Turnagain Elementary School, located in Anchorage, has had a chronic problem with cracking sections of the cast iron boilers in recent years. The building was constructed in the '50s with a large radiant floor heating system and single firetube boiler.
The original floor radiant coil specifications called for schedule 40 pipe with bent runouts, and also called to avoid the use of fittings. There was no pex tubing in 1955. In addition, the specifications called for the coils to be tested by blowing a marble measuring 1/8-in. less than the pipe diameter through each coil, welded, and hydrotested to 250 psig. Almost 50 years later, it almost goes without saying that there is zero makeup water use. It also does not appear that any repairs were made to the heating system after the 1964 earthquake that shook Alaska.
In 1990, the original boiler system was replaced with two cast iron sectional boilers. The heating system maintained the original design, which consisted of a three-way mixing valve for heating water temperature reset to the reheat coil/radiant loop. A single boiler blend pump, P-3, was added. See Figure 1 for the original heating system schematic.
The classrooms are equipped with a low water temperature zone tempering air coil (130 degrees F design EWT, 50 degrees EAT and 80 degrees LAT) and a radiant slab coil. Both the air coil and slab coil are piped in series with a single control valve. The water first enters the air coil and then the slab coil. The original reset schedule for the low temperature system was 140 degrees water at -20 degrees outside air temperature and 80 degrees water at 70 degrees outside air temperature.
Unfortunately, like many system three-way valves, over the years the three-way valve was disconnected from the control system and failed in full heating position. This provided no protection from cold return water temperatures. The blend pump, P-3, did operate based on low return water temperature to blend supply water directly back into the return. To provide some type of temperature reset, the boiler firing rate was reset based on the outside air temperature. At -20 degrees the boilers were resetting up to 190 degrees boiler water temperature and reportedly, the radiant floors in some classrooms had melted children's chocolate bars on the floor.
Another problem with the original system configuration was excessive boiler firing required to provide design heating water temperature. Boiler redundancy is a necessity in cold regions, and almost every commercial building large or small, has redundant boilers.
With two boilers in a primary only arrangement, water is continuously flowing through both the lead and lag boiler. Most boiler controls are configured to automatically fire the lag boiler due to a lead failure, and automatic isolation valves are typically not used to stop the flow of water through the offline boiler.
As a result, return water through the offline boiler is continuously mixed with hot supply water from the lead boiler. This reduces the combined supply water temperature, and the design water temperature cannot typically be reached with one boiler firing regardless of the load. This is due to the fact that all boilers have a maximum upper temperature limit. For example:
- Building return temperature is 170 degrees;
- Boiler operator controller (high limit) set to 190 degrees;
- One boiler can meet the building load; and
- Heating water setpoint is 185 degrees.
The maximum leaving water temperature of our lead boiler is 190 degrees regardless of whether it is only firing at 50% load or not. Half of our combined supply is from the lead boiler, and half is from the lag boiler. The two boiler water supply temperatures are 190 degrees and 170 degrees, or a mixed supply temperature of 180 degrees. The combined supply temperature of 180 degrees is still 5 degrees below the setpoint of 185 degrees even though the lead boiler could have met the total Btu load. Thus, to meet the setpoint of 185 degrees, both boilers will be required to fire.
Unfortunately, firing two boilers at part load is typically less efficient than firing one boiler at full load.
The SolutionArctic Sun Engineering met with the Anchorage School district project managers and maintenance staff to discuss a cure for the hydronic problems at Turnagain Elementary School. In particular, a new control strategy for the 1 MMBtu radiant system was discussed which included a variable-speed injection loop, a first for the 100-building owner. A 15% design submittal was completed which reviewed the new heating system schematic. The maintenance staff was invited to review the system schematic and review the injection pump principle. The "squirting" of hot water into the radiant loop was well received and easily understood.
In addition, winter construction was also discussed. After reviewing the system's ability to operate the air-handling systems on 100% return air during construction and the large capacitance of the radiant heating system, it was decided that there was little risk for winter construction. The design documents indicated phased piping installation to ensure that no new pumps were required to function immediately after each shutdown. The contractor's shop drawings were reviewed and phasing was discussed during the preconstruction meeting.
The hydronic system piping was reconfigured to provide a primary/secondary piping arrangement to allow for different flows and temperatures in the boiler primary loop, secondary loops, and radiant loop. See Figure 2 for the heating system schematic.
One plus of primary/secondary piping that is often forgotten is the ability to allow for different flows and temperature differences. How often do we see primary/secondary piping with a 20 degrees Delta T on the boiler loop and a 20 degrees Delta T on the secondary loop?
Since boiler shocking was a major design concern, a primary Delta T of 15 degrees was chosen. This increased the primary flow by 33% compared to the normal 20 degrees Delta T. The excessive primary flow ensures that the return temperature to the boiler is always blended with supply water, and reduces the need for a dedicated blend pump (P-3) from the original design. Boilers only know the firing rate capacity and the upper temperature limit. The only penalty for flowing more than "normal" water flow rates through the boiler is a slight increase in pressure drop. Primary piping is typically a short run, and increasing line size has a marginal cost increase.
In addition, the primary pumping arrangement also allows for no bypass water through the lag boiler and allows the lead boiler to provide design water temperature if the load is less than the boiler capacity. The Anchorage School District prefers to keep the lag boiler "warm"; they have found that this decreases the chance of gasket leaks.
In addition, a small bypass pipe with balancing valve was supplied around each primary pump check valve. This allows a small amount of supply water to flow backwards through the lag boiler. Previous design standards had added a second small primary pump parallel to the main primary pump. However, the cost for controls, space requirements, and complexity of having two pumps per boiler was replaced for this project.
Variable-speed injection was installed to replace the three-way valve for temperature control to the radiant loop. Redundant injection pumps were supplied and a 40-in. heat trap was installed to prevent thermal siphoning. Small 1/6-hp injection loop pumps were selected to move close to 1 MMBtu. For example with 180 degrees injection boiler supply and 90 degrees radiant return temperature, the injection piping transfers the same amount of Btu at approximately one-fifth the flow of traditional piping at a 20 degrees Delta T.
Injection pumps can replace large three-way control valves in many applications. Specifically, there are several pump manufacturers who now offer circulators with built-in VFDs. This allows the building management control system (BMCS) to supply a modulating signal (4-20mA) to the injection pump speed just like a control valve. Most of the injection pumps with integral VFDs also provide manual dip switches and override controls to manually set the pump speed.
However, the injection loop pump does limit the amount of "manual override" that can take place. For example, the injection pump can only inject a fixed amount of water into the radiant slab, which hopefully limits the possibility of thermal shock to the boilers especially at start-up.
The injection loop pump at Turnagain Elementary School was sized to inject up to 80% of the radiant loop flow. Normally, injection pumps are sized to inject to a maximum radiant supply temperature, which is typically significantly less than the radiant water supply flow. However, we were concerned about the unknown performance of near 50-year-old radiant floor piping. Remember, the existing three-way control valve had been disabled and the radiant slab had been supplied with high heating water temperatures.
StartupAfter startup, it was found that the original reset schedule for radiant slab still worked 50 years later. One advantage of the injection loop mixing is the ability to "resize" the control range. By varying the balancing valve in the injection loop piping and adding pressure drop, we can essentially rescale our injection pump loop to get full span on our injection pump speed.
A virtual point was added to the control system to monitor the maximum speed of the injection loop pump. The balancing valve was adjusted until at near-peak design temperatures, and the injection pump speed of approximately 90% of full speed was required. An alarm level of 100% full speed was then entered to provide indication of a problem. Tuning the span of the injection pump speed to match the system decreases injection pump cycling and provides increased control range.
Observation of the system operation was immediate after startup. Since construction occurred during the winter, we were immediately able to observe the system operating at near design temperatures, and there will be no worries about the system next winter. As for all the summer construction this year, there will be the normal break-in during our first cold snap.
The new system configuration for the radiant slab coil operated perfectly at the original design temperatures and provided good space comfort. The winter project proved doable, and we have several hydronic upgrades already in design for this coming winter. However, I will probably still stop by Turnagain Elementary on a cold day and check up on the system. There is something remarkable about seeing a 90 degrees Delta T between the supply and return piping within 6 in. of each other on a variable-speed injection loop. Explaining primary/secondary piping to an owner is one thing, but seeing it work is another. ES
EDITOR'S NOTE: The images associated with this article do not transfer to the Internet. To view the images, please refer to the print version of ES.