Pool Systems Introduction

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Systems for pumping, filtering and chemical treatment ensure that pool water is sanitary, clear and warm.

As illustrated in the Process Flow Diagram, water exits the pool via the drain and/or gutters. A surge tank balances the flow from both sources. An intake pipe near the bottom of the surge tank feeds a strainer basket that is used to catch large debris (such as leaves) prior to entering the main pump. The main pump forces water into a bank of sand filters. Filtered water then proceeds to enter multiple "loops" for chemical monitoring, heating (via the boiler) and chlorination. Finally, water is returned to the pool via small outlets at the base of the gutter tray.

Items below are listed in water flow order.

Surge Tank

The surge tank fundamentally supports the gutter system, which captures surface water and absorbs of waves formed within the pool. The water on the surface has the potential to be the dirtiest in the pool. Debris (such as leaves, pollen), algae spores and oils (from skin, sun screen lotion) collect on the surface. The sun's UV rays break down disinfectant (chlorine) most at the surface. It is important that this water be collected with priority.

When the flow from the gutters is not sufficient (due to water level and/or a lack of waves), additional flow from the drain is required to maintain the surge tank level. This is typically handled via a modulation valve on the drain inlet to the surge tank, controlled by a level monitoring mechanism (such as a float). When a modulation valve is not present, flow much be restricted on the drain return and the pool level must be maintained to ensure some amount of gutter return is present.

Key Observations

When the surge tank is too low, this is typically due to one of more of the following reasons:

  • The pool water level is too low (when a drain return modulating valve is not present). Fill the pool.
  • The drain return modulating valve is malfunctioning, if present. Service the valve.
  • The main pump flow rate is too high. Reduce the outlet flow rate (only after ensuring that other conditions are not present).

When a drain return modulating valve is not present, it is important that the drain return is restricted (the valve is partially closed). Otherwise, flow from the drain will potentially prevent adequate flow from the gutter return. In an ideal case, the surge tank should always have a modulating valve on the drain return.

Main Pump


Four Pentair Triton II Commercial TR140C-3 silica sand-filled filter tanks operate in parallel to capture particles in the water that passes through. As particles accumulate, flow is restricted. This is indicated by an increase in pressure.


Periodic back-washing is required when filter tank pressure exceeds a threshold.

The filter sand should be replaced every 5-10 years. In a marcite pool, the filter sand should be replaced following the cure period of a new marcite coating (as the process releases a large number of particles, clogging the sand filters).

Chemical Monitoring

This is detailed in the Chemical Treatment: Monitoring section, below.


The boiler is a natural gas fired 1,200,000 BTW/hr RBI LW1200 (as-built RBI drawing #42-2112).

Interlocks prevent the boiler from operating when the water level is too low, sufficient water flow is not present or the air pressure differential (as generated by the blower motors) is insufficient. Redundant over-temperature lock-outs (including those added by FFSC maintenance) prevent operation in the event that the temperature rises beyond a threshold.


Regular inspections are mandated by the state and the insurance company. These inspections occur every three years. Prior to the official inspection, a Steam/Hot Water ASME CSD-1 form must be completed by a qualified technician. This form contains details about the boiler (identified by the state license and serial numbers), identifying key controls, their set-points and operation test results. Any issues found during the technician's inspection must be remedied before the document becomes acceptable as proof of compliance per Rule 27 of the State of Michigan Boiler Code.

Two copies of the CSD-1 should be kept by FFSC. One placed in a room near the boiler, another in a central filing location.


First, always ensure that the breaker and GFCI are not tripped, and that the power switch on the side of the unit is turned on.

If the boiler does not fire in response to a call for heat, the issue is likely related to an interlock or the ignition system. Verify that the low water sensor is reset; it can be tripped by air infiltration during filter back-washing. Ensure that the pump is operating. Verify that the covers are securely fastened; a loose cover leaks air and may not close the air pressure switches. Finally, check for the presence of spark and/or pilot light. This is visible via a small window on the lower-left of the unit.

The flame sensor unit is a consumable item and may need to be replaced at a regular interval (every two years?). If the pilot valve (PV) is open, but the main valve (MV) never opens, it is likely that adequate flame is not being detected. This can be verified by measuring the microamp current output of the flame sensor and should be done by a qualified technician.

Chemical Treatment

The pool is treated with calcium hyprochlorite (chlorine) and hydrochloric (muriatic) acid automatically according to measurements.

When requested, chlorine is added via the chlorinator loop and acid is added via the acid pump. Both of these are added post-boiler to minimize chemical interaction (corrosion) with the heat exchanger.

Why acid? pH will almost never naturally drop in an outdoor pool. pH rises due to:

  • Removal of carbon dioxide from the water. Refer to the chemical reaction for carbonic acid for why carbon dioxide reduces pH. This is be caused by:
    • Splashing, which accelerates off-gassing via bubbles.
    • Algae, which consumes carbon dioxide (photosynthesis).
    • A general desire to be at equilibrium; the carbon dioxide concentration in the water approaches that of the air.
  • The addition of non-stabilized chlorine.
  • Chemical interaction with the cement-based liner (i.e. marcite).

The pH will almost certainly rise faster when the pool is heavily occupied. Decomposition of leaves and acidic rain can lower pH, but these are considered minor contributors.


The acidity and amount of chlorine in the pool are monitored by pH and ORP (oxidation reduction potential) probes, respectively. When insufficient levels of either exists, the controller requests the addition of the respective chemical. Ideally, pH should be between 7.4 and 7.6. Proper pH prevents eye/skin irritation, maximizes chlorine effectiveness and minimizes corrosion. ORP should typically be between 650 mV and 750 mV for proper water disinfection.

While ORP is a means for verifying the level of pool chlorine, it is not a direct measurement of chlorine. ORP measures chlorine's ability to oxidize pool contaminants. Care should be taken to minimize cyanuric acid (stabilizer) in the pool, as it may reduce ORP and affect chlorination control.

Water for the probes is sampled at a point just beyond the filters, prior to the boiler and chemical feeds. This is identified as the Chemtrol Unit Monitoring Loop in the process flow diagram. A flow meter identifies whether adequate flow is present; insufficient flow indicates that water in the flow cell assembly may not accurately reflect the composition of the pool water. Water from the monitoring loop is returned to the surge tank.

Addition of Acid

Hydrochloric (muriatic) acid is added when the controller has deemed the pH to be too high. A electrically actuated peristaltic dosing pump transfers acid from a reservoir into the pool water return. The relatively small amount acid flow relative to the flow of the return ensures significant dilution.


Calcium hypochlorite (chlorine) is added when the controller has deemed the ORP to be too low. A PowerBase Model 3150 chlorinator is responsible for adding chlorine by dissolving "Accu-Tab" solid tablets in water.

It has been observed that the chlorinator has a tendancy to over-shoot. Therefore, the ORP setpoint on the controller is set lower than otherwise might be expected to minimize this over-shoot. The net result is a proper amount of chlorination in the pool.