Engineers face multiple options for providing hydronic heat, along with additional decisions regarding whether that design should also serve other components such as domestic hot water. Let’s start with a straightforward question: What are the choices for types of boilers?
Fire tube boilers have a tank of water and tubes passing within that tank that the combustion gas flows through, providing heat trans-fer to water within the tank. There are two styles of fire tube: scotch marine (where the fire tubes are mounted horizontally within the tank) and vertical (where the fire tubes are mounted vertically in the tank).
Scotch marines are high-mass boilers (the ability to come to full fire from a cold condition takes a long time) with a large water volume. They cannot accept return temperatures below 140°F or rapid changes in the return temperature. Because of this, these boilers are more suited to constant flow or slowly changing primary/secondary flow. A benefit of these boilers is the ability to operate on dual fuel (natural gas and fuel oil), which is especially important for critical facilities.
Vertical fire tube boilers can be condensing or non-condensing. Most vertical tube boilers can operate in variable flow operation, with limited return temperatures changes. Typically these boilers are medium-mass boilers.
In water tube boilers, the flue gasses circulate around the tubes transferring heat to the water. Water tube boilers come in three varie-ties, incline tube, bent tube, and copper fin.
Incline tube boilers typically have a straight tube connected to a supply and return tube sheet on both ends. Inclined tube cannot accept return temperatures below 140°F or rapid changes in the return temperature. Pumping of inclined tube should be constant flow or limited slow-changing variable flow. These are typically medium-mass boilers.
Bent tube boilers typically have a supply and return header, with welded bent tubes interconnecting the two headers. Bent tube boilers cannot accept return temperatures below 140°F, but they can handle rapid changes in the return temperature. Pumping of bent tube should be constant flow or limited slow-changing variable flow. These are typically medium-mass boilers.
Copper fin boilers can be condensing with a condensing heat exchanger or non-condensing. They can either use a vertical or hor-izontal tube arrangement. Pumping of copper fin boilers should be constant volume to prevent over-temperature conditions for the copper tubes and should be designed around a minimum and maximum Delta T. These are typically low-mass boilers.
A sectional boiler is much like a plate-and-frame heat exchanger, the plates made out of cast iron separate the combustion gas and water with the plates bolted together and the heat transfer occurring through the plates. Sectional boilers can be condensing or non-condensing. Typically these boilers are medium-mass boilers but cannot handle thermal shock.
WHAT ARE THE CHOICES FOR PUMPING SYSTEMS?
With today’s push for higher efficiency and meeting of minimum code, the days of constant volume heating systems are gone (un-less fairly small). However, most boilers will not deal with low minimum flows, the possibility of thermal shock, or too high of a Delta T. These issues will require a primary (constant or variable within certain parameters) and variable secondary pumping systems.
For the boilers that can perform with variable primary flow, minimum flow should be adhered to, with bypass control valves locat-ed far from the boiler plant, and the rate of change should be per what the boiler manufacturer recommends.
The following types of pumping system paring with boiler are based on observations of boilers that have failed, lessons learned implementing systems, and successful well operating systems:
Primary secondary systems
Copper fin boilers (All Types)
Bent tube boilers
Scotch marine fire tube
Non-condensing boilers all kinds
With primary secondary pumping, the designer should look at the use of a buffer tank for non-condensing designs. The buffer tank should be sized for at least five minutes of circulation on the lowest output of a boiler (lowest turndown) that the primary loop can provide. This will limit the boilers to a maximum of six cycles an hour with proper controls. If the lowest boiler turndown is below the minimum load of the system, a buffer tank will not be needed and boiler cycling will not occur. This is instrumental to boiler effi-ciency by maintaining longer run times and avoiding short cycling.
PRIMARY VARIABLE FLOW SYSTEMS
All condensing boilers that do not have strict minimum flow requirements (above 20% of design flow) or Delta T requirements less than 60°F should use primary variable flow to maximize cold return water temperature to the boiler. For this reason, buffer tanks are not recommended, but the minimum load to minimum turndown of the boiler(s) should be scrutinized.
BE REALISTIC WITH DELTA T
Multiple times, I have seen an expensive condensing boiler provided on new and existing jobsites with 180°F supply and 140°F re-turn scheduled for the heating coils/loads or an existing system with low Delta T that does not get addressed. In order to condense the return, the temperature has to be below 140°F.
1. Do not design with 3-way valve on any boiler system, condensing or not. If you have a minimum flow requirement or are worried about maintaining supply temperature with long piping runs, provide 2-way bypass valves as far out as possible. The bypass valve will control for the minimum flow I would recommend by maintaining a minimum differential pressure across the operating boilers. For maintaining the supply temperature, place 2-way control valves at each of end of the system, with a temperature sensor prior to the valve to control within 10°F of the supply temperature setpoint. Both the minimum flow and supply temperature valves should be modulating.
2. If an existing system has low Delta T, many of the concepts to solve chilled water Delta T apply to heating hot water:
a. Three way valves — remove and provide 2-way valves instead.
b. Improper load setpoints — i.e., a condition the coil could never achieve.
c. Pumped loops with uncontrolled decouplers.
d. Coils controlling to a supply temperature setpoint with no airflow. If an AHU is off, the control valve should be closed (unless protecting for freezing, in which case it should control to the return temperature with the unit off).
In the past, boilers were staged based on return temperature with no consideration to cycling, which greatly reduces efficiency of the boiler.
This would be like going on a 600 mile road trip, but rather than running at a constant speed on the HWY you exited on every off ramp and rejoined the HWY at the next on ramp. Your gas mileage would not be very good, and chances are you would take a very long time to get where you are going.
The same is true with operating boilers under-cycling. Loads have a better chance of meeting their setpoint with a constant tem-perature rather than cycling all the time. Boiler efficiency is affected each time the heat transfer surface is heated or cooled. This affect is amplified with force draft or fan-assisted combustion with pre- and post-purge, which draws unfired air through the heat exchanger each time the boiler starts and stops.
Staging should be accomplished by minimum and maximum flow of the boiler(s) compared to the flow of the load. If low Delta T is occurring, cycling will occur. This is because you will be operating close to maximum flow of the boiler but not near maximum load of the boiler.
The second method of staging should be a running average of the return temperature, supply temperature setpoint, and the cur-rent load’s flow rate. Calculate the Btu of the load and determine how many boilers need to operate to maintain between the mini-mum and maximum output rates of the boilers that could operate.
These methods should operate in conjunction with each other, selecting the one with the most boilers to be operated. Of course, you should use delays in the staging up and down of boilers to insure the loads are real, but keeping it fast enough to ensure the boilers can stay responsive and not fail on high temp due to low flow, low load.
COMBINED DHW AND HHW SYSTEMS?
In order to improve efficiency, there has been a push to combine DHW and HHW system to reduce cycling and provide better redun-dancy on systems. Does it make sense for all cases?
If the DHW system just serves handwashing and break room sinks, no. Through observation and flow meter trending, it is disgust-ingly obvious that a majority of people do not wash their hands when the bathrooms are infrequently used in places like schools or offices. The DHW system sits idle with most of the energy spent in the recirculation loop due to heat loss. For these locations, I would recommend electric instant heaters or small electric water heater storage tanks and no recirculation with the piping runs short.
If there are shower facilities that will get used for a fraction of the building’s operating time, I would look at water heaters with a storage tank and possibly and central system with a recirculation loop. There have been advances in heat pump water heaters that provide a cooling benefit (good to use for telecom loads all year round), so investigate which fuel to use and if there is there any benefit to heat pump technology and offset gas loads.
The one place combined systems make sense is high-use DHW systems and environments where redundancy is required, such as kitchens, hotels, residences, dorms, and hospitals. When designing a combined system, provide storage capacity such as DHW or a buffer tank to improve the cycle times of the systems, and properly size and stage the boilers to provide enough turn down throughout the year.
There are many options in providing HHW and DHW. Are the boilers a good fit for the load? Is the pumping system a good fit for the boiler? Is the Delta T of the system a problem, or are the loads selected properly? Does a combined HHW and DHW system make sense?
These decisions will determine if the owner’s money is being wasted and providing for a system that operates well, meeting the expected life expectancy of the equipment and beyond.