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Building Science

Planning a Furnace-to-Ducted Heat-Pump Retrofit, Part 4: Zoning and Equipment Selection

An in-depth explanation of how to choose the right heat-pump-driven HVAC system

This article is Part 4 in a series on retrofitting fossil fuel furnaces to heat pumps with centrally ducted air handlers. By this point in the design process, we’ve collected information on customer goals, the building envelope, electric service, ductwork, and physical space constraints. We’ve used the methodology outlined in ACCA Manual J to calculate design heating and cooling loads. If possible, we’ve incorporated blower door data into our calculations. Now it’s time to use this information to evaluate zoning options and select equipment.

Zoning options

It’s possible to plan a retrofit as a “box swap” in which the new heat pump is a one-to-one replacement for the old furnace. Except for changes required to fix defects in the ductwork or accommodate the heat pump’s higher airflow, distribution and zoning remain the same as the old system. If the existing ductwork is in reasonable condition, “box swaps” are usually the simplest—and most cost-effective—approach.

But there are situations in which adding one or more ductless zones can make sense. A common example is a “bonus room” over a garage. Because it adjoins the outdoors and the garage on several sides, a bonus room has high per-square-foot heating and cooling needs. At the same time, duct runs serving bonus rooms are usually the longest and most convoluted in the house. Unless the duct system was carefully designed and installed, airflow—and thermal comfort—will be less than satisfactory.

A separate zone, with its own ductless head, offers several advantages. Heating and cooling in that space can be decoupled from the main thermostat and air handler, allowing better temperature control. Breaking off a zone from the main body of the house can also help overcome capacity limitations of equipment and ductwork. In a single-zone system, the…

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  1. ErikOlson | | #1

    Great article. Was going to ask about the possibility of using multiple units to gain a higher effective turn-down ratio, but then saw that section at the end.

    One question regarding the use of air handlers. Have noticed that the performance specs of units with full or concealed duct air handlers is lower than ceiling or floor mounted units. Is this due to the energy consumed by the blower? More on this question here:

    1. Jon_Harrod | | #2

      Thanks Erik. I believe you are correct, the more powerful fan motors are a factor. I didn't dig into Mitsubishi specs but I have an LG catalog handy, and I noticed that their ductless and low-static units draw 0.2-0.7 amps (240V), while the high-static concealed and multiposition air handlers draw 1.1-2.3 amps. Without doing the math, I could see how this could knock the SEER and HSPF ratings down a point or two.

    2. Chris_in_NC | | #3

      It takes more blower power to move air at the higher static pressure created by the ducting.

  2. charlie_sullivan | | #4

    Great information and discussion.

    I had forgotten that manual S specifies that a heat pump has to be sized based on cooling; I think a lot of people ignore that provision and just size it based on whichever requires larger equipment, cooling or heating. In a lot of cases, the loophole you describe will be fine, but with units that don't have a lot of modulation range, in climates with very modest cooling needs, or worse with single stage units, that rule could prohibit correct sizing. I'm not sure whether it's being enforced in regions where it really would prevent correct sizing, but it is a shame that most conscientious designers trying to do it correctly by the book can't do it right in some cases. I imagine that it was written that way based on the assumption that heating dominated climates would never be heated by a heat pump alone, but that was before we had enhanced vapor injection heat pumps with excellent low temperature capability. So the manual needs an update for modern technology, and even without that technological advance it ignores mild climates where you really don't need cooling at all, but do need significant heat.

    Of course, it is better to use modulating equipment such that the minimum coolant capacity at low speed is still reasonably sized for the cooling loads by one of the approaches you described, rather than just sizing for heating and ignoring the fact that it's over sized for cooling. But if it's really a heating dominated climate, the energy used for cooling can small enough that it doesn’t matter much for the overall energy use. In that case, the oversizing as cooling equipment is really only a consideration for how well the dehumidification works, and that might be better handled with different equipment anyway.

    At the other end, sizing for 90% of the heating load seems like a good choice. Then the sizing of the aux heat, deciding between 10% or bigger, is a low stakes decision in terms of energy use and equipment cost, but can be higher stakes in terms of electrical capacity, especially if it triggers a service upgrade. It should be noted that it is fully code compliant to size for 100% of the load total, not the 140% maximum that some people mistakenly treat as a minimum. As you note, in some cases people size aux heat for backup instead of supplemental heat, which can be an inexpensive way to get a backup when the service size is already big enough, but is an expensive choice when it would trigger the need for a service upgrade. And it only serves as a backup for one of the scenarios in which you would need it back up–a refrigerant leak–so it might not be worth it.

    Minor question: do you have a recommendation for a data source for binned climate data?

    1. Jon_Harrod | | #6

      Thanks, Charlie. I agree, I feel like Manual S hasn't really caught up to modern cold-climate heat pump applications. I also agree that we need to consider different equipment like dehumidifiers to handle conditions of high latent/low sensible load, which are common in the Northeast.

      I had some difficulty finding bin data (or even hourly data that I could bin myself). It turns out that Right-J has bin data for many sites available on the site/design conditions tab, though I did have to transfer it manually into a spreadsheet.

  3. maine_tyler | | #5

    >"There are a couple things to note about this graph that relate to our sizing question. One is that “mild” heating conditions—outdoor temps between about 30°F and 60°F—represent a large fraction of the heating season. Very cold conditions—less than 10°F—represent a much smaller number of hours each year."

    Perhaps the calculation you mention immediately after this paragraph cover this, but wouldn't the 'time spent' not really equal the 'energy spent.' In other words, while the majority of TIME may be spend in mild conditions, the majority of ENERGY may be used in the extremes? And would this be an argument for slight oversizing (better performance on the edges) at the expense of some cycling in the mild? I'm partly just wondering if the installers who want to oversize should be pushed back against.

    By the way, really enjoying this series. Trying to wrap my head around this complicated subject. Unfortunately, we all seem faced with installers that want to eye-ball a room and then install a big piece of equipment. But is that really as bad as it's made out to be, when it's heated dominated?

    1. charlie_sullivan | | #7

      Yes, but then again, if you size for 90%, the energy needs not met by the heat pump are a small amount of energy as well as a small amount of time, assuming you are using aux heat to supplement, not shutting down the heat pump and switching over to fossil fuel.

  4. william_sears | | #8

    Great series Jon. I wish you were closer to us here in Santa Cruz, California. Our local heat pump installer wants to just slap in a furnace replacement air handler/heat pump and call it good. No calcs, no upfront work. We have a two bedrooms above our garage that need careful thought re: cooling in summer and heating in winter. Thanks again for the detailed articles.

    1. Jon_Harrod | | #10

      Thanks William. I hope you can push your installer to do their homework. It is really in their interest to do so because it will reduce callbacks. Rooms over garages often have comfort issues. Often the fix involves some combination of insulation upgrades and mechanical work.

  5. norm_farwell | | #9

    Thanks for another fascinating article Jon. The graphs on sizing and modulation are really helpful.

    I’ve done a couple of furnace replacements and found them to be quite challenging from an install standpoint. Existing duct systems are always filthy, frequently inadequate, and usually require extensive improvements: additional return and/or supply capacity, modifications to registers, mastic sealant, and that horrendously itchy fiberglass duct wrap.

    Anyhow, looking forward to the last two installments.

  6. dadiy | | #11

    The solution I’m hoping to find is replacing the old heat exchanger in the gas furnace air handler and the outdoor AC compressor with heat pump equivalents. I’m in the Los Angeles area, so sizing is cooling driven and backup heating is a much smaller concern. For backup heat I’d like to keep the gas furnace functionality. In a power outage I can run the gas furnace off my backup battery or portable generator (a heat pump compressor pulls too much power). I also think it’s more carbon efficient overall to use a small increment of natural gas infrequently with existing equipment than it is to dig the additional metal ore (and fossil fuels) for a complete system replacement. However labor cost and skills availability is a problem as the other commenters have noted. FYI my existing backup cooling system in a power outage is an evaporative cooler at one end of the house and a whole house exhaust fan at the other.

  7. william_sears | | #12

    Jon, having left a comment here a year ago, we're now contemplating going forward with a 2 zone ducted heat pump replacement for our old gas furnace which is on its last legs. The system they are recommending is a Mitsubishi 36K BTU ducted MXZ-SM36NAM-U1 w/ PVFY-P36NAMU-E1, a Honeywell TrueZONE HZ322 zone panel w/ (2) EcoBee thermostats (one for upstairs, one for downstairs). We have a 1200 sq ft house w/two rooms above a garage and a living room/kitchen and bedroom downstairs. Some reworking of existing ductwork will happen to increase return size and upgrade old beat up ductwork. Any thoughts on equipment selection? The heat pump and air handler are advertised as variable speed but it's not clear to me that the Honeywell unit w/Ecobees supports that functionality very well. Thanks!

    1. Jon_Harrod | | #13

      Hi William,

      I try to stay away from commenting in too much detail on specific proposals. Here are a few general thoughts:
      --Ask to see the sizing calculations. I would make sure that the inputs (especially surface areas/construction type, glazing orientation, infiltration, and design temps) are correct. Without knowing much about the house, 3 tons sounds big for 1200 sf in a mild climate like Santa Cruz.
      --Ask how they decided on that equipment combination. The MXZ-SM36NAM-U1 is an advanced unit that can be connected to multiple indoor units. I wonder if a dedicated 1:1 (like an SUZ/SVZ combination) might be simpler and less expensive.
      --Mitsubishi has a new internal modulation algorithm that makes me more comfortable with using third-party thermostats like Ecobee. I don't have much experience zoning forced air using dampers, generally using multiple indoor units instead. If I were going to do this, I'd probably use Mitsubishi's Airzone controls, since they are optimized to work with Mitsubishi equipment. But if your installer has a good track record with the equipment combo listed, and their references check out, I would be inclined to go with their recommendations.

      1. william_sears | | #14

        Thanks Jon for the thoughts. The contractor has not done any Manual J type's just not how they operate around here. They looked at the 1200 sq ft and considered that we might add 500 sq ft to the house at some point in the near future (TBD...may or may not happen). Reading your previous articles, 3 tons seemed big to me too.

        Regarding dampers vs. multiple indoor you mean multiple air one for each zone?

        Thank you!

      2. william_sears | | #15

        Just used to roughly calculate heating and cooling load for our house. It's coming up with 29,700 for heating and 14,900 for cooling (1,100 for latent). I input all the windows/orientation, all conditioned rooms, etc.

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