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Is an air handler compatible with gas backup?

user-6874977 | Posted in Mechanicals on

If a heat pump/air handler system has gas as a backup for winter sub-zero temperature periods, does that necessitate replacing the air handler with an evaporator coil?


My son has recently purchased an above-energy code house in central MA (zone 5A), built in 2008. It has the original high efficiency (natural) gas furnace coupled with a Carrier AC unit that needs $1600 repair. The house is centrally ducted and the conditioned space (including the attic where the HVAC is) is approx. 1380 sq. ft. on two floors.

He would like to replace the AC unit with a heat pump so he can heat mostly with electric. For now if possible, he’d prefer to keep the gas furnace for emergency backup as opposed to an electric coil (proposals have ranged from 7 – 10 kW), as the electric requires up to a 50 amp circuit. The house has 18 PV panels on the roof. They are grid-tied. Its electric service is 150 A.

A Bosch-based system has been proposed by several contractors. Two versions, usually. One where the gas furnace remains, and the other where the gas comes out, and electric backup is installed. 

As shown in the attached NEEP charts, the performance of the system with gas backup doesn’t compete with the system using electric backup. The only difference is the evaporator coil in the first, and the air handler in the second.

My uneducated guess (I’m a musician, not an energy/HVAC specialist) is that since the furnace would have its own blower, it could possibly interfere with the operation of the air handler/heat pump on those very cold (emergency) days. If that is the problem, are there ways to have an air handler work in tandem (simultaneously) with both the heat pump and the furnace?

I would appreciate any thoughts, counsel, and input anyone cares to offer.
Thank you.

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  1. Expert Member
    BILL WICHERS | | #1

    A typical furnace can operate as an air handler by running the blower and not the heating part. That's how central air conditioning usually works: the blower in the furnace runs, but not the burner, and the A coil for the air conditioner is mounted on top of the furnace in the output path of the air.

    A heat pump is basically a reversible air conditioner, so if you can get a heat pump that can sit on top of a seperate air handler (i.e. the heat pump doesn't require it's own proprietary air handler), then you should be able to use your existing furnace as the air handler as long as it can provide the require amount of airflow for the heat pump. You would set this up as a two-stage system, with the heatpump as the first stage, and the gas heat part of the furnace as the second stage. You may need to lockout the heat pump while the gas burner is running.


  2. Expert Member
    Akos | | #2

    There are options out there now. Take a look at the Carrier/Midea 38MURAQ24AB3 (make sure it is the hyper heat version), which can integrate with a regular furnace.

    Mitsubishi also makes a P series with a coil that integrates with a gas burner:

    With the right sized hyper heat unit, I don't think any gas backup makes sense. These can now carry a place without issues even in zone 7. These are also not like the old units that would shut down when cold, the continue to provide heat to well bellow zero but at reduced rate. A bit of backup strip heat to make up for this shortfall is all that is needed but that should not be the case in zone 5.

  3. walta100 | | #3

    From a financial point of view the dual fuel systems almost never make economic sense. Assuming you are connected to city gas and the electric grid. Your prices are set by your local government at a level to allow a small profit for the suppliers and are unlikely to change wildly and are likely to move in unison.

    Consider doing the math using your local prices. Is I recall MA electric rates were some of the highest in the country. Maybe the math would be different with solar panels but my guess is you have a lot of cloudy and snow days that cut production maybe not so much.

    There is often a good amount of money to be saved if you can remove the gas service and stop paying the monthly fees. If you own a leaky old house city gas is generally the lowest cost way to get the large number of BTUs required for such buildings.

    Note the title of this thread makes zero sense. As air handlers are installed in system to provide a fan where no other fan exists and by definition all forced air furnaces include a fan so they would never be installed in the same system. If you were to add a HP to a system that had a furnace you would add an A coil in a case on top of an up flow furnace.


    1. paul_wiedefeld | | #4

      I disagree - it’s certainly not an absolute. Hybrid systems do great in the following situations:
      1. It’s easier to use a lower end heat pump
      2. Easier to use a lower end furnace too
      3. Avoiding a panel upgrade can save a lot
      4. You can size for AC load not heating load
      5. It’s an easier path to heat when the power goes out
      6. I received a hybrid system quote for much less than a hyper heat system.
      7. Some places have TOU winter rates
      8. Some places have low monthly gas rates. YMMV.
      9. Using propane/oil as the dual fuel lets you avoid the gas monthly fees, if they’re high in a particular area
      10. The places with high electricity prices often have those due to delivery charges - so the gas and electricity rates are less correlated.

  4. walta100 | | #5

    From a financial point of view the numbers do not work do the math.


    1. paul_wiedefeld | | #6

      National grid charges Boston residents $144 per year in fixed charges. If a panel upgrade costs me $2000, by keeping or adding the cheap furnace option, I avoid a 14 year payback which I may move before I ever see. By installing a cheaper heat pump, because I don't need the cold climate capabilities, I save even more. I can also install something sized for AC, which should be a little less too. You might have an instant payback keeping the dual fuel option.

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