How to Improve the Performance of Domestic Hot Water Systems

Michael Tobias
5 Minutes Read
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    Domestic hot water (DHW) systems represent a large portion of energy consumption in New York City buildings. DHW accounts for 12% of total energy used by properties subject to the Benchmarking Law (Local Law 84 of 2009), and in the multifamily residential sector this increases to 15%. In addition, most DHW systems operate with either heating oil or natural gas, making them a main source of greenhouse gas emissions in NYC. Compared with multifamily residential buildings, high-rise offices have a much lower DHW consumption, accounting for only 4% of their energy use.

    The concept of system performance has a broad definition when dealing with domestic hot water. Ideally, these systems should be designed to offer the following characteristics:

    • Delivering hot water on demand and at a suitable temperature for the corresponding application. For example, taking a shower can be uncomfortable and detrimental for your health if the water is delivered too cold, but can also be dangerous if the water is too hot.
    • There are also cases where DHW systems deliver water at the right temperature, but take time to reach that point. This tends to happen when DHW systems are first used after a long idle period: the water in the piping between the heater and the point of use has had time to cool down.
    • Consuming as little energy as possible, minimizing their operating cost. This can be accomplished by upgrading to a more efficient DHW system, by using smart controls, or by simply lowering the water temperature set point.
    • Minimizing emissions during operation, which can be achieved by reducing energy usage, and by using heating technologies that are less carbon-intensive. For example, natural gas produces less emissions than fuel oil for a given heating load, and you can even get a Con Edison rebate for the conversion.

    In a few words, a top-performing DHW system provides hot water when needed and at the right temperature, while having a low operating cost and preventing greenhouse gas emissions. Of course, achieving this in practice requires extra capital - both energy efficiency and environmental stewardship have a cost.

    Main Limitations of DHW Systems In New York City

    Building energy audits mandated by Local Law 87 of 2009 have revealed a key limitation in how DHW systems are designed and operated in NYC buildings. In many cases, the same boiler is used for space heating and domestic hot water. This is not an issue when the combined space heating and DHW load requires the full boiler capacity, but efficiency is affected drastically when only hot water is being used. The boiler is forced to run inefficiently at part load, and in some cases boilers have been found to operate at full capacity even when the space heating load is reduced or none.

    Unfortunately, 80% of multifamily buildings in NYC still use combined space heating and DHW systems, and the configuration is more common in older buildings.On average, the annual fuel consumption of these buildings is increased by 10,000 BTU per square foot, compared with a similar building with separate systems. Consider that the benchmarked area in multifamily buildings is 1.16 billion ft2: assuming 928 million ft2 are served by combined systems, the opportunity to save fuel is equivalent to 9,280 billion BTU per year!

    The two most common configurations in combined space heating and DHW systems are indirect heat exchangers and tankless coils.

    • Indirect heat exchangers operate between the space heating boiler and the DHW storage tank, serving around 25% of audited buildings, and consuming 16% more fuel than separate systems. On average, buildings with these systems consume 62.3 kBTU/ft2.
    • Tankless coils heat water by making it flow directly through a coil inside the boiler, and they serve 35% of audited buildings. Their fuel consumption is 27% higher compared with that of a separate DHW system, equivalent to 67.5 kBTU/ft2 annually.

    Dedicated DHW systems also show variation in performance according to their configuration. Buildings with a DHW system that uses a direct-fired hot water tank consume 50.7 kBTU/ft2, while buildings where the boiler and tank are separate components consume 57 kBTU/ft2. However, in both cases fuel consumption is much lower than that of combined systems.

    To summarize, DHW systems based on a fossil fuel are more efficient when they use a direct-fired hot water tank and are independent from the space heating system. Natural gas is the fuel with the lowest emissions (117 g CO2 per million BTU), which is 27% lower than the emissions resulting from heating oil (161.3 g CO2 per million BTU).

    If you have a building with combined space heating and DHW systems and are considering separate installations, make sure you get a professional assessment of your property first. If the total ownership cost of the new system is higher than the “business as usual” scenario, you may want to consider other energy efficiency upgrades first.

    Using an Electrical DHW System

    Electricity is very expensive in NYC, and the cost of resistance heating becomes prohibitive as a result. However, electrical DHW systems can greatly contribute to the emissions reduction targets of NYC, since power can come 100% from renewable sources. Resistance heaters cannot compete with the operating cost of oil or gas boilers, but high-performance heat pumps can match their running cost without the associated emissions.

    Tankless resistance heaters can be cost-effective in small applications where they are installed at the point of use. Although they use inefficient resistance heating, they are affordable and easy to install, and compensate their high running cost by only operating in short bursts when necessary. If you have a plumbing fixture where hot water is rarely used, an electric tankless heater can help you save on gas lines. NYC building audits have revealed that point-of-use tankless electric heaters are common in office buildings for this reason: they are convenient and DHW only represents 4% of energy use in the office sector.

    Final Recommendations

    Consider that domestic hot water systems have many other components beyond the boiler or heat pump. It is possible to achieve even greater savings by minimizing heat loss in piping, and by using water-saving plumbing fixtures. Keep in mind that saving hot water reduces two of your utility bills: the water bill itself, and either the electric or gas bill, depending on what you use for DHW.

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    Tags : Design Sustainability new york

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