Low Water Pressure After Installing a Water Heater
On this page (6 sections)
The Short Answer: If pressure dropped right after a new water heater went in, the cause is almost always something disturbed during installation: a shutoff valve left partly closed, debris in faucet aerators or a tankless inlet filter, or a connector that restricts flow. A water heater does not reduce supply pressure on its own. Compare hot and cold at the same faucet to locate the restriction.
Hot versus cold tells you where to look
| What you notice | Where the problem probably is |
|---|---|
| Cold is strong, hot is weak at every faucet | At or near the water heater: valves, connectors, tankless filter |
| Both hot and cold are weak everywhere | Main shutoff or pressure-reducing valve not fully reopened, or a supply issue |
| Only one or two faucets are weak | Debris in those aerators or cartridges |
| Pressure is fine at first, then drops when a second tap opens | Tankless flow limit or restriction; see below |
Common install-day causes
A valve not fully open. The installer closes the main shutoff, the heater’s cold-water inlet valve, and sometimes a water softener bypass. Any of them left partway open will cut flow. Ball valves are fully open when the handle lines up with the pipe.
Debris in aerators and filters. Shutting the water off and on dislodges scale and bits of pipe, solder or tape. They collect in faucet aerators, shower heads, and the inlet filter on tankless units. Aerators nearest the heater usually show it first.
Restrictive connectors or fittings. A connector or fitting smaller than the piping, a kinked flexible connector, or a push-fit fitting that was not seated fully can all restrict flow. See SharkBite water heater connector problems.
A new mixing valve. If the installer added a thermostatic mixing valve, the hot water at the taps may be cooler than before, so you open the tap further and it feels weaker. A restrictive or partly blocked mixing valve can also reduce flow.
A pressure-reducing valve adjusted down. If the relief valve was dripping and someone lowered house pressure to stop it, everything will feel weaker. Thermal expansion is a better thing to fix; see overflow keeps discharging.
Tankless installs
A tankless heater limits how much water it can heat at once. If you ask for more hot water than it can heat at your incoming water temperature, the unit may reduce flow to keep the temperature up, which feels like low pressure. Incoming water is colder in winter, so this is more noticeable then. Tankless units also need to be cleaned of installation debris at their inlet filter. Longer-term problems are covered in tankless water heater pressure drop and Rinnai low pressure.
What you can check safely
- Check that the main shutoff, the heater’s cold inlet valve, and any softener or filter bypass valves are fully open.
- Unscrew and rinse the aerator at a weak faucet. If it is full of grit, check the others.
- Read static pressure with a hose-bib gauge outside. If it is much lower than before the install, the main or pressure-reducing valve is the place to look.
- Call the installer. Day-one pressure problems are usually covered by the installation.
What the plumber will check
They will confirm every valve they touched is fully open, clean the tankless inlet filter and any strainers, check connector sizes against the heater’s connections, inspect push-fit fittings, and measure static and flowing pressure. If a mixing valve was added, they will check its setting and flow rating.
Frequently Asked Questions
Q: Will a bigger water heater fix low pressure? A: No. Tank size affects how much hot water is stored, not pressure. Pressure depends on the supply, valves, and piping.
Q: Why is only the hot water weak after the install? A: Because the restriction is in the hot-water path: the heater’s valves or connectors, a mixing valve, or debris that the hot-water lines carried to the faucets.
Q: Is it normal for pressure to be low for a few days after a new heater? A: No. It should be normal immediately once the air is purged from the lines. If it isn’t, something is restricting flow.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- A. O. Smith technical bulletin 45: thermal expansion (opens in a new tab)Background on pressure rise in a closed water system. Pressure testing and repairs are professional work.
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