Hot water pressure much less than cold is diagnostic (C) Daniel Friedman Improve Hot Water Pressure & Flow Rate
How to diagnose & fix poor hot water pressure & flow

  • HOT WATER PRESSURE IMPROVEMENT - CONTENTS: hot water pressure & flow diagnosis & improvement - steps to correct bad hot water pressure or flow rate. When hot water pressure is noticeably weaker than cold water pressure that clue helps us diagnose the trouble.
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Hot water pressure problem diagnosis & cures:

Tthis article describes the steps in diagnosing & correcting bad hot water pressure. If cold water pressure is acceptable but hot water flow rate or pressure are weak you'll see what to do in this article.

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4 Steps to Fix Bad Hot Water Pressure / Flow When Cold Water Pressure is OK

Tankless or demand type water heater, electric - U.S. DOE Poor hot water flow but good cold water flow at one or more plumbing fixtures usually diagnoses either a blockage at the water heating equipment itself or mineral clogging in the water heating system piping.

Heat causes mineral precipitation out of hot flowing water faster than out of cold water, that’s why often mineral clogs appear on hot water components of a plumbing supply system first.

For example in the electric demand water heater shown at left, if incoming water is high in mineral content, expect scale formation inside the heater that will eventually slow flow through the device.

[Click to enlarge any image]

When only hot water supply, pressure, or flow are poor while cold water pressure is good, then we take a different approach, focusing on hot water supply equipment, piping & controls.

Note: Building water pressure that is poor at all fixtures at both hot and cold water taps may be due to clogged building cold water supply piping. Other links at the end of this article provide guides for diagnosis and repair of other causes of bad water pressure throughout a building.

Start diagnosing weak hot water flow & pressure right at the water heater or cylinder

Diagnosing poor hot water pressure & flow when cold water pressure and flow are acceptable begins at the water heating device itself.

1. check the cold water supply

Make sure that the cold water supply into your water heating appliance is adequate and not blocked. Usually if you can confirm good cold water flow at a drain or fixture valve very close to the water heater you've answered this question, but don't rule out a partly closed or obstructed control valve or pipe connection, tee or elbow close to the water heater itself.

If your check shows that the building's cold water supply flow & pressure are poor, chances are this is not just a hot water pressure or flow problem. In that case start diagnosis

2. check the hot water heating appliance

Select the link below that describes your building's method of producing hot water: the calorifier, hot water cylinder, geyser, or water heater.

  • HOT WATER STORAGE TANKS - Electric, gas, oil heated & indirect storage water heaters, calorifiers, geysers or water cylinders - tanks containing a reservoir of hot water
  • TANKLESS COIL HOT WATER FLOW DIAGNOSIS - demand hot water systems using a heat exchanging coil installed at or in a hot water or steam boiler used to heat the building
  • TANKLESS WATER HEATER HOT WATER FLOW - demand or "instant" hot water heating systems using a stand-alone tankless water heater, usually gas or electric
  • Use our CONTACT link to send us email if your hot water is made by another method than those shown here

3. check hot water piping between the heating appliance and the plumbing fixture

Calcified water fitting due to hard water Brass PEX pipoing fitting corrosion (C) 2014 & Anon
  • BUILDING HOT WATER PIPE DE-SCALING - de-scaling building piping throughout the structure may be necessary and is discussed here but usually the worst scale deposits are in, at or close to the water heating appliance itself.

    Hard water mineral scaling is visible in our photo at above-left.
  • DEZINCIFICATION, BRASS, GUNMETAL - corrosion in copper and brass water piping can clog piping and reduce the waetr flow rate. Particularly where brass plumbing fittings are used (elbows, tees, valves), even where the rest of the water piping system is corrosion-proof plastic piping such as PEX, meringue dezincification can deposit salts inside the plumbing fitting causing clogging and poor water flow.

    Meringue dezincification showing up as a clogged brass elbow in a PEX water piping system is shown at above right.

    Because dezincification (the loss of zinc and possibly lead as a corrosive process leading to leaks) occurs at a more rapid rate in hot water piping, it is possible for clogs to occur in brass hot water pipe elbows, faucets or similar obstructions. And meringue dezincification is more likely to occur in water systems in which the water supply is at a low pH (water is acidic).
  • HOT WATER QUANTITY IMPROVEMENT - which focuses on how to get more hot water total quantity & faster hot water flow from a water heater includes a discussion

    of LARGER DIAMETER Water Supply Piping that can significantly improve hot water flow & pressure at the fixtures

4. Check the plumbing fixtures for local clogging by debris or mineral scale

Mineral scale has clogged the screen in this faucet, reducing both cold and hot water flow (C) Daniel Friedman

  • Check for clogged faucet strainers, clogged shower heads, partly closed or clogged plumbing fixture hot water stop valve.

    This step is particularly important if hot water supply flow rate is good at some fixtures and bad at others.

    Diagnostic tip:

    If both hot and cold water flow rates are slowed at a fixture then it's possible that the problem is a clogged strainer blocked by debris or mineral scale such as shown in our photo above.

    If hot water flow rate or "pressure" as people may speak, is singificantly less than cold water, since both hot and cold are being distributed through the same opening, the clogging or pressure problem is elsewhere.

    But it still might be local: a clogged pipe, elbow, or stop valve on the hot water supply piping near the problematic fixture could explain why hot water flow rate is good at some fixtures and poor at others.

    In that case also see
    for a possible cause of hot water piping blockage.

Troubleshoot poor hot water flow or pressure at conventional hot water storage systems

  • Scale in the water heater or cylinder: If only hot water pressure & flow are bad and if your hot water is made by a conventional oil, gas, or electric water heater or cylinder, look for scale deposits in the water heater itself that blocks water outflow and more important,

  • also look for scale formation in the hot water piping . Remedies for scale-clogged hot water pipes or any building water piping are discussed at WATER PIPE DE-SCALING and discussed also

    at TANKLESS WATER HEATER REPAIR GUIDE or for a blockage at the water heater outlet itself.

  • Look for malfunction in check valves used at water heaters such as that reported by this reader

    (Feb 20, 2012) Chris Covic said:

    We had this problem with our hot water. It turned out to be a glass marble in the hot water line trapped in the first 3/4" to 1/2" CPVC reducing T.

    The hot water tank is from the 90's, the cold water nipple on top of the tank contains a plastic marble that floats, the hot water nipple contains a glass marble that sinks. Both are captured inside the nipple itself with plastic retainers and are supposed to help keep the heat inside the tank.

    The plastic had failed allowing the glass marble to flow down stream in the 3/4" pipe, but was too large to fit thru the 1/2" sch 40 CPVC.

Troubleshoot poor hot water flow or pressure at Instant or Tankless hot water systems

Collapsing building © Daniel Friedman

If your hot water is made by a tankless water heater (typically electric, LP gas such as the unit shown at above left, or natural gas) or tankless hot water cylinder, see TANKLESS WATER HEATER REPAIR GUIDE for a discussion of tankless water heater clogging, pressure loss, heating coil cleaning and other maintenance requirements.

Guide to Improving Hot Water Pressure & Flow When Heated by a Tankless Coil

Tankless coils for hot waterHow to troubleshoot poor hot water flow or pressure at tankless coil hot water systems

If only hot water pressure and flow are poor, and if your hot water is made with a tankless coil see these articles for diagnosing and improving hot water from a tankless coil:

Acid flush of the tankless coil: If hot water is made by a tankless coil, often the coil itself has become blocked with minerals. A blocked tankless coil can be cleaned by flushing it with acid.

But beware, acid flushing a coil is not a permanent repair, and some plumbers will tell  you that the acid-etched pipe surface inside the tankless coil will clog more rapidly than before.

Install a water softener: If you find a pile of tankless coils like those in this photo during a building inspection you might suspect that this building has had a history of repeated clogging of tankless coils used for domestic hot water. After the tankless coil was flushed with acid, the building owner would have been better off installing a water softener to protect against subsequent mineral clogging of the tankless coil interior.

How else can we Improve the Hot Water Volume & Quantity When Water is Heated by a Tankless Coil?

See HOT WATER IMPROVEMENT for a detailed guide to improving hot water quantity and flow when a tankless coil is installed as well as using other methods of hot water heating

See MIXING / ANTI-SCALD VALVES for details about mixing valves on boilers and tankless coil systems and for an explanation of how an anti-scald or tempering valve can increase the volume of hot water available from a tankless coil hot water system.

We discuss tankless coils in detail at Guide to Tankless Coils on Heating Boilers.

Building Hot Water Pipe De-Scaling: Use of acid flushing to clear tankless coils or building water piping

Calcified water fitting due to hard waterClogged hot water piping itself can also be due to minerals and may appear worse than on the cold water piping. Some plumbers offer a service of acid flushing the entire hot (or hot and cold) water piping system. If you see minerals building up on fixtures such as the extreme case in our photo at left, you definitely need a water softener.

We discuss the procedure tankless-coil lime-removal by acid-flush procedure in detail

But any of these acid flush or component replacement “fixes” for mineral-clogged hot water piping or heaters are temporary at best. A better and long lasting repair is to have the water tested for hardness (confirming that there is a mineral problem) and then the installation of a water softener system.

But don’t kid yourself. While installing a water softener will protect water piping from further mineral clogging, it will not un-clog or remove caked-on minerals that are already clogging the system.

For a complete list of articles on diagnosing hot water problems see

What to Do to Improve Hot Water Pressure Where Long Sections of Hot Water Piping are Clogged or Hot Water Pipes are Too-Small in Diameter

sketch of water pipe diameter effectsThe following hot water piping improvement advice is adapted from

Local water piping clogs can cause bad water pressure in entire sections of hot, cold, or both hot and cold water supply pipes and fixtures in a building.

Local hot water supply pipe clogs due to minerals in the water may clog entire lengths of piping so badly that the only repair is replacement of the water supply pipes.

In other cases a plumber may offer to try an acid-wash to de-scale the hot water piping interior.

Certainly if we find that this is what’s needed, there are two approaches that can protect the investment in improved water flow.

These can both be done or  you may choose just one of them.

  • Install a water softener but remember that clogged water supply piping that has not been repaired using an acid-flush will need to be replaced. Installing a water softener won't "un-clog" mineral-clogged water pipes.

    See WATER SOFTENERS & CONDITIONERS to discuss problems with water hardness.
Improving water flow by larger pipes (C) Carson Dunlop Associates
  • Install larger diameter water piping - this method will provide piping that will deliver adequate water flow and pressure for a longer time before clogging up due to high mineral content in the water supply.

    As you can see in the sketches above and at left, courtesy of Carson Dunlop, increasing a pipe size from standard 1/2" to 3/4" in diameter more than doubles the water flow capacity in the piping!

We took this second approach on a building where we found that the hot water line clogged with a combination of minerals and iron.

On cutting open the old hot water supply pipe we saw that its interior was blocked to a mere pinhole of remaining water passage.

But the cold water line was basically clear. So we replaced the hot water line, including straight runs and all elbows, with a ¾” copper pipe. The repair has continued to function well for nearly 30 years.

Water Pipe Clog Diagnosis, Cleanout, Repair Artivcles

Contact us know if you have other questions or repair ideas or if this material leaves you with a question or suggestion.


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