Pool Heater Replacement and Gas Verification

Purpose

This procedure guarantees that a replacement gas pool heater is sized to the pool, carried by the gas supply that already exists, vented to its own instructions, and proved on measured pressures before anyone leaves. The failure it exists to stop is common: a shop swaps a dead heater for a larger one because the customer wants faster heat, the new unit fires beautifully on its own, and then starves every time the furnace and water heater call at once. The symptom arrives in January as nuisance lockouts and sooting, months after the invoice.

If you smell gas at the pad, nobody operates anything. No switch, no valve, no panel, no phone at the appliance. Move everyone back from the equipment area, call the gas utility and the shop from a distance, and do not return until the utility clears it.

Scope

Covers replacing a residential or light commercial gas-fired pool or spa heater, natural gas or propane: sizing, supply and vent survey, removal, set, connection, leak test and commissioning.

It does not cover altering gas piping, meters, regulators or venting, which is the licensed gas fitter's work under the adopted fuel gas code, nor routine heater service and the combustion measurement itself, which the heater service and combustion check SOP owns and which this procedure calls at commissioning. Heat pumps have their own SOP and no combustion side.

Roles and responsibilities

What actually constrains the changeout

Almost never the heater. Three other things decide the model, and the old nameplate is evidence for none of them.

Sizing. Delivered heat to raise a pool is gallons times 8.33 lb per gallon times the rise in degrees F, divided by the hours you want it to take; divide by the plate efficiency to get input. That is heat-up only. Holding temperature is a separate demand dominated by evaporation off an open surface, which is why a cover changes the answer more than the heater size does. Do not put a fabricated loss coefficient on a ticket - read the manufacturer's sizing chart at this pool's surface area, the design ambient and covered or uncovered, and record which row you read.

Gas supply capacity. The new input has to be carried by the pipe already in the ground, at the developed length to the appliance, on top of the total connected load of everything else on the meter. The capacity tables in NFPA 54 / ANSI Z223.1, the National Fuel Gas Code, in the edition your authority having jurisdiction has adopted and which reaches you through the permit, are read by column: the same size and length gives different capacities under a 0.3 in w.c. allowable drop than under a 0.5 in w.c. drop, and at different delivery pressures. Name the column on the ticket, or the number means nothing.

Vent and clearances come from the manufacturer's installation instructions plus the adopted fuel gas code, never from what the old unit had. One gated baseline: manifold pressure on natural gas equipment is commonly around 3.5 in w.c. and on propane around 10 in w.c., but the plate on the unit in front of you governs, and it also states a minimum inlet pressure that the manifold figure does not replace.

Procedure

1. Screen for gas odor and clip on a personal carbon monoxide monitor before you touch the old appliance. Accept when there is no gas odor at the pad, no soot at the vent outlet, and the monitor reads zero. Wrong is any of the three: leave the area, keep bystanders back, and call the gas utility from a distance without operating a switch or valve. Do not begin a changeout on an appliance you can smell, and do not treat a smell that fades as an all-clear.

2. Shut down, close the manual gas cock, open and lock the electrical disconnect, isolate and drain the water side, and let it cool. Accept when the cock handle sits crosswise to the pipe, the disconnect carries your lock and tag, the exchanger has drained and the cabinet is cool to the back of a hand. Wrong is opening a heater that just cycled: exchanger water scalds through a glove. Work practices here are 29 CFR 1910.333(b)(2), not 1910.147, which excludes electrical utilization work at its own (a)(1)(ii)(C).

3. Survey the gas supply before the model is ordered, not after it is on the truck. Record pipe material and size, developed length from the meter or tank, delivery pressure, every other appliance on the meter with its input, and the meter or regulator capacity. Accept when a named table row in the adopted fuel gas code, at that length, size, pressure and drop column, carries the proposed input on top of the total connected load. Wrong is sizing on the old heater's input. Stop rule: if the row does not carry it, the answer is a smaller unit or the gas fitter, and the unit is not installed either way. Hazard: none at this step, it is measurement and table work with the gas already off.

4. Survey the vent, the clearances and the electrical before removal. Check the proposed unit's stated clearances to windows, doors, soffits, fences and property lines, the vent arrangement its instructions require, and that the branch circuit, disconnect and bonding lug suit the new unit. Accept when every stated clearance is met at the intended location with the tape run, and the circuit matches the new plate. Wrong is assuming the old location complies because the old unit stood there. Stop and reposition or escalate rather than shortening a clearance.

5. Break the gas connection at the union downstream of the closed cock, cap the drop immediately, then free the water and electrical connections and photograph the bonding conductor before it comes off. Accept when the open gas line is capped, water unions are dry, conductors are landed nowhere, and the bonding conductor's size and lug condition are recorded. Wrong is leaving an uncapped drop while you work. Never apply a torch or flame near an opened gas line, and treat scale dust knocked out of an old exchanger as a respiratory irritant rather than blowing it toward your face.

6. Set the new unit level, plumb the water side, and make the gas connection with a sealant listed for the fuel, a sediment trap where the code requires one, and an appliance shutoff within reach. Accept when the unit sits flat, water unions seat squarely with no pipe weight on the ports, the sealant is rated for the fuel including propane, and the shutoff is reachable without a tool. Wrong is petroleum thread dope on a plastic water port, or a gas sealant not rated for propane. Support the appliance's weight with a second pair of hands rather than a knee.

7. Leak test every gas joint you made, shutoff open and burner off, with an approved leak detection solution or instrument. Accept when no joint produces a bubble over five minutes and any code-required pressure test on altered piping is done and signed by the fitter. Wrong is a flame used as a leak detector, the mechanism this step exists to eliminate. Stop rule: any bubble means close the manual cock, remake the joint, and retest from the beginning rather than tightening under gas.

8. Restore water first, then electrical, then gas, fire the unit, and prove it on measured numbers under worst-case load. Accept when the pump primes and holds, the bonding conductor is landed on a clean lug with a verified continuous path per NEC Article 680 in the adopted edition, inlet pressure stays at or above the plate minimum with the burner at full rate AND every other gas appliance in the building firing, manifold pressure matches the plate, combustion meets the heater service SOP's acceptance, temperature rise agrees with plate input and measured flow, and the pump cooldown interlock operates. Wrong is testing inlet pressure with the heater alone, which is the reading that hides an undersized supply. Stop rule: inlet below the plate minimum under simultaneous load means shut down, close the cock, leave the appliance off, and call the gas fitter - no adjustment at the heater fixes a supply problem. Fuel, water and electricity all go back at this step, so stand clear of the burner compartment at first light-off and keep the CO monitor on your collar through the run.

The record this produces

One installation block on the work order, filed to the equipment record for that appliance.

• Sizing basis: volume, target rise, hours allowed, delivered Btu per hour, plate efficiency, computed input, the sizing-chart row read, covered or uncovered.
• Supply survey: pipe size and material, developed length, delivery pressure, total connected load, meter or regulator capacity, and the code table row with its drop column.
• Vent, clearances and leak test: measured clearances against the instructions, vent arrangement, joints tested and result, plus the fitter's pressure test where piping was altered.
• Commissioning: inlet pressure heater-alone and under simultaneous load, manifold pressure, combustion result, temperature rise with the flow it was measured at, cooldown interlock, bonding continuity.
• Disposition: released, or left off with the reason, the measured value and the written notice.

The simultaneous-load inlet reading is the field this document exists to create, it is what a gas utility asks for after a lockout event, and it cannot be reconstructed later.

One changeout, filled in

A 20,000 gallon pool, 450 sq ft of surface, uncovered, fourteen year old heater dead. The customer asks for a larger unit to heat faster.

Step 3 settles the model. Heat-up demand for a 20 F rise in 24 hours is 20,000 times 8.33, which is 166,600 lb of water, times 20 F, or 3,332,000 Btu, divided by 24 hours, so 138,833 Btu per hour delivered. At a plate efficiency of 84 percent the input needed is 138,833 divided by 0.84, about 165,300 Btu per hour. The existing supply is 3/4 in steel, 60 ft developed length, sharing the meter with a furnace, a water heater and a range. The adopted code's capacity row at that size, length and drop column does not carry the requested larger unit on top of that connected load. It does carry a 250,000 Btu per hour input model, which also clears the 165,300 the heat-up math asks for with margin for surface loss. The upsize is refused in writing before the order, and the customer is told a cover buys more held temperature than any heater on that gas line will.

Steps 4 through 7 pass: clearances measured and met at the existing location, vent arrangement per the instructions, sediment trap fitted, leak solution clean at every joint over five minutes.

Step 8 fails. Fired alone, inlet pressure sits exactly at the plate minimum with no margin. With the furnace, water heater and range all firing, it reads 1.6 in w.c. BELOW the plate minimum. The paper row passed and the field measurement did not, which is the whole reason this reading is taken under simultaneous load. The stop rule is taken as written: burner off, manual cock closed, appliance left off, customer notified in writing that afternoon, gas fitter booked. The fitter finds an undersized regulator serving the run and replaces it.

Recommissioned two days later: inlet holds above the plate minimum with every appliance firing, manifold matches the plate, combustion is inside the heater service SOP's acceptance, and temperature rise agrees with 250,000 Btu per hour input at 84 percent, which is 210,000 delivered, against the measured flow. Both inlet readings go on the record, the failing one and the passing one, with the regulator change between them.

References

• NFPA 54 / ANSI Z223.1, National Fuel Gas Code, in the edition your authority having jurisdiction has adopted, which reaches you through the permit, for pipe sizing tables, sediment traps and pressure testing.
• The appliance rating plate and the manufacturer's installation instructions for input, efficiency, minimum inlet pressure, manifold pressure, clearances and vent arrangement.
• NEC Article 680, in the edition your jurisdiction has adopted, for bonding of pool heating equipment.
• 29 CFR 1910.333(b)(2) for electrical work practices at the disconnect.
• See related: the heater service and combustion check SOP for the combustion measurement and protective chain; the heat pump pool heater SOP for the non-combustion alternative.