22 pitfalls to avoid when designing or installing a
combined heating hot water and domestic
hot water system
By Ron George, CIPE, CPD
President, Ron George Design & Consulting Services
A combined heating hot water and domestic hot water system is a hybrid system that utilizes a boiler or boilers to heat water for heating the building environment, and it uses boiler water to heat domestic hot water for bathing, washing and cleaning uses. The two systems are often combined in an effort to reduce the initial cost of installation, but there are a lot of differences between the two that, if not accounted for, could result in someone getting seriously injured.
Over the years, I have investigated numerous combined systems and found that there are a lot of mistakes or pitfalls that seem to occur. Since there are many opportunities to make mistakes with combined systems, someone very familiar with how both systems are supposed to operate should design them. Heating hot water generally needs to be at a very high temperature, while domestic hot water should be at a lower temperature. If the domestic water gets too hot there are scalding dangers, so proper controls are very important.
I have been investigating scald incidents since the mid 1990s, and I have seen a significant number of these combined systems involved in scald litigation cases because they are often not designed, operated or maintained properly. There are only two plumbing code sections that address combined systems. One section calls for the piping and components in a combined system to be approved for use in potable water systems. The other code requirement calls for a thermostatic mixing valve if the system temperature exceeds 140 F.
There are many more issues than the two mentioned above that need to be addressed. Following is a list of problems or pitfalls that I have found over the years that are related to combined heating hot water and domestic hot water systems. If you can avoid these pitfalls you will have a much safer system:
Pitfall number 1: Open system vs. closed system
There are two versions of a combined heating hot water and domestic hot water system. Systems with domestic hot water flowing from the city water supply through the heating hot water system components such as pumps, control valves and heating coils are often called open systems. Open systems utilize the domestic hot water flowing through the heating coils or baseboard heaters for heating the building. The same water flows to the plumbing fixtures for bathing and washing. These systems provide a significant challenge, because the fluid in the system must be potable water. It is difficult to circulate domestic hot water through many hydronic components without having scale, corrosion, buildup of air pockets and oxidation problems
Systems with a double wall heat exchanger to separate the fluid of the heating hot water system from the domestic hot water are referred to as closed systems. Closed systems use a heat exchanger to provide a closed loop for the heating hot water. The closed loop flows through the boiler, the heating coils and the heat exchanger serving the domestic hot water. Most hydronic systems have pumps, valves, coils and components that are not approved for drinking water service. Closed hydronic heating systems allow the heating hot water to be chemically treated to prevent corrosion and scale buildup on heating surfaces and are preferred because they eliminate a lot of opportunities for systems problems.
Pitfall Number 2: System operating temperatures
The next challenge is the system operating temperatures. Heating hot water systems are generally designed to operate between 180 F and 210 F. Domestic hot water systems are designed to operate between 85 F to 140 F.
Tempered water is water having a temperature range between 85 F (29 C) and 110 F (43 C). Hot water is water at a temperature greater than or equal to 110 F (43 C) Domestic hot water for bathing and showering is usually limited to a maximum of 120 F. Domestic hot water for dishwashing and laundries can be higher. Generally, domestic hot water systems operate around 140 F and heating hot water systems operate around 190 to 200 F.
Pitfall Number 3: Not including all of the required components in the combined systems
A combined system requires many components to operate properly. If all of the components are not installed in the proper location, the system will experience problems. These components include, but are not limited to the following: the boiler, expansion tank, isolation valves, unions, dielectric waterways, circulating pumps, air eliminators or air vents, control valves, relief valves, balancing valves, heating coils, fin tube radiators, thermostats, pressure gauges, temperature gauges, flushing connections, plumbing fixtures, etc. All of these components must work in concert and must be designed to work together as a system. If any one or several of the components are not installed, or if they are undersized or installed improperly, problems and safety issues can occur.
Pitfall Number 4: Seasonal pumping and pump sizing
During the winter heating season, all of the components in a combined system will require a simultaneous peak demand in the morning when it is showering time. The circulating pump must be sized for the simultaneous peak heating and showering loads. During this time it does not make sense to circulate a large quantity of water. I often see a smaller circulating pump that is piped around the large circulating pump so it can be used in the winter months when the large circulators are not needed for building heating.
Systems with domestic hot water flowing from the city water supply through the heating hot water system components such as pumps, control valves and heating coils are often called open systems. Systems with a double wall heat exchanger separating the fluid of the heating hot water system from the domestic hot water are often referred to as closed systems.
When a significant portion of a system is being used seasonally for heating and the remainder of the system is being used year round for domestic hot water, open systems are susceptible to bacteria growth in stagnant sections of heating coil piping. Heating coils have huge potential for bacterial amplification when hydronic systems are coupled with potable hot water systems where no physical barrier or heat exchanger exists between the two fluids.
Pitfall Number 5: Dead legs
During the summer months, the fan coil units and branches to baseboard heating units are shut off with a solenoid valve or the circulating pump on these branches does not run all summer long. It is not unusual for a heating system to sit idle for more than six months in southern climates. When the first call for heating is made, there is usually a slug of brackish and foul tasting water that is high in debris, metals and bacteria content. Combined systems are by design creating very large dead legs. This is a violation in many plumbing codes. Controls on combined systems need to incorporate a periodic flushing of the zones by operating the solenoid valves and circulators on each zone on at least a weekly basis if not more often.
Pitfall Number 6: Peak load problems
The early morning is generally the coldest time of day. It is also when guests at a hotel or residents of an apartment building or condominium take their morning showers. Equipment, piping, pumps and valves must be sized to handle this simultaneous peak load. If the equipment is not sized big enough the temperature of the space will fall and the shower water temperature will fall. Either condition is likely to result in calls and complaints.
Pitfall Number 7: Sizing
Sizing problems can arise when engineers, owners or contractors try to be thrifty and save a few bucks by rounding down on their peak load calculations and downsizing pumps, piping, valves or coils. This is when the phone starts ringing with complaints of spaces being too cold or there not being enough hot water for a shower. The maintenance men usually do what comes naturally when they receive calls of not enough heat: They go to the boiler and turn the temperature up. Turning up the temperature will not cause problems for the heating coils, but it does significantly increase the risk of scalding if the maximum temperature limit stops in the showers and tub/shower valves are not readjusted.
If the shower has an old two-handle or single handle non-compensating type shower valve that compensates for changes in incoming temperature or pressure the risk of scalding is even greater. The best solution is to have a thermostatic mixing valve on the hot water supply to limit the hot water to a safe temperature. If the hot water and heating water piping are still separated, and the system uses one boiler, then a temperature actuated master thermostatic mixing valve conforming to ASSE 1017 or the appropriate CSA B-125 mixing valve can be located at the water heater to lower the hot water to a safe delivery temperature. If the combined system utilizes the same piping for heating hot water and domestic hot water, a temperature limiting valve conforming to ASSE 1070 should be used in-line to mix cold water with hot water to provide a safe temperature of hot water for bathing or showering fixtures.
Pitfall Number 8: Maintenance
The main problem with a combined system is that the system includes components and controls for two different mechanical trade disciplines. Often, if there is a service call, the service technician may be familiar with one system or the other. If the system was designed with a specific operating temperature, it is not uncommon for a service tech familiar with only one system to set the temperature to that of the system he is accustomed to.
There are also many components in the system that one trade or the other may be unfamiliar with. For example, in one case the owner called an HVAC technician to work on his combined system. The technician was used to setting hydronic systems for building heating at 190 to 200 F. He set the temperature to 190 F, and a woman was scalded when she got into her shower. The technician did not know that he needed to reset the maximum temperature limit stop on all of the ASSE 1016 shower valves when he readjusted the boiler set point temperature.
Pitfalls 9-22 will appear in the September issue of Plumbing Engineer.
Ron George is president of Ron George Design & Consulting Services. He has served as Chairman of the International Residential Plumbing & Mechanical Code Committee. He is active in plumbing code and plumbing product standard development committees with ICC, IAPMO, ASSE, ASME, ISEA and ASTM. His company specializes in plumbing, piping, fire protection and HVAC system design and consulting services. He also provides plumbing and mechanical code consulting services and he provides investigations of mechanical system failures and litigation support. To contact Ron, email: rgdc@rongeorgedesign.com.