Hot Water Circulation Optimisation (Time Control plus Insulation)

Limiting the hot water circulation pump to occupancy hours and properly insulating the circulation pipework.

How does optimising hot water circulation work?

A hot water circulation (recirculation) system keeps the water in the pipework constantly moving so that hot water arrives immediately at every tap. To do this, a circulation pump returns the cooled water from the ring main to the cylinder, where it is reheated. In many non-residential buildings this pump runs around the clock, 8,760 hours a year, even though the building is only used for a fraction of that time. Every hour of circulation means heat losses along the entire pipe length, around 25 W per metre for uninsulated pipes. Optimisation consists of two steps: a time control limits the pump running time to occupancy hours, around 2,500 h/a in office use, and full insulation of the circulation pipework reduces losses during running time. For Legionella protection, a daily flushing cycle with cylinder temperatures of at least 60 °C is retained.

Which buildings benefit from circulation optimisation?

The measure suits buildings with clearly defined occupancy hours: office buildings, schools and multi-family houses without night-time use. A documented Legionella protection concept with cylinder temperatures of at least 60 °C is a prerequisite. Care homes, hospitals and hotels with continuous round-the-clock hot water demand are unsuitable; there, at most a reduced night-time running time of around 6,000 h/a is justifiable. Because the investment is very small, the measure is one of the quick wins that can almost always be implemented as part of an energy audit.

What does circulation optimisation actually deliver?

Circulation losses fall by 30 to 50 %, which is 3 to 10 % of total hot water energy demand. The saving from time control is derived from the previous circulation loss multiplied by the share of running time saved. An office building with 12,000 kWh/a of circulation losses that reduces running time from 8,760 to 3,000 h/a saves around 7,900 kWh/a and roughly 1.6 t CO2. A time switch costs €80 to €300; pipe insulation costs €12 to €25 per metre. Payback is 1 to 3 years, with a service life of 15 years.

Energy carrier

Erdgas, Heizöl, Pellet, Fernwärme → Erdgas, Heizöl, Pellet, Fernwärme

Savings potential

30-50 % Zirkulationsverluste, 3-10 % WW-Gesamt

Worked example

An office building has so far run its hot water circulation pump continuously, 8,760 hours a year, even though the building is only used on weekdays during the day. A heat meter upstream of the circulation manifold shows circulation losses of around 12,000 kWh per year. A time switch limits the running time to occupancy hours, 3,000 hours per year; a daily flushing cycle with a cylinder temperature of at least 60 °C is retained for Legionella protection in line with ÖNORM B 5019. The saving is calculated as E_saved = 12,000 × (1 − 3,000 / 8,760) = 12,000 × 0.657 = 7,890 kWh/a. The building therefore saves around 7,900 kWh of heat per year and roughly 1.6 t CO2, for an investment of a few hundred euros for the time switch. Additional pipe insulation further reduces losses during the remaining running time.

Investment & payback

Zeitschaltuhr: 80-300 €; +Rohrdämmung 12-25 €/m · Service life: 15 a · Payback: 1-3 a

Applicability

Requirements

Exclusion criteria

Typical buildings

Frequently asked questions

Can you switch off the hot water circulation pump at night?

Yes, in office buildings, schools and multi-family houses without night-time use, the circulation pump may be switched off outside occupancy hours, provided Legionella protection is maintained. According to ÖNORM B 5019, the cylinder must hold at least 60 °C and the circulation must run daily. A time switch with a daily flushing cycle meets this requirement while significantly reducing running time.

How much energy does a time switch on the circulation pump save?

Time control reduces circulation losses by 30 to 50 %, which is 3 to 10 % of total hot water energy demand. An office building with 12,000 kWh/a of circulation losses saves around 7,900 kWh per year when running time is cut from 8,760 to 3,000 hours. The pump electricity saved during the switched-off hours comes on top.

How much does circulation optimisation cost and when does it pay back?

A time switch for the circulation pump costs €80 to €300; retrofitting insulation on the circulation pipework costs €12 to €25 per metre. With a service life of 15 years, the measure pays back in 1 to 3 years. It is therefore one of the most economical energy-saving measures in a building and can be implemented without structural work.

What role does pipe insulation play in hot water circulation?

Uninsulated hot water pipes lose around 25 W per metre; insulated pipes only a fraction of that. The losses occur throughout the pump running time and must be made up by the boiler. Full insulation to VDI 2055 complements the time control by reducing losses during the remaining operating hours. Long pipe runs in unheated basements are particularly worthwhile.

What does ÖNORM B 5019 require for Legionella protection?

ÖNORM B 5019 governs in Austria the hygiene-related design and operation of central drinking water heating systems. Among other things, it requires at least 60 °C at the cylinder outlet and daily flow through the circulation so that Legionella cannot proliferate. Time control of the circulation pump is compatible with this as long as the daily flushing cycle is documented and the temperature is monitored.

Related measures

Standards & sources

Metering points in the EDM Toolbox

Wärmemengenzähler vor Zirkulationsverteiler + Pumpenstrom. Ideal als KPI für Dashboard.

All measures