A Hydronic Approach to High-Capacity Domestic Water Heating
One of the distinct trends in custom residential construction over the last 20 years has been increased interest in luxury bathrooms. The North American plumbing industry has done a superb job of promoting such bathrooms as luxurious escapes from the cares of life. Central to that concept is surrounding oneself with lavish amounts of warm water, be it in a deep whirlpool tub, or a simulated tropical downpour showering experience.
Within the last two years, I've spoken with engineers charged with designing the mechanical systems to keep the hot water flowing in such bathrooms. One mentioned a recent project requiring 27 gallons per minute of domestic hot water supply to the master bathroom alone. Another told me of an owner's mandate of providing sufficient hot water capacity to supply every hot water outlet in the house (simultaneously) for a continuous draw of 30 minutes. Still another described a project he was retained for in which the owner spent in excess of $100,000 for the master bathroom alone. Finally, a hydronic heating professional in the Philadelphia area told me of a new home he was involved with that contained 17 bathrooms!
While these may be "extreme"
Times Have ChangedFor years, the ability to produce domestic hot water (DHW) has been viewed as an ancillary load for a hydronic space-heating system. The reasoning to the homeowner went something like this:
Since you've decided to use hydronic space heating, and therefore need a boiler, why not equip that boiler with a tankless coil or indirect water heater for domestic hot water?
This reasoning was sound in years past when the average house didn't consume the copious quantities of DHW that many new homes now require. However, the marked increase in DHW demand now presents a great opportunity for savvy hydronic heating professionals. It does, however, require a significant change in mindset"
Consider the AlternativesSo, why is the hydronic approach the solution to this situation? Well, lets consider the alternatives.
Providing high-capacity DHW in a house equipped with forced-air heating or heat pumps usually requires multiple direct-fired domestic water heaters. In some cases, one or more commercial-grade water heaters are used to provide the necessary recovery rates. Such tanks often cost as much or more than a boiler of equivalent capacity. The multiple tank approach also requires additional fuel connections, and venting means beyond those used for space-heating appliances. The multiple tank approach also results in greater surface area between the heated water and surrounding air, which leads to greater standby heat loss. The thermal efficiency of direct-fired water heaters is typically lower than that of boilers of similar capacity. Finally, direct-fired domestic water heaters are usually connected as dedicated purpose devices. They cannot easily provide heat to other loads, such as space heating, snow melting, or pool heating.
Contrast this with the possibilities offered by hydronic heating technology. A typical high-capacity DHW subsystem can be built around a multiple boiler system and high-capacity indirect tank, as shown in Figure 1.
Here's an example of how the multiple boiler approach compares to a standard commercial gas-fired water heater.
Imagine the DHW demand in a chalet at a ski resort where several skiers have just returned from the slopes and want to shower up before heading out for the evening. Since the chalet has four or five bathrooms, they all go into use at about the same time. Let's assume the DHW load under these conditions is 15 gallons per minute flow at a fixture delivery temperature of 110°F. In the basement, there's a 100-gallon commercial-grade direct-fired DHW tank with a 200,000 Btu/hr gas input rate burner full of 140°F water.
Working in a few assumptions, such as a cold water temperature of 50°F and an 80% volume draw down prior to a significant drop in delivery temperature, this tank will provide just under 11 minutes of draw at this flow rate before a significant drop in delivery temperature occurs. After that, someone's going to be standing in a less than cozy shower, and you're probably going to hear about it.
Full AfterburnerNow, suppose that chalet had three 150,000 Btu/hr boilers configured as a multiple boiler system, and a single 80-gallon storage tank connected using a generously sized flat plate heat exchanger. The total output of 450,000 Btu/hr could sustain the 15-gallon per minute load at 110°F indefinitely, even without considering the reserve of hot water in the storage tank.
At other times, the multiple-boiler system could drop back to a fraction of full capacity to meet space-heating loads while still maintaining high seasonal efficiency, significantly higher than a single 450,000 Btu/hr boiler could maintain at the relatively low duty cycles required by space heating in such a building.
When the high-capacity DHW demand is satisfied, the multiple boiler system is ready to handle other large capacity loads, such as a significant area of snow melting. It could even be used to quickly warm a 20,000-gallon swimming pool by 30°F in about 12 hours.
This approach is like having a Cessna with an afterburner. The extra power is instantly available when a high demand for DHW occurs, but otherwise doesn't create a significant drop in thermal efficiency as the system cruises along under much lower but prolonged space-heating loads. This approach also provides the inherent "insurance"
No BottlenecksIf you're planning a high-capacity DHW system using multiple boilers, it's critically important not to create bottlenecks to heat transfer or flow between the boiler system and the domestic water. Don't even think about using 3/4" tubing and a 1/25-horsepower circulator between the boiler plant and DHW heat exchanger.
In situations where an indirect tank can't provide the necessary heat transfer, consider use of a generously sized stainless steel flat plate heat exchanger along with a standard storage tank, as shown in Figure 2.
During a call for domestic water heating, the boiler staging control targets a relatively high supply temperature, usually in the range of 180°F -200°F. This provides high rates of heat transfer at the tank heat exchanger. All other heating loads are temporarily suspended, at least for a set time, while maximum heat output is focused on the DHW tank. The boiler staging controller then returns to outdoor reset control as needed.
Note the placement of the boiler supply temperature sensor on the outlet of the hydraulic separator. This ensures that the boiler staging controller receives the proper feedback on the water temperature supplied during domestic water heating as well as space-heating loads. It also accounts for any mixing within the hydraulic separator due to differences between boiler plant flow and system flow.
Properly controlled multiple modulating boiler systems also provide excellent load-tracking characteristics. The greater the number of boilers, the greater the system turndown ratio becomes. For example, using four boilers, each with a turndown ratio of 5:1, yields a system turndown ratio of 20:1. This allows a powerful boiler plant to deliver a tiny fraction of its full output for minor space-heating loads, while still retaining excellent seasonal efficiency.