Of course, customer choices to leave windows open or to set thermostats high affect energy use and complicate heat pump program evaluations. But those problems exist for any heating system.
Customer choices are both consequential and especially hard to predict in the context of partial heat pump conversions — conversions where an existing heating system remains in place and continues to serve a portion of the heating load. There are many possible ways that people could choose to mix heat pump use with an existing heating system, ranging from ad-hoc comfort-driven operation to routine simultaneous operation to rigid cutover rules implemented with automatic “integrated controls“. Integrate controls typically activate only the heat pumps until the outside temperature reaches a temperature below which the fossil system is necessary to keep up and/or is more cost efficient than the heat pump.
Early evaluation research had suggested that many consumers were using their heat pumps for heating mostly in moderate weather conditions. As a result, heat pumps were not achieving the fuel displacement that planners had assumed. In response, in preparing their most recent three-year (2022-4) plan, MassSave Program Administrators conducted the Energy Optimization Fuel Displacement Impact and Process Study, which included a rigorous analysis of consumer use of heat pumps installed in 2019. Based on the findings of that study, they implemented a new requirement for use of integrated controls in order to receive incentives for partial replacement projects.
It remains to be seen how customers will actually use integrated controls and how much integrated controls will actually increase fuel displacement. We can look forward to further evaluation research. The results will be important because a relatively small change in typical cutover practices can make a big difference. Cutovers in the mid-20s save almost twice as much fuel as cutovers in the mid-30s as the chart below shows.
| ANALYSIS FOR PARTIAL REPLACEMENT OF FUEL HEATING WITH MINISPLIT HEAT PUMP | |
| Expected annual net winter electric increase from MHSP partially displacing oil (from Eversource Benefit Cost Analysis: EA2c268 combined with Midstream, EA2c004; 91% winter load share from EA2c268 applied to both) | 3,302 kwh |
| Expected fuel savings – thermal value of oil or gas (EA2c268) | 44.95 MMBTU |
| Net lifetime GHG saving if displacing oil heat (directly using average marginal AESC .333 kgCO2/kwh; EPA 74.54 kgCO2/MMBTU) | 38.3 MTCO2e |
| Net lifetime GHG saving if displacing gas heat (directly using average marginal AESC .333 kgCO2/kwh; 53.06 kgCO2/MMBTU) | 21.8 MTCO2e |
| Benefit cost-analysis assumes average cutover temperature from heat pump to fuel heat in mid 20s (see Fuel Displacement Study Table A-2 through A-4, planned, and compare with Table 4-5, modeled) | Roughly 25 degrees |
| If true average cutover temperature is in mid 30’s, operating GHG savings would be reduced by 50% (see Table 4-5 in Fuel Displacement Study: savings from baseline of 80MMBtu drops roughly 50% from mid 20s to mid 30s) | 20 MTCO2e (oil), 10 MTCO2e (gas) |
| Approximate GHG impact of 100% life-time GHG leak – enough to wipe out GHG savings from a partial gas displacement. Mass Save 2.8 tons average installed for minisplit partial equates to > 10lb. of GHG 2000 refrigerant charge, based on example 1.5 ton pump with 7.5 lbs charge. | 10 tons CO2e |
Some real world observations:
Few consumers are completely removing conventional heating systems when installing heat pumps. Particularly with mini-splits, as it is near impossible to have supply (hot/cold air) in every room. This is especially true in bathrooms, with limited wall or ceiling space. Who wants a cold bathroom in winter?
Integrated controls? Would you be surprised if these were being bought by contractors, the invoice used to get rebates…but they are never installed, rather are returned to the supply house?
3302 kWh net winter increase…that’s not what I’m seeing in the field. I have a customer that saw her electric bill go from $800 after heat pumps. That was every winter month this last season. Her oil bill would have been <$400/month. That increase is far more than $3302 for the entire winter. After spending 10's of thousands of dollars it was rather a disappointment. To make matters worse, as the hot water boiler was off she burst a heating pipe, which soaked the controls on the boiler – so now there is no backup and she has little money to repair it after spending the money on installing heat pumps.
Solar might help, as it can be installed with 0$ down and the net after payments is almost always a lower electric bill – why isn't solar being paired with heat pumps to make it more economically advantageous?
Are any of these calculations using a biofuel/heating oil blend in their calculations? Almost all progressive dealers are using a 10-20% blend, therefore reducing their GHG emissions with zero cost to the consumer. While not a total reduction in emission, it's better than nothing, which is what you get when consumers are reluctant to embrace conversion to heat pumps for a whole host of reasons.