Does it make sense to have 2 tankless water heaters?

Quick Answer

Having two tankless water heaters can make sense in certain situations where a single tankless unit is not able to meet the hot water demand. The main factors to consider are the flow rate and temperature rise requirements for your household. Two smaller units may be more cost-effective than one large one. Installation considerations like gas line capacity and venting are also important when using multiple units. Ongoing maintenance costs may be higher with two units instead of one.

What are the benefits of having two tankless water heaters?

Some potential benefits of having two tankless water heaters include:

  • Increased hot water capacity – Having two units allows you to meet higher flow rate demands than a single unit may be capable of.
  • Cost savings – In some cases, two smaller units may be less expensive than one large unit with the same combined flow capacity.
  • Redundancy – If one unit needs service, you still have hot water from the other one.
  • Versatility – Units can be located closer to points of use rather than having one large central unit.
  • Energy efficiency – Tankless heaters only heat water on demand, avoiding standby losses associated with tank style heaters.

The increased flow capacity is one of the main reasons homeowners opt for two tankless heaters. For example, a single tankless may be rated at 6 gallons per minute (GPM). Two of those units would give you 12 GPM combined. This may be necessary for larger homes with multiple bathrooms where peak demands might exceed the flow capacity of one heater.

Having redundancy in the system can also be appealing for some homeowners. If one tankless unit needs service or repairs, you would still have hot water available from the other one. This avoids being without hot water during any downtime for maintenance.

What are the downsides of having two tankless units?

Potential downsides of having two tankless water heaters include:

  • Higher upfront costs – Installing two units costs more than a single unit upfront.
  • Increased maintenance – You may need to service two units instead of just one.
  • More complicated installation – Having adequate gas line capacity and venting for two units instead of one adds complexity.
  • Takes up more space – Two units will have a larger footprint than a single larger unit.
  • Higher operating costs – You lose some efficiency compared to one larger single unit.

The installation considerations are important to weigh when thinking about having two tankless water heaters. Making sure your gas line can handle multiple units running at full capacity is crucial. Proper venting is also key – whether using direct vent systems or conventional venting, having adequate capacity for two units needs to be factored in.

Operating and maintenance costs may be higher as well. You have twice as many components that could potentially need service or repair. Things like descaling may need to be done on two units instead of one. These ongoing costs need to be considered.

How do you determine if you need two tankless water heaters?

Determining if two tankless heaters are needed depends on several factors:

  • Peak water flow rate demand – How many fixtures operating simultaneously?
  • Outlet water temperature – What temperature rise is needed?
  • Heater flow capacity – Flow rate of a single unit vs. two units combined.
  • Number of bathrooms and hot water uses – More need higher capacity.
  • House size – Larger homes have higher demand.

The key is calculating your peak water usage in gallons per minute (GPM) and temperature rise needed. Compare this to the rated capacity of one tankless unit vs. having two units.

For example, if your peak demand is 12 GPM and you need a 50°F temperature rise, a single 9 GPM unit may not meet that need. Two smaller 6 GPM units together could handle the 12 GPM demand.

Larger homes with multiple bathrooms, appliances, fixtures running simultaneously need more flow capacity. More bathrooms means more hot water being used at the same time.

Do an accurate assessment of your peak needs before deciding between one or two tankless heaters. Also factor in any plans to add bathrooms or upgrades that may increase demand.

What size tankless water heaters should you get?

When installing two tankless water heaters, properly sizing each unit is important:

  • Determine peak demand flow rate and temperature rise needed.
  • Add up flow capacity of units to meet peak needs.
  • Don’t significantly oversize units – leads to efficiency loss.
  • Allow for some capacity growth if additions planned.
  • Check manufacturer sizing guides for flow rates at temperature rise.
  • Consider 9-12 GPM units for typical whole house applications.

Ideally each unit combined should meet your peak demand without greatly exceeding it. Oversizing units means they will short cycle on and off more, reducing operating efficiency.

Many whole home tankless heaters fall in the 9-12 GPM size range from most manufacturers like Rinnai, Navien, and others. Two units in this flow range is commonly used to supply hot water to larger homes with higher demand.

Be sure to check the manufacturer literature for the flow capacity at your needed temperature rise. Flow rates are often based on a 77°F rise so capacity decreases at higher temperature increases you may need.

Properly sizing the units for your peak demand is key to maximize efficiency and performance when installing two tankless water heaters.

How do you install two tankless water heaters?

Tips for installing two tankless water heaters:

  • Have a licensed professional do the installation.
  • Check gas line capacity – may need service line upgrade.
  • Ensure adequate space for clearance between units.
  • Determine optimal venting strategy – single chimney manifold or sidewall.
  • Isolate units with shutoff valves for serviceability.
  • Consider combining into a bank of heaters manifold system.
  • Set up correct water piping in parallel from the cold inlet.
  • Insulate inlet and outlet piping especially when combined manifold.

Proper spacing between the two units is necessary based on manufacturer recommendations, usually around 8 inches clearance minimum. Venting can be done with a single chimney manifold joining the two exhausts or sidewall direct venting each one separately.

Combining the units into a central bank manifold system can make water piping and service easier compared to mounting them separately. Shutoff valves allow isolating one heater for maintenance if needed. Insulating the cold inlet and hot outlet manifold piping helps maximize efficiency.

A qualified professional should install the units according to local code requirements. Permits are usually required for installation of tankless water heaters. Proper gas and venting capacity needs to be ensured before the units are mounted and plumbed in.

How much does it cost to install two tankless water heaters?

The costs to install two tankless water heaters can range from $2500 to $5000 depending on several factors:

  • Tankless water heater costs – $600-$1500 each for mid-efficiency models.
  • Venting – $500-$1000 if direct venting both units separately.
  • Gas line upgrades – $500-$1000 if gas service needs upsizing.
  • Combination manifold – $500-$1000 to combine units.
  • Labor – $40-$100 per hour for 6-15 hours of work.
  • Permits – $50-$200 may be required.

The tankless water heater units themselves will likely be the biggest cost. Expect to pay $600-$1500 each for mid-efficiency condensing tankless models from major brands. Higher end units with wifi connectivity or greater efficiency ratings run $1500-$3000 each.

Venting two units separately out a sidewall is more expensive than manifolding into a single chimney vent. Any gas line upgrades necessary to support two units adds cost as well. Combining into a unified manifold bank requires additional materials.

Labor will also be more intensive than a single unit install, likely requiring 6-15+ hours in many cases. Securing any required permits also adds marginal cost.

In general, expect total installed costs of $2500-$5000 to have two tankless heaters professionally installed. This provides a fully functional system with adequate capacity, efficiency, and redundancy.

What maintenance is needed with two tankless heaters?

Some maintenance tips for two installed tankless heaters:

  • Inspect inlet water filter screens – clean/replace as needed.
  • Check for error codes – diagnose and address any issues.
  • Flush heat exchangers annually with descaling solution.
  • Clean the burners if any combustion deterioration occurs.
  • Confirm exhaust venting remains clear.
  • Drain any drip legs or low points where water could collect.
  • Inspect gas and water connections for any leaks.
  • Have units periodically serviced by a professional.

Like all heaters, periodic maintenance is key for performance and longevity. Items like flushing the heat exchanger to prevent mineral scale buildup should be done annually.

Inspecting the burners for any fouling or debris buildup is also wise to maintain peak combustion efficiency. Checking for obstruction exhaust venting is important as well.

Isolating each heater and cleaning inlet water filters can be done fairly easily. Checking for error codes and troubleshooting issues as soon as they occur prevents bigger problems.

Professional preventative maintenance service by a qualified technician every 2-3 years is recommended. This helps spot potential problems and do thorough maintenance for optimum operation.

Following the manufacturer recommended maintenance schedule is wise when operating two tankless water heaters together. This ensures both units function reliably for many years.


Having two tankless water heaters in your home can provide benefits like increased flow capacity, redundancy, and installation flexibility. It comes at a higher upfront investment and potentially increased maintenance compared to a single unit.

Carefully calculating your peak hot water flow rate and temperature rise demands is crucial to properly size the units. Adequate gas, venting, water piping, and clearances all need to be ensured during installation. Ongoing maintenance like flushing heat exchangers and inspecting burners and exhaust venting needs to be performed on both units.

If your home’s hot water requirements exceed the capacity of one tankless heater, combining two smaller units may be a cost-effective solution. This provides the flow rate and redundancy needed for large homes with high demand. Just be sure to factor in the installation considerations and maintenance requirements. With proper sizing and maintenance, two tankless water heaters can be a viable option for whole home hot water.

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