What size pool will 135,000 BTU heat?

It suits pools up to roughly 15,000 gallons in mild climates, less in cold regions or with a spa attached. Larger pools need a higher-BTU model.

Is it a direct replacement for an older Hayward H-Series?

Often yes. The Universal H-Series shares the footprint and plumbing layout of many earlier H-Series heaters, but check the model and gas line size before ordering.

Does it need electricity as well as gas?

Yes. It uses electronic ignition and a blower, so it needs a 120V or 240V supply plus a natural gas line installed by a licensed technician.

Can it be converted to propane?

This W3H135FDN version is set up for natural gas. Propane use requires a conversion kit and a qualified installer.

How does it compare with a condensing heater?

Condensing heaters run more efficiently and cost less to operate, but they cost more upfront and need different venting. This unit is simpler and cheaper to install.

What maintenance does it need?

Clean or replace the filter regularly, keep the burner area clear, and have the unit serviced yearly. Low water flow is a common cause of pressure-switch faults.