For Massachusetts homeowners with an existing boiler and central air conditioning, upgrading to heat pumps doesn't necessarily mean abandoning the hydronic heating system already in the home.

A recent Denommee Plumbing, HVAC & Electric project in Dunstable, Massachusetts provides a good example.

At this circa-2000, approximately 3,170-square-foot, four-bedroom, three-bath single-family home, Denommee replaced two existing central air-conditioning systems and hydro-air handlers with two Bosch IDS Edge variable-capacity cold-climate heat pump systems.

But the existing boiler wasn't removed.

Instead, each new Bosch air handler was paired with an Aquecoil hydronic hot-water coil, allowing the existing boiler system to remain available as a backup heat source.

The finished system gives the homeowner:

Cold-climate heat pumps as the primary heating source.
High-efficiency heat-pump cooling during summer.
Existing boiler-generated hot water available for backup heat.
Separate upstairs and downstairs HVAC systems.
Ecobee smart controls.

It's a particularly useful example of how a thoughtfully designed heat-pump conversion can incorporate useful parts of an existing Massachusetts home's mechanical system rather than automatically requiring everything to be removed and replaced.

Dunstable Heat Pump Project at a Glance

The result is not simply an air-conditioning replacement.

It's a dual-source heating strategy that allows modern cold-climate heat-pump technology and the home's existing hydronic heating infrastructure to work within the same overall HVAC design.

Why This Dunstable Home Was a Good Candidate for a Heat Pump Upgrade

Many Massachusetts homes built during the 1990s and early 2000s use some variation of central air conditioning combined with a boiler-based heating system.

In a hydro-air configuration, hot water from the boiler circulates through a hydronic coil located within an air handler. The air handler moves air across the heated coil and distributes warm air through the home's ductwork.

This Dunstable home already had two hydro-air handlers and two outdoor air-conditioning units serving separate areas of the house.

That created an interesting opportunity when the HVAC equipment was ready for replacement.

Instead of installing another pair of conventional AC systems and continuing to rely exclusively on the boiler for heating, the homeowner could add cold-climate heat pumps capable of providing both heating and cooling.

And because the existing boiler and hydronic system could remain in service, Denommee could retain hydronic backup heat through new Aquecoil coils installed with the Bosch air handlers.

Can You Add a Heat Pump to a Home With a Boiler?

Yes. A home with a boiler can potentially be upgraded with heat pumps without removing the boiler.

The appropriate design depends on the home's existing heating distribution, ductwork, heat-loss requirements, electrical system and other site-specific conditions.

This project illustrates one possible approach particularly well.

The existing boiler continues to produce hot water for the hydronic heating system. New Aquecoil hot-water coils incorporated into the Bosch air-handler installations allow that hydronic energy to provide backup heat through the home's existing ducted distribution system.

Meanwhile, the Bosch IDS Edge heat pumps are configured as the primary source of space heating.

That's fundamentally different from simply installing a heat pump and electric resistance backup heat.

No electric heat strips were included in this project.

Instead, the home retains its existing boiler as another available heating resource.

Upstairs: 2-Ton Bosch IDS Edge Cold-Climate Heat Pump

The upstairs system uses a Bosch IDS Edge BOHA-24MTB-M18S variable-capacity heat pump paired with a Bosch BIVB-24RCB-M20X ECM air handler.

An Aquecoil hydronic coil provides the connection to the home's existing boiler system, while an Ecobee smart thermostat provides system control.

Upstairs System Specifications

Outdoor unit: Bosch BOHA-24MTB-M18S
Indoor unit: Bosch BIVB-24RCB-M20X
Hydronic backup: Aquecoil add-on hot-water coil
Refrigerant: R-454B
AHRI Reference: 218032211
SEER2: 17.7
HSPF2: 9.2
Rated cooling capacity: 23,000 BTU

The AHRI-rated heating capacities are particularly useful for homeowners trying to understand cold-climate heat-pump performance:

47°F: 24,000 BTU
17°F: 17,600 BTU
5°F: 20,000 BTU

Those ratings demonstrate an important point: heat-pump performance shouldn't be evaluated from nominal tonnage alone.

Heating capacity changes with outdoor conditions, which is why equipment selection needs to consider the actual heating requirements of the home and the certified performance characteristics of the equipment.

Downstairs: 3-Ton Bosch IDS Edge Cold-Climate Heat Pump

The larger downstairs zone uses a Bosch BOHA-36MTB-M18S variable-capacity outdoor heat pump paired with a Bosch BIVB-36RCB-M20X ECM air handler and Aquecoil hydronic backup coil.

Downstairs System Specifications

Outdoor unit: Bosch BOHA-36MTB-M18S
Indoor unit: Bosch BIVB-36RCB-M20X
Hydronic backup: Aquecoil add-on hot-water coil
Refrigerant: R-454B
AHRI Reference: 218032212
SEER2: 17.2
HSPF2: 9.4
Rated cooling capacity: 34,000 BTU

AHRI-rated heating capacity:

47°F: 36,000 BTU
17°F: 27,800 BTU
5°F: 36,000 BTU

Rather than forcing one large HVAC system to serve the entire 3,170-square-foot house, this installation maintains the home's two-system configuration, with equipment selected separately for the upstairs and downstairs areas.

What Is the Bosch IDS Edge?

The Bosch IDS Edge is an inverter-driven heat-pump system designed to provide both heating and cooling.

Unlike traditional single-stage HVAC equipment that generally operates in an on/off fashion, variable-capacity technology allows the system to adjust operation according to changing demand.

For homeowners, the important distinction isn't simply technological.

A properly designed variable-capacity system can respond to changing loads rather than treating a mild spring afternoon and a much colder winter day as identical operating conditions.

In this Dunstable project, that technology is paired with ECM indoor air handlers, creating an all-electric primary heating and cooling system while preserving hydronic backup capability.

What Is an Aquecoil—and Why Use One With a Heat Pump?

The Aquecoils visible above the Bosch air handlers are a key part of this installation.

An Aquecoil is a hydronic heating coil that uses hot water as a source of heat. Hot water produced by the home's boiler circulates through the coil. Air passing across the heated coil can then be distributed through the duct system.

For this project, Denommee drained the affected hydronic piping, installed the new air handlers and Aquecoils, reconnected the hydronic circuits to the existing boiler system, and refilled the affected piping.

This creates an elegant solution for this particular home:

Bosch heat pump → primary heating

Existing boiler + Aquecoil → hydronic backup heating

The homeowner gains cold-climate heat-pump technology without discarding a functional boiler-based heating resource.

Why Not Just Remove the Boiler?

Electrification doesn't necessarily have to be an all-or-nothing decision.

Some homeowners want to move completely away from fossil-fuel heating. Others want to substantially increase heat-pump use while retaining an existing boiler or furnace.

The appropriate strategy depends on the house and homeowner's objectives.

In this case, retaining the boiler allowed Denommee to create a system that uses the new heat pumps as primary heat while preserving hydronic backup.

That can be especially appealing to homeowners who already have substantial investment in a functional boiler and hydronic infrastructure.

It also demonstrates why homeowners researching heat pumps shouldn't assume that their only choices are:

Keep the boiler and don't install heat pumps

or

Install heat pumps and throw the boiler away.

There can be other approaches.

No Electric Resistance Heat Strips Were Required for This Design

This is another important distinction in the Dunstable installation.

Electric heat strips were not included.

Electric resistance heat is sometimes incorporated into heat-pump air handlers as supplemental or emergency heat. In this system, however, backup heat is available through the Aquecoil and existing boiler.

That makes the hydronic system an intentional part of the overall heating strategy rather than obsolete equipment simply left behind in the basement.

Reusing an Existing Refrigerant Line Set—But Only After Testing

The project also demonstrates something homeowners frequently ask about during HVAC replacement:

Can the existing refrigerant lines be reused?

Sometimes—but they shouldn't simply be assumed to be suitable.

For the upstairs system, Denommee's scope called for the existing refrigerant line set to be flushed, pressure tested and reused only if it successfully passed testing.

For the downstairs system, Denommee installed a new refrigerant line set.

That distinction illustrates a broader principle of replacement HVAC work:

Reuse should be based on suitability and testing, not convenience alone.

Why Nitrogen Pressure Testing and a Deep Vacuum Matter

Installing expensive HVAC equipment is only part of a quality heat-pump installation.

The refrigeration circuit must also be properly prepared and commissioned.

For both Bosch systems, Denommee's installation scope included:

Nitrogen pressure testing of the refrigeration circuits.

Evacuation below 500 microns before final charging and operation.

Refrigerant charging to manufacturer specifications.

Complete system commissioning.

Equipment warranty registration.

A deep vacuum helps remove air and moisture from the sealed refrigeration system before operation. Pressure testing helps verify system integrity before refrigerant commissioning.

These procedures aren't particularly glamorous, and homeowners may never see them.

But they're exactly the kind of behind-the-scenes installation practices that matter to long-term HVAC performance.

The Systems Use R-454B Refrigerant

Both new Bosch IDS Edge systems use R-454B refrigerant, reflecting the transition taking place across the HVAC industry toward newer lower-global-warming-potential refrigerants.

For homeowners replacing older central air-conditioning equipment, refrigerant type has become another consideration when comparing replacement options.

The important point isn't simply that one refrigerant has a different name.

New refrigerants bring updated equipment and installation requirements, making contractor training, proper tools and adherence to manufacturer procedures increasingly important.

Condensate Management and Flood Protection Matter Too

The indoor equipment shown in the project photograph illustrates another part of HVAC installation homeowners rarely think about until something goes wrong: condensate water.

Air-conditioning and heat-pump systems remove moisture from indoor air during cooling operation. That water has to be collected and discharged properly.

Denommee's scope included installation of drain pans, condensate piping and flood shutoff protection as required.

These components don't improve an efficiency rating or appear prominently on an equipment brochure.

But in an attic, basement or finished portion of a home, managing condensate properly can be extremely important.

Existing Ductwork Still Matters When Converting to Heat Pumps

Installing new high-efficiency equipment doesn't automatically correct deficiencies elsewhere in an HVAC system.

The existing ductwork remains an important part of system performance.

For this Dunstable project, attic ductwork repairs were specifically identified as work that could be quoted separately if requested.

That's worth emphasizing for homeowners considering a similar conversion.

A heat pump's rated efficiency and capacity don't eliminate the need for appropriate:

• Airflow
• Supply distribution
• Return-air pathways
• Duct sizing
• Duct sealing
• Insulation
• Static-pressure considerations

The equipment and air-distribution system have to function together.

This is why replacing HVAC equipment should involve more than selecting a unit based solely on the square footage of the house.

Electrical Work Is Part of a Heat Pump Conversion

Replacing conventional air conditioning with heat pumps can change electrical requirements because the outdoor equipment now operates for heating as well as cooling.

Denommee's electrical team rewired the units as required for this heat-pump upgrade.

The project did not include an electrical service or panel upgrade because one wasn't part of the required scope.

That's another advantage of having HVAC and electrical professionals working within the same company: electrical requirements can be coordinated as part of the heat-pump project rather than becoming an unexpected handoff between unrelated contractors.

Permits, Inspections and Commissioning Were Included

Denommee's project scope included the required permits and inspections, along with removal and disposal of the equipment being replaced and project-related debris.

After installation, both systems were commissioned to manufacturer requirements and registered for their applicable equipment warranties.

The project includes:

One-year Denommee labor and installation warranty

10-year registered Bosch parts warranty

Applicable Aquecoil manufacturer warranty

Applicable Ecobee manufacturer warranty

For homeowners comparing HVAC proposals, these details matter.

The lowest equipment price and the most complete installation aren't necessarily the same thing.

First-Year Heat Pump Maintenance Is Included

Denommee also includes an end-of-first-year heat-pump cleaning, maintenance and performance service with this installation.

That gives our HVAC service team an opportunity to return after the systems have operated through real-world heating and cooling conditions.

It's the same principle we've discussed in our other heat-pump projects: installation day isn't the end of responsible HVAC ownership.

A return visit can help assess how the equipment is operating after its first full year in the home.

Mass Save and NEHPA Incentives Helped Reduce the Project Cost

This Dunstable project qualified for two incentives identified in the project scope:

Mass Save Partial-Home Heat Pump Rebate: $5,344

NEHPA Instant Incentive: $1,300

That's a combined $6,644 in project incentives.

Because incentive programs, qualification requirements and available amounts can change, homeowners should verify current eligibility and program terms when planning their own project rather than assuming the incentives associated with a previous installation will still apply.

This project is nevertheless a useful real-world example of how available programs can influence the economics of upgrading from conventional air conditioning to cold-climate heat pumps.

Peace of Mind Service Program Membership Included

The installation also includes membership in Denommee's Peace of Mind Service Program.

The program is designed to extend the service relationship beyond installation day, with benefits including:

• Priority scheduling
• Waived standard dispatch fees
• Annual whole-home plumbing, HVAC and electrical safety and performance assessments
• Member-only pricing
• Loyalty Credits toward qualifying future work

For a home like this one—with a boiler, hydronic piping, two heat-pump systems, two air handlers, smart controls, plumbing and a complete electrical system—whole-home service can be particularly valuable.

Modern home systems increasingly interact with one another.

Maintaining them shouldn't require homeowners to start from scratch every time they need help.

Is a Heat Pump With Hydronic Backup Right for Your Massachusetts Home?

This Dunstable installation doesn't mean every boiler-heated home should use exactly the same design.

It demonstrates that homeowners have more options than they may realize.

If you have an existing boiler and central air conditioning, questions worth discussing with an experienced HVAC designer include:

Can my existing ductwork support heat-pump heating and cooling?

Is my boiler worth retaining?

Could hydronic coils provide backup heat?

How much heating capacity does the heat pump provide at 17°F and 5°F—not just at 47°F?

Does my electrical system have adequate capacity?

Can any existing refrigerant piping be safely reused?

Should the home remain divided into multiple HVAC systems or zones?

What current rebates or incentives apply?

Those questions lead to a system designed around the actual house rather than simply replacing old equipment with whatever fits in the same space.

Frequently Asked Questions About Heat Pumps With Hydronic Backup

Can a Bosch heat pump work with an existing boiler?

Potentially, yes. In this Dunstable installation, Bosch IDS Edge heat pumps were paired with Aquecoil hydronic coils connected to the home's existing boiler. The heat pumps provide primary heat while the hydronic system remains available for backup. The appropriate configuration depends on the individual home's mechanical systems and heating requirements.

What is a hydro-air heating system?

A hydro-air system uses hot water—typically supplied by a boiler—to heat a coil within an air handler. A blower moves air across that coil and distributes heated air through ductwork.

Can an existing hydro-air system be converted to a heat pump?

In some homes, yes. This project replaced two existing hydro-air handlers and conventional AC condensers with Bosch heat-pump air handlers and outdoor units while retaining hydronic backup through new Aquecoil coils.

Does a cold-climate heat pump still work at 5°F?

Cold-climate heat pumps are designed to continue providing heating at low outdoor temperatures, but capacity varies by equipment and conditions. The AHRI-certified data for the systems installed in this project lists heating capacity at 47°F, 17°F and 5°F. Proper system design should consider low-temperature capacity rather than nominal tonnage alone.

Does this system use electric heat strips for backup?

No. Electric resistance heat strips were not included in this Dunstable project. Hydronic coils connected to the existing boiler provide the available backup-heating strategy.

Why install two heat pumps instead of one large system?

This home already used separate upstairs and downstairs HVAC systems. The new installation maintains that general configuration with a 2-ton upstairs system and a 3-ton downstairs system rather than relying on one piece of equipment to serve the entire approximately 3,170-square-foot house.

Can an old AC refrigerant line be reused with a new heat pump?

Sometimes, depending on the equipment, line dimensions, condition, refrigerant requirements and manufacturer requirements. For this project, Denommee planned to flush and pressure-test the existing upstairs line set and reuse it only if it successfully passed testing. A new line set was installed for the downstairs system.

Considering a Heat Pump Upgrade in Dunstable, MA?

If your Massachusetts home already has a boiler, central air conditioning or hydro-air heat, don't assume installing a heat pump means everything you already own has to be removed.

The better question is:

Which parts of the existing system still have value, and how can they fit into a modern heating and cooling strategy?

For this 3,170-square-foot Dunstable home, Denommee designed a solution using two Bosch IDS Edge cold-climate heat pumps for primary heating and cooling while retaining the existing boiler through Aquecoil hydronic backup heat.

Our in-house HVAC and electrical teams handled the equipment replacement, hydronic integration, refrigeration work, electrical requirements, controls, permitting, commissioning and warranty registration as part of the coordinated project.

If you're considering replacing aging central air conditioning, adding heat pumps or determining whether your existing boiler can remain part of your home's future heating system, Denommee Plumbing, HVAC & Electric can evaluate the entire house and help you understand your options.

Get an online HVAC price estimate or schedule a free in-home consultation with Denommee to start planning your heat-pump upgrade.