How to Check Water Hardness on a Service Call

Why this matters

Hard water does not break anything on the day you install it. It lays down scale a little at a time on every surface that heats water or evaporates it, and two years later the tankless heater is throwing flow errors, the boiler exchanger is kettling, the dishwasher's heating element has a mineral jacket, and the pool heater's exchanger is half blocked. Each of those calls gets diagnosed as an equipment fault and the equipment gets the blame. A hardness test takes five minutes and a cheap kit, and it is the difference between fixing the cause and replacing a part that will scale up again.

The unit, and what counts as hard

Hardness is dissolved calcium and magnesium, the minerals that come out of solution as scale when water is heated or evaporated. It is reported two ways:

• Grains per gallon (gpg), the unit softener and appliance manuals use. One grain is 1/7,000 of a pound.
• Milligrams per liter as calcium carbonate (mg/L as CaCO3), the unit water labs and utility reports use. Parts per million (ppm) is the same number for water.

One gpg equals 17.1 mg/L. Multiply gpg by 17.1 to get mg/L; divide mg/L by 17.1 to get gpg.

The USGS classification, used widely: soft 0 to 60 mg/L (0 to 3.5 gpg), moderately hard 61 to 120 (3.5 to 7), hard 121 to 180 (7 to 10.5), very hard over 180 (over 10.5). Scale problems start showing on heated equipment in the hard range and are routine in the very hard range.

What it does, trade by trade

A pool is a separate measurement. Pool calcium hardness is calcium only, measured in the pool water after evaporation has concentrated it, and judged with the saturation index together with pH, alkalinity and temperature. Fill-water hardness tells you how fast the pool will drift, not where it is now.

Step 1: Ask before you test

On the phone or at the door, find out three things: whether the water is from a utility or a private well, whether there is a softener or other treatment, and what the customer has noticed (white crust on faucets and shower glass, spotty dishes, a heater that "isn't what it used to be"). For utility water, the utility's annual water quality report or its website often lists a hardness value, which tells you what to expect before you test. See related: How to Read the Utility's Water Quality Report.

What breaks if you skip it: you test the wrong tap. A house with a softener has two waters, and the one you need depends on the question.

Step 2: Choose the sample point for the question you are answering

• To decide whether treatment is needed, test raw water: a tap upstream of the softener, often the outdoor hose bib or a sample valve at the well tank, or any tap with the softener in bypass.
• To decide what the equipment has been living on, test at or near the equipment's inlet, with treatment in whatever state you found it.

If there is a softener, test both and write down which is which. What breaks if you skip it: you condemn a softener that is working, or miss one that has been in bypass for a year.

Step 3: Draw the sample correctly

Use a cold tap. Run it for a minute or two until the water temperature steadies, so you are testing water from the main, not water that sat in the line. Rinse the kit's sample tube with the water first, then fill to the line.

Never sample from the hot side. Heated water has already dropped some of its hardness as scale, so it reads low, and a tank or tankless outlet can be hot enough to scald. What breaks if you skip it: a falsely low result that clears a heater that is scaling.

Step 4: Run the titration

A drop-count titration kit (an EDTA reagent that changes the sample's color when all the hardness has reacted) is the field standard. Add the buffer and indicator the kit specifies, then add titrant one drop at a time, swirling, and count drops until the color changes completely. On common kits each drop equals 1 gpg for the sample size the kit specifies; read your kit's card, because some use a different sample volume and a different drop value.

Test strips are faster and coarser; they are fine for "soft or hard" and not for comparing against an equipment limit.

Read the reagents' safety data sheets. The buffers are commonly ammonia-based irritants, so wear safety glasses and gloves, cap the bottles between drops, and do not sniff the vial; rinse any splash from the skin or eyes with water for 15 minutes. What breaks if you skip it: a vague strip result where a number decides the recommendation, and a reagent splash on the first job you hurry.

High dissolved iron, common in well water, can blur the color change. If the sample has an orange tint or the fixtures are stained, note it and test iron separately; it is its own treatment problem.

Step 5: Convert, classify, and compare to the equipment's own number

Convert to whichever unit the equipment manual uses, classify it, and compare it to the number in the manual: many tankless and boiler manuals state a maximum hardness or tie their descaling interval to hardness bands, and many dishwashers have a hardness setting in their menu. The manual's number governs that piece of equipment, not the USGS class.

What breaks if you skip it: "very hard" is a description; "39 mg/L over what this heater allows" is a recommendation the customer can act on.

Step 6: Record it and recommend

The ticket gets the sample point, the treatment state, the raw result, the converted result and the comparison. The recommendation follows from the comparison: return a bypassed softener to service, set the dishwasher's hardness menu, add a descaling interval to the maintenance agreement, or quote treatment.

Worked through: the tankless heater with a flow error

A tankless gas heater on a private well is throwing a low-flow error and the customer says showers swing hot and cold. The kitchen faucet has white crust. There is a softener in the basement.

The tech finds the softener's bypass valve turned to bypass; the customer remembers a plumber doing that "a while back" during a repair. The tech tests the kitchen cold tap in that state, puts the softener back in service, starts a manual regeneration, and retests the same tap when the cycle finishes.

The heater manual, for this example, allows up to 200 mg/L.

HARDNESS - well, softener on site
Sample A: kitchen cold, softener in BYPASS
  14 drops x 1 gpg/drop = 14 gpg
  14 gpg x 17.1 = 239 mg/L as CaCO3, very hard
Sample B: same tap, softener in service,
          after regeneration
  1 drop = 1 gpg = 17 mg/L, soft
Heater manual limit: 200 mg/L (200 / 17.1 =
  11.7 gpg)
  A: 239 - 200 = 39 mg/L over, about 20 pct
  B: 17 is under 200, within the limit
Iron: no tint, no staining; not tested

What it means. The heater has been living on Sample A, roughly 20 percent above its own limit, for as long as the softener has been bypassed. The low-flow error fits a scaled exchanger. The softener works when it is in service, so the fix is not new treatment.

The work. Descaling follows the heater's own procedure: shut off the gas and electrical supply to the heater first, close the isolation valves, and let the unit cool before opening the service ports, because the exchanger and its water can still be hot enough to burn. After the flush, the restart step carries its own check: open the water first and confirm flow through the unit with no leaks at the service valves, then restore gas and power, and confirm the burner lights and outlet temperature holds at a steady flow before the customer runs a shower.

What would change the answer. If Sample B had also come back hard, the softener would need its own diagnosis (salt bridge, exhausted resin, a failed control) before descaling, or the heater scales again in months. If there had been no softener, the recommendation would be treatment or a descaling interval set from the manual's hardness bands, and the customer decides which. On softened water a tank heater's anode can be consumed faster, so a tank in the same house gets its anode checked at the next flush.

References

• USGS, Hardness of Water (classification by mg/L as calcium carbonate)
• Manufacturer installation and maintenance instructions for the specific heater, boiler or appliance, including any water quality limits
• Safety data sheets for the test kit reagents
• See related: How to Read the Utility's Water Quality Report; Private Wells and the Water Nobody Tests; The Well Water That Explained the Failing Heater