In this brief discussion we take a quick look at the differences between designing and operating a dedicated domestic hot water (DHW) boiler plant and a combined heat and hot water (CHHW) plant producing the domestic hot water indirectly via a heat exchanger. In drawing this comparison we need to assume the storage, circulation, and distribution portions of the system are the same so that we can focus only on the differences in the heating source and its configuration. Furthermore, we shall consider these two options on the basis of first cost, operating efficiency, and other operating costs relating to maintenance, service, and longevity of the equipment.
When considering first cost there is a premium for a potable water rated boiler whether it is made of stainless steel or Cupro-Nickel, and sizing a combined heating and hot water plant according to ASHRAE’s recommendations the boiler capacity included in the plant added specifically for the domestic hot water load is likely only 50% of what would be added in the case of a dedicated DHW boiler.
These cost savings for a combined heating and hot water plant are somewhat offset by the cost of the heat exchanger, the extra pumping, and the extra piping, valving, and auxiliary components required, but not entirely thus the CHHW plant will cost less initially to install.
There are a number of considerations when comparing both systems for energy efficiency. Firstly, what type of boiler technology is used in either system and in what configuration is the rest of the system. A high efficiency, non-condensing domestic hot water boiler will operate at roughly the same efficiency as would a boiler in a CHHW system, with some additional loss of efficiency through the heat exchanger. Furthermore, a condensing domestic hot water boiler when piped with cold water to the inlet will operate at an efficiency of about 5% to 10% higher than the high efficiency, non-condensing type and roughly 10% to 15% higher than a CHHW system.
There is one additional factor that is extremely important when discussing the efficiency of two such DHW systems, which is that of the thermal mass in the boiler. While it is true that boilers in a CHHW system benefit from having treated water circulated through its heat exchanger; outside of the heating season there is very little thermal mass on the boiler side that causes the boiler to stage and cycle rapidly in an effort to match the load. A rapidly cycling boiler can be as much as 15% less efficient than one cycling a normal amount and a lot of thermal stress is placed on the boiler, which can drastically reduce the life of the boiler. It also requires that more time and cost is spent initially and on an ongoing basis for the control system. A CHHW system is inherently more difficult to control as there are more components involved and it operates under drastically different conditions at different times of year.
It is largely these latter factors alone that makes a dedicated DHW boiler (stainless steel condensing) the system of choice for a single multi-residential building. However, there may be building or system applications where a different type of DHW system should be implemented. A professional engineer or hydronic system expert should be contacted to review your particular application and system to determine the best approach for your building.