How to Figure the Fill Volume of a Piping Loop
Why this matters
A closed loop has to be filled with a known volume when it gets antifreeze, a chemical treatment or a cleaner, and the volume sets both the order and the dose. Order too little glycol and the loop ends up weaker than the freeze protection the customer was sold, which nobody discovers until a cold snap splits a coil. Order too much and the temptation is to pour it all in, which overshoots the concentration and cuts the fluid's ability to carry heat. Nobody has the loop's volume written down. You build it from a tally: every pipe size's length times its gallons per foot, plus every piece of equipment's water content from its label or manual. The pipe is the part people calculate carefully, and on most real loops it is not the part that decides the answer.
The tally
LOOP FILL TALLY
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PIPE (gal/ft from the pipe's published ID)
Size Type Feet x gal/ft = gal
____ ____ ____ ______ ____
____ ____ ____ ______ ____
____ ____ ____ ______ ____
Pipe total ____
EQUIPMENT (water content from label/manual)
Item Source gal
Boiler / heat exch. ___________ ____
Indirect tank coil ___________ ____
Buffer tank ___________ ____
Air separator, other ___________ ____
Equipment total ____
LOOP TOTAL = pipe + equipment ____
Fluid maker's % for protection temp ____ %
Concentrate = total x % ____ gal
Water = total - concentrate ____ gal
Metered at fill: ____ gal Refract.: ____ %
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The steps
1. Walk the loop and list every pipe size with its length. Supply and return both, every zone, every branch. Measure what you can and estimate concealed runs from the floor plan, and mark which lengths are estimates. Fin-tube baseboard elements are copper tube, so count their length as pipe of that size. PEX radiant tubing goes in as its own line with its own size, because a radiant floor can hold more water than all the other piping put together.
Hazard at this step: walking a loop means touching piping near a running boiler. Supply piping can be hot enough to burn on contact; feel near a pipe before you grip it, and wear gloves in a mechanical room.
2. Get the gallons per foot from the pipe's inside diameter, not its name. The formula, gal/ft = 0.0408 x ID x ID, and the reason nominal size overstates or understates it, are owned by Gallons Per Foot of Pipe by Inside Diameter. A nominal figure is fine for a first rough tally. For the order, use the published ID for the actual type: copper Type L and Type M of the same size hold different amounts, and PEX of a given size holds noticeably less than copper.
3. Get each piece of equipment's water content from its own documentation. The boiler's water content is on its data plate or in its installation manual; so is an indirect water heater's coil volume, a buffer tank's capacity and an air separator's volume. Write the source next to each figure. Do not estimate a boiler from its size: two boilers with the same output can hold very different amounts, one a few quarts in a compact exchanger and one many gallons in a cast iron block.
4. Leave out what is mostly air at fill. A diaphragm or bladder expansion tank holds only a little system fluid at fill pressure; its job is to take up expansion. Do not add its full rated volume to the tally. The fluid's expansion as it heats is the expansion tank's sizing question, and the loop total from this tally is the input to it.
5. Add it up, then figure the mix. Concentration comes from the fluid maker's chart for the freeze or burst protection you need, stated as percent by volume of concentrate. Concentrate = loop total x that percent. The rest is water. Write both.
6. Round the order up, and the pour exactly. Buy at least the concentrate the tally calls for, rounded up to the container size. Then pour the computed amount, not the whole order. Leftover concentrate goes back on the truck. Pouring the whole round-up into the loop raises the concentration above what you designed.
7. Meter the fill and confirm the mix. Count containers or read a meter as the loop fills, and compare to the tally. After the fluid has circulated, check the concentration with a refractometer or the test method the fluid maker specifies. The meter checks your volume; the refractometer checks your mix.
Worked example: a boiler loop with an indirect tank and a buffer
A loop has 120 ft of 1 in copper and 60 ft of 3/4 in copper (the 60 ft includes the baseboard elements). It serves a boiler, an indirect water heater and a buffer tank.
First pass, nominal sizes:
Line Item Value Unit
1 1 in: 120 ft x 0.041 4.92 gal
2 3/4 in: 60 ft x 0.023 1.38 gal
3 Pipe total = L1 + L2 6.30 gal
About 6.3 gallons of pipe before equipment. A tech who stops here and orders for a 6 gallon loop is going to run out of fluid very early in the fill.
Second pass, published Type L IDs:
Line Item Value Unit
4 1 in Type L: 120 x 0.0429 5.15 gal
5 3/4 in Type L: 60 x 0.0251 1.51 gal
6 Pipe total = L4 + L5 6.65 gal
Using the real bore adds 0.35 gallons (6.65 minus 6.30), about 6 percent of the pipe. Worth doing, and also not the point.
Equipment, from the documentation on site (these are the values read for this example; yours come from your equipment):
Line Item Value Unit
7 Boiler (data plate) 2.4 gal
8 Indirect coil (manual) 1.2 gal
9 Buffer tank (label) 15.0 gal
10 Equipment total 18.6 gal
11 LOOP TOTAL = L6 + L10 25.25 gal
The equipment holds 18.6 of the 25.25 gallons, 74 percent of the loop (18.6 / 25.25 = 0.737). The buffer tank alone is 59 percent (15.0 / 25.25 = 0.594). The careful Type L correction moved the total by 0.35 gallons; forgetting the buffer tank would have moved it by 15. That is the general lesson of a loop tally: get every piece of equipment on the list before refining the pipe.
The mix. Say the fluid maker's chart calls for 30 percent by volume for the protection this customer needs (read your own fluid's chart; the percentage depends on the product and the temperature).
Line Item Value Unit
12 Concentrate = 25.25 x 0.30 7.58 gal
13 Water = 25.25 - 7.58 17.67 gal
The order is rounded up to whole containers of concentrate. If it comes in 5 gallon pails, that is two pails, 10 gallons. Pour 7.58 gallons, not 10: all 10 in a 25.25 gallon loop would make 10 / 25.25 = 40 percent, a third stronger than designed (40 / 30 = 1.33), with the heat-transfer penalty that comes with it.
The fill. The meter reads 25.8 gallons by the time the loop is full and purged, against 25.25 tallied. The extra half gallon is in the purge cart's hoses and the fill line, and a 2 percent gap is inside the uncertainty of the estimated concealed runs. After circulation, the refractometer reads 30 percent. Both checks close.
What a failed check looks like. Suppose the meter had read 31 gallons. That is 5.75 gallons over the tally (31 minus 25.25), 23 percent, which is too much to be hoses. Look for equipment nobody listed (a second zone's panel radiators, a radiant manifold behind a wall, an old tank left in the loop), and recompute before adding any more concentrate. If the extra is real loop volume, the 7.58 gallons already poured are now in 31 gallons, about 24 percent (7.58 / 31 = 0.245), which is short of the design and needs bringing up to the new total's 30 percent: 9.3 gallons of concentrate in all, 1.72 more than is in it. Room has to be made by draining mix that already carries concentrate, so the swap is larger than the gap: drain about 2.3 gallons of the current mix and add about 2.3 gallons of concentrate (1.72 / (1 - 0.245) = 2.28), then recheck with the refractometer.
The hazards of the fill itself
- Cool and isolate before opening anything. Turn the boiler off at its service switch, lock out its breaker, close its gas valve, and let the loop cool before opening drains or fill connections. Drained loop water can scald.
- Fill pressure stays below the relief valve. Fill and purge at the pressure the boiler maker specifies and always below the setting stamped on the boiler's relief valve. A purge cart can easily exceed it; watch the gauge while it runs.
- The fluid must match the heat exchangers in the loop. Where any exchanger in the loop has potable water on its other side (the indirect tank coil in this example), the fluid has to be one that the plumbing and mechanical codes as adopted by your jurisdiction and the exchanger's listing permit for that use. Ethylene glycol is toxic and is not used where it could reach drinking water. Read the fluid's label and safety data sheet for its eye and skin protection.
- The fill connection keeps its backflow protection. The makeup water connection to a boiler loop carries a backflow preventer required by code as adopted; it stays in service during and after the fill.
- Old fluid is a waste stream. Drained glycol or chemically treated water goes where local rules allow, never into a storm drain.
- Restore and prove. Restore power and gas only after the loop is full, purged and at its cold fill pressure, then fire the boiler and watch the gauge rise with temperature and stay below the relief setting. Hydronic work is licensed in many jurisdictions; if you do not hold the license it requires, the tally is yours to do and the fill is not.
What changes the answer
- The loop has radiant tubing. PEX in a slab or under a floor can be hundreds or thousands of feet. Get the tubing lengths from the manifold tags or the design, because the tubing can outweigh everything else in the tally.
- The loop is open to a cooling tower or uses a heat pump. The volume method is the same, but the fluid's protection and compatibility come from the heat pump or chiller maker's requirements, which may cap the concentration.
- Part of the loop is valved off seasonally. Fill and dose the whole loop with every zone valve open, or the closed zone fills later with plain water and dilutes the mix.
How to verify you got this right
- Every pipe line has a type and a published ID, and estimated lengths are marked.
- Every piece of equipment has a source for its water content.
- The metered fill is within a few percent of the tally, with any gap explained.
- The refractometer reading matches the design percentage after circulation.
- The tally sheet stays with the job record, so the next fill or dose starts from it.
References
- See related: Gallons Per Foot of Pipe by Inside Diameter
- Boiler, indirect water heater and buffer tank installation manuals for water content
- Heat-transfer fluid maker's concentration charts and safety data sheets