How Frisco's Hard Water Destroys Tankless Heat Exchangers

Frisco is supplied primarily through the North Texas Municipal Water District, drawing from surface reservoirs in the Trinity basin. Surface water in this region carries dissolved calcium and magnesium picked up from limestone geology, and the resulting hardness typically measures between 8 and 11 grains per gallon. Converted to the units on a lab report, that is roughly 137 to 188 parts per million as calcium carbonate.

Grain per gallon (gpg)

The standard US hardness unit. One grain per gallon equals 17.1 mg/L of calcium carbonate equivalent. Water under 3 gpg is soft, 3-7 is moderately hard, 7-10 is hard, and above 10 is very hard.

So Frisco sits at the boundary between hard and very hard, and it moves within that range seasonally as reservoir levels and blend ratios change. It is not the worst water in Texas — parts of the Hill Country running on limestone aquifers are considerably worse — but it is comfortably hard enough that a tankless water heater installed and then ignored will not reach its rated service life.

Calcium carbonate is unusual among common salts in that it becomes less soluble as water gets hotter. Most things dissolve better in hot water; calcium carbonate does the opposite. This property, called retrograde solubility, is the entire reason water heaters scale and cold pipes do not.

Dissolved calcium bicarbonate in cool supply water is stable. Heat it and the equilibrium shifts: bicarbonate breaks down, carbon dioxide comes out of solution, and solid calcium carbonate drops out. It drops out preferentially at the hottest surface it can find, because that is where the solubility limit is exceeded first.

A tankless heat exchanger is a bundle of thin-walled passages — copper on most non-condensing primaries, stainless on condensing secondaries — with combustion gas on one side and potable water on the other. The design goal is to move a great deal of heat across a very thin wall very quickly. A 199,000 BTU unit is doing in a few feet of tubing what a storage tank does over 40 gallons and an hour.

Calcium carbonate has a thermal conductivity somewhere around 2 to 3 W/m·K. Copper is around 400. Putting a layer of scale on the water side of that exchanger is roughly equivalent to insulating it, and the burner does not know that. It fires at the same rate, the heat cannot escape into the water as efficiently, and the exchanger metal itself runs hotter.

• Heat transfer efficiency falls — the unit burns more gas to hit the same setpoint
• Flue gas temperature rises, because heat that should have gone into the water goes up the vent instead
• Flow rate through the exchanger drops as scale narrows the passage, and the flow sensor sees it
• Exchanger wall temperature climbs, which accelerates further scale formation — the problem compounds
• Localized hot spots develop where scale coverage is uneven, and thermal stress at those points is what eventually cracks the exchanger

Scale damage is gradual, which is why it so often goes unaddressed. Nothing breaks on a Tuesday. Performance degrades over years in increments small enough that the household adjusts around it without noticing.

1. 1

   Year 1 to 2 — nothing visible

   Deposition is underway but the layer is thin enough that output is within a few percent of new. This is precisely the window in which the first descaling is cheap and takes an hour.

2. 2

   Year 2 to 3 — the first real symptoms

   Hot water takes noticeably longer to arrive. The unit occasionally overshoots or undershoots on low-flow draws. Winter performance feels weaker than the first winter did, and it is easy to blame the weather.

3. 3

   Year 3 to 5 — intermittent faults

   The unit throws occasional flow or overheat codes and clears them on a power cycle. Flow rate at full demand is measurably below the unit's rating. Two simultaneous showers now feels marginal where it used to be comfortable.

4. 4

   Year 5 to 8 — hard failure

   Persistent lockouts. Rumbling or kettling noise as trapped water flashes to steam under the scale layer. At this point the exchanger has usually been thermally stressed enough that descaling restores some function but not all of it, and the warranty claim will be denied for lack of maintenance records.

Descaling circulates a mild acid solution — food-grade white vinegar at around 5% acetic acid, or a proprietary sulfamic or citric descaler — through the heat exchanger in a closed loop using a submersible pump and two hoses. The acid dissolves calcium carbonate into soluble calcium acetate or citrate and carbon dioxide. The loop runs 45 to 60 minutes, then the system is flushed with clean water.

With the isolation valve kit that should have been installed with the unit, this is straightforward. Two service valves, two hoses, a bucket and a pump. Without those valves, the plumber has to cut into the water lines to create service points, which is why the valve kit pays for itself at the first flush.

1. Shut off gas and power, close both isolation valves to take the unit out of the house loop
2. Connect the pump discharge to the cold service port and the return hose to the hot service port
3. Open the service valves and circulate descaling solution for 45-60 minutes
4. Drain the solution, then flush clear water through in the same loop until the discharge runs clean and pH neutral
5. Remove, rinse and reinstall the cold-side inlet screen, which is where most flow-restriction complaints actually originate
6. Restore gas and power, purge air from the lines, and verify flow rate and outlet temperature against the unit's rating

Descaling is remedial: it removes what has already formed. Water treatment is preventive: it stops the deposition. At Frisco hardness both approaches work, and the right answer depends on how much you want to spend up front versus how much you want to think about it later.

One honest caveat on softeners: fully softened water is slightly more aggressive toward copper than hard water is, because the protective scale film that would normally form does not. This is rarely a practical problem at residential temperatures and correct setpoints, but it is a reason to set the softener to leave a small residual hardness rather than driving it to zero.

Read the warranty document that came with the unit. Every major manufacturer excludes damage caused by scale, lime or mineral deposits, and every one of them reserves the right to require evidence that the maintenance schedule in the manual was followed. Heat exchanger coverage on a residential condensing unit is commonly 12 to 15 years — a genuinely good warranty, and one that is straightforward to void.

• Keep dated invoices for every descaling, showing the model and serial number of the unit
• Keep the original installation permit and inspection record
• If you have water treatment, keep the installation record for that too — it strengthens the file
• Register the unit with the manufacturer at installation; several shorten coverage significantly on unregistered units
• Photograph the rating plate so the serial number is never a question later

A replacement heat exchanger on a whole-house condensing unit, with labor, commonly runs $700 to $1,400 — often close enough to the cost of a new unit that replacement is the better decision. That is the number annual descaling is protecting against.

• Measured flow at full hot demand is noticeably below what the unit delivered when new
• Outlet temperature oscillates during a long shower rather than holding steady
• Rumbling, percolating or kettling from the unit while it fires
• Intermittent flow-sensor, overheat or exhaust-temperature error codes that clear on a reset
• Gas consumption trending up year over year with no change in household hot water use
• White or greyish flakes in the discharge when the inlet screen is pulled
• The unit takes longer to reach setpoint than it used to, especially on small draws

At 8 to 11 grains, annual descaling is the baseline and it is not negotiable if you want the twenty-year life the unit is capable of. Whether you add treatment is a spending decision, not a technical one — treatment does not eliminate the need for periodic service, it extends the interval and reduces the wear happening between services.

If the unit serves a large household, runs at a high setpoint, or is a non-condensing model with a copper primary exchanger running at higher wall temperatures, lean toward treatment. If it is a modest household on a condensing unit at 120F, disciplined annual descaling is genuinely sufficient.

Frisco Tankless Water Heater Pros performs descaling with isolation-valve service loops and can test your hardness on site to tell you which side of that line you are on. Call (972) 430-9024.