Low Pressure After a New Water Heater
The Short Answer: Day-one low pressure after a heater swap is almost never “buy a bigger tank.” It is isolation or shutoff valves left at 45°, solder/PEX shavings in the inlet screen or aerators, a 1/2” flex on 3/4” ports, a PRV cranked down, or a tankless board throttling GPM. Gauge the house. If cold is strong and hot is weak, the restriction is in the new work.
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Search Intent & The Real Problem
The old tank came out Friday. Saturday the shower is a trickle. That is an install defect until proven otherwise. A tank does not boost municipal psi. A tankless adds a heat exchanger, two service valves, and a filter — so flow drops if those parts are wrong. Long-term tankless restriction is a different article: tankless pressure drop, Rinnai low pressure. This page is the job that was just finished.
What other guides get wrong
They lead with a booster pump and a “higher capacity” heater. Capacity is gallons of stored heat, not psi. They also blame the T&P valve for low shower pressure. A dripping T&P wastes water; it does not throttle a cartridge. Flushing a brand-new tank to “remove sediment” will not open a half-closed ball valve.
Step-by-Step Troubleshooting
Step 1: Cold versus hot, one fixture versus whole house
| What you feel | Meaning |
|---|---|
| Cold strong, hot weak | Heater path: valves, flex, screen, mixing valve |
| Cold and hot both weak | PRV, meter washer, well, softener, main not fully open |
| One fixture only | Aerator or cartridge packed with install debris |
| Fine yesterday, dead today | Valve left closed or a connector collapsed |
- Required tools: Hose-bib gauge, two faucets.
Static at an outside bib should sit about 40–80 psi. Under 40 psi many tankless units will not activate. Over 80 psi you will also fight T&P drips — that is a PRV, not the new heater.
Step 2: Every valve the installer touched
- What to look for: Cold inlet on the tank not parallel to the pipe. Tankless isolation valves at 45°. A water-softener bypass left half-open. A new PRV on the house main closed down “so the T&P would not drip.”
- Required tools: Eyes. Open ball valves fully.
I restore more “dead” post-install showers with a quarter-turn than with a new exchanger.
Step 3: Debris and undersized connectors
- What to look for: Teflon, solder balls, PEX shavings in faucet aerators and in a tankless inlet filter. A 1/2” stainless flex on 3/4” nipples because that is what was on the truck. Push-to-connect fittings that did not go to depth — see SharkBite connector problems.
- Required tools: Bucket, aerator key, flashlight.
Clean the nearest aerator first. If it is packed with metal chips, every fixture will be.
Step 4: Mixing valve, dip tube, and “it feels like no pressure”
- What to look for: A new mixing valve on the hot outlet set to 105°F so people open the tap wider (more GPM, same heat). That feels like low pressure. A missing dip tube makes short hot then cold — that is temperature, not psi.
- Required tools: Contact thermometer on tank/outlet vs shower.
Step 5: Tankless-specific day-one throttling
- What to look for: Unit fires, then flow dumps when a second fixture opens. Inlet water 40°F and a 7 GPM heater asked to make 120°F. That is physics, not a clog. Isolation valves and the screen still come first.
- Required tools: Inlet-temp reading, honest GPM count.
If the trickle started months later, leave this page and use the tankless pressure-drop sequence.
When DIY Ends: Calling a Licensed Pro
Stop if static is over 80 psi, you have no isolation valves on a tankless, a flex is scorching-hot and collapsed, or the closet is wet. You may open valves, clean aerators, and rinse a customer-accessible inlet screen. PRV replacement, heat-exchanger work, and any gas-line resize are licensed. A heater that trickled on hour one was often piped in 1/2” or left with the service valves at 45° — that is a callback, not a new unit.
Frequently Asked Questions
Q: Why is my water pressure low after installing a water heater? A: Closed or half-open valves, install debris in screens, an undersized flex, a cranked PRV, or a tankless throttling flow. A new heater does not raise city pressure.
Q: Will a bigger water heater fix low pressure? A: No. Tank size is stored gallons. Pressure is the supply, the valves, and the pipe diameter. Fix those before you change BTU or gallons.
Q: Can I cap the T&P valve to get more pressure? A: Never. Capping a T&P is a scald and rupture hazard. It will not restore shower flow.