How to Measure Heater Temperature Rise and Check Flow

Why this matters

Most "the heater keeps shutting off" calls are water problems, not heater problems. A loaded filter or a pump turned down to its quiet speed starves the heat exchanger, the water inside overheats, and the high limit or the pressure switch does exactly what it is supposed to. The opposite case, too much flow, quietly erodes copper exchangers and lets a gas heater's flue gas condense on its own tubes. Temperature rise across the heater is the water-side number that tells you which side of the line you are on, and it is a check a pool tech can do with the panels closed. Anything on the gas or refrigerant side routes per 'The Pool Tech's Boundary at the Heater: Gas, Power and Refrigerant'.

The boundary and the hazards on this check

This check never opens the heater. You read pipes and the heater's display, nothing more. No burner tray, no gas valve, no refrigerant ports, no electrical compartment. If a step tempts you inside the cabinet to get a number, the number is not yours to get; write down what you saw and route it.

The heater is firing while you read it. Keep hands and face away from the vent or top exhaust of a gas heater: it is hot enough to burn and it carries combustion products. Do not hold your head over it to listen. On a heat pump, keep hands off the fan guard and out of the airflow. Outlet piping close to a gas heater can be hot to touch on a starved exchanger, which is the case this check exists to find; touch it with the probe, not your palm.

Gas smell at the heater means stop. Do not operate any switch, do not start or stop the heater, leave the area, keep the customer away, and call the gas utility or 911 from a distance.

What the rise tells you

Water carries heat at a fixed rate. For water, the relation is:

output (BTU/hr) = 500 x flow (gpm) x rise (F)

The 500 is 8.33 lb per gallon times 60 minutes per hour times 1 BTU per lb per degree F. Rearranged, it gives you the flow through the heater from two thermometers:

flow (gpm) = output / (500 x rise)

So for a heater putting out a steady amount of heat, a high rise means low flow and a low rise means high flow. That is the whole check. The heater maker's manual gives the minimum and maximum flow, and often the expected rise, for each model; those are the limits you judge against. The output figure has to be the heat actually delivered to the water, not the input:

• Gas heater: nameplate input times the efficiency on the label or in the manual. A heater that is under-firing delivers less than that, which reads as a lower rise and inflates the computed flow; the check below catches that case.
• Heat pump: output swings widely with air temperature and humidity, and the nameplate figure is at one rating condition. Use the maker's performance chart at today's air temperature and humidity, or the check is meaningless.

Measure at the heater's inlet and outlet connections. On a gas heater with an internal bypass, the outlet water is already mixed with bypassed water by the time it leaves the heater. The energy balance still holds at the connections, so the flow you compute is the total flow through the heater, which is what the maker's flow limits refer to.

Start state

Bring: a contact pipe thermometer or two thermocouple probes with insulating sleeves, the filter's recorded clean baseline from the equipment tag, the heater maker's manual or its flow and rise table, and the heater's nameplate.

Set up before you fire anything:

• Clean the filter first if it is at or near its cleaning point. Per 'Filter Cleaning and Media Replacement', that is 8 to 10 psi above baseline. Measuring rise on a loaded filter measures the filter.
• Confirm the pump speed on a variable-speed pump, and write it down. The rise is only meaningful at a known speed, and the speed the heater sees on its schedule is the one to check.
• Check the valves. Every valve between the pump and the pool in its normal heating position, any external bypass around the heater in the position the installer set.

The procedure

1. Clean the filter and record the pressure. As above. Write the clean pressure on the check line.

2. Set the pump to the speed the heater runs at on its schedule. If the customer heats at a lower speed than you would choose, check at theirs; that is the condition the heater lives in.

3. Fire the heater from its own controls and wait 15 minutes. The exchanger and the water in it need to reach steady output, and a reading taken in the first few minutes catches the warm-up, not the heater.

4. Read inlet and outlet with the same instrument and the same method. Probe on bare pipe as close to the heater's inlet and outlet connections as you can reach, sleeved so ambient air does not cool it, left until the number settles. Using one method on both pipes means any offset in the method cancels in the subtraction. An infrared gun on white PVC reads the pipe surface badly and is not this check.

5. Subtract and compute the flow. rise = outlet minus inlet. Then flow = output / (500 x rise), with output from the label efficiency or the heat pump chart.

6. Judge against the maker's limits.

• Computed flow below the maker's minimum, rise high: low flow. Look upstream: filter, pump speed, a partly closed valve, a clogged pump basket or skimmer, an external bypass left open too far.
• Computed flow above the maker's maximum, rise low: overflow. The fix is an external bypass set per the manual, or a lower pump speed, not leaving it.
• Computed flow plausible for the pump and plumbing, rise low, water slow to warm: the water side is doing its job and the heat is not arriving. That is a gas-side or refrigerant-side question, and it routes.

7. Leave the heater as you found its controls, and log the check line.

Worked example: a heater cycling off on a cool evening

A gas pool heater, nameplate input 400,000 BTU/hr, label efficiency 82 percent, on a 20,000 gal pool with a variable-speed pump and a cartridge filter. The customer reports the heater runs a few minutes and shuts off. For this example, say the manual lists 40 gpm minimum and 120 gpm maximum flow for the model; read the real figures from your unit's manual.

Output. 400,000 x 0.82 = 328,000 BTU/hr.

First reading, as found. Filter at 24 psi against a tag baseline of 14 psi, 10 psi over. Pump scheduled at 1,500 rpm for the evening heat. Fired, and it shut off on its high limit at about 6 minutes, short of the 15-minute wait. A heater that cannot reach 15 minutes is itself the finding, and the last reading before the trip is the one to use: inlet 80 F, outlet 106 F, rise 26 F. The rise was still climbing when it tripped, so the steady value would only have been higher and the flow lower. Flow = 328,000 / (500 x 26) = 328,000 / 13,000 = about 25 gpm. Below the 40 gpm minimum.

The fix, water side only. Filter cleaned to 15 psi. Speed raised for the heating schedule to 2,400 rpm.

   HEATER WATER-SIDE CHECK, gas, 400,000 BTU/hr input

                     as found      after
   filter psi        24            15   (tag base 14)
   pump rpm          1,500         2,400
   fired / read      6 min, trip   15 min
   inlet F           80.0          81.0
   outlet F          106.0         91.5
   rise F            26.0          10.5
   output BTU/hr     328,000       328,000
   flow gpm          25            62
   maker min-max     40 to 120 gpm (example)
   verdict           LOW FLOW      IN RANGE

Second reading. Inlet 81 F, outlet 91.5 F, rise 10.5 F. Flow = 328,000 / (500 x 10.5) = 328,000 / 5,250 = about 62 gpm. Inside 40 to 120 gpm. Cross-check from the other direction: at 62 gpm the heater should raise water 328,000 / (500 x 62), about 10.6 F, matching the 10.5 F measured. The heater ran 30 minutes without cycling.

What the shop does with it. The evening schedule stays at 2,400 rpm when the heater is called, and the ticket records why, so the next tech does not turn it back down to save noise. The cycling was the high limit protecting a starved exchanger, and the limit was right to trip.

What changes the reading

• A heat pump. Same formula, but output comes from the performance chart at today's conditions. Say the chart gives 110,000 BTU/hr at today's air temperature and humidity, and the rise reads 3.5 F: flow is 110,000 / (500 x 3.5), about 63 gpm. A heat pump's rise is small, so a degree of probe error is a large share of it; read carefully and read twice.
• The efficiency figure is an estimate. A few points of efficiency move the computed flow by the same few percent. That matters near a limit, not in the middle of the range.
• Low rise with a strong flow and a slow warm-up. The heater is delivering less heat than its label. Under-firing, a dirty burner, a scaled exchanger, a refrigerant problem on a heat pump: all inside the cabinet, all routed per the boundary card.
• Pressure switch trips on startup but the rise is fine once running. Some heaters need more flow to make the pressure switch than to run; adjusting the switch is not a water-side fix, so record the flow and route the switch itself.

References

• See related: The Pool Tech's Boundary at the Heater: Gas, Power and Refrigerant (what a pool tech does and routes at a heater)
• See related: Filter Cleaning and Media Replacement (cleaning point before the check)
• See related: The Equipment Pad Flow Path: From Skimmer to Return
• Heater manufacturer's installation and operation manual (flow limits, rise, bypass setting, heat pump performance chart)