge that later chokes the coils and the pump. The minerals in the make-up water drop out as scale on the hottest surfaces, the boiler tubes and heat exchangers, where scale insulates and cuts heat transfer. The more make-up the loop takes, the faster both happen. A system that is constantly fed is a system that is constantly corroding.

This is why the make-up water meter earns its place. A well-maintained closed loop should need less than about 5 percent of its volume replaced in a year. Watch the meter, not your gut. A meter climbing faster than that is telling you the loop has a leak somewhere, a weeping relief valve, a packing gland, a pinhole, and the leak is doing double damage: losing treated water out one end and pulling raw, oxygenated water in the other. Constant make-up is not a quirk to live with. It is corrosion you are paying to run.

What does air do to a hydronic loop?

Air in a hydronic loop wrecks four things at once, and most callbacks on a new system that runs cold trace back to air nobody got out. First, noise: trapped air gurgles and bangs through the pipe and the radiators, and the owner hears it from day one. Second, corrosion: air is oxygen, and the oxygen feeds the rust the loop was supposed to starve. Third, heat transfer: air is an insulator, so an air-bound coil cannot hand off its heat even at full flow, which reads as a low delta-T you will chase for hours if you do not know the air is there. The balancing guide covers that chase in depth.

Fourth, and the one that stops a job cold, is loss of circulation. A centrifugal pump throws water by its weight and cannot throw air, so a pocket of air at a high point or in the pump volute breaks the flow. The loop goes air-bound, the pump spins against vapor, and the far end gets nothing. A circulator running against a slug of air also runs its seal dry, which is a pump-guide failure mode that starts as an air problem.

Air gets into a closed loop three ways: it comes in with the fill water and the make-up, it comes out of solution when the water heats and the pressure drops, and it gets pulled in through any point of the loop that goes to vacuum. The fix is not a one-time bleed. It is a system that separates and vents air continuously, because the loop keeps making a little air every time it heats and cools.

The air separator and the high-point vents

The air separator is the device that pulls entrained air out of the moving water and sends it up to a vent. A microbubble separator runs the flow through a coalescing media that gives the tiny bubbles a place to cling, where they merge into bubbles big enough to rise out of the stream and collect at the top, then vent. It is a multi-pass device. Over many laps of the loop it scrubs the dissolved air content down, commonly to under about 0.5 percent of system volume, which is the point where the water stops giving the system trouble.

Location is not arbitrary. Air comes out of solution most readily where the water is hottest and the pressure is lowest, because that is where gas is least soluble. So the separator goes at that spot, which on most loops is the outlet of the boiler or the heat source, before the water cools and the pressure climbs. Put it there and it catches the air the moment the water releases it. Put it on the cold return and it works far harder for less.

The separator does the bulk work, but the high points still need vents. Air collects at the top of every riser and at each coil, and an automatic or manual air vent at each high point lets it out so the pump never has to push it around. Getting the air out cuts the corrosion and restores the heat transfer at the same time. A loop that keeps making air after a thorough purge has a real fault to find: a vent sucking air on the suction side of the pump, a fill valve cycling and adding aerated water, or a section going to vacuum. Find it, because you cannot balance or treat a loop that is still swallowing air.

Microbubble air separator

A coalescing device, best placed at the hot, low-pressure heat-source outlet, that merges tiny bubbles so they rise and vent

Air vent

A manual or automatic valve at each system high point that lets collected air out so the pump does not have to move it

Why does a hydronic system need an expansion tank?

A hydronic system needs an expansion tank because water expands when it heats and water does not compress. Fill a closed loop cold, then heat it to operating temperature, and the same mass of water now wants more volume. With nowhere to go, that expansion has only one outlet: the pressure relief valve, which lifts, dumps treated water, and then the system cools, the pressure drops, and the fill valve adds fresh oxygenated make-up to replace what blew out. A loop with no working expansion tank corrodes itself through its own relief valve, one heating cycle at a time.

The modern answer is a sealed tank with an air cushion held behind a flexible membrane. A diaphragm tank uses a fixed membrane bonded across the shell; a bladder tank uses a replaceable balloon that holds the water and keeps it off the steel. Both keep the air charge and the system water apart, which matters because the old plain steel compression tank let air dissolve into the water and had to be sized larger and watched for waterlogging. The membrane tank gives the expanding water somewhere to push without mixing air back into the loop.

Two numbers set the tank up. The size has to accept the volume the system water gains between cold fill and operating temperature, which depends on the loop volume, the temperature swing, and the fill and relief pressures, and is worked from the fluid expansion data, not guessed. The precharge is the air pressure on the dry side, set before water goes in, and it has to match the cold fill pressure at the tank, commonly 12 psi on a small system and set to the system height on a taller one. Get the precharge wrong and a correctly sized tank still cannot do its job.

Diaphragm tank

An expansion tank with a fixed membrane separating the air charge from the system water

Bladder tank

An expansion tank with a replaceable bladder that holds the water off the steel shell, separate from the air charge

Precharge

The air pressure set on the dry side of the tank before filling, matched to the cold fill pressure at the tank

The point of no pressure change and pump-away piping

Where the expansion tank ties into the loop is not a convenience choice, it is a design decision that sets the pressure everywhere else. The connection point is the one spot in the system whose pressure does not change when the pump starts, because the tank holds it at the fill pressure no matter what the pump does. The trade calls it the point of no pressure change. The pump cannot change the pressure at the tank; it can only add its head to one side of that point and subtract from the other.

Tie the tank in near the suction side of the pump, and when the pump starts it adds its full head downstream, into the loop, so the whole system pressure rises and stays above the fill point. That is pumping away from the expansion tank, and it is the arrangement you want, because a system held above its fill pressure keeps the air dissolved, keeps the high points from going to vacuum, and feeds the pump suction the head it needs to avoid cavitation. The pump guide covers the suction-side consequence in detail.

Tie the tank in on the discharge side instead, so the pump pumps toward it, and the pump's head gets subtracted from the loop downstream. The system pressure drops when the pump runs, the top of the loop can fall below atmospheric and pull air in through every vent, and the loop air-binds on the very mornings it should be running hardest. Same tank, same pump, opposite result, decided entirely by which side of the point of no pressure change the tank lives on. Connect the tank near the pump suction and pump away from it.

Propylene or ethylene glycol: which belongs in the loop?

Glycol goes in the loop when part of the system can freeze: a rooftop unit, an air handler with an outdoor coil, a garage or attic run, a snowmelt slab, or any chilled water line exposed to winter. Plain water freezes and splits a coil, and a split coil over a holiday weekend is the most expensive water-side failure there is. The choice is between two glycols, and toxicity usually settles it.

Propylene glycol is the default for HVAC in occupied buildings. It is classed by the FDA as generally recognized as safe, the same family used in food and cosmetics, so if a heat exchanger ever leaks it into potable water the consequence is far smaller. Ethylene glycol carries a lower freeze point and lower viscosity at the same percentage, so it transfers heat better and pumps easier, but it is toxic, with a sweet taste that draws kids and animals, and ingestion can cause kidney failure. Ethylene shows up in industrial process loops and sealed equipment where no one can drink it. In a building with people, propylene is the safe call, and many plumbing codes and project specs require it for that reason.

Either way it has to be inhibited glycol made for HVAC, never automotive antifreeze. That is its own section, because it is the mistake that quietly destroys systems. Pick the glycol for the toxicity risk first, then live with the heat-transfer and pumping penalty that comes with it, which the next section and the balancing guide both account for.

Propylene glycol

The low-toxicity glycol (FDA generally recognized as safe) used in occupied-building HVAC; higher viscosity, slight heat-transfer penalty

Ethylene glycol

The lower-viscosity glycol with better heat transfer and a lower freeze point, but toxic, so it is kept out of occupied potable-risk systems

How much glycol: freeze protection versus burst protection

There are two protection levels and they call for very different amounts of glycol. Freeze protection keeps the fluid liquid and pumpable at the lowest expected temperature, which you need on any line that has to keep circulating in the cold. Burst protection only keeps the fluid from expanding hard enough to split the pipe; the fluid is allowed to turn to slush and stop flowing, as long as it does not crack the metal. Burst protection needs far less glycol than freeze protection, because slush expands less than the equipment can tolerate.

Pick the level by what the line has to do. A chilled water loop that runs through winter, or a heating coil that must hold against an outdoor design temperature, gets freeze protection at that temperature. A drained-down or dormant system that only has to survive a cold snap without splitting gets burst protection. Sizing burst protection like freeze protection is a common and expensive overshoot, because more glycol costs more, pumps harder, and carries less heat.

The percentages below are approximate for propylene glycol and exist to show the shape of the curve, not to size a real system. Get the actual percentage for your fluid and your low temperature from the glycol manufacturer's chart, because the numbers are specific to the product and the inhibitor package. The penalty is real at every step: a glycol mix holds less heat per pound and pumps thicker than water, so the design flow has to climb to move the same load, which is exactly the flow correction the balancing guide applies. Run a glycol loop on water-based numbers and every flow comes in short.

Inhibited glycol and the maintenance it needs

The glycol that goes in an HVAC loop has to be inhibited glycol formulated for the job, and the difference is the inhibitor package mixed into it. Inhibited industrial glycol carries chemicals that passivate the metal surfaces so they resist corrosion, plus buffers that neutralize the organic acids glycol forms as it slowly oxidizes. Uninhibited glycol protects against freeze and nothing else, and it turns on the system. As glycol oxidizes it makes acid, and with no buffer the fluid goes acidic and starts eating the very metal it is supposed to protect.

Automotive antifreeze is the trap. It is glycol, and it is cheap and on the shelf, so it ends up in boilers it has no business being in. Its inhibitor package, often silicate-based, is built for an engine cooling system, not a heating loop, and it can drop out, gel, foul heat exchangers, and clog the small passages in control valves and coils. Use it once and you can spend a season chasing the fouling it leaves. The rule is simple: HVAC-inhibited glycol only, never automotive antifreeze, no exceptions on a system you want to keep.

Inhibited glycol is not fill-and-forget. The inhibitor depletes over time, faster when the loop keeps taking fresh make-up water or untreated glycol, and once it is gone the fluid degrades fast into corrosive acid. So you test it. A glycol refractometer reads the concentration in the field, which confirms both the freeze protection and that the mix has not been diluted by make-up water. A pH check shows whether the inhibitor is still alive: a pH drifting toward neutral or acidic means the package is depleting. On larger systems a reserve alkalinity titration against the baseline tells you how much buffer is left and whether the fluid needs topping up or changing out. Test on the schedule the fluid maker and the water-treatment vendor set, and log the numbers so the trend is visible before the fluid turns.

Inhibited glycol

HVAC glycol carrying a corrosion-inhibitor and acid-buffer package; the only kind that belongs in a hydronic loop

Glycol refractometer

The field tool that reads glycol concentration from a drop of fluid, confirming freeze protection and flagging dilution

Reserve alkalinity

A titration that measures the buffer left in the fluid; falling against baseline means the inhibitor is depleting

Water treatment: inhibitor, pH, and biocide

Even a loop with no glycol needs the water treated, because plain make-up water left alone corrodes steel and grows biology. Closed-loop water treatment runs on three things: a corrosion inhibitor to protect the metal, a pH held in the right band, and a biocide where the system can grow microbes. The water-treatment vendor sets the program and the target ranges to suit the metals in the loop and the make-up water chemistry, and the program is matched to the system, not pulled off a generic shelf.

Corrosion inhibitors come in a few chemistries. Nitrite forms a passive oxide film on steel and iron and is a common workhorse, often run with other chemicals to round out the protection. Molybdate works similarly and shows up where nitrite is not wanted, sometimes blended with it. Both want the water held in a controlled band, with closed-loop pH commonly kept on the alkaline side, often somewhere around 8.0 to 10.5 depending on the metals and the program. Hold the pH and the inhibitor residual in range and the steel stays passive; let either drift and the protection fails quietly.

Biology is the other front. A loop that runs warm, especially one carrying glycol, which microbes can feed on, can grow bacteria and slime that clog small passages and drive under-deposit corrosion. A biocide keeps it in check where the risk is real. The way to know any of this is working is to test the water against the vendor's targets, the same closed-loop checks an inspection program looks for: pH, conductivity, dissolved iron, and the corrosion-inhibitor residual. Dissolved iron climbing in the sample is the loop telling you it is rusting, before you ever see the magnetite.

Corrosion inhibitor

A chemical such as nitrite or molybdate that forms a protective film on the loop metal and is held to a target residual

Biocide

A treatment that controls bacteria and slime, which clog passages and drive under-deposit corrosion, especially in glycol loops

The dirt separator and the magnetic separator

Corrosion makes a solid, and that solid is the next problem. As steel rusts in the loop it sheds iron oxide, the black magnetite sludge that collects in strainers, settles in low spots, and coats the inside of coils, radiators, and fan coils. Magnetite is heavy and abrasive. It sands the pump seal and impeller, the same way construction debris does in the pump guide, and a layer of it on a coil acts like insulation that cuts the heat transfer the coil was sized for. A loop full of magnetite runs cold and pumps hard for no return.

A dirt separator pulls the suspended solids out of the moving water the way the air separator pulls out air, by slowing and swirling the flow so the particles drop into a collection chamber you blow down. A magnetic dirt separator adds a magnet, usually a sleeved rod in the flow, that grabs the magnetite directly, because the iron oxide is magnetic and a plain dirt separator catches it only as it settles. The magnet holds the sludge against the wall until you pull the rod and wipe it clean. The trade-off is that a magnetic separator is built for magnetite and only partly catches the non-magnetic dirt, so the two functions are often combined.

Put the dirt separator where it sees the flow before the sensitive parts, commonly ahead of the pump and the heat exchanger, and blow it down on a schedule. The volume of black sludge that comes out of the blowdown is a direct read on how much the loop is corroding. A separator that keeps loading heavily after the system has been clean for a while is pointing back at an oxygen problem, almost always make-up water the loop should not be taking.

Magnetite

Black iron-oxide sludge from corroding steel; abrasive to the pump and insulating on coils, and the thing a magnetic separator targets

Magnetic dirt separator

A separator with a magnet that captures magnetite from the flow, blown down or wiped clean on a schedule

Fill and flush before the treatment goes in

A new loop is full of trash, and treating it before it is clean wastes the chemicals and fouls the system. Field-built piping carries cutting oil, pipe dope, thread chips, weld slag, mill scale, and general dirt, and the first water through it sweeps all of that toward the first restriction, a strainer, a valve, or a coil. The order is flush first, then treat. Run clean water through the system with the temporary startup strainers in place, pull and clean them until they come out clean twice, and only then charge the inhibitor and the glycol.

Treat a dirty loop and the debris keeps fouling the strainers you have to clean, every cleaning drains treated water, and the trash that gets past coats the new metal you were trying to protect. The flush is the same cleanliness discipline the pump and balancing guides both insist on, because all three jobs share the loop and all three fail on a dirty one. A flush done right also carries off the loose mill scale that would otherwise become the loop's first load of magnetite.

Once the loop is clean, fill it with treated water or pre-mixed glycol to the design percentage, bring it up to pressure through the fill valve, and run the purge to get the air out before the system runs in earnest. Mixing glycol on site means using deionized or soft make-up water if the spec calls for it, because hard water minerals fight the inhibitor and drop out as scale. Clean loop, right water, then the chemistry. Do it in that order and the treatment protects metal instead of chasing dirt.

Chilled, hot, and condenser water are not the same job

The water side changes with what the loop carries. Hot water heating runs warm and closed, so its main fights are air, which comes out of solution fastest when the water is hot, and the expansion that comes with the temperature swing. Get the air separator at the boiler outlet and the expansion tank sized and precharged, and most hot water troubles are solved on the water side.

Chilled water runs cold and closed, and the cold brings two extra problems. Any part of the loop exposed to freezing needs glycol, which is why chilled water systems with outdoor air handlers or rooftop coils carry it. And cold pipe sweats, so condensation and insulation are part of the job in a way they never are on a hot loop. Chilled water also tends to run year-round in big buildings, so the corrosion clock runs every hour, and the plant delta-T that the balancing guide chases lives or dies on clean, air-free, properly treated water.

Condenser water is the different animal, because it is usually an open loop tied to a cooling tower. Open to the air, it is constantly picking up oxygen, evaporating and concentrating its minerals, and growing biology in the tower basin, so it scales, fouls, and corrodes far faster than any closed loop and it has its own treatment program built around blowdown, scale and corrosion control, and biocide, plus Legionella control. That open-tower chemistry is a topic of its own in the cooling pillar and sits outside this closed-loop guide. The line to draw is simple: closed loops fight the oxygen they should never get more of, and the open condenser loop fights the oxygen and minerals it gets all day.

Commissioning the water side

Commissioning the water side is a short sequence that proves the loop was set up to last, and it produces numbers, not opinions. It comes after the flush and the fill and before the system is handed over, and it ties together every part of this guide into a set of readings someone can check later.

Confirm the cold fill pressure at the fill valve matches the design, high enough to lift water to the top of the system plus margin. Confirm the expansion tank precharge, checked on the dry side with the tank isolated and the water side drained, matches that fill pressure, because a tank precharged wrong cannot accept the expansion no matter how well it is sized. Run the purge and confirm the air is out, clean water and no bubbles from every high-point vent and from the separator. On a glycol system, read the concentration with the refractometer and confirm it meets the design percentage for the freeze or burst protection the job needs. Pull a water sample and confirm the pH and the inhibitor residual are in the vendor's range.

Those five readings, fill pressure, precharge, air out, glycol percentage, and water chemistry, are the water-side acceptance. A system that passes them starts life protected. A system handed over without them is handed over on hope, and the first cold snap or the first season of corrosion finds out which numbers were never set.

Maintaining it once it is in service

Whoever fills and treats the loop hands the owner a short list of things that need a look on a schedule, and the system rewards or punishes whether anyone keeps it. None of it is hard. All of it gets skipped on systems that fail early.

The glycol gets tested for concentration and for inhibitor health, because the protection it gives drifts as the loop takes make-up water and as the inhibitor depletes. The water chemistry gets tested for pH and inhibitor residual against the vendor's targets, and the dissolved iron watched as the early read on corrosion. The expansion tank precharge gets checked, because a tank slowly loses its air charge through the valve over the years and a waterlogged tank stops protecting the loop. The air vents get checked, because a stuck vent either stops clearing air or weeps and adds make-up. And the dirt separator gets blown down, with the volume of magnetite that comes out tracked as a read on how the loop is aging.

The make-up water meter ties the whole list together. A loop that holds its water needs almost none, and a meter that barely moves says the system is sealed, treated, and stable. A meter climbing past about 5 percent of system volume a year is the single most useful warning the owner has, because it means a leak is bleeding treated water out and pulling raw oxygenated water in, which undoes the glycol, the inhibitor, and the air control all at once. Teach the owner to read the meter, and they will catch the slow failures before they become the loud ones.

Chilled and condenser water treatment in a data center

In a data center the water side runs to a harder standard, because the cooling never stops and the loops carry the load that keeps the hall alive. The closed chilled water loops feeding the CRAH coils get the full closed-loop program: tight air control, sized and precharged expansion, corrosion inhibitor held in range, dirt and magnetic separation, and glycol where any part of the loop sees the cold. Magnetite in a data center loop is not just a coil-fouling nuisance; it is abrasive sludge headed for pumps that are not allowed to fail, so the magnetic separators and the blowdown discipline matter more, not less.

The make-up water meter carries extra weight here, because a leak that bleeds treated water and pulls oxygenated make-up into a round-the-clock loop corrodes faster than the same leak on a building that idles at night. The water chemistry gets sampled on a real cadence, and the dissolved iron trend is watched, because a corrosion problem caught in the lab is a treatment adjustment and the same problem caught in the field is a fouled coil and a hot aisle.

The open condenser water and cooling tower side runs its own aggressive program for scale, corrosion, biology, and Legionella, and that open-loop chemistry sits in the cooling pillar alongside the tower and the chiller work. This guide is the closed-loop companion to the airflow, containment, and balancing in that pillar. The water has to be clean, air-free, and treated for the coils to hand the server heat to the plant at all, and a spotless airside balance cannot save a loop quietly rusting itself shut.

What to document

The water-side setup is only as good as the record that proves it, and the commissioning numbers are the baseline every future reading gets measured against. A glycol percentage drifting down, a precharge falling, a make-up count climbing, a dissolved iron creeping up, none of them mean anything without the day-one number to compare to. Capture the fluid, the protection, the pressures, the chemistry, and the equipment so the next person can pick up the loop years later and know what it should read.

For each loop record the fluid and glycol type and percentage, the freeze or burst target it was set for, the cold fill pressure and the expansion tank model and precharge, the corrosion inhibitor and its residual target, the dirt and air separators fitted, and the make-up meter reading at handover. Log every water and glycol test with its date and result so the trend is visible. The table below is the core loop record; on a multi-loop plant, one row per loop with the test dates noted alongside.

Common mistakes

• Living with constant make-up water instead of finding the leak, so every gallon brings fresh oxygen and minerals that corrode the loop.
• Filling and running before the air is separated and vented, leaving the loop noisy, air-bound, and unable to circulate or transfer heat.
• Setting the expansion tank precharge wrong or letting it waterlog, so a correctly sized tank cannot accept the expansion and the relief valve weeps.
• Pumping toward the expansion tank instead of away from it, so system pressure drops when the pump runs and the high points pull in air.
• Pouring automotive antifreeze into a boiler, where its engine-coolant inhibitors gel, foul heat exchangers, and clog valves and coils.
• Using uninhibited glycol, or never testing the inhibited glycol, so the package depletes and the fluid turns acidic and corrodes the metal.
• Skipping the dirt or magnetic separator, letting magnetite sand the pump and insulate the coils until the system runs cold.
• Charging the treatment into a loop that was never flushed, so construction debris fouls the strainers and coats the new metal.

Field checklist

Standards and references

ASHRAE owns the hydronic system and water-treatment framework, with its handbooks covering closed-loop system design and water treatment, and ASHRAE Standard 180 addressing the inspection and maintenance of building HVAC systems, including the closed-loop water quality checks, pH, conductivity, dissolved iron, and inhibitor residual, that prove the program is working. Name the document that governs the point and confirm the edition, because these revise on their own cycles, and treat the design targets as design guidance rather than a substitute for the project specification.

The fluid and inhibitor manufacturer is the controlling authority on the glycol. The freeze and burst percentages, the heat-transfer correction, the inhibitor program, and the test methods, including reserve alkalinity and concentration by refractometer, come from the manufacturer's data for the specific product, and those numbers override any rule of thumb. The water-treatment vendor sets and holds the closed-loop chemistry program and the target ranges for the metals and make-up water on the job.

On the make-up connection, the plumbing code governs the backflow protection between the building water and the loop, with the adopted code edition and the local amendments controlling the device and the installation; the backflow itself is a topic the plumbing trade owns. Above all of it sits the project specification and the engineer of record, who set the glycol type and percentage, the tank sizing, the treatment program, and the acceptance. Cite the body that owns the point, verify it against the adopted edition, and let the contract documents control when they are tighter than common practice.

Units, terms, and conversions

The water side carries its own vocabulary and a couple of unit systems, so the same quantity reads differently across a fill-valve label, a glycol chart, and a metric drawing.

Pressure is psi in the field and kPa or bar in metric sources, and the static height of a loop converts to pressure at about 2.31 feet of water per psi for cold water, which is the figure that sets the fill pressure and the tank precharge. Glycol concentration is given as a percentage by volume and read with a refractometer, distinct from freeze protection, the temperature the fluid stays liquid, and burst protection, the lower temperature it survives as slush without splitting the pipe. System volume is in gallons or liters, and the under-5-percent-a-year make-up figure is referenced to that volume. pH is unitless on its 0 to 14 scale, and the corrosion-inhibitor residual is read in the units the vendor's test kit gives.

Make-up water

Fresh water the fill valve adds to hold loop pressure; every gallon brings oxygen and minerals, so a rising count signals a leak

Freeze vs burst protection

Freeze protection keeps the fluid pumpable at the low temperature; burst protection only keeps it from splitting the pipe as slush, and needs far less glycol

Point of no pressure change

The expansion tank connection, whose pressure the pump cannot change; tie it near the pump suction and pump away from it

Precharge

The air pressure on the dry side of the expansion tank, set to the cold fill pressure before the loop is filled

Inhibitor residual

The amount of active corrosion inhibitor left in the water, tested against the vendor's target range

Magnetite

Black magnetic iron-oxide sludge from corroding steel, captured by a magnetic dirt separator and blown down

Related tools

Calculators and readiness checks for this work

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