How can a tankless water heater pump more gpm

Ever turned on two showers at once and suddenly the water goes cold? That’s usually a GPM problem. Tankless water heaters don’t actually “store” hot water, so how much hot water you get at one time depends on how well the system is set up. Let’s break it down in plain language. A tankless water heater can pump more GPM (gallons per minute) by using the right size unit, increasing the temperature rise efficiency, improving water flow balance, or installing more than one unit. You can’t magically push more GPM out of a small heater—it has limits.

What GPM Really Means for Tankless Water Heaters

GPM is how much hot water flows per minute.
More fixtures running = more GPM needed.

For example:

Shower: ~2 GPM
Sink: ~1 GPM
Washing machine: ~2 GPM

Two showers + a sink = 5 GPM needed

If your unit is rated for 3–4 GPM at your local water temperature, it simply can’t keep up.

Why Your Tankless Heater Feels “Weak” on GPM

Most people blame the heater, but the issue is usually one of these:

Unit is undersized
Incoming water is very cold
Too many fixtures running at once
Old plumbing limiting flow
No flow control or balancing

Tankless heaters don’t boost pressure—they only heat what flows through them.

Ways to Increase GPM from a Tankless Water Heater

1. Install a Properly Sized Unit (This Is Huge)

Tankless heaters are sized based on:

Number of fixtures
Incoming water temperature
Desired output temperature

If your groundwater is cold, the heater must work harder, which reduces GPM.

Example:
A heater rated at 8 GPM may only deliver 4–5 GPM in cold climates.

If your current unit is small, no adjustment will fix that.

2. Lower the Temperature Setting Slightly

Higher temperature = lower GPM.

If your heater is set very high (like 140°F), it heats water slower.

Dropping it to 120°F often increases usable GPM and is safer too.

3. Use Flow-Efficient Fixtures

Modern low-flow showers and faucets help a lot.

Old showerhead: 3–4 GPM
New efficient one: 1.5–2 GPM

Same comfort, less demand on the heater.

4. Balance Water Use in the Home

Tankless systems work best when hot water demand is spread out.

Tips:

Avoid running dishwasher and showers together
Stagger laundry and bathing times
Install thermostatic mixing valves

This doesn’t increase GPM—but it feels like it does.

5. Install a Second Tankless Unit (Best for Big Homes)

For large households, one unit often isn’t enough.

Two options:

Parallel units (both share the load)
Dedicated units (one for bathrooms, one for kitchen)

This is the most reliable way to increase total GPM.

6. Maintain the Heater (Often Overlooked)

Mineral buildup slows heat transfer.

Annual flushing:

Improves efficiency
Restores lost GPM
Extends heater life

Hard water makes this even more important.

Pros & Cons of Increasing Tankless Water Heater GPM

Pros	Cons
Better comfort with multiple fixtures	Higher upfront cost
Endless hot water when sized right	May need electrical or gas upgrades
Energy efficient	Limited by incoming water temp
Saves space	Not ideal for very large demand without multiple units

Real-World Examples

Small Apartment:
One shower + sink = 3 GPM
A 4–5 GPM unit works perfectly.

Family Home:
Two bathrooms + kitchen = 6–8 GPM
Needs a larger unit or dual setup.

Cold Climate House:
Incoming water at 40°F
Even an 8 GPM heater may only deliver 4–5 GPM unless oversized.

FAQs (People Also Ask)

Can I increase GPM by adjusting water pressure?

No. Pressure and GPM are different. Tankless heaters don’t increase pressure.

Does a recirculation pump increase GPM?

No. It improves wait time for hot water, not flow rate.

Is gas better than electric for higher GPM?

Yes. Gas tankless heaters usually support higher GPM than electric ones.

Why does my GPM drop in winter?

Cold incoming water needs more heating, which lowers flow rate.

Can one tankless heater run three showers?

Only if it’s sized correctly and incoming water isn’t too cold.

Final Verdict

A tankless water heater can only pump more GPM within its design limits. If you’re running out of hot water, the fix is usually proper sizing, better flow management, or adding another unit—not tweaking settings endlessly.