Expansion tanks: the $150–$350 line item your quote probably includes
It's the small grey tank on the cold line — and the most-questioned line on water heater quotes. Most of the time, it's a legitimate code requirement.
What it is and why code wants it
Heated water expands. In an open system the extra volume pushes back into the city main. But if your home has a check valve, backflow preventer, or pressure-reducing valve — most homes built or re-piped since the 1990s do — that expansion has nowhere to go. Pressure spikes stress the tank, trip the relief valve, and hammer fixtures. The expansion tank is an air cushion that absorbs it, and code requires one on these "closed systems" in most jurisdictions.
2026 cost
| Item | Cost |
|---|---|
| Expansion tank (unit) | $40 – $150 |
| Installed with a water heater replacement | $150 – $350 |
| Installed as a standalone visit | $250 – $450 |
Adding it during a replacement is the cheap moment — the plumber is already on the cold line. Standalone installs eat a service-call minimum.
When you can legitimately skip it
Two cases. First: your system is genuinely open — no check valve or PRV anywhere on the service line — and your jurisdiction doesn't mandate one regardless. Second: you already have one, it's correctly sized, and it holds its air charge (a knuckle-tap on the bottom half should ring hollow; a thud means it's waterlogged and due for replacement anyway — they last 5–10 years).
How to verify the claim
Ask the plumber to point at the check valve or PRV that closes your system. It's a 30-second show-and-tell at the meter or main shutoff. If they can point at it, pay for the tank without grumbling; it's protecting the new heater's warranty too — some manufacturers require one on closed systems.
Buying the tank yourself
The unit is a $40–$150 part, and the two names plumbers actually install are Amtrol and Watts. With an accessible ¾" tee on the cold line, a wrench and thread sealant, it's a fair DIY job — check the Amtrol ST-5 (covers most 40–50 gallon heaters) or the Watts DET-5 equivalent.
Prices and availability are accurate as of the date and time shown and can change. The price shown on Amazon at the time of purchase is the one that applies.
Two cautions. Set the air charge to your house pressure with a hand pump before installing — the factory charge is rarely right. And if the tank would hang off a long horizontal run without support, hand the job back to the plumber; a waterlogged tank on old pipe is its own failure mode.
Questions people ask
Is an expansion tank required when replacing a water heater?
In most jurisdictions, yes — whenever the home has a closed plumbing system (check valve, backflow preventer, or pressure-reducing valve on the service line). Expect $150–$350 installed during a replacement.
How long do expansion tanks last?
5–10 years. Tap the bottom half: hollow ring means healthy, dull thud means waterlogged and due for replacement.