How to Increase Water Pressure in Your Rinnai Tankless Water Heater

You turn on the shower, expecting the usual blast, and instead get a sad drizzle. The cold water side is fine, but the hot side—fed by your Rinnai tankless—barely trickles. You’re not alone. This is one of the most common complaints I hear from homeowners with tankless units, and the fix is often simpler than you’d think.

In this guide, I’ll walk you through the real causes of low hot water pressure in a Rinnai tankless water heater, and the exact steps to fix each one. You’ll learn how to check your inlet filter, why a 3/4-inch gas line matters, when to descale, and how to test your flow rate like a pro. By the end, you’ll know whether you can fix it yourself or if it’s time to call a plumber.

One tool I keep on my own workbench is the Chromex Tankless Water Heater Flush Kit. It comes with a 5-gallon bucket, a 1/6 HP pump, two 6-foot hoses, and NSF-certified citric acid descaler. If you suspect scale buildup (and you’ll know after reading this), this kit makes the descaling job a 30-minute task instead of a headache.

First, Understand What “Low Pressure” Actually Means

Water pressure is measured in pounds per square inch (PSI). Most homes run between 40 and 60 PSI. A tankless water heater needs a minimum flow rate (usually 0.5 to 1.0 gallons per minute) to turn on, and it needs enough pressure to push water through the heat exchanger and out to your fixtures.

If your hot water pressure is lower than your cold water pressure, the problem is almost always inside the heater or the hot water pipes. If both hot and cold are low, the issue is in your main supply line or a pressure regulator.

So the first step is to measure both pressures. You can buy a simple pressure gauge for about $10 at a hardware store. Attach it to an outdoor spigot for cold, and to a washing machine outlet for hot (if you have one). Compare the numbers.

• Hot and cold both low: Check your main shutoff valve and pressure regulator.
• Hot only low: Move to the next steps.

Don’t skip this. I’ve seen people replace their entire heater when the real problem was a half-closed main valve.

Step 1: Clean the Inlet Water Filter

Rinnai tankless units have a small mesh screen on the cold water inlet. Its job is to catch sediment, rust flakes, and scale particles before they enter the heat exchanger. Over time, that screen gets clogged, and the flow drops dramatically.

Here’s how to clean it:

1. Turn off the gas and power to the unit.
2. Close the cold water isolation valve (the lever on the inlet pipe).
3. Open a hot water faucet to relieve pressure.
4. Unscrew the inlet union nut (use a wrench, but don’t grip too hard—the brass is soft).
5. Pull out the filter screen. Rinse it under running water, or soak it in vinegar for 10 minutes if it’s crusty.
6. Reinstall everything and open the valves.

Do this every 3-6 months if you have hard water, or once a year if you’re on city water. I keep a spare filter on hand because they’re cheap and easy to lose.

If the filter was your problem, you’ll notice the pressure return immediately. If not, move on.

Step 2: Check the Isolation Valves

Rinnai units come with two isolation valves (one on the inlet, one on the outlet). These valves are used for servicing—you close them to isolate the heater. But sometimes they don’t open all the way.

Those valves have a small lever. If it’s not fully parallel with the pipe, it’s partially closed. That restricts flow and reduces pressure.

Check both valves. Make sure the levers are in the full-open position. You might even try closing and reopening them a couple times to clear any debris that’s stuck in the seat.

Also, check the main gas shutoff valve. If it’s not fully open, the burner won’t produce maximum heat output, which can cause low pressure when you’re using hot water at multiple fixtures. The gas valve should be parallel to the gas line.

Step 3: Measure Your Flow Rate

If you’ve cleaned the filter and confirmed the valves are open, the next step is to measure the actual flow rate. This tells you if the heater is getting enough water to even operate.

Grab a 5-gallon bucket and a stopwatch. Place the bucket under a hot water faucet, turn it on full, and time how long it takes to fill 1 gallon. Divide 60 by that time to get gallons per minute (GPM).

For example, if it takes 15 seconds to fill 1 gallon, your flow rate is 4 GPM (60/15 = 4). That’s plenty for a Rinnai. If it takes 30 seconds, you’re at 2 GPM—still enough for one fixture, but marginal for a shower plus a sink.

Compare that to the flow rate at the cold water side. If hot is significantly lower, the restriction is in the heater or the hot water pipes. If both are low, the problem is upstream.

Also, check the pressure drop across the heater. You can install a pressure gauge on the inlet and outlet service ports (if your unit has them). A drop of more than 10 PSI indicates a blockage inside the heat exchanger.

Step 4: Descale the Heat Exchanger

Scale buildup is the number one cause of low pressure in tankless heaters, especially in areas with hard water. Calcium and magnesium deposits coat the inside of the heat exchanger tubes, narrowing them and restricting flow.

You’ll know it’s scale if you’ve done steps 1-3 and the pressure is still low. Also, if you hear a rumbling or see the unit shutting off and on rapidly, that’s classic scale.

Descaling is a DIY job if you have the right kit. I use the Chromex flush kit because it has everything you need: a pump, hoses, and the descaler. Here’s the process:

1. Turn off the gas and power.
2. Close both isolation valves.
3. Connect the two hoses to the service ports (the ones with the small valves).
4. Put the pump in the bucket with the descaler solution.
5. Run one hose from the pump to the inlet service valve, and the other from the outlet service valve back to the bucket.
6. Open the service valves, turn on the pump, and let it circulate for 30-45 minutes.
7. Turn off the pump, close the service valves, disconnect the hoses, and reopen the isolation valves.
8. Flush the unit with fresh water for a few minutes to remove any remaining descaler.

I recommend descaling every 12 months, or more often if you have very hard water. The citric acid in the Chromex solution is safe for the heat exchanger and won’t harm your plumbing. It’s NSF certified for potable water systems, so you don’t have to worry about residue.

After descaling, the pressure should improve noticeably. If it doesn’t, the heat exchanger might be too far gone, and you may need a new unit—but that’s rare if you descale regularly.

Step 5: Inspect Your Pipes and Fixtures

Sometimes the problem isn’t the heater at all. It’s the pipes between the heater and your faucet.

Old galvanized steel pipes corrode on the inside, and over time, they can build up scale that reduces flow. If you have a 1/2-inch pipe running to a shower that’s 30 feet away, you’ll get less pressure than a 3/4-inch pipe.

Also, check the aerators on your faucets and the showerhead. These have small screens that catch debris. If they’re clogged, they’ll reduce flow even if the pressure is fine.

Remove the aerator, clean it with vinegar, and see if that helps. For showerheads, soak them in vinegar overnight.

If your pipes are old, you might consider replacing them with PEX or copper. That’s a bigger job, but it can solve chronic low pressure.

Compare Your Options: Quick Fix vs. Permanent Fix

Method	Difficulty	Time	Cost	Effectiveness
Clean inlet filter	Easy	10 min	$0	High if clogged
Open isolation valves	Easy	5 min	$0	High if partially closed
Measure flow rate	Easy	5 min	$0	Diagnostic only
Descale heat exchanger	Moderate	45 min	$30-60 for kit	High if scale
Replace pipes	Hard	Days	$200-1000	Permanent if pipes are the issue
Install pressure booster pump	Moderate	2-4 hours	$150-400	Only if house pressure is low

I’ve listed a pressure booster pump as an option, but don’t jump to that until you’ve ruled out everything else. A pump won’t help if the heater is clogged with scale.

Step 6: Check Your Home’s Main Water Pressure

If you’ve done all the steps above and the pressure is still low, it’s time to look at your home’s main pressure.

Most homes have a pressure regulator where the water line enters the house. It’s usually set to 50-60 PSI. If it’s set too low (say, 30 PSI), you’ll get weak flow everywhere, including hot water.

You can adjust the regulator yourself if you have a wrench and a pressure gauge. Turn the adjusting screw clockwise to increase pressure, but don’t exceed 80 PSI—that can damage your pipes and appliances.

If you don’t have a regulator, your pressure might be too high or too low depending on your municipality. In that case, call a plumber to check it.

Also, check if your home has a whole-house sediment filter. If the cartridge is clogged, it will reduce pressure to the entire house, including the tankless heater. Replace the cartridge if it’s overdue.

Step 7: Consider Water Softening

If you have hard water (over 7 grains per gallon), scale buildup will keep coming back no matter how often you descale. A water softener removes the calcium and magnesium before they enter the heater, which prevents scale in the first place.

I know water softeners are an investment, but they pay off in the long run. They extend the life of your tankless heater, improve efficiency, and reduce the frequency of descaling. If you’re on well water, a softener is almost essential—well water often has high mineral content.

For more on this, check out water softener impact on your hot water heater.

Frequently Asked Questions

Why is my Rinnai tankless water heater not producing enough hot water pressure?

Low pressure usually comes from a clogged inlet filter, scale in the heat exchanger, or partially closed isolation valves. Check those first. If the filter is clean and valves are open, descale the unit. If that doesn’t help, inspect your pipes for corrosion or blockages.

Can I increase water pressure by adjusting the Rinnai settings?

No. Rinnai tankless heaters don’t have a pressure adjustment. They operate based on the incoming water pressure and flow rate. You can adjust temperature and flow rate limits, but not pressure. If the pressure is low, you need to fix the cause, not the settings.

How often should I descale my Rinnai tankless water heater?

At least once a year, but if you have hard water, do it every 6 months. If you notice a drop in pressure or the unit is turning on and off quickly, that’s a sign you need to descale sooner.

Will a larger gas line increase hot water pressure?

No. A larger gas line won’t increase pressure; it only affects the heater’s ability to heat water at high flow rates. If you’re experiencing low pressure, it’s a water flow issue, not a gas issue. However, a properly sized gas line is essential for the heater to reach its rated flow rate. If the gas line is too small, the burner won’t get enough fuel, and you’ll get lukewarm water at high flow, which can feel like low pressure.

Is low hot water pressure dangerous for my tankless heater?

Not directly, but it can be a symptom of a bigger problem. Scale buildup can cause overheating and damage the heat exchanger. If the unit is running with low flow, it might not activate at all, which means no hot water. It’s best to fix the issue promptly to avoid costly repairs.

What You Can Do Right Now

• Clean the inlet filter—it’s the most common and easiest fix.
• Check the isolation valves are fully open.
• Measure your flow rate to see if the problem is in the heater or the house.
• Descale the heat exchanger with a flush kit at least once a year.
• Inspect aerators and showerheads for clogs.
• Test your main water pressure with a gauge.
• Consider a water softener if you have hard water.

If you’ve tried all these and the pressure is still low, it might be time to call a professional. But in most cases, a little maintenance goes a long way. For more on tankless water pressure, read this tankless water pressure guide or check required water pressure for your unit.