Why Is My Boiler Running in the Summer? Understanding Indirect Hot Water in NH Homes

It is 80 degrees outside, your thermostat is not calling for heat, and then you hear the boiler fire up in the basement.

For many New Hampshire homeowners, that seems strange. Why would the heating system run in July, August, or September when the house does not need heat?

In many homes, the answer is simple: your boiler may also be making your hot water.

New Hampshire has many homes with boilers that heat baseboards, radiators, or radiant floors. Depending on how the system is set up, that same boiler may also provide the hot water used for showers, sinks, laundry, and dishwashing.

Understanding which type of system you have can help you determine whether summer boiler operation is normal and whether your current setup still makes sense for your home.

Three Common Ways Boilers Make Hot Water

Homeowners with boiler heat will usually encounter one of three systems:

• A tankless coil built into the boiler
• An indirect water heater connected to the boiler
• A combination, or combi, boiler

They all use the boiler to produce domestic hot water, but the way they operate is very different.

Why Older Boilers May Run Throughout the Summer

Many older New Hampshire boilers use a tankless coil.

A tankless coil is a heat exchanger installed inside the boiler. Cold water passes through the coil and picks up heat from the hot boiler water before traveling to your faucets.

The important thing for homeowners to understand is that many of these systems need to keep the boiler warm even when the home does not need heat.

As the boiler cools, its controls eventually tell the burner to fire again. That can happen whether or not someone is currently taking a shower or using a faucet.

So if an older boiler periodically turns on throughout a hot summer day, it may simply be maintaining the temperature needed to produce hot water.

That does not automatically mean something is wrong. However, keeping a boiler hot all summer can use more fuel than a system that only fires when hot water is actually needed.

What Is the Tank Sitting Next to My Boiler?

If you have a separate cylindrical tank beside your boiler, you may have an indirect water heater.

It often looks like a conventional water heater, but it does not have its own burner. Instead, the boiler heats the water stored inside the tank through a heat exchanger.

When the tank needs more heat, it calls on the boiler. The boiler fires, reheats the stored water, and then shuts back down.

Because the tank stores hot water, a compatible boiler does not necessarily need to remain hot throughout the day.

That is one of the biggest differences between an indirect water heater and an older tankless-coil system.

Why Homeowners Often Like Indirect Water Heaters

Indirect water heaters can be a strong fit for homes that already rely on a boiler for heat.

A properly designed system can provide:

• Fast hot water recovery after showers or heavy use
• Stored hot water for households with higher demand
• Less unnecessary summer boiler operation in compatible systems
• One heating appliance serving both home heating and hot water

For larger households, recovery can be especially important.

A water heater’s gallon capacity only tells part of the story. How quickly the system can reheat that water also affects whether your family runs out during busy periods.

What About a Combi Boiler?

A combi boiler combines space heating and domestic hot water in one compact appliance, often without a separate hot water storage tank.

Instead of storing a large amount of heated water, the system heats domestic water as it is needed.

This can work well for some homes, particularly where mechanical-room space is limited. However, hot water demand becomes very important.

A smaller household with moderate usage may have very different needs than a home where several people shower around the same time, laundry is running, and a large tub is being filled.

That is why homeowners should not choose between a combi boiler and an indirect water heater based only on efficiency ratings or equipment size.

The better question is: How does your household actually use hot water?

Is It Normal for My Boiler to Run in the Summer?

If your boiler makes your domestic hot water, some summer operation may be completely normal.

What matters more is whether its behavior has changed.

For example, a boiler that fires after several morning showers may simply be reheating an indirect water heater. An older boiler that occasionally cycles throughout the day may be maintaining temperature for a tankless coil.

It is worth having the system checked if you begin noticing:

• Hot water running out much faster than before
• Water temperatures changing unexpectedly
• Frequent short boiler cycles
• Leaks or unusual noises
• A noticeable increase in fuel use

Those changes can tell you more about the health of the system than the simple fact that the boiler runs during warm weather.

Should You Replace an Older Tankless Coil?

That depends on more than the coil itself.

If your boiler is otherwise in good condition, adding an indirect water heater may provide better hot water performance and reduce the need for some systems to remain hot throughout the summer.

But if the boiler is nearing the end of its useful life, investing heavily in the existing setup may not make sense.

Before making a decision, consider:

• The age and condition of the boiler
• How much hot water your household uses
• Whether you regularly run out of hot water
• Your current fuel and operating costs
• Whether a larger heating-system upgrade is already being considered

This is where looking at the entire heating and hot water system matters more than replacing one component at a time.

The Bottom Line

Hearing your boiler fire in August does not necessarily mean there is a problem. In many New Hampshire homes, the boiler has two jobs: heating the house in winter and producing hot water all year.

The important thing is knowing which type of system you have.

An older tankless coil may require the boiler to stay warm throughout the summer. An indirect water heater stores hot water and may allow a compatible boiler to remain off until more heat is needed. A combi boiler takes another approach by producing hot water as you use it.

Once you understand how your system works, it becomes much easier to decide whether its summer operation is normal, whether your hot water performance is adequate, and whether an upgrade would actually provide meaningful value.

After 50 years of working in New Hampshire homes, Al Terry Plumbing & Heating can help homeowners understand how their boiler and hot water systems work together and determine which options make the most sense for their home.