What's the Best Heating System for My Massachusetts ADU?
When homeowners ask us how to heat their new ADU, we hear this worry over and over: "Will it really keep me warm in a cold Massachusetts winter without costing me a fortune?" We understand that no one wants to put their money into an ADU and then worry about their heat bill every January.
At BuildX, we have put heating systems into lots of ADUs across the state of Massachusetts, from Cape Cod all the way down to the South Shore and we've seen for ourselves what works best, what doesn't work and which heating systems leave ADU owners frustrated when the winters get cold. Our energy experts test every system that we put in, and the information we have collected gives us a consistent winner.
Quick Answer: Heat pump systems are clearly the best choice when they are installed correctly and operated according to directions. A heat pump system will give the ADU owner a 30 to 50 percent lower cost of operation when compared to a traditional heating system. It will also heat and cool the ADU with just a single unit, and meets all Massachusetts energy code requirements. For the heat pump system to operate successfully, the owner needs to set their preferred temperature and then leave it alone. Heat pumps work best when they are allowed to run slow and steady, and should not be cranked up and down like a conventional furnace. The ADU owner should put the heating system in a conditioned crawl space but not the attic, and make sure the building envelope is tight.
In This Article
Heat Pumps Outperform Traditional Heating Units in ADUs
Those who choose to build an ADU in the State of Massachusetts usually select one of four different heating options. These are heat pumps (mini-split or ducted), gas furnaces with a separate AC unit, electric baseboard heat, or a hybrid system combining more than one of these options. The cost of installation can range from $2,000 for basic baseboard to over $12,000 for a fully ducted heat pump. The cost to operate each system can actually vary even more substantially and that is why the beginning installation price does not tell the full story.
Heat pumps move heat around but do not generate it and that makes them much more efficient than other systems that burn fuel or use electric resistance. If the heat pump is installed correctly in an insulated ADU that has little air leakage, the heat pump can keep the occupants constantly comfortable and use just a small portion of the energy of baseboard heaters or gas furnaces.
The heat pumps that we put in will operate at temperatures down to 15 degrees below zero, and have an electric backup for extreme temperatures. In a Plymouth ADU that was completed last year, the three people living in an all-electric unit have an average monthly electric bill of about $250, which covers heating, cooling, hot water, and all other types of usage.
We always use name-brand equipment from manufacturers such as LG and Mitsubishi that have national service networks, parts that are easily available, and that are proven to perform well in cold climates. We have seen many homeowners buy cheap equipment to save $2,000 or $3,000 and then end up needing to completely replace their heating system just a few years later when a circuit board fails and they cannot find a replacement part.
A heat pump operates differently than a traditional furnace and this is probably the most common source of frustration for some homeowners. When you are operating a traditional furnace, you can come into the ADU after a weekend in the cold, turn the thermostat up to 70 degrees and in about 10 minutes the ADU will be warm.
Heat pumps don't work in the same way. Instead they are made to run at a lower temperature and take more time to warm up, so you should just set your temperature once and then leave it alone. When you turn the thermostat up and down it will actually reduce the efficiency of your heat pump and you will be less comfortable. For example, if you set the temperature to 55 degrees when you are going away for the weekend, plan to turn it back up a couple of days before you come back. That way, the heat pump can use that time to gradually bring the ADU back to your ideal temperature. We always recommend that you install a thermostat that has a remote control so that you are able to control settings and adjust temperatures before you get home. This type of approach that is slow and steady is what will make our heat pump most efficient, and fighting against that will just create frustration and lead to higher costs.
Where you install your equipment is more important than most ADU owners realize. We strongly suggest that you place all equipment in a conditioned crawl space and not an unconditioned attic. Attics in the State of Massachusetts can get as hot as 120 degrees with up to 90 percent humidity in the summer. So even with R-8 ducts that are insulated, the metal boots at the ceiling penetrations will likely drip condensation the entire summer. That condensation will soak into cellulose insulation, and eventually will stain the sheetrock, and can even cause mold to form. We have seen this type of damage occur and it can easily cost the ADU owner thousands of dollars to fix.
A conditioned crawl space keeps the equipment in a controlled environment, reduces energy losses, and eliminates condensation problems. If you are building a new ADU, plan to keep your heating equipment in a conditioned crawl space. The property equipment placement will reduce loss of energy, get rid of any future condensation issues and save headaches later.
Cost Differences Between Heating Systems
There are many cost differences between heating systems. For example, the cost of installation is different depending on the system chosen. The cost per month to operate the different systems varies even more. Below are average expenditures for a typical 900-square-foot ADU in Massachusetts.
| System Type | Installation Cost | Monthly Operating | Key Considerations |
|---|---|---|---|
| Heat Pump (Mini-Split) | $4,000 to $6,000 | $60 to $90 | Both heats and cools; it is zoning capable; has a learning curve |
| Ducted Heat Pump | $8,000 to $12,000 | $70 to $100 | Whole-home comfort; equipment is hidden; needs duct space |
| Gas Furnace + AC | $7,000 to $10,000 | $100 to $140 | Fast heat; but requires two systems and needs gas line |
| Electric Baseboard | $2,000 to $3,000 | $180 to $250 | Cheap to install; but expensive to run; no cooling equipment |
The difference in the operating cost between electric baseboard heat and a heat pump can be more than $1,500 a year. Over a period of 10 years, that is a difference that would more than cover the higher cost of the heat pump installation.
When Heat Pumps Are Not Right for Everyone
Even though we usually install heat pump systems in the ADUs we build, heat pumps are not right for every situation and we do not recommend that option for everyone. For instance, if you already have propane or natural gas on your property and you must have a gas stove for cooking, we can work with that situation. We would then recommend just using the heat pump for space conditioning and adding a gas stove or gas dryer instead of changing to a gas-fired heating system. If you change from a heat pump to a gas boiler it will add a lot of cost to the project without adding much benefit for most ADU applications.
The other situation where a homeowner might want an alternative is when sensitivity to indoor noise is a problem. Indoor heat pumps are quiet but they are not silent. If absolute quiet is something that matters, especially in a home office or a guest suite, a fully ducted system that has an air handler in a separate space might be worth the extra cost.
New compact systems that combine heating and hot water systems are becoming more common and they can handle both space conditioning and hot water from just one outdoor unit. These are very useful for ADUs where space is limited and simplicity matters.
Some builders that we work with use products from Hisense that will connect a heat pump outdoor unit to both an air handler for heating and cooling and an indirect hot water tank. A similar product called The Sanco2 system has a dedicated outdoor unit and indirect tank, and is priced around $5,000 for the equipment. These systems will eliminate the need for a separate heat pump water heater that takes up floor space inside your ADU.
Massachusetts Energy Codes Can Affect Your ADU Options
In early 2024, the energy requirements changed, and some of the previous restrictions were loosened for ADUs and they went back to previous code levels. (Refer to current ADU requirements at mass.gov/info-details/accessory-dwelling-units.) This means that mixed-fuel options can now meet energy modeling requirements where before they could not.
Under these current rules, your modeling options include installing all-electric with a heat pump with a HERS rating around 55, mixed fuel like a gas stove with heat pump with a HERS of around 52, and also in some cases even standard electric water heaters where heat pump water heater noise is an issue if the overall energy model meets the new requirements.
For most homeowners building a new ADU, all-electric with a heat pump is still the easiest path to comply with codes and also has the lowest operating costs. But if you have specific requirements that make mixed fuel a better option for you it is now a possibility.
Questions to Ask Your ADU Builder
Before you decide on an ADU builder, there are several important questions you should ask about their approach to heating.
First, make sure you know what brand of heat pump equipment they typically install and what the rating is for cold-weather. Units in Massachusetts should be rated to at least minus 15 degrees from manufacturers such as Mitsubishi and LG.
Also find out where the equipment will be located. The ideal location should be in a conditioned crawl space rather than an unconditioned attic placement which would raise concerns.
Ask if your installer is manufacturer-certified because the proper installation is just as important as the quality of the equipment being installed.
And finally, find out what the building envelope specification is. A tight and well-insulated ADU is needed for a heat pump to perform best so inquire about air-sealing targets and levels of insulation.
For most new Massachusetts ADUs, a properly installed heat pump system will give the homeowner the best combination of efficiency, comfort, and low operating costs. The key to achieving this balance is high-quality equipment, the proper placement, and a complete understanding of how to operate the system for the best performance.
If you are still in the research phase, please read our other articles on ADU insulation requirements and meeting net-zero energy requirements to learn how heating fits into the bigger picture of ADU energy performance.
When you are ready to discuss your specific project, schedule a consultation with our team to review heating options for your site and budget. We will walk through choices of equipment, placement considerations, and help you understand what to expect for both installation and operating costs.
