Hot water recirculation at home: what the wait costs and which scheme to choose
While the far tap runs cold, everything standing in the hot pipe goes down the drain: a 20 mm PP-R pipe holds about 0.14 L per metre [1]. Recirculation keeps hot water near the tap but uses heat and electricity itself. The calculator counts both sides — the water and the wait without recirculation and the losses of every scheme — and shows what is cheaper for your pipe.
Updated 2026-10-30 · every regulation and price links to its source at the bottom of the page. A guide, not a construction design.
Work out your pipe
Measure the length along the pipe, not in a straight line: from the water heater to the farthest hot tap, with every rise and detour. «Times a day» means how often the far tap is opened after a pause, when the pipe has cooled down.
Enter your own water and energy prices in your currency to compare costs; with the prices empty the calculator compares water and energy only.
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How much water stands in the pipe and how long you wait
The pipe volume is the bore area times the length. A PP-R pipe PN 20 of 20 mm has a 13.2 mm bore, of 25 mm — 16.6 mm [1]. The wait is the volume divided by the tap flow: an efficient basin tap gives up to 1.5 gallons a minute (about 5.7 L/min), a standard one 2.2 gallons (about 8.3 L/min) [2].
Even in a 1,200 sq ft house the farthest tap can be more than 1.5 gallons and more than 90 seconds away; the US DOE Zero Energy Ready Home program allows no more than 0.6 gallons (about 2.3 L) before hot water arrives [3].
| Pipe (outer × wall) | Bore | Water per metre | 10 m of pipe: wait at 5.7 L/min |
|---|---|---|---|
| PP-R PN 20, 20 × 3.4 | 13.2 mm | 0.137 L | about 14 s (1.37 L) |
| PP-R PN 20, 25 × 4.2 | 16.6 mm | 0.216 L | about 23 s (2.16 L) |
| PP-R PN 20, 32 × 5.4 | 21.2 mm | 0.353 L | about 37 s (3.53 L) |
| PP-R SDR 7.4, 20 × 2.8 | 14.4 mm | 0.163 L | about 17 s (1.63 L) |
| PP-R SDR 7.4, 25 × 3.5 | 18.0 mm | 0.254 L | about 27 s (2.54 L) |
| PP-R SDR 7.4, 32 × 4.4 | 23.2 mm | 0.423 L | about 44 s (4.23 L) |
Recirculation schemes: how they work and when they fit
A demand pump is started by a button, a timer or a motion sensor: it sends the cooled water from the hot pipe back to the heater and stops when the water is hot; without a return pipe the water goes back through the cold pipe, which warms up a little [4]. A pump running continuously can use more energy on pumping and pipe losses than it saves on wasted water [4]; in Germany a circulation pump must have automatic switching on and off [5].
Instead of a loop, a self-regulating heat-trace cable on the flow pipe can make up the heat loss and hold the pipe temperature without a return pipe or pump, as the manufacturer states [6]. The cable uses exactly what the insulated pipe loses, so the calculator counts it from the same loss.
A small hot water circulator draws about 7 W [7]. The main cost of any loop is not the pump but the heat the pipe gives off to the house.
| Scheme | How it works | When it fits | Watch out |
|---|---|---|---|
| Loop, pump on all day or thermostat | A separate return pipe from the far tap to the heater, water circulates all the time | Large house, many far outlets, hot water needed any time | Highest heat loss: the loop is hot 24 hours [4] |
| Loop, pump on a timer | The same, the pump runs mornings and evenings only | A regular family routine | Outside the timer hours you wait again |
| Loop, pump on demand | A button or motion sensor runs the pump until hot water reaches the tap [4] | A far bathroom used a few times a day | The same wait, but no water to the drain |
| Valve under the far sink | A thermostatic valve joins the hot and cold pipes, the pump at the heater, return through the cold pipe | An existing house without a third pipe | The cold tap there runs warm at first [4] |
| Heat-trace cable | A self-regulating cable under the insulation of the flow pipe [6] | One long branch, no room for a return pipe | Insulated pipe only; heats with electricity |
| Separate heater at the far tap | A small tank or instant heater next to the tap | One far outlet such as a guest bathroom or summer kitchen | Another appliance: check its standby loss in the data sheet |
Example: 12 m and 25 m of pipe
Both cases: 20 mm insulation, 60 °C from the heater, 5 °C mains water, 20 °C indoors, a 6-hour timer with two starts, a 7 W pump, a 5.7 L/min tap. The first — 12 m of PN 20 pipe of 20 mm and 8 cold starts a day, the second — 25 m of 25 mm pipe and 12 starts. Energy per day is heat plus electricity.
| 12 m, 20 mm | 25 m, 25 mm | |
|---|---|---|
| Water in the pipe / wait | 1.64 L / about 17 s | 5.41 L / about 57 s |
| No recirculation: to the drain | 13.1 L a day, 0.84 kWh a day | 64.9 L a day, 4.15 kWh a day |
| Loop all day | 4.02 kWh a day | 9.34 kWh a day |
| Loop on a timer | 1.29 kWh a day | 3.28 kWh a day |
| Loop on demand | 0.61 kWh a day | 3.02 kWh a day |
| Valve under the sink, timer | 0.67 kWh a day | 1.66 kWh a day |
| Heat-trace cable, timer | 0.58 kWh a day | 1.51 kWh a day |
Hot water temperature and Legionella
Legionella bacteria live and grow in water at 20 to 50 °C, best at 35 °C [8]. Cooled water in a long pipe and a barely warm loop are worse than they look.
In the UK hot water should be stored at 60 °C or more and distributed so that it reaches 50 °C at the outlets within one minute [9]. The Ukrainian code asks for at least 50 °C anywhere in a circulation system, no more than a 5 °C drop from the heater, and allows a few minutes at 75–80 °C for thermal disinfection [10].
Do not lower the heater temperature to save on the loop: the Legionella risk goes up. Against scalding, fit a thermostatic mixing valve at the outlet instead.
Insulation: without it the loop heats the walls
In Germany the minimum insulation of hot water pipes is 20 mm for bores up to 22 mm and 30 mm up to 35 mm, at a conductivity of 0.035 W/(m·K) [11].
Our calculation for a 20 mm pipe at 57.5 °C in a 20 °C room: bare, it loses about 20.5 W per metre, with 20 mm insulation about 6.7 W. For the surface we take a heat transfer coefficient of 8.7 W/(m²·K), as for an inside wall surface [12].
Common mistakes
- A pump on all day «so it is always hot» — the most expensive option; use a timer, thermostat or button [5].
- An uninsulated return pipe — the heat goes into walls and floors, not to the tap.
- An oversized pipe to one far tap: 32 mm instead of 20 mm holds 2.6 times more cold water at every start.
- Several branches without balancing valves: the hot water takes the shortest way [13].
- A lower heater temperature to save energy — good conditions for Legionella [8].
- A cable on a bare pipe, or a cable and a loop on the same pipe — paying twice.
Frequently asked questions
Do I need hot water recirculation in a house?
When the pipe to the far tap holds more than about 2–3 L — over 20–30 seconds of waiting. The US DOE program sets 0.6 gallons (about 2.3 L) [3].
How much electricity does a recirculation pump use?
A small circulator draws about 7 W [7], 0.17 kWh a day running non-stop. The heat lost by the loop costs far more: about 9 kWh a day for 25 m of pipe running all day in the example.
Pump loop or heat-trace cable?
A cable is simpler to fit (no return pipe) and heats only the flow pipe, but with electricity. If gas or a heat pump heats your water, a timer or demand loop is usually cheaper — check with your own prices.
Can I have recirculation without a third pipe?
Yes: a thermostatic valve under the farthest sink joins the hot and cold pipes, and a pump at the heater sends the water back through the cold pipe [4]. The catch — the cold tap there runs warm at first.
Sources
- teplota.ua (INT) · common practice (not a rule) · checked 2026-10-30 · source
- EPA WaterSense: Bathroom Faucets (US) · recommendation · checked 2026-10-30 · source
- PNNL Building America Solution Center (DOE): Demand Pumping System, Technical Description (US) · recommendation · checked 2026-10-30 · source
- ENERGY STAR (EPA / DOE): Demand Hot Water Recirculating System (US) · recommendation · checked 2026-10-30 · source
- gesetze-im-internet.de, § 64 (DE) · regulation · checked 2026-10-30 · source
- assets.danfoss.com, Description; Specification data (INT) · common practice (not a rule) · checked 2026-10-30 · source
- cdn.shopify.com (INT) · common practice (not a rule) · checked 2026-10-30 · source
- who.int, Overview / Prevention (INT) · recommendation · checked 2026-10-30 · source
- HSE (Health and Safety Executive): Legionnaires' disease — Hot and cold water systems (UK) · recommendation · checked 2026-10-30 · source
- dreamdim.ua (UA) · regulation · checked 2026-10-30 · source
- gesetze-im-internet.de (DE) · regulation · checked 2026-10-30 · source
- eurobud.ua (UA) · regulation · checked 2026-09-30 · source
- dreamdim.ua (UA) · regulation · checked 2026-10-30 · source