Quick answer
The correct pool equipment plumbing order is pump → filter → heater → chlorinator (or chemical feeder), with any solar panels plumbed in before a gas heater or heat pump rather than after. Filtering before heating keeps debris out of the heat exchanger, and placing the chlorinator last — with a check valve between it and the heater — keeps freshly generated, highly concentrated chlorine from sitting against the heater’s metal components and corroding them. Getting this sequence backward is one of the few plumbing mistakes that can quietly damage a $2,000-$7,000 heater over months rather than failing obviously on day one.
The full equipment order, and why each step matters
| Order | Equipment | Why it goes here |
|---|---|---|
| 1 | Pump | Pulls water from the pool; everything downstream depends on flow rate from here |
| 2 | Filter | Removes debris before it reaches the heat exchanger's narrow internal passages |
| 3 | Solar panels (if installed) | Works on temperature difference vs. incoming water — most effective on the coolest water in the loop |
| 4 | Gas heater or heat pump | Raises water temperature after filtering (and after solar has already done what it can) |
| 5 | Check valve | Blocks chlorinated/acidic water from draining backward into the heater when the pump stops |
| 6 | Salt chlorinator or chemical feeder | Generates/doses chlorine last, so it reaches the pool fresh instead of sitting against heater metal |
Two sequencing rules do almost all the work here. First, filter before you heat — sending unfiltered water through a heater is a documented way to clog the heat exchanger and void the manufacturer’s warranty, which is why every major brand’s install manual puts the filter upstream of the heater, not downstream. Second, sanitize last — a salt chlorinator or liquid/tablet feeder dosing acid or chlorine needs to sit after the heater so concentrated chemical never has a reason to contact the heat exchanger before it’s diluted into the rest of the pool’s water volume.
If you’re running a bypass valve to protect the heater from a pump that outflows its rated maximum, that valve gets cut into the plumbing immediately around the heater itself — see that guide for sizing — it doesn’t change where the heater sits in the overall pump-filter-heater-chlorinator sequence.
Why the chlorinator goes last, specifically
A salt chlorinator generates chlorine directly at the cell, and that freshly generated chlorine is corrosive to metal at the concentrations present right at the source — plumbing the cell before the heater routes that concentrated output straight through the heat exchanger. Plumbing it after the heater instead means already-heated water passes through the heat exchanger clean, and the chlorine only gets generated once that water is on its way back to the pool, where it dilutes into the much larger water volume instead of sitting against heater metal.
The same logic applies to a liquid or tablet chemical feeder dosing chlorine or acid directly into the plumbing — dosing has to happen downstream of the heat exchanger for the same corrosion-avoidance reason, not just with salt systems.
Hayward AquaRite Salt Chlorination System
- If you're adding a salt system to an existing gas or heat-pump setup, cut the cell in after the heater, not before — see the check-valve section below before finishing the plumbing.
- Pairs with any heater in our pool heat pump picks or gas heater picks.
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The check valve: what it does and where it goes
Most heater manufacturers require a corrosion-resistant swing check valve on the return side, installed between the heater and the chlorinator, with its flow arrow pointing away from the equipment pad toward the pool. The reason is backflow, not forward flow: once the pump shuts off, residual water in the plumbing between the chlorinator and the heater can sit motionless rather than draining, and per pool-service forum reporting (TroubleFreePool), even a gallon or two of water carrying a high chlorine concentration sitting against the heat exchanger between pump cycles is enough to cause measurable corrosion over a season. The check valve physically blocks that backflow path. Some installers use a Hartford-loop-style plumbing arrangement as a substitute, but a dedicated check valve is the simpler, more commonly specified fix, and it’s inexpensive relative to the cost of a corroded heat exchanger — see our heat exchanger replacement picks for what that repair costs if the corrosion isn’t caught early.
Where solar panels fit if you have them
If your setup includes solar panels alongside a gas heater or heat pump, the solar panels go before the gas/heat-pump unit in the sequence, not after — every major solar pool-heating manufacturer’s manual specifies this. Solar heat transfer depends on the temperature difference (delta-T) between the water entering the panel and the panel’s own surface temperature; sending already-heated water from a gas heater or heat pump up to the panels first narrows that temperature gap and can make the panels nearly ineffective, or in some conditions even cool water that a downstream heater would then have to reheat. A booster pump, if the panels sit on a roof and need extra pressure to get there, is plumbed in after the main pump and filter but before the panels — not after the gas heater.
FlowGuard Check Valve, 1.5-inch PVC
- Confirm the flow arrow points toward the pool return, not back toward the heater, before solvent-welding it in place.
- If you're adding this during a full re-plumb, our installation guide covers the rest of the union and pad setup.
The bottom line
The correct order is pump, filter, heater, then chlorinator or chemical feeder last — with solar panels (if present) plumbed before the gas heater or heat pump, not after. Filtering before heating protects the heat exchanger from debris, and sanitizing last — backed by a check valve between the heater and the chlorinator — keeps concentrated chlorine from corroding heater components during the pump’s off cycles. None of this shows up as an obvious failure the day it’s plumbed wrong; it shows up as a slowly corroding heat exchanger or a warranty claim denied months later, which is why it’s worth checking the sequence against this diagram before — not after — the unions are sealed.
Sources: InyoPools — What Order Should I Install My Pool Equipment, TroubleFreePool — Check valve between filter and heater, Florida Solar Design Group — Solar Pool Heater Plumbing: Before or After Gas and Electric Heaters