The Story behind a Newton Accessory Apartment: HVAC

Heated or cooled air will enter the unit through two ceiling-mounted vents – one in the main living space and the other in the bedroom. They are very low-profile and will hardly be noticed up there. They also provide dehumidification. They are operated by remote control.

Work continues on the Accessory Apartment we’ve been blogging about over the last couple of months. The project is really starting to come together!

A few weeks ago we installed the HVAC system now that the walls are up and the unit is insulated. The main section of the home has a gas fired furnace and forced hot air in addition to air conditioning units. However, it wasn’t efficient to expand that system to the new accessory apartment. A far better option was to add a heat pump – a ductless system, also referred to as a “mini split,” that provides BOTH heating and cooling. The crew is very happy to be installing trim with the A/C on!

Air source heat pumps use a refrigeration circuit to extract energy from the ambient air and deliver it as heat. Today’s heat pumps can operate effectively even in extremely hot and cold conditions, making them a reliable way to efficiently heat and cool a home in the Boston area.

The condenser is very quiet and is located outside the home on a stand to keep it out of the snow during the winter months.

To get a little technical (and maybe magical) the coefficient of performance, or COP, of a heat pump describes how much useful output (i.e. heat) it can generate from one unit of input energy. For example, a COP of 4.5 (or 450%) means that for every 1 watt of electricity it uses a heat pump would produce 4.5 watts of heat. Even the most efficient gas-fired boilers and furnaces have a COP around 0.9, or 90%, meaning they produce less useful energy than they consume.  The COP of our system is 4.25 at 47 degrees outdoors and 2.53 at 17 degrees.  That’s way better than the best boiler or furnace at 90%!

Even when accounting for the efficiency of generating and distributing electricity, heat pumps use less energy and result in fewer carbon emissions than natural gas, despite gas being dubbed a “clean” fuel, and especially as the regional and national electric grid continues to transition to renewable energy. The math is even more favorable if the home has rooftop solar – which our accessory apartment and main home had already installed.

Not only are heat pumps more efficient and therefore save homeowners’ money in the long run, Eversource and National Grid are able to offer customers incentives to install heat pumps in their homes through the MassSave program. Hello discounts!

So with solar panels already on the roof and lowering the electric bills for this home, we installed two heat pump zones in the apartment: one that covers the kitchen, living room and staircase, and a second one in the bedroom. Heat for the bathroom comes in from the rest of the unit when the door is open and is also supplemented by in-floor radiant heat (so feet will be nice and toasty in the middle of the night!).

The ceiling-mounted ERV. It captures air that’s exhausted from the home and uses that air to create the energy needed to ventilate the apartment.

From an efficiency and carbon footprint perspective, we give heat pumps a huge thumbs up. This is just one more way that we’re making sure this new in-law apartment will run efficiently. All the appliances that will be installed in the kitchen are also electric and highly efficient. From a carbon footprint perspective, we’ve done as much as possible to reduce the environmental impact of this remodel…right down to insulating with cellulose instead of foam and minimizing the amount of lumber we used.

We should also mention that, because the apartment is so well insulated and air sealed, we needed a way to bring in fresh air. We installed a low-profile Panasonic Energy Recovery Ventilator (ERV) which supplies fresh air to replace exhausted air – helping to balance the air pressure within the home. What’s great about and ERV is that it also preconditions the incoming air using the energy from the air that’s being exhausted. No need to throw that energy away!