Many HVAC systems use hydronic piping as a means of providing space heating and cooling. Individual fan coils serve each zone, while a central chiller and boiler take on full HVAC loads as needed. Two main system configurations are possible: the same hydronic piping circuit can be used for both functions, or a separate hydronic piping can be used for heating and cooling.
- Two-pipe system: When heating and cooling share hydronic piping, each fan coil has only 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.
As with most engineering decisions, each system configuration has advantages and disadvantages. This article will provide an overview of two- and four-pipe systems, comparing them to a more modern alternative: water source heat pumps.
Our MEP engineers can find the best HVAC configuration for your building.
Two-pipe HVAC systems
A two-pipe system uses half the hydronic piping required by a four-pipe system, resulting in lower cost and shorter installation time. The system is also more compact, reducing the need for space in mechanical rooms. Maintenance is also simpler on a two-pipe system, thanks to the reduced number of piping fittings and valves.
The main limitation of a two-pipe HVAC system is the lack of operational flexibility. The hydronic piping circuit running through the building connects to the boiler or chiller, depending on general needs, and all areas of the building 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 option for tropical climates, where buildings often operate for an entire year without needing 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 construction system.
Four-pipe HVAC system
This system configuration uses twice as much piping as a two-pipe HVAC system and is therefore more expensive and takes longer to install. Additionally, a four-pipe system requires more space to accommodate two loops of hydronic piping running through the building. The increased number of accessories, 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 in a two-pipe system. For example, fan coils can provide simultaneous cooling and dehumidification by using both chilled and hot water coils at the same time:
- The chilled water coil is used at maximum capacity to remove as much moisture from the air as possible, even if the air is cooled below the required temperature.
- Any excessive cooling is then compensated by the heating coil, providing air with an acceptable temperature and humidity.
A two-pipe system does not allow for this flexibility, since the temperature and humidity of the air are fixed as it flows through the fan coil. Increased dehumidification requires more cooling, and a higher air temperature results in higher humidity.
Another significant advantage of a four-pipe system is that different areas of the building can be cooled or heated simultaneously. Simply use the corresponding hydronic circuit in the fan-coils that serve these areas.
How two- 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. Because electricity in New York is very expensive, a ton-hour of cooling generally costs more than a 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 their return on investment.
Of course, there may be exceptions to the above rule. If a building has a modern high-efficiency refrigerator and an older 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 loop of hydronic piping. Water source heat pumps can operate in cooling or heating mode with a common water circuit.
- Heat pumps extract heat from areas that require cooling and the heat is rejected into the water circuit.
- Space heating is possible simultaneously, and this thermal energy can be extracted from the same water circuit by heat pumps in heating mode.
With this system configuration, heating and cooling loads balance each other, resulting in much greater operational efficiency. The chiller and boiler are never required to operate simultaneously: the chiller operates when the cooling load is greater and the boiler operates when the heating load is greater.
To further reduce operating costs, high-efficiency boilers and chillers can be used, but consider that efficiency is reported differently for each type of equipment:
- Gas or oil boilers use Annual Fuel Utilization Efficiency (AFUE), which is reported as a percentage. For example, a gas boiler with an AFUE of 95% provides 95% of the heat of combustion to the water flowing in the hydronic piping.
- Chillers use the Energy Efficiency Ratio (EER) to report their efficiency under standard test conditions, and the Integrated Energy Efficiency Ratio (IEER) to reflect their efficiency after accounting for seasonal factors and load variability. The EER and IEER are not percentage values, but rather a relationship between refrigeration production in Btu/hour and electricity input in watts – similar to the fuel consumption value of a car.
The most efficient boilers on 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 than their water-cooled counterparts.
It is also possible to use a geothermal heat pump to replace the boiler and chiller. These units are as efficient as a water-cooled chiller and can equal the cost of running a gas boiler when in heating mode, although they run on electricity. However, underground heat pumps require specific groundwater conditions to be viable. They can be an excellent choice in new constructions where the chiller and boiler have not been installed, or when both the chiller and boiler are old and inefficient. If your existing refrigerator and boiler are already efficient, upgrading to a geothermal heat pump may not be financially viable.
