Comparing Two-Pipe and Four-Pipe HVAC Systems with Water-Source Heat Pumps

Michael Tobias
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    Many HVAC systems use hydronic piping as a means to provide space heating and cooling. Individual fan-coils serve each zone, while a central chiller and boiler assume total HVAC loads as needed. Two main system configurations are possible: the same hydronic piping circuit can be used for both functions, or separate hydronic piping can be used for heating and cooling.

    • Two-Pipe System: When heating and cooling share hydronic piping, each fan-coil only has one supply pipe and one return pipe.
    • Four-Pipe System: When heating and cooling have separate hydronic piping, fan-coils have two supply pipes and two return pipes.

    Like in most engineering decisions, each system configuration has advantages and disadvantages. This article will provide an overview of two-pipe and four-pipe systems, while comparing them with a more modern alternative: water-source heat pumps.

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    Two-Pipe HVAC Systems

    A two-pipe system uses half the hydronic piping required by a four-pipe system, which results in a lower cost and a shorter installation time. The system is also more compact, reducing the space requirements of mechanical rooms. Maintenance is also simpler in a two-pipe system, thanks to the reduced number of piping fixtures and valves.

    The main limitation of a two-pipe HVAC system is lack of operating flexibility. The hydronic piping circuit that runs through the building connects to either the boiler or the chiller depending on overall needs, and all building areas must operate in the same mode; heating some areas while cooling others is not possible with this system configuration.

    Two-pipe HVAC systems are a great choice for tropical climates, where buildings often go for an entire year without requiring space heating. The boiler is normally omitted in these cases, unless it is required for hot water, but in that case it is a completely different building system.

    Four-Pipe HVAC System

    This system configuration uses twice as much piping as a two-pipe HVAC system, and thus it is more expensive and takes longer to install. In addition, a four-pipe system requires more space to accommodate two hydronic piping circuits that run through the building. The increased number of fixtures, valves and connection points also results in a more demanding system in terms of maintenance.

    However, four-pipe HVAC systems offer performance features not available with a two-pipe system. For example, fan coils can deliver simultaneous cooling and dehumidification by using the chilled and hot water coils at the same time:

    • The chilled water coil is used at maximum capacity to remove as much moisture as possible from the air, even if the air is cooled below the required temperature.
    • Any excessive cooling is then compensated with the heating coil, delivering air with an acceptable temperature and humidity.

    A two-pipe system does not allow this flexibility, since air temperature and humidity are fixed once it flows through the fan-coil. Increased dehumidification requires more cooling, and a higher air temperature results in a higher humidity.

    Another significant advantage of a four-pipe system is that different building areas can be cooled or heated simultaneously. It is just a matter of using the corresponding hydronic circuit in the fan-coils serving those areas.

    How Two-Pipe and Four-Pipe Systems Use Energy

    In New York City most space cooling is accomplished with electricity, while space heating typically relies on natural gas or heating oil. Since electricity in NYC is very expensive, one ton-hour of cooling generally has a higher cost than one ton-hour of heating. For this reason, cooling system upgrades tend to offer a higher return per dollar spent, and property management companies can focus on them first to maximize return on investment.

    Of course, there can be exceptions to the rule above. If a building has a modern high-efficiency chiller and an old boiler, the cost per ton-hour of heating may be higher. An energy audit is the best way to identify the most cost-effective building upgrades.

    Water-Source Heat Pumps: The Best Features of Both Systems

    If a system uses water-source heat pumps instead of fan-coils, it can offer the advantages of a four-pipe system while relying on a single hydronic piping circuit. Water-source heat pumps can operate in either cooling or heating mode with a common water loop.

    • Heat pumps extract heat from areas requiring cooling, and the heat is rejected into the water loop.
    • Space heating is possible simultaneously, and this thermal energy can be extracted from the same water loop by heat pumps in heating mode.

    With this system configuration, heating and cooling loads balance each other out, resulting in a much higher operating efficiency. The chiller and boiler are never required to work simultaneously: the chiller operates when the cooling load is higher, and the boiler operates when the heating load is higher.

    To reduce operating costs even more, high-efficiency boilers and chillers can be used, but consider that efficiency is reported differently for each type of equipment:

    • Gas- or oil-fired boilers use the Annual Fuel Utilization Efficiency (AFUE), which is reported as a percentage. For example, a gas boiler with an AFUE of 95% delivers 95% of the combustion heat to the water flowing in the hydronic piping.
    • Chillers use the Energy Efficiency Ratio (EER) to report their efficiency at standard test conditions, and the Integrated Energy Efficiency Ratio (IEER) to reflect their efficiency after considering seasonal factors and load variability. The EER and IEER are not percentage values, but rather a ratio of cooling output in Btu/hour to electricity input in watts - similar to the gas mileage value of a car.

    The most efficient boilers in the market have an AFUE above 95%, while the most efficient water-cooled chillers have an EER above 20. Air-cooled chillers are less efficient that their water-cooled counterparts.

    It is also possible to use a ground-source heat pump to replace both the boiler and the chiller. These units are just as efficient as a water-cooled chiller, and can match the running cost of a gas boiler when in heating mode, even though they work with electricity. However, ground-source heat pumps require specific groundwater conditions to be viable. They can be a great choice in new constructions where no chiller and boiler have been installed, or when both the chiller and boiler are old and inefficient. If the existing chiller and boiler are already efficient, the upgrade to a ground-source heat pump may not be financially viable.

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    Tags : Design Sustainability

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