In addition to thermal performance estimating, several other factors need to be considered when evaluating the suitability of bringing a new heat source into an existing hydronic heating system. This section discusses those factors.
Seasonal Performance Estimates
Modern heat sources, such as modulating/condensing boilers, air-to-water heat pumps and geothermal water-to-water heat pumps, deliver higher thermal efficiencies when operated at lower water temperatures.
If the required supply water temperature of existing zone circuits is relatively high, such as 160 +ºF at design load, it’s unlikely that most current-generation air-to-water or water-to-water heat pumps can supply the system at design load conditions. Heat pumps using R290 (propane) or CO2 (carbon dioxide) refrigerants are capable of higher temperature operation, but at present have limited availability in North America.
However, even distribution systems that require high water temperature at design load can be served by air-to-water or geothermal water-to-water heat pumps. Those heat pumps can provide much of the space heating energy required under partial load conditions, with an existing (or new) boiler providing the higher temperature water required at or near design load conditions.
Simulations of modern “cold climate” air-to-water heat pumps have shown that they can supply upwards of 90 percent of the total seasonal energy required for space heating, even when the existing distribution system requires 180ºF water at design load conditions.
Figure 4-1 shows the results of such simulations for a house with a design space heating load of 36,000 Btu/hr in a location with an outdoor design temperature of -9ºF. The required indoor temperature at design load is 70ºF.
The simulations were performed for three different locations in New York State (Plattsburg, Albany, and Brooklyn). These locations represent a range of climates from approximately 4,900 ºF•days in Brooklyn, to just over 8,000 ºF•days in Plattsburg. The air-to-water heat pump modeled in the simulations had an enhanced vapor injection refrigeration system, and a nominal capacity of 4 tons. The simulation also included a year-round domestic water heating load of 60 gallons per day, heated from 50ºF to 120ºF.
The graph in figure 4-1 shows the percentage of the space heating + domestic water heating load supplied by the air-to-water heat pump as a function of the supply water temperature required at design load conditions.
The simulation model was based on the use of outdoor reset control of the water temperature supplied to the hydronic distribution system. This well-established control technique is crucial in leveraging the ability of the heat pump under partial load conditions. It allows the water temperature leaving the heat pump to be just high enough to meet the heating load of the building, with no “excess” temperature that would reduce the efficiency of the heat pump. The reset schedule was also adjusted to account for modest internal heat gains. This helps improve the accuracy of the simulation, especially in low-energy houses.
The results show that the space heating contribution by the heat pump is relatively insensitive for systems that have design load supply water temperatures under 140ºF. This contribution decreases for systems that require higher water temperatures at design load. However, even systems that require 180ºF water at design load can have the majority of their seasonal space heating energy supplied by the heat pump.
Lowering Water Temperature Requirements Through Load Reduction
Since the thermal efficiency of modern hydronic heat sources improves with decreasing supply water temperature, any measures that decrease the required supply water temperatures of existing systems, while still maintaining building comfort, will boost efficiency and decrease operating costs.
One way to reduce the required supply water temperature of an existing hydronic distribution system, or a zone within that system, is to reduce the heating load through building weatherization. Measures such as improved insulation, reduced air leakage or window upgrades can reduce heating (and cooling) loads. Such measures also reduce the total space-heating energy required by a building during every subsequent year. In many cases, they are the most cost-effective changes that can be made to reduce heating and cooling costs, especially when the original building is old, poorly insulated or leaky.
The change in supply water temperature at design load is proportional to the change in design load. The reduced supply water temperature under design load conditions can be determined using formula 4-1:
Formula 4-1:
$$T_{\text{new}} = T_{\text{in}} + \left( \frac{Q_{\text{new}}}{Q_{\text{existing}}} \right) \times (T_{\text{De}} - T_{\text{in}})$$
Where:
Tnew = supply water temperature at design load after building envelope improvements (ºF)
Tin = desired indoor air temperature (ºF)
Qnew = design heating load after building envelope improvements (Btu/hr)
Qexisting = existing design heating load (before improvements) (Btu/hr)
TDe = existing supply water temperature at design load (before improvements) (Btu/hr)
Example: Consider a building with an existing design heating load of 100,000 Btu/hr, based on maintaining an interior temperature of 70ºF. The existing hydronic distribution system uses standard fin-tube baseboard and requires a supply water temperature of 180ºF at design load conditions. Assume that improvements to the building’s thermal envelope have reduced the design load from 100,000 Btu/hr to 70,000 Btu/hr. The new supply water temperature to the existing distribution system at design load is calculated as:
$$T_{\text{new}} = 70 + \left( \frac{70,000}{100,000} \right) \times (180 - 70) = 147^\circ F$$
Figure 4-2 shows the relationship between supply water temperature and outdoor temperature based on the original design load, and the reduced load after the building envelope improvements were made.
Reducing the design heating load from 100,000 Btu/hr to 70,000 Btu/hr reduces the required supply water temperature under design load conditions from 180ºF to 147ºF. Although this is certainly an improvement, it’s still above what some renewable energy heat sources can consistently provide. For example, most current generation air-to-water or geothermal water-to-water heat pumps cannot heat water to 147ºF
However, it’s important to remember that the 147ºF supply water temperature determined in the previous example is only required during design load conditions. Under partial load conditions, the supply water temperature could be significantly lower, as shown by the sloping lines in figure 4-2. For example, when the outdoor temperature is 40ºF, the required supply water temperature indicated by the lower sloping line in figure 4-2 is about 104ºF. This is easily within the operating range of current generation hydronic heat pumps.
Lowering Water Temperature Requirements by Adding Heat Emitters
Another way to reduce the water temperature in a hydronic system (or zone) is by adding one or more heat emitters to a zone. Any heat emitter that increases the total surface area separating the heating water from the room air will reduce the required supply temperature, while maintaining a given rate of heat output. This concept, along with representative numbers, is illustrated in figure 4-3.
The type of heat emitter that’s added to the system does not necessarily have to be the same as the existing heat emitters. For example, one or more panel radiators could be added to a circuit with existing fin-tube baseboard. A length of fin-tube or a fan-coil could be added to a system with cast iron radiators. An area of lower temperature radiant floor, wall or ceiling heating could be added to an existing system originally designed to operate at a relatively high temperature, but doing so would require a mixing device to reduce water temperature to the radiant panel.
When a decision is made to add heat emitters, the “topology” of the existing system can also be modified. A long circuit containing several series-connected heat emitters can be split into two or more parallel circuits. This concept is shown in figure 4-4.
In this example, the original series circuit of the baseboard was cut into four segments. Those segments were then reconnected to a manifold station using 1/2" PEX or PEX-AL-PEX tubing. These flexible tube options are easier to install and less expensive than reconnecting with rigid copper tubing. Fittings for transitioning between copper tubing and either PEX or PEX-AL-PEX tubing are readily available.
Dividing the original series circuit into branches also opens the possibility of zoning. Individual zones can be controlled using non-electric thermostatic radiator valves or manifold valve actuators. Figure 4-5 shows an example of the latter.
This system uses manifold valve actuators operated by zone thermostats. All wiring is routed through a Caleffi Z-one™ relay center. A variable speed pressure-regulated circulator set for constant differential pressure mode automatically adjusts its speed as different zones turn on and off.
Dealing With "Outlier" Zones
In a multi-zone system, there’s a possibility that one or more of the zones requires a fluid supply temperature significantly higher than the other zones. The methods of analysis presented in section 3 allows such zones to be identified.
Consider a four-zone system where the supply fluid temperatures at design load are as follows:
Zone 1: supply fluid temperature at design load = 123ºF
Zone 2: supply fluid temperature at design load = 165ºF
Zone 3: supply fluid temperature at design load = 118ºF
Zone 4: supply fluid temperature at design load = 120ºF
The average supply fluid temperature for zones 1, 3, and 4 is 120.3ºF. The maximum deviation of any of these temperatures from this average is 2.7ºF. Given this limited “spread” of required supply temperatures, it’s likely that these three zones could be served from a common source device, at the same temperature, without creating detectable differences in comfort in their respective spaces. However, the supply temperature for zone 2 is well above this average. If it were supplied at, say, 121ºF, it’s unlikely that comfort could be maintained under design load conditions.
Upon seeing this “outlier” zone, the designer could inform their client about the situation and make recommendations for correcting the problem through weatherization of the building’s thermal envelope associated with that zone, or by adding heat emitters, or a combination of these modifications.
Identifying these issues prior to installing a new or alternative hydronic heat source and advising clients of the consequences and possible corrective actions is professional. It helps ensure satisfaction with the modernized system. More than a new heat source would be required the example shown in Figure 4-6.
Protecting Conventional Boilers From Sustained Flue Gas Condensation
When a modern heat source such as an air-to-water heat pump is combined with an existing conventional boiler, and the water temperature in the system is controlled based on outdoor reset, there will be many hours of operation where the water temperature on the return side of the distribution system is well below the dewpoint temperature of the boiler’s flue gasses. If this water returns directly to a conventional boiler, that boiler and its venting system can be seriously damaged by corrosion.
The galvanized steel vent connector piping shown in figure 4-7 was corroded to the point of failure due to sustained flue gas condensation. That failure occurred after only 6 months of boiler operation.
This type of failure could result in the release of toxic flue gasses, such as carbon monoxide, into occupied spaces and must be avoided.
Although the exact boiler inlet temperature that can create sustained flue gas condensation varies with the fuel and fuel/air ratio, a generally accepted guideline is to maintain the inlet water temperature to a boiler burning natural gas at or above 130ºF.
Figure 4-8 shows how a conventional boiler transitions from “non-condensing” to “condensing” mode operation as the boiler’s inlet water temperature decreases.
There are several ways of protecting a conventional boiler against sustained flue gas condensation. They involve different hardware and control methods. One of the simplest approaches is to install a high-flow-capacity thermostatic mixing valve such as the Caleffi ThermProtec™, as shown in figure 4-9.
When the water temperature leaving the ThermoProtec™ valve is below the calibrated temperature of its sensing element, the valve’s cold port is closed. This blocks any flow into or out of the load. Heated water leaving the boiler recirculates through the valve and back into the boiler. The full heat output of the boiler is temporarily focused on heating the water in this recirculating piping loop, allowing the water temperature to quickly rise above the dewpoint of the flue gasses. As this temperature rise progresses, the cold port of the ThermoProtec valve begins to open, and the valve’s bypass port begins to close. This allows flow to develop between the boiler and the load, while keeping the boiler operating in a “non-condensing” mode.
Evaluating Water Quality in Existing Systems
Older hydronic systems, especially those converted from steam to hot water, often contain sludge formed by oxidized iron. This sludge contains very fine particles that can become lodged between moving parts in valves and circulators, which impedes or totally stops the required motion. Figure 4-10 shows an accumulation of iron oxide particles on the permanent magnet rotor of a modern circulator.
Figure 4-11 shows debris and scale that have jammed within a valve.
Dirt and magnetic particles can be removed using a magnetic dirt separator, such as the Caleffi XF “extra filtration” separator shown in figure 4-12.
The dirt separator should be installed upstream of both the existing and new heat source, as well as upstream of the circulators serving those heat sources.
The “chemistry” of the water in the existing system should also be considered when adding a new heat source. A sample of the water should be drawn and sent to a company specializing in water treatment for hydronic systems. There it can be analyzed, and a water treatment plan developed based on the results of that analysis.
The water chemistry of systems that have been operating with an antifreeze solution should also be checked, especially if those systems have operated at higher temperatures with minimal (or no) maintenance of the antifreeze solution. Over time, glycol-based antifreeze becomes acidic, which can accelerate certain forms of internal corrosion. If the fluid sample has a dark color or pungent odor, the system should be drained and internally cleaned with a hydronic detergent/descaling solution. After cleaning, draining and rinsing, the ideal refill fluid would be a mixture of demineralized water mixed with new antifreeze.
For more information on water quality in hydronic heating systems, see idronics #18.
Flow Requirements For Heat Pumps
When evaluating an existing hydronic system for a new heat source, such as an air-to-water heat pump or mod/con boiler, it’s important to remember that these modern heat sources have minimum flow rate requirements. This is in contrast to cast iron boilers that can usually operate at very low flow rates. Older cast iron boilers equipped with tankless coils for domestic water heating can even be fired with no flow.
In many older systems, the water and metal content of a cast iron boiler or steel fire-tube boiler is such that no buffer tank is needed for a multi-zone application. The boiler is able to operate at low flow rates, such as might occur when only one zone is active. However, hydronic heat pumps typically require 2-3 gallons per minute of flow per ton (12,000 Btu/hr) of output capacity. Directly connecting such a heat pump to an extensively zoned hydronic distribution system, such as shown in figure 4-13, is likely to create short cycling issues, especially for heat pumps with fixed speed compressors.
A buffer tank is typically required when retrofitting a hydronic heat pump that has a fixed speed compressor to an extensively zoned hydronic distribution system, as shown in figure 4-14.
The buffer tank allows the heat pump’s heat output rate to be much higher than the load from a single zone circuit. A properly sized buffer tank can prevent short cycling. The heat pump is configured to respond to the temperature within the buffer tank, rather than directly from room thermostats.
When a heat pump with a variable-speed “inverter” compressor is used, it may be possible to use a hydraulic separator between the heat pump and zoned load circuits, as shown in figure 4-15.
This configuration is based on the ability of the heat pump to reduce its heat output rate to potentially match the requirement of a single zone circuit, and thus not require the added thermal mass of a buffer tank. The SEP4 hydraulic separator allows the heat pump flow rate to be different from that of the distribution system. It also provides air, dirt and magnetic particle separation for the system. The latter functions are especially important when retrofitting older systems that may contain dirt or magnetite from cast iron or steel components.
For more information on air-to-water heat pumps systems, see idronics #27