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Heat & Tap
An assortment of threaded plumbing fittings

Water Heater Accessories and Install Parts

A water heater is a system, not a box. This is the part of the job people find out about halfway through.

By Brijon M., Owner and writer, MadGrey LLC·Last updated October 6, 2026·How we pick →

Four parts go in with most water heaters: an expansion tank (required by code on a closed system), a drain pan sized to the tank, an anode rod check, and for tankless a flush kit plus isolation valves. A recirculation pump fixes the wait at a distant fixture, and a leak detector is the cheapest insurance in the house.

This is the section of the site we think matters most, and it is the one almost nobody else covers properly. Search for any water heater and you will find a hundred reviews of the appliance. Search for what goes in with it and you find manufacturer PDFs and forum threads.

Which is strange, because this is where the surprises live. The expansion tank your inspector requires. The drain pan that turns out to be the wrong diameter. The isolation valves nobody fitted, which means the tankless unit cannot be flushed at all. None of these are expensive. All of them are annoying to discover on install day.

What goes in with what

Parts by water heater type
PartGas tankElectric tankHeat pumpTankless
Expansion tankIf closed systemIf closed systemIf closed systemUsually not — no stored volume to expand
Drain panRecommendedRecommendedRequired (condensate)Wall-hung pan if above finished space
Anode rodYes, check annuallyYes, check annuallyYesNo tank, no anode
Flush kitTank flush via drain valveSameSameYes, plus isolation valves
DescalerOccasionally, for elementsOccasionallyOccasionallyYes, annually
Recirculation pumpOptional comfortOptional comfortOptional comfortOften needed — fixes the ignition delay
Leak detectorRecommendedStrongly if above finished spaceStrongly — condensate tooRecommended
Seismic strapsRequired in seismic zonesRequired in seismic zonesRequired in seismic zonesNot applicable to a wall-hung unit

The parts, in order of how often they catch people out

Why the expansion tank is the one to understand

Water expands when heated. In an open system that expansion pushes back into the municipal main and nothing happens. In a closed system — one with a pressure-reducing valve, a check valve or a backflow preventer where the water enters the house — there is nowhere for it to go, so the pressure rises.

IPC Section 607.3 requires a thermal expansion tank on the cold water supply downstream of those devices in that situation, and specifically does not allow the relief valve to be used to control thermal expansion. Most modern houses have a pressure-reducing valve. Expansion tank code requirements, and how to size one.

The tankless-specific list

If you are having a tankless unit installed, insist on these at install time. Retrofitting them later costs more than fitting them now:

  1. Isolation valves on the hot and cold connections. Without them there is literally nothing to attach a flush hose to, and the annual descale becomes impossible.
  2. A condensate drain route, if the unit is condensing. A pump if gravity will not do it.
  3. A sediment or inlet screen you can reach, because that is the first thing to check when flow drops.
  4. A flush kit, bought at the same time so the first annual service actually happens. Flush kits.

The one part everyone skips and should not

The anode rod. It is what stands between the water and the steel, it is consumed over time, and replacing it is a single hour of work that can add years to a tank. In hard water, reported replacement intervals run from as little as 3 months to 18.

Nobody does it, which is why identical tanks in identical houses fail at 8 years and 15 years. Anode rods.

What goes in with it

A water heater is a system, not a box. These are the parts that go in with it, and the ones people discover they need halfway through the job.

  • Expansion tank

    Required by code on a closed system, which is most homes with a pressure-reducing valve.

  • Drain pan

    Cheap insurance anywhere the tank sits above something you care about.

  • Anode rod

    The part that decides when the tank leaks. Replacing it is the highest-value hour of maintenance there is.

Questions people actually ask

›What accessories do I need with a new water heater?
On a storage tank: an expansion tank if your plumbing is a closed system, a drain pan sized to the tank diameter, and new flexible connectors and a shutoff. On a tankless unit: isolation valves, a condensate route if it is condensing, and a flush kit.
›Is an expansion tank required by code?
Where a pressure-reducing valve, check valve or backflow preventer makes your plumbing a closed system, yes. IPC Section 607.3 requires a thermal expansion tank on the cold supply downstream of those devices and does not permit the relief valve to be used for the purpose.
›Do I need a drain pan under my water heater?
If the heater sits above anything that water would damage, yes, and codes commonly require one in those locations. On a heat pump water heater it is effectively mandatory because the unit produces condensate continuously.
›What is a flush kit and do I need one?
A pump, two hoses, a bucket and a descaling solution — everything needed to circulate descaler through a tankless heat exchanger. If you own a tankless water heater, yes: manufacturers recommend descaling annually for residential use.
›Are hot water recirculating pumps worth it?
If you wait more than about 20 seconds for hot water at a fixture you use daily, yes. A comfort pump on a timer draws about 25 watts while running and removes the wait and the water you currently run down the drain.
›How much do water heater accessories add to the job?
We do not publish prices, but the ranking is useful: an expansion tank and a drain pan are modest, a flush kit is a one-off that pays for itself the first time, a recirculation pump is more, and a whole-house automatic shutoff valve is the most expensive item on this hub.

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