Lessons from the Field: Month Three of the Whole-Home ASHP Pilot

MassCEC’s whole-home air-source heat pump pilot program has been running for about three months now. This blog is part of our commitment to provide monthly updates on the pilot (see our launch posting and our June and July posts for more information). The goal of these updates is to share results along the way and get input as we go.  This month I’m going to dive into two technical issues that keep coming up: electrical service upgrades and designing around minimum capacity.

But first a general update!

We’re now up to sixteen applications: five new construction projects and eleven existing building projects. However, there’s not a bright line between the two types of projects; several of our existing building projects are part of significant home renovation projects, which is a great time to update your heating and cooling system to heat pumps.

We now have installers from twenty-six companies listed as participating installers for this pilot program. We’re starting to see repeat installers. Net Zero Heating & Air Conditioning, LLC has submitted three applications and Gates HVACR LLC has submitted two applications. Three projects were part of the same Habitat for Humanity development installed by M.J. Moran, Inc., and Keyes North Atlantic, Inc. submitted for two separate units in the same building.

We got our first project completion and CMC Energy Services completed our first inspection using our new inspection rubric at Alison Gustafson’s home in Newburyport. The project passed with no issues, and Affinity Heating & AC used both strategies I mentioned in the July post to keep her heat pumps above the snowline (a ground-mounted stand and wall brackets).

Electrical Upgrades

Now for a more technical deep dive. Three of our projects so far have required an electrical service upgrade. This is a common issue in older housing stock. Many older homes only have 100 amp service (some even have 60 amp service), which was plenty for when they were built, but it may not be enough for homes to electrify their heating, not to mention their car, their stove, their hot water, etc. Costs to upgrade your electrical service from 100 amp service to the 200 amp service that is more standard in modern construction start around two to three thousand dollars, but the upgrade could be more depending on your home (e.g, if you have underground electrical service). It’s also possible that other limitations such as breaker availability may require more modest upgrades such as a new sub-panel. 

A single-head heat pump might add 10-15 amps of rated load (which might go on a 20 amp circuit breaker), and a multi-head might add 30 amps or more, which may bump the home’s total load over the capacity of an existing 100 amp panel. Electric vehicle Level 2 chargers (standard for home charging) can vary, but typically require 16 to 30 amps. An electric clothing dryer could add another 15-20 amps, and heating your hot water with electricity could require 15 amps for a heat pump water heater to 20-25 amps for a standard electric water heater. So you can see how all this electrification can start to add up.

I want to note that you do not take the sum of the circuit breakers in a panel and add them together to get the total size of the service because the mechanical equipment load on each circuit breaker is typically smaller than the breaker size, and because not all the equipment in a home will be running at the same time.  Code requirements for sizing electrical service specify the calculations to use.  For example, MassCEC’s technical consultant for this pilot, Bruce Harley, has three tons of heat pumps, an electric water heater, and an electric car charger at his home, and he still has 100 amp service. The highest load he’s ever measured at his energy efficient home is only 48 amps.  I’m not saying that we need to upgrade every 100 amp panel in the state, but I do want to highlight that as we move to “electrify everything,” upgrading electrical service is going to be a significant barrier for older housing stock.

Minimum Capacity

The next thing I want to talk about is a common sizing issue that we’ve seen on most of the applications: oversizing. The general thinking seems to go: Isn’t it good to have some oversizing buffer for these whole-home systems, especially since these units are variable speed so they can ramp up and down? There’s some merit to that approach, but heat pump installers also need to think about the minimum capacity of the systems they are installing. If it’s a milder day and the indoor space needs less heat than the minimum capacity of the heat pump, it will cycle off and on, which is less efficient and causes wear and tear on the unit. In the very worst case, if the heat pump is so oversized that the home’s maximum heat load is actually less than the minimum capacity of the heat pump, that heat pump will cycle all winter even on the coldest days!

An oversized minimum capacity is a more common issue with multi-head systems because there is a bigger difference between the lowest possible load in one room and the lowest capacity of the outdoor unit. One source of confusion that has come up in a few of our design reviews is what the minimum capacity for these units is. For example, on the submittal for the MXZ-4C36NAHZ it looks like the minimum capacity of the unit is 7,200 btu/hr at 47°F, but the NEEP data for that same unit says that its minimum capacity is 22,500 btu/hr at 47°F. That’s a big difference! Bruce Harley confirmed with Mitsubishi engineers that the Mitsubishi submittals are meant to indicate the capacity of the smallest indoor unit that can be connected to the multi-head and are not meant to indicate the minimum capacity of the outdoor unit.

If you’re worried about oversizing, one solution is to have multiple outdoor units. Bruce Harley recommends one single-head for the main space in a home and a smaller multi-head for the other room.