Partial Conversions

In my previous posts about the economics of home heating electrification using heat pumps, I’ve focused on whole home heat pump conversions and I’ve not given careful enough attention to the economics of partial conversions.

While I’ve done a whole home conversion myself, and while most agree that universal whole-home electrification is the ideal long term endpoint, we have to expect more partial conversions in the short run. That is the expectation built into the Commonwealth’s major planning documents, and partial conversions do offer some real advantages.

Alternative Home Scenarios

Different homes favor different electrification approaches. Home features that can make a difference include:

  • Presence or absence of a modern, high-capacity system of ventilation ducts that can support the air flow that ducted heat pumps need for efficient operation.
  • Performance of the building envelope (insulation, windows, air sealing) — lower performance envelopes increase the size of needed heat pumps and also make it harder to deliver comfort in all parts of the house.
  • Size and layout of the home — larger homes with multiple rooms on multiple floors create heat distribution challenges. Existing heating systems may solve these challenges well (radiator in every room); but a heat pump system with heads in every room may be inefficient and/or prohibitively expensive.
  • Existing air conditioning system — a central air system may offer the ducting necessary to support a whole home heat pump conversion; on the other hand, an absent or limited air conditioning system may motivate the installation of heat pumps for air conditioning.
  • Existing heating system fuel type — oil and propane systems are generally more expensive to run than natural gas systems and an electrification investment may pencil out more favorably.
  • Age and quality of air conditioning and heating systems — the closer to end of life and the lower the quality of existing systems, the more favorable the case for electrification.

Alternative conversion approaches include the following:

  • A whole-home, ducted heat pump system will be feasible where there are adequate existing ducts and the performance of the building envelope is good. Owners who are both affluent and carbon-motivated are likely to make the electrification investment if their home has these advantages. The overall uptake is likely to be low.
  • A whole-home, ductless system may be attractive in smaller units with an open interior layout that favors air mixing throughout the unit.
  • For most single family homes and most 2-4 unit homes, where each unit includes multiple rooms, some type of hybrid system is the most likely choice. The configuration possibilities in hybrid systems are many, but they include:
    • Install ductless heat pumps in only one or two heavily used rooms and enjoy both good heat and good cooling just in those rooms; activate the fossil heating system based on a very low thermostat setting to prevent pipe freezing.
    • Install ductless heat pumps adequate to heat more or less the whole house on days when heating demands are moderate; automatically cut over to the fossil system when temperatures go below a fixed outdoor temperature, for example 30 degrees. A lower cut-over temperature may be workable in homes that are well-insulated or have an open layout.
    • Install ductless heat pumps adequate to heat the whole house even with the lowest outdoor temperatures, but allow the electric utility to remotely force the house over to fossil heat as needed to manage demand. The utility will turn on the fossil system to protect the grid and avoid paying high spot electric prices in winter electric peaks that will develop as heat pumps become more prevalent. The utility will pay the homeowner for the privilege of managing the electric-to-fossil switch. Again, this is most likely to be feasible in homes that are well-insulated or have an open layout.

Pros and Cons of Partial Conversions

Advantages of partial home conversions in which an existing fossil system is preserved and used only at lower temperatures include:

  • Economic accessibility — installation of one or a few ductless units will usually be less expensive than a whole home installation.
  • Air conditioning motivation — people seeking to replace air conditioners or to add or expand the comfort of air conditioning can choose heat pumps at a relatively modest additional cost.
  • Operating savings (?)– using heat pumps only for some heating needs will, of course, reduce the increase in electric bills, but more importantly, the possible cost disadvantages of heat pumps are greater at lower temperatures where their operating efficiency may decline. Additionally, in the future, homeowners may be able to receive demand management payments from utilities if they will allow the utility turn off the heat pumps when the grid is overloaded.
  • Carbon emissions (?) — in general, heat pumps are efficient enough that they still lower carbon emissions even when using electricity generated with natural gas. This is important, because over the next decade or so, fossil fuels will continue to supply an important portion of our electric power; so turning on a heat pump will usually require that more natural gas be burned to generate the power. However, at lower temperatures, the actual operation efficiency of a heat pump may be low enough that, as compared to a gas furnace, it has negligible carbon benefits, or even carbon costs. The tipping point is a coefficient of performance of 1.9, not far below the rated low temperature performance of many pumps (and actual performance may run below rated performance).
  • Grid protection — both regional transmission systems and local street electricity distribution systems will be taxed as heat pumps become more common. A heat pump running at low temperatures will draw several times the power of an air conditioner serving the same home (it is pumping heat across a much higher indoor/outdoor difference). Hybrid systems can reduce the low temperature load on the grid.

The main disadvantage of partial conversions is that they do not position us for a fully electrified future. It seems unlikely that before 2050 we can expect most people to go through two major heating system transitions: we are stretching to achieve even one transition for each of the 2 million plus homes in Massachusetts. That means that many of the partial conversions will remain in place in 2050, necessitating either the preservation of our local gas distribution system or an ugly expansion of on-site stored fuels.

How much carbon emissions remain locked-in with partial conversions depends on how much of the heating load one expects the heat pumps to shoulder.

Temperatures Measured at Hanscom Field, 2013-2022

Temperature RangePercent of Year Round HoursPercent of Heating Degree Hours*
Below 10F1%5%
Total (differs from detail due to rounding) 64%99%
Source: METAR data via WeatherSpark, analyzed here.
*Heating Degree Hours are from base 65. Using a lower HDH base increases the share of HDH in the lower temperature ranges. For example, at base 55, 47% of HDD hours are below 30 degrees as compared to 39% at base 65 shown in the chart. The more efficient the home, the lower the appropriate HDH base — internal gains become relatively more significant when the home is more efficient; the heating-free balance point moves lower.

The chart above shows that temperatures below 30 account for 39% of the heating load for homes near Hanscom Field; temperatures in the 30s account for 34%. With this temperature profile, to cut the burning of fossil fuel by 50%, a hybrid system needs to consistently supply the heat above 33F.

To maintain whole home comfort in the 30s, a typical home with a fragmented living area will probably need very good insulation and/or an expensive multi-head heat pump system. Anecdotal reports suggest that many hybrid system owners end up turning on the fossil systems at higher temperatures. I’ll be impressed if the partially converted homes as a group achieve much better than a 50% reduction in carbon emissions. To be positive, that’s a glass half full.

Partial Conversions in our Climate Plans

For better or for worse, the Commonwealth’s major climate planning documents contemplate a future with many hybrid systems.

  • Our current 3 year Mass Save plan contemplates 36,590 partial heat pump conversions, but only 10,062 whole home (“full displacement”) conversions.
  • Our 2025/30 plan envisions 610,000 homes with hybrid systems in 2030, but only 180,000 with whole home heat pumps.
  • Even our 2050 plan expects that many homes will have fossil heat at least as backup: I was discomfited to notice on page xiii of the latest 2050 Clean Energy and Climate Plan a 2050 goal with 80% of homes electrified “(including those with on-site fuel backups).”

The emphasis on partial conversions reflects a choice of scenario by the climate planners. They discuss it at length in the 2025/30 plan and Appendix A to that plan. The planners note the cost advantages of partial conversions over whole-home conversions and the common motivation to improve air conditioning. They see partial conversions as the most promising approach to increase adoption rates. They struggle with the problem that full electrification will be delayed, but accept partial electrification noting the work force shortage and consumer unfamiliarity with heat pumps:

[T]ransitioning buildings to whole-home heat pumps can be costly in some circumstances, and the opportunities to replace existing heating appliances are infrequent. While heat pump technology available today can and increasingly does provide whole-home heating in the Massachusetts climate, heat pumps can also be deployed to replace an air conditioner or provide cooling for those who currently do not use air conditioners, while partially electrifying a building’s space heating. This type of retrofit is typically less expensive compared to a whole home retrofit and a phased approach increases the adoption rate in the early years. While partial electrification does avoid costs in the 2020s, these savings must be traded off against increased investments to fully electrify buildings after 2030. However, given the current lack of experienced workforce and lack of popular knowledge about heat pumps, a phased approach focuses the near-term efforts on expanding the market for heat pumps, potentially delivering cost and performance benefits in the longer term. This allows for significant efforts needed to augment workforce training, supply-chain scale, contractor knowledge, and consumer awareness of heat pump technology, bearing in mind the urgent need to drive decarbonization efforts forward quickly. Building market capacity toward realistic near-term goals while maintaining long-term goals for widespread electrification is critical for providing good-faith guidance and coordinating gas and electric utility planning.

Clean Energy and Climate Plan for 2025 and 2030, page 28.

The hope reflected in Appendix A to the 2025/30 plan is that after 2030, whole home conversions will dramatically increase. It’s not obvious that consumer decision factors will have changed much by then, but it’s not unreasonable to hope that in the 2030s consumers and installers will have greater familiarity and comfort with heat pumps. The technology may improve a bit. And, its possible that the relationship between gas and electric prices will evolve in a way that favors whole-home electrification.

Return to heat pump outline

Published by Will Brownsberger

Will Brownsberger is State Senator from the Second Suffolk and Middlesex District.

One reply on “Partial Conversions”

  1. I find myself skeptical of the idea that partial conversions would be a good way to offset winter peak grid load.

    I skimmed but the paper you cited seemed not to factor in the wholesale price effect of burning gas at home for heat: “gain, at scale, there may be a natural gas system unit price increase; we are beginning to examine this.” And did it consider the emissions implicit in switching plants from gas to oil as price climbs?

    It may be simplistic but, putting aside renewable growth for the moment, it seems to me we either burn gas for heat in homes or we burn it in power plants to generate electricity to heat our homes. Suppose the heat pump had no efficiency advantage, so it’s one to one in terms of gas btu per unit home heat. Then a full conversion vs. a partial is effectively just moving where we burn some of the gas.

    The stickler is whether we have power plants to burn it in if we move it to “the grid”. Seems like we do. In the summer our (ISO region) power plants can get up to as much as 13 or 14 GW peak of gas generation. As our little cold snap starts to kick in today, I see us burning oil and dropping down to around 5 GW, presumably because the gas is being used for residential heat. So I see a 8 or 9 GW window where existing plants could burn gas on days like this instead of it being burnt in home boilers and instead of switching over to oil. And I think the efficiency assumption isn’t generous enough, maybe. And we will be adding wind power which correlates well with heat load, so there’s that too.

    If we can’t get people to full heat pumps for cost or labor reasons that’s another thing, but home gas boilers strike me as a terrible way of offering the grid spare capacity, so maybe that shouldn’t be held up on the advantage side of the ledger.

    Btw. imagine how bad the economics of heat pumps and our emissions plans would be if we’d allowed more gas pipelines to be built, what with the marcellus shale gas looking for more markets now and being cheap (most of the time). We can’t have carbon prices maybe but kudos to all those who resisted those pipelines.

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