Plenty of homeowners call after a repipe and ask why the shower runs hot then lukewarm, or why their tankless heater trips out when two faucets open at once. The piping upgrade did its job, but the water heater did not get the memo. Repipe plumbing changes the hydraulics of a home. That can be great for flow and longevity, yet it exposes mismatches with aging water heaters, undersized gas lines, or a recirculation loop that was never set up correctly. I have seen it play out dozens of times: new pipes, old assumptions. Let’s straighten out the most common questions so you get the hot water you paid for.
What a repipe actually changes inside your home
When a home is repiped, you are replacing the distribution network that carries water from the meter and heater to fixtures. The materials change, the pipe diameters often change, and sometimes the layout itself shifts from long branch runs to a central manifold with home runs. That affects two things you feel every day: pressure and temperature stability.
Copper, PEX, and CPVC each have their own friction profile. PEX has a smoother interior than old galvanized or mineral-choked copper, so it often delivers higher flow at the same pressure. Switching from a long, undersized 1/2 inch branch system to a PEX home-run manifold can shave 10 to 40 seconds off hot water wait times, depending on distances. It also helps fixtures share pressure more gracefully, which is why that second-floor shower stops going cold when the washing machine opens.
Those are wins. The catch is that your water heater was sized and vented for the old piping’s behavior. After a repipe, total demand may climb, minimum flow thresholds change, and heat loss along the way drops. If the heater is marginal, the upgrade exposes it.
Will my old water heater work with new piping?
Usually, but there are trip wires. A conventional tank heater tends to be forgiving, as long as it is in decent shape and properly vented. A tankless is less forgiving. Here is what I look at after any repipe:
- Minimum flow to activate a tankless. Many non-condensing tankless units need 0.4 to 0.6 gallons per minute to ignite. A new manifold with efficient, low-flow fixtures can put a single lavatory faucet right on the edge. The result is tepid water that flips hot then cold when you tweak the handle. Newer tankless models can fire at 0.26 gpm, which solves the problem. Some older units do not. Total hot water flow increase. Lower friction in new piping can lift flow to showers and tubs. A 2.5 gpm shower that used to run at 2.1 may now reach its full rating. Two showers open together now ask for 5 gallons per minute instead of 4.2. If your tankless was sized at 4.5 gpm at a 70 degree rise, you will feel a temperature drop. A 40 or 50 gallon tank can also fall behind sooner. Gas line sizing to the heater. Repipe projects sometimes expose that the water heater’s gas line was undersized. Tankless units need 120,000 to 199,000 BTU/hr, and many older homes have a 1/2 inch run feeding multiple appliances. The new piping may allow the unit to hit higher flow, but the gas train cannot support full fire, so the outlet temperature sags or the unit throws an error under load. Venting and condensation. Swapping to a high-efficiency tankless without checking vent materials or slope is a classic headache. New heaters produce cooler, wetter exhaust that requires special venting. If the repipe moves the heater or changes the room’s airflow, draft dynamics change. Recirculation loop behavior. If you have a hot water recirc system, a repipe often reshapes the loop. Suddenly the pump overpowers a short circuit, the check valve orientation is wrong for the new branch, or the thermostatic valve is now far from the heat source. You get hot-cold oscillation or constant pump short-cycling.
If your heater is young, properly sized, and well maintained, the repipe should make it look better. If it is more than 10 years old for a tank, or more than 12 to 15 for a tankless, the repipe may be the right moment to replace it.
How pipe material affects heater performance and lifespan
Material selection in repipe plumbing changes water chemistry exposure and heat loss characteristics. That can lengthen or shorten the life of your heater, sometimes by years.
Copper has higher thermal conductivity than PEX or CPVC. Hot water loses more heat along copper runs unless insulation is added, which means the heater cycles more to maintain temperature. The plus is durability against UV and high temperatures, and it handles mechanical abuse better. Copper systems tend to have fewer concerns with permeation or oxygen diffusion, as long as water chemistry is not aggressive. In some municipal districts with low pH or high conductivity, I have seen pinhole corrosion in 8 to 12 years, usually at elbows. A whole house repipe to copper in those conditions is asking the heater to deal with more dissolved copper ions, which can chew up sacrificial anode rods faster.
PEX is a strong match for modern water heaters. Its lower heat loss reduces cycling, and the flexible runs lessen the need for elbows, trimming friction. But PEX is sensitive to high water temperatures and low chlorine levels. Run a tankless too hot, or disable mixing valves, and you can bake PEX near the heater. Keep your storage temperature at 130 to 140 with proper mixing at the fixtures, and route at least 18 inches of metallic stubs from the heater before transitioning to PEX. Oxygen barrier PEX (often stamped PEX-b with an EVOH layer) keeps dissolved oxygen from attacking cast iron pumps and components in hydronic loops, but for domestic water you mainly care about temperature rating and local code.
CPVC tolerates heat well, but it is brittle and hates mechanical stress or UV. If your water heater vibrates during ignition, or the flue backdrafts and warms the wall, CPVC can crack at glued joints over time. I do not like CPVC within the first few feet of a heater. Keep a metallic nipple and union set, then transition.
Short version: PEX with proper standoffs and a mixing valve gives most tankless and tank systems a friendly downstream environment. Copper is robust, but it should be insulated and matched to water chemistry. Avoid CPVC right at the heater.
Does a repipe change hot water wait times?
Yes, and typically for the better. Two factors matter most: the internal volume of pipe between the heater and the fixture, and whether you have a recirc system.
A 1/2 inch line holds about 0.016 gallons per foot. A 3/4 inch line holds roughly 0.033 gallons per foot. If your old layout ran a 3/4 inch trunk 40 feet to the far bath then 20 feet of 1/2 inch to the shower, you had around 1.7 gallons of cooled water to push out. At 2.5 gpm, that is 40 seconds before hot. Repiping with a manifold and 1/2 inch home run of 50 feet cuts the purge volume to around 0.8 gallons, roughly 20 seconds. If the repipe includes smaller, insulation-wrapped home runs, the delay can feel even shorter, especially if the heater fires fast.
The caveat is minimum fire on tankless units. If you open a faucet slowly to avoid splashing, you may not hit the activation threshold, so you wait forever because the heater never lights. This is not a piping flaw. It is a controls and flow interaction. Pick fixtures with a stable flow profile, and set anti-scald mixing valves properly so you can open the handle fully without blasting hot water.
Will a new manifold or smaller branches starve my heater?
Unlikely. Heaters are not fed by the branches, they feed them. Where people get into trouble is on the cold side. If the repipe reduced the size of the cold feed to the heater, or added long runs with many elbows, you might choke the supply during heavy draw. Most pros keep a 3/4 inch cold feed to the heater, sometimes 1 inch if other appliances pull hard on the same trunk. On the hot side, 3/4 out of the heater to a manifold is normal, then 1/2 to fixtures. For homes with high-flow soaking tubs or body-spray showers, run a dedicated 3/4 inch home run to that one fixture and keep the rest 1/2.
I have measured pressure drop on new 1/2 inch PEX home runs at around 1 to 3 psi for a single lavatory. That will not upset a water heater. Keep the heater’s inlet screen clean, set the inlet ball valve fully open, and do not install restrictive check valves right at the inlet unless the system design demands it.
What size water heater should I have after a repipe?
The repipe does not change your household’s peak demand, it reveals it. If your old galvanized pipes strangled the house, you might have unknowingly adapted by not running two showers and a dishwasher at the same time. Once the repipe restores flow, people use it. Size your heater for realistic simultaneous use, not the old constraints.
For tank heaters, a family of four with two bathrooms often does well with a 50 gallon, 40,000 BTU unit, as long as showers are standard low-flow and laundry does not run during the morning rush. If you have a deep-soak tub, consider 65 to 75 gallons or a higher recovery 50 with 50,000 BTU. For tankless, look at your temperature rise. If your incoming winter water is 45 degrees and you want 115 at the shower, that is a 70 degree rise. A 180,000 BTU unit may give you 4 to 5 gpm at that rise. Two showers plus a sink can exceed that. Step to 199,000 BTU or install two smaller units in parallel if the home frequently runs three fixtures at once.
One practical approach is to log usage for a week. Note how many fixtures run at the same time during busy hours. Combine their rated flows, then add a 10 to 20 percent margin. That is your target output.
Common problems after a repipe and how to fix them
I keep a mental list of patterns that show up after repipe projects. They are fixable, and the cure usually involves one change, not a full redo.
- Lukewarm or fluctuating hot water on tankless with single small draws. Solution: lower the outlet setpoint a few degrees and install a faucet with a steady flow cartridge, or upgrade to a model with a lower activation threshold. Check for partially closed isolation valves or clogged inlet screens. Two showers cause a temperature slide. Solution: confirm the gas line sizing to the heater, then check the unit’s maximum output at your winter temperature rise. If it is undersized, either limit fixture flow with balanced 1.8 to 2.0 gpm showerheads or upsize the unit. If venting is marginal, the heater may be derating itself, so do a combustion analysis. Recirculation pump runs constantly or never gets hot to the end. Solution: verify check valve orientation, add a balancing valve on the return, and insulate the loop. For crossover-style comfort systems that use a thermostatic bridge under a sink, ensure the bridge is on the farthest fixture and rated for the new temperature. Some need 120 to 140 degrees to close tightly. Noisy pipes when the heater fires. Solution: add standoffs and isolation where the new PEX passes near the heater and expansion tank. Thermal expansion can make PEX tick as it slides. A small expansion tank on the cold inlet and a few more clamps often quiets a mechanical symphony. Discolored water after a repipe. Solution: flush the heater and the new lines thoroughly. Soldering flux, PEX plastic dust, and disturbed minerals from the old system take time to clear. If the anode rod is already compromised, high oxygen after a repipe can accelerate reaction and create a metallic tint. Inspect and replace the anode if needed.
Do I need a mixing valve after a repipe?
If you want safety and stable temperatures, yes. Set the water heater storage temperature at 130 to 140 degrees for Legionella control, then use a thermostatic mixing valve on the hot outlet to blend down to 120 at fixtures. Many jurisdictions require this. A repipe is the perfect time to install a quality ASSE 1017 master mixing valve and ASSE 1070 point-of-use valves for tubs and showers that need tighter control.
Without mixing, you end up lowering the tank to 120 to avoid scalding. That increases bacteria risk and reduces effective capacity. Mixing lets you store hotter water and stretch usable gallons from a tank, which matters if multiple teens shower back-to-back.
What about expansion tanks and check valves after a repipe?
Newer neighborhoods often have pressure-reducing valves or backflow devices at the meter. That makes your home a closed system. When water heats, it expands, and pressure spikes unless there is a place for that expansion to go. Install a properly sized expansion tank on the cold inlet of the heater and precharge it to match your static water pressure. If you skip this, relief valves weep, faucets chatter, and the heater works harder.
Check valves deserve respect. Put them where they belong, not everywhere. A check on the recirc return, a check at the pump if required by the manufacturer, and a check in certain mixing valve setups makes sense. Adding checks on random branches can trap heat and create dead legs that support bacteria growth.
Will a tankless save me money after a repipe?
It can, but only if you run it within its sweet spot and maintain it. For homes with modest simultaneous use and long gaps between draws, tankless avoids standby loss. A well-insulated tank comes closer than people think, though, especially if your climate is mild and the heater sits in conditioned space. The bigger gains come from recirculation control. A recirc pump on a demand button or occupancy sensor saves more energy on a tank or tankless than any other tweak I see. Timers and aquastats help, but they still run more than necessary. Demand control only runs the pump when someone needs hot water.
If your water is hard, budget for descaling. A tankless with a scaled heat exchanger uses more gas to produce the same outlet temperature, and it short cycles. After a repipe, flows are stronger, which can move scale around and plug filters. Add a service valve kit and plan to flush with vinegar or a mild citric acid solution every 6 to 18 months, depending on grains per gallon.
How do low-flow fixtures play with tankless after a repipe?
The combination of low-flow aerators and smooth new piping is where many folks run into igniter thresholds. If the sink aerator is 0.35 to 0.5 gpm and your tankless needs 0.5 gpm to fire, the faucet becomes a coin flip. Swap to a 1.0 gpm aerator at the hand-washing sinks that matter, and leave the ultra-low at utility sinks where tepid is fine. Showers at 1.75 gpm typically do well with modern tankless units. Go below 1.5 and you start flirting with nuisance shutoffs if the unit is older.
For tank heaters, low-flow extends your shower time and reduces temperature sag. There is no downside beyond personal preference. If you run body sprays or rain heads, these use cases overwhelm both tanks and tankless unless they are sized for it.
I moved my water heater during the repipe. What should I watch?
Relocations change venting, gas runs, electrical bonding, and the length of hot and cold lines. A few inches is trivial, but a move across the garage is a different story. Keep the heater close to the gas meter or a trunk that can deliver full BTU without pressure drop. Maintain proper vent rise and clearance from windows and eaves. If you switch from center of house to an exterior wall, tankless units often become easier to vent, while tanks might require a power-vent model.
Regarding hot water wait times, moving the heater farther from the kitchen often triggers calls. If the new route adds 30 feet of 3/4 inch hot, you just added a gallon to purge. Consider a small dedicated recirc line for the kitchen, or route a 1/2 inch insulated home run.
Will a repipe and new heater fix my temperature swings?
Usually, yes, if you also address the valves at the shower. Pressure balancing or thermostatic mixing valves at the fixtures make a noticeable difference. A repipe reduces pressure drops when other fixtures open, and a modern valve stops the cold side from stealing the show. If your home has two-handle showers without modern cartridges, upgrade them. I have replaced antique valves after a repipe because the added flow exposed how sensitive they were to pressure swing.
Also confirm the water heater’s outlet temperature stability under changing flows. Many newer tankless models use proportional gas valves and complex sensors to hold a setpoint within 2 to 3 degrees. Older ones lag and overshoot. If you feel waves every 10 to 15 seconds, that rhythm is the heater, not the pipes.
Do I need water treatment after a repipe?
If your water had high hardness before, it still does. The repipe does not change minerals. Hard water eats anodes, adds scale, and narrows passageways. A whole-home softener or a scale inhibitor can protect both new piping and the water heater. Smart dosing matters. Over-softening to zero grains can make the water feel slick and accelerate corrosion in some metals. Aim for 3 to 5 grains if you are coming from 15 to 25 grains. If you have aggressive water with low pH, a neutralizer can protect copper and brass. For PEX systems with brass manifolds, that West Linn plumbing solutions still matters.
If you only care about taste and chloramine, a point-of-use carbon system at the kitchen is cheaper and avoids starving the home of residual disinfectant. If you install a whole-home carbon filter, keep the system clean and sized so you do not create a bacteria incubator.
What permits or inspections are required?
Most jurisdictions require permits for a whole house repipe and for water heater replacement or relocation. Inspectors focus on seismic strapping, expansion tanks, venting, T&P discharge routing, gas drip legs, and dielectric unions where applicable. If you transition to PEX, they will check for the correct fitting system, support spacing, and UV exposure.
I have been to jobs where the repipe was perfect but the heater failed inspection because the T&P drain terminated uphill or outside without an air gap. Do not neglect the last five feet. That is where many red tags happen.
How to prepare your home and expectations
A repipe is disruptive. Walls open. Water shuts off for hours at a time. Good crews stage the job so you have water by night, but until fixtures are trimmed and flushed, expect cloudy or aerated water. If you are keeping your old heater, ask the crew to flush it fully after they finish pressurizing. Debris migrates to the heater’s inlet screen and the aerators.
Schedule a system tune after the repipe: verify static and dynamic pressure, test heater firing across flows, set the mixing valve, confirm the expansion tank precharge, and balance any recirc loop. That two-hour visit is cheap insurance against weeks of small annoyances.
A quick homeowner checklist for repipe and water heater compatibility
- Verify heater sizing against real simultaneous use at your winter temperature rise. Confirm gas line size, venting, and combustion air support the heater’s full output. Install a master mixing valve, expansion tank, and service valves at the heater. Choose fixtures with stable low-flow performance that meet your heater’s activation thresholds. If using recirculation, balance the loop, insulate the lines, and control the pump with demand or smart scheduling.
How pros diagnose issues after the fact
When a homeowner calls with “ever since the repipe,” I run a simple sequence. First, measure inlet static pressure, then dynamic with two fixtures running. A pressure drop of 15 psi or more hints at a supply restriction or an undersized main. Next, check heater inlet screens, then fire the heater at low and high flows while monitoring outlet temperature and flue temperature. If the burner does not hit target, I look at gas pressure under load. Most tankless units want 4 to 10 inches water column at the manifold. If the gas dips below spec when other appliances run, it is a fuel delivery issue, not a piping issue.
I also check for cross-connection. A mis-plumbed fixture or a failed single-handle cartridge can blend cold into hot and ruin temperature stability everywhere. Turn off cold stops at fixtures one by one and watch the hot go steady again. It is Repipe Plumbing a fast way to rule out a rogue mixer.
On recirc complaints, I kill the pump and watch return line cooling rates. A fast drop means poor insulation or a long exposed run. Install slip-on insulation and fix the easy losses before chasing exotic answers.
When to replace the heater during a repipe
If your tank is over 10 years old, or shows rust at seams, replace it during the repipe. You will have walls open and plumbers on site. Labor is already mobilized, and you can integrate mixing, expansion, and recirc cleanly. If you have a 15-year-old tankless, the improvements in modulation range and minimum fire on newer models are worth considering. I have replaced many 0.6 gpm activation units with 0.26 gpm models, and the low-flow nuisance disappeared instantly.
If budget forces a delay, at least add isolation and service valves, new flexible connectors, a fresh anode if the tank supports it, and a compliant T&P discharge. Those steps extend life and reduce future labor.
Final word from the field
Repipe plumbing is not just new tubes in the walls. It is a reset of how water moves, and it shines a bright light on the water heater’s capabilities. When the piping and heater are matched, the house feels calm. Showers hold temperature. Sinks run hot quickly. Pumps hum less. Gas bills reflect useful heat instead of wasted standby.
Treat the repipe and the water heater as a team. Size for how you live, not how the old pipes limited you. Keep the details tight at the heater, from mixing to expansion to service valves. Choose fixtures that play well with your heater’s minimum flow. If something feels off after the upgrade, measure, do not guess. Most issues are simple and upstream of tearing into walls.
The best calls I get after a repipe are the boring ones. “We forget the plumbing exists now.” That is the right outcome.
Business Name: Principled Plumbing LLC Address: Oregon City, OR 97045 About Business: Principled Plumbing: Honest Plumbing Done Right, Since 2024 Serving Clackamas, Multnomah, Washington, Marion, and Yamhill counties since 2024, Principled Plumbing installs and repairs water heaters (tank & tankless), fixes pipes/leaks/drains (including trenchless sewer), and installs fixtures/appliances. We support remodels, new construction, sump pumps, and filtration systems. Emergency plumbing available—fast, honest, and code-compliant. Trust us for upfront pricing and expert plumbing service every time! Website: https://principledplumbing.com/ Phone: (503) 919-7243