Table of Contents
- Quick Summary
- What Is Water Pressure and How Is It Measured?
- How to Measure Your Home's Water Pressure
- RO System Pressure Requirements
- Whole-House Filter Pressure Drop
- Water Softener Pressure Impact
- Solutions for Low Water Pressure
- Solutions for High Water Pressure
- Municipal vs. Well Water Pressure
- Recommended Pressure Management Products
- FAQ
Quick Summary
Residential water pressure should fall between 40-80 PSI, with 45-65 PSI being the ideal range for most homes. Below 40 PSI, showers feel weak, appliances fill slowly, and reverse osmosis systems produce inadequate water. Above 80 PSI, you risk pipe damage, leaks, and voided appliance warranties. Each filtration component adds pressure drop: sediment filters (1-2 PSI), carbon blocks (3-5 PSI), UV sterilizers (2-3 PSI), and water softeners (5-15 PSI). A 3-stage whole-house system can reduce pressure by 10-15 PSI total, which must be accounted for when sizing. RO systems need minimum 40 PSI inlet pressure (60-80 PSI optimal) - below 40 PSI, a booster pump ($150-250) is required. Well water systems operate on pressure tank cycles of 40-60 PSI (cut-in/cut-out), which may need adjustment for filtration compatibility.
What Is Water Pressure and How Is It Measured?
Water pressure is the force that pushes water through your plumbing system, measured in pounds per square inch (PSI) or bar (1 bar = 14.5 PSI). Pressure is created by elevation (gravity-fed systems), municipal pumping stations, or private well pumps. Understanding your pressure is essential because every filtration system - from a simple faucet filter to a whole-house multi-stage setup - either requires minimum pressure to function or reduces pressure as water passes through it.
PSI vs. GPM: The Critical Distinction
Water pressure (PSI) and flow rate (GPM - gallons per minute) are related but distinct. PSI is the force driving water; GPM is the volume that actually flows. A system can have adequate PSI but insufficient GPM if pipes are undersized, filters are clogged, or fixtures are flow-restricted. Conversely, high GPM at low PSI produces a weak, flowing stream rather than a forceful spray. When evaluating filtration systems, both metrics matter: the filter specifications list pressure requirements (minimum PSI), while your experience is determined by the resulting GPM at your fixtures.
Residential Pressure Standards
The International Plumbing Code (IPC) and most local codes require residential water pressure to be maintained between 40-80 PSI. The optimal range for comfort and equipment longevity is 45-65 PSI. Here's how different pressure levels affect your home:
| PSI Range | Classification | Effects |
|---|---|---|
| Below 30 | Very Low | Toilets may not flush properly. Shower is a trickle. RO systems won't produce water. Most appliances fail to function. |
| 30-40 | Low | Shower performance is poor. Washing machine fill times are excessive. RO output is severely reduced. Faucet flow is weak. |
| 40-50 | Acceptable | Adequate for most fixtures. RO works but below optimal. Some users find showers marginally acceptable. |
| 50-65 | Ideal | Strong showers, fast appliance fill, optimal RO performance. Best balance of comfort and pipe safety. |
| 65-80 | High-Normal | Very strong flow. Acceptable but approaching the stress threshold for older plumbing and some appliances. |
| 80-100 | Excessive | Pipe stress, leak risk at joints, water hammer noise. Voids warranties on many appliances and water heaters. Install PRV immediately. |
| Above 100 | Dangerous | Risk of pipe rupture, significant leak damage, appliance failure. Immediate PRV installation required. |
How to Measure Your Home's Water Pressure
Accurate pressure measurement requires only a simple gauge and takes less than 5 minutes.
Tools Needed
- Hose Bib Gauge ($8-15): A pressure gauge that threads onto any standard 3/4" outdoor hose spigot. Look for a gauge with a 0-200 PSI range and a lazy hand (peak pressure indicator) that records the highest pressure reached - useful for detecting nighttime pressure spikes from municipal systems.
- Multi-Test Gauge ($15-25): Includes adapters for faucet threads and washing machine supply valves in addition to hose bib threading. Useful for measuring pressure at multiple points in the home.
Choose an outdoor hose spigot closest to where your main water line enters the home - this gives the reading with the least interference from internal plumbing. If you have a pressure reducing valve (PRV), measure on the house side of the PRV to see what your fixtures actually receive. Wrap the gauge threads with 2-3 layers of plumber's tape and thread onto the hose bib hand-tight, then snug 1/4 turn with a wrench. Do not overtighten - brass gauge threads strip easily.
Ensure no water is running anywhere in the home (no toilets filling, no ice maker cycling, no dishwasher running). Open the hose bib fully and read the gauge. This is your static pressure - the pressure in the pipes when no water is running. Record this value. Typical municipal static pressure: 50-80 PSI. Typical well system static pressure: 40-60 PSI (or whatever your pressure tank is set to).
With the gauge still attached, turn on a nearby faucet or run the washing machine. Watch the gauge reading drop - this is your dynamic pressure (also called working pressure). The difference between static and dynamic pressure indicates how much your plumbing system loses under load. A drop of 5-10 PSI is normal. A drop of 20 PSI indicates flow restrictions: possibly undersized pipes, a partially closed valve, or significant pipe corrosion in older galvanized systems.
If you suspect pressure varies across the home, use a multi-test gauge to measure at an upstairs bathroom faucet and the washing machine supply valve. Pressure typically drops 3-5 PSI per floor in multi-story homes due to elevation (water loses approximately 0.43 PSI per foot of vertical rise). Large pressure differences between floors may indicate undersized riser pipes or partially blocked supply lines.
Reverse Osmosis System Pressure Requirements
Reverse osmosis systems are the most pressure-sensitive filtration devices in residential use. The RO membrane requires adequate inlet pressure to force water through the semi-permeable membrane against osmotic pressure.
Minimum Pressure: 40 PSI
At 40 PSI inlet pressure, a standard RO system produces water, but output is slow (approximately 50% of rated GPD capacity) and rejection rates may be 5-10% lower than optimal. Below 40 PSI, production becomes impractical - the storage tank fills slowly or not at all, and TDS rejection drops below acceptable levels. If your home's pressure measures below 40 PSI at the kitchen sink, you need a booster pump.
Optimal Pressure: 60-80 PSI
In the 60-80 PSI range, RO systems operate at peak efficiency. A standard 75 GPD membrane produces 70-75 gallons per day, rejection rates reach 95-98%, and the storage tank refills in 2-3 hours after depletion. This is the range most manufacturers design for and warranty against.
Maximum Pressure: 100 PSI
Standard RO systems specify a maximum inlet pressure of 80-100 PSI. Above 100 PSI, the membrane housing, fittings, and storage tank bladder experience excessive stress. Leaks develop at push-fit connections, the auto-shutoff valve may fail to close properly, and membrane life shortens due to compaction of the thin-film composite layer. If your inlet pressure exceeds 80 PSI, install a pressure regulator upstream of the RO system to reduce pressure to 60-70 PSI.
Booster Pumps for Low Pressure
An RO booster pump ($150-250) increases inlet pressure by 30-40 PSI using a diaphragm pump powered by a transformer plugged into a standard outlet. The pump installs in-line between the cold water supply and the RO unit. Most booster pumps include a pressure switch that activates only when the RO faucet is opened, preventing continuous operation. Popular models include the Aquatec CDP-6800 and the Permeate PP-600, both delivering up to 90 PSI output from as low as 30 PSI input. Booster pumps are essential for well water systems operating at 40-50 PSI and for homes with pressure at the lower end of municipal ranges.
Permeate Pumps for Efficiency
A permeate pump ($60-100) is not a pressure booster - it uses the energy of the RO drain water to reduce back-pressure on the membrane, improving efficiency without electricity. Permeate pumps increase production by 30-50% and reduce wastewater by up to 80% at any given inlet pressure. They are particularly valuable at lower pressures (40-55 PSI) where they provide production improvements similar to a booster pump at lower cost and without electrical wiring. The Aquatec ERP-1000 is the standard permeate pump used with most residential RO systems.
Whole-House Filter System Pressure Drop
Every filtration stage introduces resistance that reduces water pressure. When designing a whole-house system, you must account for cumulative pressure drop across all stages to ensure adequate pressure remains at your fixtures.
Pressure Drop by Filter Type
| Filter Stage | Pressure Drop (PSI) | Factors Affecting Drop |
|---|---|---|
| Sediment filter (5-20 micron) | 1 - 2 PSI | Higher at new (when clean and flows fastest) versus 3-4 PSI when clogged. Micron rating matters - 1 micron drops more than 20 micron. |
| Carbon block (GAC) | 3 - 5 PSI | Denser carbon blocks (higher quality) drop more pressure. Drop increases as carbon adsorbs contaminants and surface pores clog. |
| Carbon granule (GAC loose bed) | 1 - 3 PSI | Less drop than carbon block but also less contact time and potential for channeling. |
| UV sterilizer | 2 - 3 PSI | Drop comes from the quartz sleeve and chamber geometry, not from filtration. Relatively constant over lamp life. |
| KDF media (copper-zinc) | 2 - 4 PSI | Depends on granule size and bed depth. Increases as media oxidizes and develops surface coating. |
| Sub-micron filter (0.5-1 micron) | 4 - 8 PSI | Fine mechanical filtration creates significant flow resistance. These filters clog faster and require more frequent replacement. |
Cumulative Pressure Drop: Sizing Your System
A typical 3-stage whole-house system (sediment carbon block carbon block or UV) introduces 7-12 PSI of pressure drop when filters are new, and 12-18 PSI when filters are approaching replacement. If your inlet pressure is 60 PSI, a loaded 3-stage system could leave only 42-48 PSI at your fixtures - still acceptable but noticeably weaker than unfiltered pressure.
For homes with inlet pressure below 50 PSI, consider these strategies to minimize pressure loss:
- Use oversized filter housings: A 20" x 4.5" "Big Blue" housing has 4- the surface area of a standard 10" x 2.5" housing, cutting pressure drop by 50-70% for the same filtration rating.
- Choose higher micron ratings where appropriate: A 5-micron sediment filter catches most visible particles with half the pressure drop of a 1-micron filter. Use 5-micron as a pre-filter and save sub-micron filtration for point-of-use.
- Use GAC instead of carbon block for initial carbon stages: Loose granulated activated carbon has lower pressure drop than bonded carbon block, though it requires a heavier housing to contain the media.
- Limit stages: Each stage adds drop. A 2-stage system (sediment carbon) loses less pressure than a 4-stage system with equivalent total filtration capacity.
Water Softener Pressure Impact
Water softeners add 5-15 PSI of pressure drop across the resin bed, depending on several factors:
Resin Bed Depth and Tank Size
A standard 9" - 48" softener tank (32,000 grain capacity) with 1.0 cubic foot of resin creates approximately 8-12 PSI of drop at 8 GPM flow. Larger 12" - 52" tanks (48,000 grains) with 1.5 cubic feet drop 6-10 PSI at the same flow because the wider diameter reduces flow velocity through the bed. The relationship is inverse: wider tanks with shallower beds have less pressure drop but require more frequent regeneration for the same hardness removal.
Flow Rate Through the Softener
Pressure drop increases with the square of flow rate (approximately). At 4 GPM (single shower), drop might be 4 PSI. At 12 GPM (multiple simultaneous showers dishwasher), drop increases to 12-15 PSI. Size your softener not just for grain capacity but also for peak flow rate. A family of four needs a softener rated for at least 8-10 GPM service flow to avoid pressure problems during morning rush hours.
Resin Condition
As ion exchange resin ages (typically 8-12 year lifespan), it can become fouled with iron, sediment, or organic coatings that increase resistance. Resin beads can also fragment over time, creating fines that pack at the bottom of the tank and restrict flow. If pressure drop across your softener increases gradually over years, resin replacement or cleaning with resin restorer (sodium bisulfite or citric acid solution) may be necessary.
Bypass Valve Consideration
Install a bypass valve around your softener (most units include one). This allows you to maintain water to the house during softener maintenance, resin replacement, or if the softener develops a flow restriction. During bypass, water hardness returns but pressure is fully restored - useful as a diagnostic tool if you suspect the softener is causing pressure problems.
Solutions for Low Water Pressure
Solution 1: Booster Pump ($200-400 installed)
A whole-house booster pump installs on the main water line and increases pressure throughout the home. Shallow well jet pumps and municipal booster pumps (Grundfos MQ3-45, Davey BT14-30) deliver 30-60 PSI boost and activate on demand via a flow switch. For municipal water, choose a pump with a storage tank/pressure combination (constant pressure system) to avoid pulsating flow. For well water, a constant pressure variable-speed pump (Grundfos SQE, Goulds AquaBoost) maintains steady pressure regardless of demand. Installation requires electrical wiring and potentially a pressure tank - hire a professional unless you are experienced with plumbing and electrical work.
Solution 2: Replace Clogged or Old Pipes
Homes built before 1960 may have galvanized steel pipes that corrode internally, reducing pipe diameter and creating flow restrictions. A 3/4" galvanized pipe with heavy internal corrosion may have an effective diameter of 1/2" or less. Replacing with copper or PEX restores original flow capacity. This is expensive ($3,000-8,000 for whole-house repipe) but permanently solves pressure and flow issues while eliminating lead joints and corrosion byproducts.
Solution 3: Remove Unnecessary Restrictions
Check for partially closed main shutoff valves (the valve should be fully open - turned counterclockwise until it stops), pressure reducing valves that are set too low or failing closed, and clogged sediment traps or pre-filters on the main line. A PRV that has failed in the closed direction can reduce pressure to 30 PSI or lower even with 80 PSI municipal supply. PRV replacement costs $100-200 in parts and 1-2 hours of labor.
Solution 4: Increase Pipe Diameter
If your home has 1/2" supply pipes serving multiple bathrooms, upgrading the main trunk to 3/4" or 1" reduces flow velocity and pressure drop. This is most cost-effective during renovations when walls are already open. For a single problematic fixture, replacing the fixture supply line with a larger diameter braided stainless steel line may help marginally.
Solutions for High Water Pressure
Install a Pressure Reducing Valve (PRV) - $100-200
A PRV (also called a pressure regulator) installs on the main water line where it enters the home, after the main shutoff valve and before any branches. It uses a spring-loaded diaphragm to reduce incoming pressure to a user-adjustable setpoint (typically 50-60 PSI). PRVs protect plumbing, appliances, and filtration systems from pressure spikes and consistently high pressure.
Select a PRV rated for your pipe size (3/4" or 1" NPT for most homes) and maximum inlet pressure (most handle up to 150 PSI inlet). Brands like Watts LF25AUB-Z3, Cash Acme EB-45, and Zurn Wilkins 600XL are reliable choices with 5-10 year lifespans. Choose a lead-free brass model to comply with current plumbing codes.
Turn off the main water supply. Cut the pipe 6-12 inches downstream of the main shutoff valve. Install the PRV with an arrow indicating flow direction pointing toward the house (not toward the street). Include unions on both sides for future replacement. Install a pressure gauge on the house side of the PRV to monitor adjusted pressure. Turn water back on and check for leaks.
Most PRVs have an adjustment nut or screw on top. Turning clockwise increases outlet pressure; counterclockwise decreases it. With a gauge installed downstream, adjust to 55-60 PSI for optimal home performance. Check pressure at multiple fixtures to confirm consistent delivery. The adjustment may take several iterations - small turns (1/4 turn) produce 5-10 PSI changes.
Municipal Water vs. Well Water Pressure Systems
Municipal (City) Water Pressure
City water arrives at your home under pressure created by pumping stations and elevated storage tanks (water towers). Typical pressure at the street main: 50-100 PSI, though some systems deliver as high as 150 PSI in low-elevation areas near the treatment plant. Municipal pressure fluctuates with neighborhood demand - lowest during morning and evening peak usage, highest overnight. The city is responsible for maintaining adequate pressure at the meter; you are responsible for managing pressure within your home via PRVs if needed. Municipal water rarely requires booster pumps unless you are at a high elevation within the service area or the main pressure is chronically low.
Well Water Pressure Systems
Private wells use a submersible pump (or jet pump for shallow wells) combined with a pressure tank to deliver water on demand. The system operates on a pressure switch with two setpoints: cut-in (pump turns on, typically 30-40 PSI) and cut-out (pump turns off, typically 50-60 PSI). This means well water pressure fluctuates between these two values constantly - when the tank is full, you have 50-60 PSI; as water is used, pressure drops to 30-40 PSI before the pump cycles on again.
For filtration compatibility, well systems present unique challenges:
- Low-end pressure: At the cut-in pressure (30-40 PSI), some whole-house filters and RO systems operate at their minimum or below. A booster pump or constant-pressure pump system may be needed.
- Cycling: The pressure fluctuation causes variable flow rates through filters, which can lead to inconsistent filtration performance and accelerated media exhaustion.
- Pressure tank sizing: An undersized pressure tank causes rapid pump cycling (short cycling), which damages the pump and creates pressure pulses. Minimum recommended tank size: 20 gallons drawdown for a typical residential system.
- Constant pressure conversion: Converting a standard well system to constant pressure (using a variable frequency drive pump like the Grundfos SQE) maintains steady 55-60 PSI regardless of demand, dramatically improving filtration performance and eliminating pressure fluctuations. Cost: $800-1,500 installed.
Recommended Pressure Management Products
MEASURETOOL Water Pressure Test Gauge (0-200 PSI with Lazy Hand)
$10 - $14
3/4" female hose thread gauge with 0-200 PSI range and red peak-pressure indicator hand. Chrome-plated brass case with 2.5" face for easy reading. Accuracy: -3-2-3%. The lazy hand records the highest pressure reached since last reset - leave it attached overnight to capture municipal pressure spikes. Includes Teflon tape. Essential diagnostic tool for any homeowner with filtration systems.
Watts LF25AUB-Z3 Pressure Reducing Valve (3/4")
$85 - $120
Lead-free bronze PRV with integral strainer, thermal expansion bypass, and stainless steel adjustment stem. Adjustable outlet range: 25-75 PSI. Maximum inlet: 300 PSI. Includes union connections for easy installation and future service. Rated for 400,000 cycle life. Suitable for municipal water up to 150 PSI inlet. 3/4" NPT female threads. Meets ANSI/NSF 372 low-lead standard. A professional-grade PRV that will protect your entire plumbing and filtration system for 10 years.
Aquatec CDP-6800 RO Booster Pump
$150 - $190
Diaphragm booster pump designed specifically for reverse osmosis systems. Increases inlet pressure by 30-40 PSI using a 115V transformer. Flow-activated via integrated pressure switch - runs only when RO faucet is open. Maximum output: 90 PSI. Compatible with standard 1/4" and 3/8" RO tubing. Noise level: 45 dB (comparable to refrigerator hum). Includes pump, transformer, mounting bracket, and hardware. Essential for RO installations with inlet pressure below 45 PSI. 2-year manufacturer warranty.
Aquatec ERP-1000 Permeate Pump
$65 - $85
Non-electric pump that uses RO wastewater energy to reduce membrane back-pressure. Improves production rate by 30-50% and reduces wastewater ratio from 4:1 to 1:1 or better. Works at any inlet pressure but provides the most noticeable improvement at 40-55 PSI where RO performance is marginal. Quick-connect fittings for 1/4" tubing. No electricity required - purely mechanical operation. Pays for itself in reduced water waste within 1-2 years for households with high RO usage.
Our Methodology
Every product on Filter Tested undergoes 4-6 months of research-based analysis in real-world conditions. We verify all manufacturer claims against independent lab results and NSF certification databases. Products are scored across 8 categories including filtration performance, flow rate, certifications, installation complexity, and total cost of ownership. Learn more about how we test.
Related Reading
Frequently Asked Questions
Most whole-house filtration systems require minimum inlet pressure of 30-40 PSI to function. However, practical considerations matter more than manufacturer minimums. At 40 PSI inlet, a 3-stage whole-house system with 10-15 PSI pressure drop leaves only 25-30 PSI at your upstairs shower - an unsatisfactory experience. For acceptable fixture performance with a whole-house filter, you need minimum 45 PSI static pressure at the filter inlet, which allows 30 PSI at fixtures after filter drop. Below 45 PSI, consider point-of-use filters (under-sink, faucet-mounted) instead of whole-house, or install a booster pump upstream of the filtration system. If you already have a whole-house filter and experience low pressure, try switching to 20" Big Blue housings (lower pressure drop), reducing micron ratings (5 micron instead of 1 micron), or removing one stage if water quality allows.
Yes - every filter creates some pressure drop. The amount depends on filter type, micron rating, media density, flow rate, and how long since the filter was changed. A clean 5-micron sediment filter in a standard 10" housing drops 1-2 PSI at 5 GPM. A 0.5-micron carbon block drops 4-8 PSI at the same flow. A loaded (dirty) filter can drop 2-3- as much pressure as a clean one. For a typical 3-stage whole-house system with clean filters, expect 7-12 PSI total drop. The key is ensuring your inlet pressure is high enough to accommodate this drop while still delivering acceptable pressure at fixtures. If your inlet is 60 PSI and your system drops 12 PSI, you have 48 PSI remaining - adequate but noticeable. Monitor pressure quarterly and change filters when pressure drop increases by 50% from baseline.
Pressure drop under multiple simultaneous draws indicates either insufficient pipe capacity or a flow restriction somewhere in the supply path. When you open one faucet, water flows through the path of least resistance at moderate velocity. When you open additional fixtures, the total flow demand increases, water velocity increases in the main supply pipe, and friction losses increase exponentially (pressure loss is proportional to the square of flow velocity). If your main supply line is undersized (1/2" serving multiple bathrooms), or if a filter, softener, or partially closed valve creates a bottleneck, the pressure at each fixture drops dramatically under combined load. Solutions: upgrade main supply to 3/4" or 1", install a larger pressure tank (well systems), clean or replace clogged filters, or install a booster pump to increase total system capacity.
If installing a booster pump is not practical, several alternatives can improve RO performance: (1) Install a permeate pump ($65-85) - non-electric, uses drain water energy to improve production by 30-50% and works at any pressure. (2) Increase storage tank pre-charge - the air bladder in the RO tank should be pressurized to 5-7 PSI when empty. If it has leaked down to 2-3 PSI, less water stores in the tank and delivery pressure is weak. Check with a tire gauge and inflate to 7 PSI. (3) Remove flow restrictor - RO systems have a flow restrictor on the drain line that controls wastewater ratio. For low-pressure installations, replacing the standard restrictor with a lower-ratio restrictor reduces back-pressure on the membrane. Check with your RO manufacturer for low-pressure restrictor options. (4) Upgrade to a tankless RO system - models like the Waterdrop G3P800 have built-in pumps and do not rely on storage tank pressure, delivering consistent flow regardless of inlet pressure (within their operating range).
Municipal water pressure spikes have several causes: (1) Tower fill cycles - elevated storage tanks fill during low-demand hours (2-4 AM), temporarily increasing distribution pressure by 10-20 PSI. (2) Zone valve switching - utilities reconfigure distribution zones during maintenance, which can redirect high-pressure flow into normally lower-pressure areas. (3) Fire hydrant testing - flushing hydrants creates transient pressure waves that travel through the pipe network. (4) Water main breaks and repairs - after a main repair, air pockets and flow reconfiguration cause temporary pressure surges. (5) Thermal expansion in your home - your water heater warms cold water, causing it to expand. With a PRV or check valve preventing backflow to the street, this expansion increases pressure in your home's pipes by 10-40 PSI above the street pressure. Install a thermal expansion tank ($30-50) on the cold water inlet of your water heater to absorb this expansion and protect your plumbing.
Whole-house booster pumps typically increase pressure by 30-60 PSI, depending on the model and flow rate. The Aquatec CDP-6800 (designed for RO systems) adds 30-40 PSI at 1-2 GPM. Municipal booster pumps like the Grundfos MQ3-45 deliver up to 65 PSI boost at 15-25 GPM, suitable for whole-home service. Variable-speed constant-pressure pumps maintain a set outlet pressure (e.g., 60 PSI) regardless of demand from a single dripping faucet to multiple showers running simultaneously. When sizing a booster pump, calculate your needed output pressure (target 55-60 PSI at fixtures) minus your current inlet pressure. If you have 35 PSI inlet and want 60 PSI, you need a 25 PSI boost. Add 5-10 PSI for filter/softener pressure drop if installed downstream of the pump. Always install a pressure tank with the booster to prevent short cycling and provide buffer capacity.
Tankless (on-demand) water heaters create flow restrictions that can affect filtration systems installed on the same line. Most tankless units have internal flow restrictors that limit flow to 3-5 GPM for gas units or 1.5-2.5 GPM for electric units, creating 2-8 PSI of pressure drop depending on flow demand. If your whole-house filter is installed upstream of a tankless heater, the combined pressure drop of filter heater may cause insufficient flow to activate the heater or deliver adequate hot water pressure. Best practice: install whole-house filters on the cold water main before the line splits to the water heater, or use a dedicated filter line for cold drinking water that bypasses the tankless heater. For RO systems (which feed from cold lines only), tankless heaters have no direct impact unless they create back-pressure through shared plumbing.