Scale in a Water Heater and Boiler: Where It Comes From and What It Costs

Scale does not appear out of nowhere. It arrives with the water you filled the system with, and it stays in the system for good. Below is a breakdown of the chemistry of the process and of verifiable loss figures.

Where Scale Comes From

Natural water contains dissolved calcium and magnesium salts. In the groundwater and river water of the Tashkent region, the main form is calcium bicarbonate, Ca(HCO₃)₂. This compound exists only in solution. When heated, it decomposes:

Ca(HCO₃)₂ → CaCO₃↓ + CO₂↑ + H₂O

Calcium carbonate is insoluble. It precipitates on the hottest surface: the wall of a heat exchanger, a heating element, the tube of a fire-tube boiler. The second component of deposits is calcium sulfate, CaSO₄. It comes from sulfates, which in the Chirchik and Akhangaran rivers, according to a 2024 study, range from 3.2 to 522 mg/L, with concentration increasing downstream.

An important point. Calcium and magnesium are not consumed and do not disappear. As much as entered the system with the makeup water remains inside, either in the water or on the metal.

Why Temperature Decides Everything

The solubility of calcium carbonate behaves unusually. For most salts it rises with temperature; for CaCO₃ it falls. The hotter the surface, the more actively precipitation proceeds.

Hence the practical rule. Scale does not deposit throughout the volume, but at specific points, in the most heavily loaded areas:

• the heat exchanger wall of a wall-mounted boiler, where the metal temperature is higher than the water temperature
• the surface of the heating element in an electric water heater
• the furnace tube and smoke tubes in medium-capacity boilers
• the plates of a brazed DHW heat exchanger, where the gap between plates is measured in fractions of a millimeter

The German standard VDI 2035 states this directly: the higher the temperature, the higher the risk of deposit formation.

What Scale Does to Heat Transfer

Scale acts as thermal insulation. In its steam systems tip sheet, the U.S. Department of Energy notes that the thermal conductivity of scale is an order of magnitude lower than that of clean steel, and even a thin layer noticeably slows heat transfer.

Then a chain of consequences begins. Heat does not pass into the water, so the metal heats up more. The burner runs longer to reach the set temperature. The flue gas temperature rises. With a thick layer of deposits, the metal overheats and loses strength.

What It Costs: The Figures and Their Spread

Precision is needed here. A claim of the type "1 mm of scale adds 10 percent to fuel consumption" is widely circulated on the internet, but no such figure appears in primary engineering sources. Other figures do.

The U.S. Department of Energy gives a table of fuel loss as a function of the thickness and composition of deposits:

• 0.4 mm of ordinary scale: a loss of about 1 percent
• 0.8 mm of ordinary scale: about 2 percent
• 1.2 mm of ordinary scale: about 3 percent
• 1.6 mm of ordinary scale: about 3.9 percent

Composition changes the picture considerably. With a high iron content, the same 1.6 mm gives 6.2 percent. With a combination of iron and silicon, 0.8 mm gives 7 percent. The general conclusion of the document: fuel consumption in fire-tube boilers can increase by up to 5 percent because of deposits.

So the correct formulation is as follows. A layer of about a millimeter costs roughly 2 to 7 percent of fuel, depending on the composition of the deposits. The spread is large, and it is explained not by disagreement among the authors but by different water chemistry.

What Tests of Residential Water Heaters Showed

In 2009, the Battelle institute, commissioned by the Water Quality Research Foundation, conducted accelerated tests of water heaters on water with a hardness of 26 grains per gallon, which is approximately 445 mg/L CaCO₃, or about 8.9 meq/L. Results over a period equivalent to two years of operation:

• a gas storage water heater lost efficiency, dropping from 70.4 to 67.4 percent
• scale accumulation was about 528 g per year versus 7 g per year on softened water
• a tankless gas water heater dropped from 80 to 72 percent over a period equivalent to 1.6 years, and after chemical flushing recovered only to 77 percent
• an electric storage water heater showed almost no change in efficiency, but accumulated about 907 g of deposits per year on the heating element and the tank
• showerheads and faucet aerators on hard water clogged within a matter of days of equivalent operation

Note the electric water heater. Its efficiency formally does not fall, because the entire heating element is inside the water and the heat goes into the tank anyway. But scale on the heating element raises the coil temperature and shortens its service life. The losses here are measured not in kilowatt-hours but in replacements.

Where the Losses Are Not in Kilowatts but in Hardware

The economics of scale are not limited to fuel. Deposits create three separate cost items:

• reduced service life of heat exchangers and heating elements due to local overheating
• the cost of chemical flushes and the associated downtime
• the manufacturer's refusal to honor the warranty if the water quality does not meet the requirements of the manual

For hot water heating systems, VDI 2035 directly links the permissible hardness of the makeup water to the capacity of the heat source and the specific system volume. For a capacity of 50–200 kW the limit is on the order of 11.2 °dH, for 200–600 kW about 8.4 °dH, and above 600 kW less than 0.3 °dH. The larger and more heavily loaded the system, the stricter the water requirements.

What this means for a project in Tashkent

• The water in Tashkent is hard. In approximately 85 percent of the city and in all other regions of the republic, the water is hard and of low quality. Uzbekistan's sanitary requirements allow total hardness of up to 7 meq/L, and with justification up to 10 meq/L. That is 19.6–28 °dH, which is very hard water by the WHO classification.
• Start with a water analysis at the specific project, not with equipment selection. You need total hardness, iron, sulfates and suspended solids. Old trunk mains cause secondary contamination, iron and mechanical impurities even where the raw water is acceptable.
• Do not count only fuel. For a gas boiler, efficiency losses are real; for an electric water heater, the main damage is the service life of the heating element; for a flow-through DHW heat exchanger, it is clogged channels.
• Mechanical filtration is installed before softening. Suspended solids and iron oxides put the ion exchange resin bed out of service faster than hardness itself.