Pool Heater Pressure Drop: How It Affects Pump Size

What pressure drop means in a heater listing

Pressure drop is the resistance a pool heater creates when water passes through it. Every heater contains an internal passage, a heat exchanger, a burner chamber or a set of collector panels, and those passages are narrower than the pipe that feeds them. The pump has to push against that resistance, and the water loses some of the pressure it gained at the pump. In a listing, that loss is described as pressure drop, usually measured in feet of head.

The same resistance appears in a pump listing under a different name. A pump's max head is the highest column of water it can push in feet. The heater's pressure drop adds to all the other resistance in the loop, the filter, the pipe runs, the fittings and the return fittings. When a pool owner reads a heater listing and a pump listing together, the pressure drop is the heater's share of the head the pump must overcome.

Where the pressure drop figure appears

The pressure drop figure is not usually in the heater's title. A heater title states the output in BTU or kW, the fuel or the heat pump type, and words like pool and spa. The pressure drop lives in the maker's spec sheet and manual, next to the heater's flow rate range.

A heater listing may state a minimum and a maximum flow rate in GPM. The minimum is the slowest flow that keeps the heater working, and the maximum is the fastest flow the heat exchanger can accept. The listing may also state a maximum working pressure. Those three figures, the flow range and the working pressure, are the numbers to bring to the pump question.

How heater types compare in listing terms

Gas and propane heaters use a compact heat exchanger above a burner. Their listing terms emphasize BTU input and the fuel name, and the internal passages are short. The pressure drop figure on the spec sheet is the heater's share of the loop resistance. A natural gas pool heater listing and a propane pool heater listing read the same way.

Heat pumps move water through a larger internal coil inside a tall cabinet. The listing terms emphasize BTU output, air-source wording and COP. The water side of a heat pump has a larger coil, and the pressure drop appears on the spec sheet as part of the same water loop. The pool heat pump listing states the flow range the same way a gas heater does.

Solar heaters place the collector outside the pump loop. The listing describes panels, collectors, mats or domes, and the water travels through long panel passages. The pressure drop appears in the collector specifications, and the pipe run from the pump adds head of its own. A solar pool heater listing carries the collector figures, not a single pressure drop number.

How pressure drop changes the pump question

A pump listing shows a max head in feet and a flow range in GPM. A heater listing shows a flow range and a pressure drop. The two are read together: the pump has to deliver the heater's minimum flow while still overcoming the heater's pressure drop plus everything else in the loop. That combination, flow at a given head, is what a pump performance curve shows.

The performance curve is the working picture of a pump. A pump can move its maximum flow at low head, or push against high head at reduced flow, but not both at the same time. When a heater joins the loop, the working point moves up the curve. The matching question, whether a given pump can hold the heater's minimum flow at the loop's total head, is for the maker's sizing page and the installer.

Why the heater and pump are one system

A heater is never a stand-alone purchase in the listing sense. The heater's flow range assumes a pump on the same loop, and the pump's head figure assumes whatever resistance sits downstream. Reading a heater listing without the pump leaves the pressure drop figure without context. Reading a pump listing without the heater leaves the head figure short of the real loop.

This article cannot do the sizing math. Sizing a pump to a heater involves the total dynamic head of the whole loop, and that calculation belongs to the maker's sizing page. Hayward publishes a total dynamic head calculator on its site, and the Pentair heater pages describe how a heater is chosen with the pool's layout in mind. The pool owner's job is to collect the stated figures from both listings and hand them to the installer.

Where to get the figures and who decides the match

Every figure in this article comes from the listings and the maker's spec sheets. The pressure drop, the flow range and the working pressure are stated by the maker, and this site repeats them without conversion. A pool owner should never invent a pressure drop for a heater; the figure is either in the spec sheet or it is not stated.

The match between a pump and a heater is decided by the maker's sizing page and by a licensed installer. The installer brings the loop layout, the pipe sizes and the filter resistance, and the maker brings the heater's stated figures. A pool owner who is still choosing between heater types can compare the listing terms, but the final pump size is a system decision. A variable speed pool pump gives the most flexibility to adjust flow after the heater joins the loop, and the inground pool pump guide covers pump selection in detail.

What to pick for your situation

Questions

Does every pool heater add pressure drop to the loop?

Every heater contains internal passages narrower than the pipe that feeds it, so every heater adds some resistance. The amount is stated as pressure drop on the spec sheet. A listing that does not state it leaves the figure unknown.

Is pressure drop the same as feet of head?

Pressure drop and feet of head describe the same resistance. A pump listing states max head in feet. A heater spec sheet may state pressure drop in feet of head or in psi. The two are the same kind of figure, read together.

Which heater type adds the most resistance?

The listing terms describe the internal passages: compact heat exchangers for gas and propane, larger coils for heat pumps, long panel passages for solar. The stated pressure drop on each model is the only figure that matters, and this article does not rank types by measured resistance.

Where do you find a heater's pressure drop?

In the maker's spec sheet and manual, next to the flow rate range. A title usually states BTU or kW and the fuel or type, not the pressure drop.

Does adding a heater mean the pump must be larger?

Not by itself. The pump's max head and flow range, and the heater's new flow range and pressure drop, decide the match. The maker's sizing page and the installer determine whether the existing pump still fits the loop.

What is a heater's flow rate range?

The minimum and maximum GPM the heater accepts. The pump must hold at least the minimum flow while overcoming the heater's pressure drop and the rest of the loop's resistance.