The counterintuitive part: a bigger pipe makes you wait longer
Hot water does not travel down the pipe and arrive. The hot water pushes the cold water that was already sitting in the pipe out ahead of it, and you wait for that slug to leave. The wait is therefore the volume of the run divided by the flow rate, and the water wasted is exactly the volume of the run. Nothing else enters into it.
That makes pipe diameter work backwards from intuition. Fifty feet of half inch type L copper holds about 0.61 gallons, which at 1.8 GPM is a twenty second wait. Fifty feet of three quarter inch copper holds about 1.26 gallons over the same distance, so the same tap at the same flow waits forty-two seconds and throws away twice as much water. Upsizing a branch to "get better flow" buys you pressure you probably did not need and costs you a wait you notice every day. The pipe volume calculator works the volume side out for any material and size, and the pressure drop calculator covers the flow side that upsizing is usually meant to address.
Where the money in a recirculation loop actually goes
Three quantities decide whether a loop pays, and they are not the same size as each other.
| Quantity | Typical scale for a house | What moves it |
|---|---|---|
| Water and sewer saved | Tens of dollars a year | Draws per day, run length, the combined water and sewer rate |
| Pump electricity | A few dollars to fifty a year | Pump watts, and above all how many hours a day it runs |
| Extra standby heat | Tens to hundreds of dollars a year | Loop length, insulation, hours held hot, fuel price |
The third row is the one that surprises people, because a pump is a visible device with a nameplate and standby loss is invisible. A loop that is hot around the clock is a long thin radiator running twenty-four hours a day in a crawl space. Insulating it does not make the loop less useful and typically removes about two thirds of that number, which is why the comparison block on this page shows bare against sleeved rather than leaving it as advice.
The four ways a loop gets controlled
Continuous circulation is the baseline and the worst case. A time clock cuts the hours to the two or three windows a household actually uses, which cuts both the pump energy and the standby loss in the same proportion. Adding a thermostat on the return leg lets the pump stop once the loop is warm and restart when it cools, so the pump duty falls further while the pipe stays hot. Demand control is different in kind: nothing circulates until somebody presses a button or trips a sensor, the pump moves one loop volume, and the pipe is allowed to go cold in between. That last arrangement gives up the instant delivery in exchange for nearly all of the standby saving, and it is the only control strategy that regularly comes out ahead on running cost alone.
Crossover valve retrofits, which return water through the cold line instead of a dedicated return, avoid the cost of new pipe and introduce a different nuisance: the cold line at that fixture is warm for a while after the pump runs.
What this calculator is not weighing
It prices water, pump electricity and standby heat, and it stops there. It does not price the convenience, which is the entire reason anyone fits a loop and which no arithmetic reaches. It does not price a drought restriction or a well with a limited yield, where the gallons matter for reasons a tariff does not capture. It does not model the heat that the loop dumps into the building, which is a small credit in winter and a small penalty in summer. And it says nothing about the plumbing itself: a return leg, a check valve, a pump on a heater and any tie-in near the water service are licensed work in most places, and a loop plumbed without a proper check valve can push hot water into cold lines in ways that are more than a nuisance.
For the heater the loop is hanging off, the water heater sizing calculator covers whether it can meet the peak hour at all, and the water heater cost calculator covers the whole annual bill this standby figure is a slice of. For the loop itself, work the insulation number properly on the pipe insulation heat loss calculator.
Questions people ask
How much water does a household really waste waiting for hot?
Multiply the volume of the run by the number of times a day somebody starts it cold. A fifty foot half inch copper run holds about 0.61 gallons, so eight cold starts a day is about five gallons a day and around 1,800 gallons a year. Longer runs and larger pipe scale that up quickly: the same eight draws on a hundred feet of three quarter inch pipe is more than 7,000 gallons a year. Both figures are real, and both are worth less in money than most people expect, because water and sewer together are commonly in the region of ten to fifteen dollars per thousand gallons. The number that decides a loop is almost never the water; it is what the loop spends staying hot.
Will a recirculation pump raise my energy bill?
Yes, in two separate ways, and the smaller one is the pump. A small circulator drawing 25 watts continuously uses about 219 kWh a year. The larger cost is that the loop is kept hot, so the pipe loses heat all day instead of only around draws, and every BTU that leaves the pipe was paid for at the heater. A hundred feet of bare three quarter inch copper at a sixty degree difference sheds roughly 2,700 BTU an hour, which run continuously is well over twenty million BTU a year before efficiency. Insulation and a control strategy that lets the loop go cold between uses are what keep that from being the dominant line on the page.
Does insulating the hot water pipes shorten the wait?
Barely, and not for the reason people expect. The wait is the volume of water in the pipe divided by the flow rate, and insulation changes neither. What insulation does is keep the water already in the pipe warm for longer after a draw, so a second draw within the next while may find warm water instead of cold and skip the purge entirely. That effect is real and it is worth having, but it is measured in how often you avoid a wait, not in how long the wait is when you do have one. The reliable way to shorten a wait is a shorter run, a smaller bore, or hot water that is already there.
Is a demand pump better than a timer?
On running cost, usually yes, because it addresses the larger of the two costs. A timer reduces the hours the pump runs and the hours the loop is held hot in the same ratio, so a six hour window costs a quarter of continuous operation. A demand pump runs for perhaps thirty seconds per draw and lets the loop cool completely in between, which removes almost all of the standby loss. What you give up is the thing the loop was for: hot water is not instant, you press a button and wait about as long as it takes the pump to move one loop volume, which is a lot faster than a purge but is not zero. The comparison block on this page shows both so the trade is visible rather than argued.
Can I fit a recirculation loop myself?
Some parts of it, in some jurisdictions, and the parts that matter are usually not among them. A dedicated return leg is new pipe run through the structure and tied into the heater, a crossover retrofit puts a valve under a fixture that connects hot to cold, and both involve check valves whose failure mode is hot water where it should not be. Anything at or near the water service, the relief valve or a backflow point is licensed work almost everywhere. What is safely a homeowner job is measuring the run, pricing the water on your own bill, and insulating exposed pipe. Take the rest to a plumber who can see the installation, and get the control strategy specified at the same time rather than added afterwards.