Domestic Hot Water Can Account for 48% of Your Building’s Fuel Consumption

There is a trend in increased use of domestic hot water in apartments. As younger tenants move in, more showers and laundry are the rule. These increased loads directly increase the buildings fuel consumption and the result is increased expense.

Understanding how the domestic systems operate will help you address this rising use. In this newsletter, I will focus on the domestic water re-circulation line.

The domestic water re-circulation system is a separate pipe and circulation pump that keeps the domestic water moving around the building to reduce waiting time and wasted water at the hot water fixtures. In many buildings, this water circulates 24 hours a day, 7 days a week. This constant circulation turns into heat loss, and as you can imagine, will contribute to your already rising fuel bills.

Most re-circulation systems result in heat loss within the building because the piping is not insulated. Oversized re-circulation pumps erode the piping with excessive water flow. As buildings age and more maintenance is required, airlocks and sludge buildups are common. The sludge accumulates over time and the airlocks can be caused by draining and refilling the system during repairs.


A copper fitting and hose bib is installed at the re-circulation pump.

This allows the water to be flushed through the line at full force and clear sludge and airlocks. The oversized circulators can be changed to a correctly sized circulator.

The heat loss of the re-circulation piping can be addressed in different ways:

Insulate the piping.

Good idea but the piping is in the walls, has been for 30 years and that’s how it will probably stay. So now what? This depends on the type of building and the tenant use.

A time clock.

This shuts the re-circulation pump off for a few hours every night. This may be inconvenient for shift workers in the building.

An aquastat.

This cycles the re-circulation pump off when the water returning from the building is warm enough. This is more responsive to the building demands than a time clock.

An aquastat and time clock.

These are best used together to provide the advantages of the domestic water re-circulation and heat loss reduction. In this system, the re-circulation pump runs constantly but the domestic water temperature is reduced in the setback mode just like a heating boiler. Notice I said the temperature is reduced not turned off. Nothing gets tenants angry like no hot water. The average domestic water temperature is 130°F. In setback mode this temperature can be reduced to 115°F. Every 3°F the water temperature is lowered results in a 1% fuel savings. Lowering to 115°F shows a 5% reduction in fuel use for this boiler during setback. All of these solutions work when they are correctly applied to the right building.