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How to improve hot water pressure & flow: This article explains how to diagnose the sudden loss of hot water pressure, quantity, or flow in a building.

Thanks to Carson Dunlop Associates, a Toronto Home Inspection Firm and Home Inspection Educator, for permission to use sketches shown in this article. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

Hot water problems and diagnosis articles:

• If you have no hot water at all, but cold water comes out of the hot water faucets, see NO HEAT - NO HOT WATER: HEATER DIAGNOSIS.
• If your problem is gradual or long-standing poor hot water quantity, pressure, or flow, start your diagnosis HOT WATER IMPROVEMENTS or see HOT WATER QUANTITY IMPROVEMENT.
• If hot water pressure has suddenly been lost but you still have some, poor hot water flow, see HOT WATER PRESSURE LOSS
• If you want to speed the delivery of hot water to plumbing fixtures see HOT WATER DELIVERY SPEED UP
• If your concern with total water pressure and flow, both hot and cold, see WATER PRESSURE LOSS DIAGNOSIS & REPAIR, and if you simply run out of water, both hot and cold, see WATER QUANTITY IMPROVEMENT.

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Diagnosing Sudden Loss of Hot Water Pressure & Flow in a Building

Question about diagnosing the cause of sudden loss of hot water pressure:

We suddenly lost hot water pressure [only] in the whole house, a week ago (7 years old townhouse: basement, main floor and upper floor). We drained the hot water tank, checked the various valves in the house but still same weak pressure.

Do you think it could be mineral deposits in the pipes? Or a clogged pipe? What is the most common cause for it? My plumber doesn’t seem to find out the cause of the sudden lost of Hot Water Pressure in the whole house. Are there specialized plumbers for this situation? I assume not all plumbers have the training. I am kind of confused. - R. S.

Answer:

We assume you mean that hot water pressure and flow are suddenly badly diminished in a building, not that pressure or flow have stopped entirely.

We also note that you refer to a hot water system that uses a tank, not a tankless coil to produce the building's hot water. Tankless coil clogging is discussed separately at Clogged Pipes / Tankless Coil De-Scale.

If mineral clogging of the building plumbing system is a problem, usually we notice that hot water pressure is worse than cold water pressure, but also that the hot water pressure declines over time, rather than appearing to suddenly diminish severely. So our diagnosis here is going to look for things that might produce a sudden severe reduction in hot water pressure - some sort of blockage or valve problem in the building water piping.

If hot water pressure suddenly stops entirely but cold water flow continues just fine, then a control valve may have been left closed, or a valve may have failed internally (appearing to be turned on but actually "off" internally due to a broken valve stem part), or a valve or pipe elbow may have become partly blocked by a chunk of mineral scale or by other debris. Here are some hot water pressure diagnosis suggestions:

Often the problem with loss of both hot and cold water pressure at plumbing fixtures is local clogging at a fixture such as faucet strainer and shower head. But if if you SUDDENLY lost hot water pressure everywhere in the building we have some different ideas:

It is unlikely that fixture or strainer or shower head clogging would explain a hot water pressure sudden loss at all plumbing fixtures; usually that clogging builds up over time. Indeed a scrap of mineral crud could break loose and suddenly clog an individual fixture strainer, but the chances of that happening at the same time at all individual fixtures just doesn't seem likely.

Possible Point Stoppages or Clogs in Building Hot Water Pipes, Valves, or Heaters

But a similar problem could have occurred right at the water heater, or in water heater piping near the point of hot water origin. For example, a failed, or clogged control valve, pipe section, pipe elbow, or even the outlet at the hot water tank itself could be come blocked. Clogged piping diagnosis is discussed in more general terms (besides a clogged tankless coil) at Water pipe clog diagnosis and repairs at Water pipe clog repair guide.

But let's distinguish between clogging of runs of water supply piping in a building, and a specific point clog or blockage. Point clogs or stoppages in water piping can occur anywhere in the water pipe system, but are more common at these points:

• Debris blockage at an individual water control valve where a clump of debris or mineral debris can partly block the valve opening
• Damage blockage at an individual water valve that might appear to be fully "open" (valve handle turned fully counter-clockwise) but internally in some valve designs if the valve stem breaks, the handle may turn but the valve may remain closed (or open, or in whatever position it was last found)
• Debris blockage in an elbow or bend in water piping - more likely to have occurred closest to the debris source such as near the water heater
• A clogged water filter - would normally affect both hot and cold water pressure unless someone installed a water filter just upstream of water entering the hot water heater.
• A clog or problem at the water pressure tank - would normally affect both hot and could water pressure

Three Types of Plumbing Valves Used on Water Piping: Globe Valve, Gate Valve, Ball Valve Compared

As Carson Dunlop Associates' illustrations below show, a broken water control valve stem can obstruct or block water flow through the valve. Globe valves (below left) tend to most restrict water flow compared with the gate valve (center) and ball valve (right). But a broken stem on a gate valve can also leave the valve stuck partly or even fully closed (or open), regardless of how you may be able to turn the valve handle.

Draining the Hot Water Heater Tank to Fix Poor Hot Water Pressure?

Simply draining the hot water tank might not fix the problem. For example if a chunk of mineral debris has escaped the tank and is clogging a pipe, elbow, or valve. But if during draining you found clots of mineral scale, that adds to the suspicion of a scale problem and blockage.

For example, if a chunk of scale happened to flow into the piping leaving the hot water heater tank, or into a valve in the piping system, that could cause a blockage.

Watch out: as Carson Dunlop Associates' illustration (left) shows, for safety reasons we normally expect to see a shutoff valve only on the cold water line into the water heater, not on the hot water heater tank outlet side.

Clues Pointing to Mineral Debris Clogging in Hot Water Piping

• Was the water heater recently more noisy than usual? (WATER HEATER NOISES).
• Is your water supply high in mineral content?

Take a look at the water heater scale discussion at WATER HEATER SCALE - De-Liming Procedure. Also, Water Heater Flush Procedure discusses how to flush scale and debris - which you may have already tried, BUT maybe without recalling the extent of scale you found you may need to take another look at the chance of scale or debris clogging a valve or pipe or pipe elbow at the water heater.

Also check cold water supply INTO the water heater. If you open the water heater drain and turn back on the cold water supply to the water heater, and if you do NOT see powerful water flow out of the heater tank drain, then the drain valve could itself be blocked by debris (most common) or alternately you might have poor water flow INTO the water heater.

Check all of the plumbing control valves feeding the water heater to be sure no one closed one of them, and if there are shutoff valves on the hot water piping system ahead of most of the building plumbing fixtures, check those for operation and blockage too.

Also take a look at the diagnostics at WATER PRESSURE LOSS DIAGNOSIS & REPAIR or at HOT WATER IMPROVEMENTS where we list several articles diagnosing lost hot water pressure where cold water pressure is OK.

Questions & Answers regarding this article

Questions & answers about diagnosing & repairing sudden loss of hot water pressure in buildings

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HOT WATER PRESSURE LOSS

HOT WATER QUANTITY IMPROVEMENT
  Alternative Hot Water Sources & Methods
  Anti-Scald Valves & Hot Water Quantity
  CLOGGED PIPING & Hot Water Flow
  Extra Tanks to Increase Hot Water
  Insulate Hot Water Piping
  Insulate Hot Water Tank?
  Larger Diameter Water Supply Piping
  Water pipe clog diagnosis
  Water pipe clog repair guide

WATER PRESSURE & FLOW MEASUREMENT

• Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
• Thanks to reader - R.S. (anonymous by req.) for discussing hot water loss problems - August 2010

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