{"id":173,"date":"2026-05-24T14:45:54","date_gmt":"2026-05-24T09:45:54","guid":{"rendered":"https:\/\/watertankcalculator.com\/guides\/?p=173"},"modified":"2026-10-10T10:37:59","modified_gmt":"2026-10-10T05:37:59","slug":"why-is-my-water-tank-taking-so-long-to-refill","status":"publish","type":"post","link":"https:\/\/watertankcalculator.com\/guides\/why-is-my-water-tank-taking-so-long-to-refill\/","title":{"rendered":"Why Is My Water Tank Taking So Long to Refill?"},"content":{"rendered":"\n<p>A water tank that takes hours to refill is being held back by one of five things: weak supply pressure, a float valve that restricts the flow, a pipe that is too small or too long, a blockage or half-closed valve, or a pump working against more height than it suits. You can tell which one in about ten minutes by timing the flow at two points: the first tap after your meter or pump, and the tank inlet.<\/p>\n\n\n\n<p>That test matters because the fixes cost very different amounts. Cleaning a strainer is free. A new float valve costs little. A new pump or a longer pipe run costs a lot, and neither helps if the real restriction is somewhere else.<\/p>\n\n\n\n<div class=\"wp-block-group has-background has-global-padding is-layout-constrained wp-container-core-group-is-layout-30e86add wp-block-group-is-layout-constrained\" style=\"border-radius:8px;background-color:#eef6fb;padding-top:1.25rem;padding-right:1.5rem;padding-bottom:1.25rem;padding-left:1.5rem\">\n<p><strong>Quick answer<\/strong><\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Refill time = tank volume \u00f7 flow rate.<\/strong> 1,000 litres at 20 L\/min takes 50 minutes. At 5 L\/min it takes 3 hours 20 minutes.<\/li>\n\n\n\n<li><strong>Slow at the first tap too?<\/strong> The supply or the pump is the limit.<\/li>\n\n\n\n<li><strong>Strong at the first tap, weak at the tank?<\/strong> Look at the pipe run, a valve or strainer on the way, and the float valve.<\/li>\n\n\n\n<li><strong>Used to be fine, now slow?<\/strong> Something has clogged or worn: strainer, scale, float valve seat, or the pump.<\/li>\n\n\n\n<li><strong>Slow only at certain hours?<\/strong> Mains pressure is dropping when your neighbours draw water.<\/li>\n<\/ul>\n\n\n\n<div class=\"wp-block-buttons is-layout-flex wp-block-buttons-is-layout-flex\">\n<div class=\"wp-block-button\"><a class=\"wp-block-button__link wp-element-button\" href=\"https:\/\/watertankcalculator.com\/calculators\/flow\/tank-refill-time-calculator\">Open the Tank Refill Time Calculator<\/a><\/div>\n<\/div>\n<\/div>\n\n\n\n<h2 class=\"wp-block-heading\">How long should a tank take to refill?<\/h2>\n\n\n\n<p>Before deciding the refill is slow, check what the numbers say it should be. The sum is simple division:<\/p>\n\n\n\n<p class=\"has-background\" style=\"background-color:#eef6fb\"><strong>Refill time (minutes) = Volume to fill (litres) \u00f7 Flow rate (litres per minute)<\/strong><\/p>\n\n\n\n<figure class=\"wp-block-table is-style-stripes\"><table class=\"has-fixed-layout\"><thead><tr><th>Volume to fill<\/th><th>At 5 L\/min<\/th><th>At 10 L\/min<\/th><th>At 20 L\/min<\/th><th>At 40 L\/min<\/th><\/tr><\/thead><tbody><tr><td><strong>500 L<\/strong><\/td><td>1 h 40 min<\/td><td>50 min<\/td><td>25 min<\/td><td>13 min<\/td><\/tr><tr><td><strong>1,000 L<\/strong><\/td><td>3 h 20 min<\/td><td>1 h 40 min<\/td><td>50 min<\/td><td>25 min<\/td><\/tr><tr><td><strong>2,000 L<\/strong><\/td><td>6 h 40 min<\/td><td>3 h 20 min<\/td><td>1 h 40 min<\/td><td>50 min<\/td><\/tr><tr><td><strong>5,000 L<\/strong><\/td><td>16 h 40 min<\/td><td>8 h 20 min<\/td><td>4 h 10 min<\/td><td>2 h 5 min<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p>Two points about reading this table. First, only the volume you are replacing counts. A 2,000 L tank that is half full needs 1,000 L, so use that row. Second, there is no single &#8220;normal&#8221; flow rate, but there is a useful floor. UK water companies aim to deliver about 10 metres of head (1 bar, 14 psi) at the property boundary, and at that pressure the first tap in the house should fill a 4.5-litre bucket in 30 seconds, which is 9 L\/min (<a href=\"https:\/\/www.watersafe.org.uk\/advice\/common_plumbing_questions1\/pressure\/what_is_the_minimum_water_pressure_that_a_water_supplier_must_supply\/\" target=\"_blank\" rel=\"noreferrer noopener\">WaterSafe<\/a>). If your tank inlet delivers well under that, something is holding it back.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Step 1: Measure the flow at the tank inlet<\/h2>\n\n\n\n<p>Every diagnosis starts with one number: how many litres per minute actually arrive at the tank. There are two ways to get it.<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Bucket test.<\/strong> With the tank part empty, so the float valve is fully open, hold a bucket of known size under the inlet and time how long it takes to fill. Litres \u00f7 seconds \u00d7 60 gives litres per minute. A 10-litre bucket in 40 seconds is 15 L\/min.<\/li>\n\n\n\n<li><strong>Level-rise test.<\/strong> Close the outlet so nothing leaves, then time how long the water takes to rise by a measured height. Volume is the tank&#8217;s floor area times the rise. In a round tank 1.1 m across, a 10 cm rise is about 95 litres, so if that takes 6 minutes the flow is about 16 L\/min.<\/li>\n<\/ul>\n\n\n\n<p>The <a href=\"https:\/\/watertankcalculator.com\/calculators\/flow\/water-flow-rate-calculator\">Water Flow Rate Calculator<\/a> does the conversion from a volume and a time. For the level-rise method, the <a href=\"https:\/\/watertankcalculator.com\/calculators\/volume\/cylindrical-tank-volume-calculator\">cylindrical<\/a> and <a href=\"https:\/\/watertankcalculator.com\/calculators\/volume\/rectangular-water-tank-volume-calculator\">rectangular<\/a> tank volume calculators work out the litres in a given depth.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Step 2: Measure it again closer to the source<\/h2>\n\n\n\n<p>Now repeat the bucket test at the first tap after the water meter, or at the pump outlet if the tank is pump-fed. Then compare the two results:<\/p>\n\n\n\n<figure class=\"wp-block-table is-style-stripes\"><table class=\"has-fixed-layout\"><thead><tr><th>What you find<\/th><th>Where the restriction is<\/th><th>Go to<\/th><\/tr><\/thead><tbody><tr><td>Weak at the first tap and at the tank<\/td><td>The supply itself, or the pump<\/td><td>Cause 1 or Cause 5<\/td><\/tr><tr><td>Strong at the first tap, weak at the tank<\/td><td>Between the two: pipe, valve, strainer, height<\/td><td>Cause 1, 3 or 4<\/td><\/tr><tr><td>Strong from the open pipe, weak through the float valve<\/td><td>The float valve<\/td><td>Cause 2<\/td><\/tr><tr><td>Strong at night, weak in the morning and evening<\/td><td>Mains pressure falling at peak demand<\/td><td>Cause 1<\/td><\/tr><tr><td>Was fine for years, now slow<\/td><td>Something clogged or worn<\/td><td>Cause 2, 4 or 5<\/td><\/tr><tr><td>Slow since the day it was installed<\/td><td>Design: pipe size, pipe length, height or pump choice<\/td><td>Cause 1, 3 or 5<\/td><\/tr><tr><td>Inlet flow is good but the level barely rises<\/td><td>Water is leaving while it fills<\/td><td>Cause 6<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\">Cause 1: Not enough supply pressure for the height<\/h2>\n\n\n\n<p>Pressure is what pushes water through the pipe, and height uses it up. Every metre you lift water costs about 0.1 bar (1.42 psi). A main that arrives with 10 metres of head has only 4 metres left by the time the water has climbed to a tank inlet 6 metres up, and those 4 metres have to overcome all the friction in the pipe and the float valve.<\/p>\n\n\n\n<p>The effect on fill rate is large. This is the same main and the same 20 metres of 15 mm bore plastic pipe, with only the tank height changed:<\/p>\n\n\n\n<figure class=\"wp-block-table is-style-stripes\"><table class=\"has-fixed-layout\"><thead><tr><th>Tank inlet height above the main<\/th><th>Head left to drive flow<\/th><th>Flow through the pipe<\/th><th>Time to fill 1,000 L<\/th><\/tr><\/thead><tbody><tr><td>Ground level<\/td><td>10 m<\/td><td>About 27 L\/min<\/td><td>About 37 min<\/td><\/tr><tr><td>6 m (roof of a two-storey house)<\/td><td>4 m<\/td><td>About 17 L\/min<\/td><td>About 1 hour<\/td><\/tr><tr><td>9 m (roof of a three-storey house)<\/td><td>1 m<\/td><td>About 8 L\/min<\/td><td>About 2 hours<\/td><\/tr><\/tbody><\/table><figcaption class=\"wp-element-caption\">Hazen-Williams equation, C = 150, pipe friction only. Bends, valves and the float valve reduce every figure further.<\/figcaption><\/figure>\n\n\n\n<p>Three things follow from this:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Mains pressure is not constant.<\/strong> It falls when everyone on the street draws water at once, usually early morning and evening. If your tank fills in an hour at night and three hours at breakfast time, this is the cause. Time the fill at both ends of the day to confirm it.<\/li>\n\n\n\n<li><strong>Intermittent supply runs at low pressure.<\/strong> Where water comes for a few hours a day, pressure at the far end of the network is often too low to reach a roof at all.<\/li>\n\n\n\n<li><strong>Flow does not fall in step with pressure.<\/strong> Halving the pressure available cuts flow by about 30%, not 50%. A modest pressure drop therefore causes a modest slowdown, and a severe slowdown means pressure has nearly run out.<\/li>\n<\/ul>\n\n\n\n<p>The <a href=\"https:\/\/watertankcalculator.com\/calculators\/pressure\/water-pressure-calculator\">Water Pressure Calculator<\/a> converts a height into pressure, and <a href=\"https:\/\/watertankcalculator.com\/guides\/water-pressure-explained-psi-bar-and-kpa\/\">water pressure explained in psi, bar and kPa<\/a> covers the units. If the taps are weak as well as the refill, see <a href=\"https:\/\/watertankcalculator.com\/guides\/my-water-tank-pressure-is-low\/\">my water tank pressure is low<\/a>.<\/p>\n\n\n\n<p class=\"has-background\" style=\"background-color:#eaf7ee\"><strong>The fix:<\/strong> if pressure is fine off-peak, fill then, using a timer on the pump or by leaving the inlet open overnight. If it is never enough to reach the roof, let the mains fill a ground-level tank under its own pressure and pump from that tank to the roof. <a href=\"https:\/\/watertankcalculator.com\/guides\/overhead-vs-underground-water-tank-which-is-better\/\">Overhead vs underground water tanks<\/a> explains the two-tank arrangement.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Cause 2: The float valve is the bottleneck<\/h2>\n\n\n\n<p>All the water entering the tank passes through a hole in the float valve that is much smaller than the pipe feeding it. British Standard float valves, for example, use seats with bores from 1\/8 inch to 3\/8 inch (about 3 to 10 mm), with the small bores made for high-pressure supplies and the large ones for low-pressure supplies (<a href=\"https:\/\/www.wrasapprovals.co.uk\/downloads\/regulations\/1111.2.pdf\" target=\"_blank\" rel=\"noreferrer noopener\">WRc test code sheet for BS 1212 valves<\/a>). That makes the float valve the narrowest point in most tank inlets, and it goes wrong in four ways:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>The wrong seat for the pressure.<\/strong> A high-pressure seat on a low-pressure supply, such as a gravity feed from another tank, lets through a fraction of the flow the pipe could carry. This is a common reason for a tank that has been slow since day one.<\/li>\n\n\n\n<li><strong>A valve smaller than the pipe.<\/strong> A half-inch valve on a one-inch supply pipe throws away most of what the larger pipe was bought for.<\/li>\n\n\n\n<li><strong>Grit or scale in the seat.<\/strong> A grain of sand lodged in a 3 mm hole blocks a large share of it.<\/li>\n\n\n\n<li><strong>An arm that does not drop fully.<\/strong> A stiff pivot or a bent arm keeps the valve partly closed even when the tank is empty.<\/li>\n<\/ul>\n\n\n\n<p><strong>How to test it:<\/strong> push the float arm fully down by hand and watch the flow. Then, if you can, close the isolating valve, unscrew the float valve and run the open pipe into the tank for a minute. If the open pipe delivers far more, the valve is the restriction. Replace it with one matched to the pipe size and to your supply pressure.<\/p>\n\n\n\n<p class=\"has-background\" style=\"background-color:#eef6fb\"><strong>The last part of every fill is slow, and that is normal.<\/strong> A float valve closes gradually as the level rises, so the final stretch always takes longer than the rest. Judge the fill rate from the middle of the fill, not the end.<\/p>\n\n\n\n<p>For how the mechanism works and how valves are sized, see <a href=\"https:\/\/watertankcalculator.com\/guides\/what-is-a-float-valve-and-how-does-it-control-tank-filling\/\">what a float valve is and how it controls tank filling<\/a>. A float valve that chatters or bangs as it closes is covered in <a href=\"https:\/\/watertankcalculator.com\/guides\/water-tank-making-noise-causes-and-what-to-check\/\">water tank making noise<\/a>, and one that fails to shut is covered in <a href=\"https:\/\/watertankcalculator.com\/guides\/water-tank-overflowing-what-causes-it-and-how-to-fix-it\/\">water tank overflowing<\/a>.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Cause 3: The pipe is too small or too long<\/h2>\n\n\n\n<p>Water rubbing against the pipe wall uses up pressure, and the loss climbs steeply as the pipe gets narrower or the flow gets faster. The table shows how many metres of head are lost in every 10 metres of smooth plastic pipe:<\/p>\n\n\n\n<figure class=\"wp-block-table is-style-stripes\"><table class=\"has-fixed-layout\"><thead><tr><th>Internal diameter<\/th><th>At 10 L\/min<\/th><th>At 20 L\/min<\/th><th>At 30 L\/min<\/th><\/tr><\/thead><tbody><tr><td><strong>15 mm<\/strong><\/td><td>0.77 m<\/td><td>2.77 m<\/td><td>5.87 m<\/td><\/tr><tr><td><strong>20 mm<\/strong><\/td><td>0.19 m<\/td><td>0.68 m<\/td><td>1.45 m<\/td><\/tr><tr><td><strong>25 mm<\/strong><\/td><td>0.06 m<\/td><td>0.23 m<\/td><td>0.49 m<\/td><\/tr><tr><td><strong>32 mm<\/strong><\/td><td>0.02 m<\/td><td>0.07 m<\/td><td>0.15 m<\/td><\/tr><\/tbody><\/table><figcaption class=\"wp-element-caption\">Head loss per 10 m of straight pipe, Hazen-Williams equation with C = 150 (new plastic). 1 m of head is about 0.1 bar.<\/figcaption><\/figure>\n\n\n\n<p>Read across the 15 mm row. To push 20 L\/min through 40 metres of it would take about 11 metres of head, which is more than the whole 10 metres a typical main delivers. So that flow cannot happen, and the system settles at a lower one. The same 40 metres in 25 mm pipe would lose less than 1 metre.<\/p>\n\n\n\n<p>Four practical points:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>One pipe size up roughly doubles the flow<\/strong> at the same pressure. Going from 20 mm to 25 mm bore gives about 1.8 times as much.<\/li>\n\n\n\n<li><strong>Measure the bore, not the label.<\/strong> Plastic pipe is usually sold by its outside diameter, so the hole down the middle is smaller than the size printed on it, sometimes by a lot in thick-walled pressure pipe.<\/li>\n\n\n\n<li><strong>Old galvanised iron is worse.<\/strong> Even when new it is rougher than plastic (C = 120 against 140\u2013150), which means about 1.5 times the loss at the same flow, and a pipe&#8217;s roughness &#8220;can change considerably with age&#8221; (<a href=\"https:\/\/epanet22.readthedocs.io\/en\/latest\/3_network_model.html\" target=\"_blank\" rel=\"noreferrer noopener\">US EPA EPANET manual<\/a>). Rust also narrows the bore.<\/li>\n\n\n\n<li><strong>Every bend and valve adds to it.<\/strong> A run with a dozen elbows behaves like a noticeably longer straight pipe.<\/li>\n<\/ul>\n\n\n\n<p>To model your own run, enter the internal diameter, length, pressure and height rise in the <a href=\"https:\/\/watertankcalculator.com\/calculators\/flow\/pipe-size-flow-rate-calculator\">Pipe Size and Flow Rate Calculator<\/a>. It uses the same equation as the table above.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Cause 4: A blockage or a half-closed valve<\/h2>\n\n\n\n<p>If the tank used to fill quickly and now does not, and nothing about the system has changed, something has narrowed. Check these in order, because the first ones are free:<\/p>\n\n\n\n<ol class=\"wp-block-list\">\n<li><strong>Every valve on the route is fully open.<\/strong> The stop valve at the meter, any isolating valve near the tank, and any valve a plumber may have turned during other work. A gate valve with a worn stem can look open when the gate inside has dropped.<\/li>\n\n\n\n<li><strong>Strainers and filters.<\/strong> There is often a mesh strainer at the meter, at the pump inlet and inside the float valve. After mains repairs they fill with grit.<\/li>\n\n\n\n<li><strong>Scale.<\/strong> In hard-water areas limescale builds up inside pipes and fittings and slowly shrinks the bore. <a href=\"https:\/\/watertankcalculator.com\/guides\/water-hardness-and-how-does-it-affect\/\">What water hardness is and how it affects your system<\/a> explains why, and the <a href=\"https:\/\/watertankcalculator.com\/calculators\/treatment\/water-hardness-calculator\">Water Hardness Calculator<\/a> classifies your water.<\/li>\n\n\n\n<li><strong>Sediment.<\/strong> Sand and silt from the supply settle in low points and in the valve seat. If you also find it in the tank, see <a href=\"https:\/\/watertankcalculator.com\/guides\/why-is-there-sediment-at-the-bottom-of-my-water-tank\/\">why there is sediment at the bottom of your water tank<\/a>.<\/li>\n\n\n\n<li><strong>Kinks and crushed sections.<\/strong> Flexible connectors and buried plastic pipe are the usual places.<\/li>\n\n\n\n<li><strong>Trapped air.<\/strong> In a low-pressure gravity line, a high point in the pipe can hold an air pocket that the water cannot push out. Opening the highest fitting to bleed it restores the flow.<\/li>\n<\/ol>\n\n\n\n<h2 class=\"wp-block-heading\">Cause 5: The pump cannot deliver at your height<\/h2>\n\n\n\n<p>The two big numbers on a pump&#8217;s label are its maximum flow and its maximum head, and it never delivers both at once. Maximum flow is what it moves with no lift at all. Maximum head is the height at which flow falls to zero. Your tank sits somewhere between, and the flow you get there comes from the pump&#8217;s performance curve, not from the label.<\/p>\n\n\n\n<p>The head the pump works against is the vertical lift, plus the friction in the pipe, plus any pressure needed at the outlet. Add those up with the <a href=\"https:\/\/watertankcalculator.com\/calculators\/pressure\/pump-head-pressure-calculator\">Pump Head Pressure Calculator<\/a>, then find that head on the curve in the pump&#8217;s data sheet and read the flow beside it. If the flow you measured in Step 1 is close to that figure, the pump is doing all it can and the design is the problem. If it is well below, the pump or its suction side needs attention. <a href=\"https:\/\/watertankcalculator.com\/guides\/how-pump-head-works-and-why-it-matters-for-water-tanks\/\">How pump head works<\/a> goes through reading a curve.<\/p>\n\n\n\n<p>When a pump that once filled the tank well has slowed down over months, the usual reasons are:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>A clogged suction strainer or foot valve.<\/strong> The pump is being starved.<\/li>\n\n\n\n<li><strong>An air leak on the suction side.<\/strong> Suction pipework runs below atmospheric pressure, so it has to be airtight, not only watertight. A joint that never drips can still pull in air.<\/li>\n\n\n\n<li><strong>The water level has dropped.<\/strong> A surface pump can lift water about 7.6 m (25 ft) at most at sea level, against a theoretical limit of 10.3 m (34 ft), and less at altitude or with warm water (<a href=\"https:\/\/www.pumpsandsystems.com\/10-factors-affect-suction-lift\" target=\"_blank\" rel=\"noreferrer noopener\">Pumps &amp; Systems<\/a>). Many small pumps manage less. If the sump or well level falls in the dry season, flow drops and the pump may start to cavitate, which sounds like gravel passing through it.<\/li>\n\n\n\n<li><strong>Wear.<\/strong> A worn impeller moves less water at the same speed.<\/li>\n<\/ul>\n\n\n\n<p>If you do need a different pump, the <a href=\"https:\/\/watertankcalculator.com\/calculators\/flow\/pump-horsepower-flow-rate-calculator\">Pump Horsepower and Flow Rate Calculator<\/a> gives the power needed for a chosen flow and head. For whether a pump belongs in the system at all, see <a href=\"https:\/\/watertankcalculator.com\/guides\/pump-system-vs-gravity-feed\/\">pump system vs gravity feed<\/a>.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Cause 6: Too little fall, or water leaving as it fills<\/h2>\n\n\n\n<p><strong>Gravity-fed tanks.<\/strong> If your tank fills from a higher tank, spring or reservoir, the only pressure available is the height difference: 0.1 bar per metre. Three metres of fall through 30 metres of smooth pipe gives roughly 12 L\/min in a 15 mm bore, 25 L\/min in 20 mm and 44 L\/min in 25 mm, before fittings and the float valve take their share. With so little pressure to spend, pipe size decides almost everything, and a low-pressure float valve seat is essential. The <a href=\"https:\/\/watertankcalculator.com\/calculators\/pressure\/gravity-feed-flow-rate-calculator\">Gravity Feed Flow Rate Calculator<\/a> works this out for your height and pipe, and <a href=\"https:\/\/watertankcalculator.com\/guides\/how-gravity-feed-water-systems-work\/\">how gravity feed water systems work<\/a> covers the principles.<\/p>\n\n\n\n<p><strong>Water leaving during the fill.<\/strong> The level rises at inflow minus outflow. A shower running, a washing machine filling or a leak on the outlet side all slow the rise without any fault on the inlet. To rule out a leak, turn off every tap and watch the water meter or the tank level for an hour. <a href=\"https:\/\/watertankcalculator.com\/guides\/how-to-read-a-water-meter\/\">How to read a water meter<\/a> shows the leak check step by step.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Fixes, cheapest first<\/h2>\n\n\n\n<figure class=\"wp-block-table is-style-stripes\"><table class=\"has-fixed-layout\"><thead><tr><th>Fix<\/th><th>Solves<\/th><th>Cost and effort<\/th><\/tr><\/thead><tbody><tr><td>Open every valve fully; clean strainers<\/td><td>Blockages (Cause 4)<\/td><td>Free, under an hour<\/td><\/tr><tr><td>Fill at night or off-peak<\/td><td>Peak-hour pressure drop (Cause 1)<\/td><td>Free, or the price of a timer<\/td><\/tr><tr><td>Replace the float valve with the right size and seat<\/td><td>Float valve restriction (Cause 2)<\/td><td>Low, a basic plumbing job<\/td><\/tr><tr><td>Replace the worst section of pipe with a larger bore<\/td><td>Friction (Cause 3)<\/td><td>Moderate, depends on access<\/td><\/tr><tr><td>Service the pump: strainer, foot valve, suction joints<\/td><td>Pump decline (Cause 5)<\/td><td>Low to moderate<\/td><\/tr><tr><td>Add a ground-level tank and transfer pump<\/td><td>Mains too weak for the roof (Cause 1)<\/td><td>High, but the lasting fix<\/td><\/tr><tr><td>Replace the pump with one chosen from its curve<\/td><td>Pump wrong for the height (Cause 5)<\/td><td>High<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\">Will the tank fill inside your supply window?<\/h2>\n\n\n\n<p>Where mains water comes for only a few hours, the question is not how fast the tank fills but whether it fills enough before the water stops. You do not have to fill the whole tank each time, only replace what was used since the last window:<\/p>\n\n\n\n<p class=\"has-background\" style=\"background-color:#eef6fb\"><strong>Flow needed (L\/min) = Water used between windows (L) \u00f7 Length of window (minutes)<\/strong><\/p>\n\n\n\n<p class=\"has-background\" style=\"background-color:#eaf7ee\"><strong>Example:<\/strong> a family uses 540 L a day and gets water every second day for 2 hours. They need to take in 1,080 L in 120 minutes, which is <strong>9 L\/min<\/strong>. At 5 L\/min they collect only 600 L per window and the tank level falls a little further every cycle until it runs dry.<\/p>\n\n\n\n<p>If your measured flow is below the figure you need, a bigger tank will not help, because it is the intake that is short. If it is comfortably above, the tank size is what limits you. <a href=\"https:\/\/watertankcalculator.com\/guides\/how-to-calculate-the-right-water-tank-size-for-your-home\/\">How to calculate the right water tank size for your home<\/a> covers that side, and the <a href=\"https:\/\/watertankcalculator.com\/calculators\/household\/how-long-will-my-tank-last-calculator\">How Long Will My Tank Last Calculator<\/a> has a field for water still coming in, so you can see whether the level is gaining or losing.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Mistakes that cost money<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Buying a bigger tank to cure a slow fill.<\/strong> A 5,000 L tank at 8 L\/min takes about 10 and a half hours to fill from empty. More volume at the same flow only means a longer wait. Fix the flow first, then check the size with <a href=\"https:\/\/watertankcalculator.com\/guides\/how-to-tell-if-your-water-tank-is-the-wrong-size-for-your-home\/\">how to tell if your water tank is the wrong size<\/a>.<\/li>\n\n\n\n<li><strong>Replacing the pump before testing the pipe.<\/strong> If 40 metres of narrow, rusted pipe is the restriction, a stronger pump gains little. Friction rises with nearly the square of the flow, so doubling the flow through the same pipe takes about 3.6 times the friction head.<\/li>\n\n\n\n<li><strong>Trusting the number on the pump label.<\/strong> It is the flow at zero lift. Measure what arrives at the tank.<\/li>\n\n\n\n<li><strong>Judging by the end of the fill.<\/strong> The float valve slows the last part on every tank.<\/li>\n<\/ul>\n\n\n\n<p class=\"has-background\" style=\"background-color:#fff4e0\"><strong>Do not connect a pump straight to the mains to pull more water.<\/strong> It lowers the pressure in the street pipe for everyone, and low pressure in water mains is one of the conditions that lets contamination into them (<a href=\"https:\/\/journals.plos.org\/plosmedicine\/article?id=10.1371\/journal.pmed.1001892\" target=\"_blank\" rel=\"noreferrer noopener\">Ercumen et al., PLOS Medicine, 2015<\/a>). Many utilities restrict or ban it. In the UK, for example, a pump or booster drawing more than 12 litres per minute from a supply pipe needs the water company&#8217;s consent before it is installed (<a href=\"https:\/\/www.legislation.gov.uk\/uksi\/1999\/1148\/regulation\/5\/made\" target=\"_blank\" rel=\"noreferrer noopener\">Water Fittings Regulations 1999, regulation 5<\/a>). The accepted method is to let the mains fill a ground-level tank and pump from that. Check your own utility&#8217;s rules first.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Frequently asked questions<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">How long should it take to refill a 1,000-litre water tank?<\/h3>\n\n\n\n<p>Divide 1,000 by your flow rate in litres per minute. At 20 L\/min it takes 50 minutes, at 10 L\/min 1 hour 40 minutes, and at 5 L\/min 3 hours 20 minutes. If the tank is only half empty, halve those times. The <a href=\"https:\/\/watertankcalculator.com\/calculators\/flow\/tank-refill-time-calculator\">Tank Refill Time Calculator<\/a> handles any volume, flow unit and starting level.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Why does my tank fill fast sometimes and slowly at other times?<\/h3>\n\n\n\n<p>Because the supply pressure is changing. Mains pressure falls when demand on the street is high, typically in the morning and evening, and recovers at night. Time the fill at both ends of the day. If the difference is large, fill off-peak, or let the mains fill a ground-level tank at whatever rate it manages and pump from there.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Can a float valve slow down tank refilling?<\/h3>\n\n\n\n<p>Yes. The opening in the valve seat is only a few millimetres across, so it is often the narrowest point on the inlet. A seat made for high pressure fitted to a low-pressure supply, a valve smaller than the pipe, grit in the seat or a stiff arm will each cut the flow. Compare the flow through the valve with the flow from the open pipe to find out.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Why does my tank fill fine in winter but slowly in summer?<\/h3>\n\n\n\n<p>Summer demand is higher, so mains pressure is lower for more of the day. Wells, boreholes and sumps also sit at lower levels in the dry season, which means a pump has to lift further and delivers less. On a gravity-fed system, every metre the source level drops takes about 0.1 bar off the inlet pressure.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">What pipe size gives the best refill rate?<\/h3>\n\n\n\n<p>It depends on the length of the run and the pressure you have. As a guide, each step up in bore roughly doubles the flow at the same pressure: 20 mm to 25 mm gives about 1.8 times as much. For runs over about 20 metres, or wherever pressure is low, a 25 mm bore or larger is worth the extra cost. The friction table above shows the loss for each size.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Will a bigger pump fill my tank faster?<\/h3>\n\n\n\n<p>Only if the pump is what limits the flow. If the pipe is narrow or long, most of a bigger pump&#8217;s extra effort goes into friction, since doubling the flow takes about 3.6 times the friction head. Measure the flow, work out the total head, and compare with the pump&#8217;s curve before buying.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">My tank never fills completely. Is it too big?<\/h3>\n\n\n\n<p>Not necessarily. A tank fills whenever more water comes in during each supply window than you use between windows. If it never reaches the top, either the intake per window is less than your use, or the mains pressure runs out before the water reaches the float valve height. If the tank empties regularly, see <a href=\"https:\/\/watertankcalculator.com\/guides\/my-water-runs-out-in-2-days\/\">my water runs out in 2 days<\/a>.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">Sources<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li>WaterSafe (UK), <a href=\"https:\/\/www.watersafe.org.uk\/advice\/common_plumbing_questions1\/pressure\/what_is_the_minimum_water_pressure_that_a_water_supplier_must_supply\/\" target=\"_blank\" rel=\"noreferrer noopener\">What is the minimum water pressure that a water supplier must supply?<\/a> Companies aim for at least 10 m head at the boundary, with a guaranteed minimum of 7 m static head; the first tap should fill a 4.5-litre bucket in 30 seconds.<\/li>\n\n\n\n<li>US EPA, <a href=\"https:\/\/epanet22.readthedocs.io\/en\/latest\/3_network_model.html\" target=\"_blank\" rel=\"noreferrer noopener\">EPANET 2.2 User Manual, The Network Model<\/a>. Hazen-Williams head loss formula and roughness coefficients for new pipe: plastic 140\u2013150, galvanized iron 120.<\/li>\n\n\n\n<li>WRc Evaluation and Testing Centre, <a href=\"https:\/\/www.wrasapprovals.co.uk\/downloads\/regulations\/1111.2.pdf\" target=\"_blank\" rel=\"noreferrer noopener\">Test code sheet 1111.2, float operated valves<\/a>. BS 1212 seat bores of 1\/8, 3\/16, 1\/4 and 3\/8 inch by pressure class.<\/li>\n\n\n\n<li>Jim Elsey, <a href=\"https:\/\/www.pumpsandsystems.com\/10-factors-affect-suction-lift\" target=\"_blank\" rel=\"noreferrer noopener\">factors that affect suction lift<\/a>, Pumps &amp; Systems. Theoretical suction lift of 34 ft at sea level, 25 ft in practice; suction pipework must be airtight.<\/li>\n\n\n\n<li>UK Government, <a href=\"https:\/\/www.legislation.gov.uk\/uksi\/1999\/1148\/regulation\/5\/made\" target=\"_blank\" rel=\"noreferrer noopener\">The Water Supply (Water Fittings) Regulations 1999, regulation 5<\/a>. Consent required for a pump or booster drawing more than 12 litres per minute.<\/li>\n\n\n\n<li>Ercumen A, Arnold BF, Kumpel E, et al., <a href=\"https:\/\/journals.plos.org\/plosmedicine\/article?id=10.1371\/journal.pmed.1001892\" target=\"_blank\" rel=\"noreferrer noopener\">Upgrading a Piped Water Supply from Intermittent to Continuous Delivery and Association with Waterborne Illness<\/a>, PLOS Medicine, 2015. Persistent low pressure in intermittent systems enables contamination in pipelines.<\/li>\n\n\n\n<li>NIST, <a href=\"https:\/\/www.nist.gov\/pml\/special-publication-811\" target=\"_blank\" rel=\"noreferrer noopener\">Special Publication 811<\/a>. Unit conversions: 1 m of water is about 9.81 kPa (0.098 bar, 1.42 psi).<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>A water tank that takes hours to refill is being held back by one of five things: weak supply pressure, a float valve that restricts the flow, a pipe that is too small or too long, a blockage or half-closed valve, or a pump working against more height than it suits. You can tell which [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":56,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[],"class_list":["post-173","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-problem-solution"],"_links":{"self":[{"href":"https:\/\/watertankcalculator.com\/guides\/wp-json\/wp\/v2\/posts\/173","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/watertankcalculator.com\/guides\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/watertankcalculator.com\/guides\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/watertankcalculator.com\/guides\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/watertankcalculator.com\/guides\/wp-json\/wp\/v2\/comments?post=173"}],"version-history":[{"count":2,"href":"https:\/\/watertankcalculator.com\/guides\/wp-json\/wp\/v2\/posts\/173\/revisions"}],"predecessor-version":[{"id":466,"href":"https:\/\/watertankcalculator.com\/guides\/wp-json\/wp\/v2\/posts\/173\/revisions\/466"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/watertankcalculator.com\/guides\/wp-json\/wp\/v2\/media\/56"}],"wp:attachment":[{"href":"https:\/\/watertankcalculator.com\/guides\/wp-json\/wp\/v2\/media?parent=173"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/watertankcalculator.com\/guides\/wp-json\/wp\/v2\/categories?post=173"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/watertankcalculator.com\/guides\/wp-json\/wp\/v2\/tags?post=173"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}