The heat is on
Initially, Presidential City maintained all its bodies of water at the same temperature during the winter. The resort’s new goal was to maintain the hot tubs at 38.9 C (102 F) degrees year-round; and the pools at 28.9 C (84 F) in the summer and 4.4 C (40 F) degrees in the winter.
The six existing pool heaters, which ranged from 400 MBH to 1.2 MMBH, had been failing at a rate of one every 18 months. Luckily, there was some redundancy built into the system for the pools and spas to remain at temperature, but the maintenance costs were extraordinary.
Initially, management only wanted to fix the existing pool heaters, but this was not possible. Repairing or retrofitting the existing heaters was cost prohibitive due to the piping design being inadequate and not being installed as per the pool heater manufacturer’s specifications. Additionally, only one of the six pool heaters was of a high-efficiency condensing model, which can withstand low influent water temperatures. Yet, in the winter, influent water temperatures were as low as 4.4 C (40 F). The heat exchangers of the conventional, non-condensing heaters were constantly condensing; and condensate from gas-fired appliances is caustic, and will destroy conventional heat exchangers over time. The pool’s salt content was also degrading the copper heat exchangers at an alarming rate. To buy some time, these issues were temporarily fixed before a new heating system could be designed and installed.
Post Brothers’ main request was for the pools to remain at temperature, without fail. It was imperative to not have any inconvenience for their tenants. Once given the nod to proceed with a replacement, the design process began.
Robust design
Before starting the design, Post Brothers insisted the most important results of the replacement should be reliability, redundancy, and efficiency.
To begin the design phase, two separate calculations were needed, one for the hot tubs and one for the pools. Considerations included the volume and surface area of the water, ambient air temperature, and wind factor. Combined with the desired setpoint of the hot tubs and the pools, these figures provide a maximum heat loss. In this case, the plan was to use multiple modulating, condensing boilers. Condensing boilers are not only designed for low influent water temperatures, but they provide variable firing rates, allowing the system to efficiently accommodate the vast difference between the heat load in summer and winter.
After deducting the efficiency loss across the 10 titanium shell-and-tube heat exchangers, it was determined that four, 800 MBH condensing boilers would be sufficient to provide the necessary heat. Combined, the boilers have an input of 3.2 MMBH, but because each boiler has 10 stages of input, the system can fire as low as 80,000 BTUH in situations where the call for heat is minimal. Unlike the existing pool heaters, which operated independently of one another and were dedicated to specific bodies of water, the four boilers operate together to produce heat for the entire system. The bank of heat exchangers distribute heat from the boiler loop to the pools for much greater redundancy.
