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Community and Q&A

Air-source heat pump vs. AC/furnace

ctkanes | Posted in Mechanicals on

I am contemplating some different HVAC options in Connecticut (zone 5). I currently have an 80% efficiency Carrier Infinity NG furnace (8 years old) that can be paired with a Carrier Greenspeed heat pump. I can do that for $11,400 before incentives, or replace my existing and dying 10 SEER AC unit with a new 16 SEER for $8,000.

I can also pair a new AC unit with a high efficiency gas furnace, ranging from $15,000 for Performance level or basic Infinity equipment to $18,400 for equipment with Greenspeed technology. Our current equipment is 5-ton, and our home is 3,500 square feet. We’ve tightened the envelope and insulated up to R-49.

We are in this home to stay, so we can wait for payback to occur. I just don’t know costly it will be to heat our home using an air source heat pump, or how much we’d save going with a higher efficiency gas furnace over keeping what we currently have. Does anyone have any thoughts or suggestions?
Thank you!

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  1. user-968787 | | #1

    In order to calculate any kind of payback or pros / cons analysis you should probably get a heat load calculation completed.

    Heat loss link

  2. rocket190 | | #2

    Good lord those prices seem high unless you need extensive reworking of your ducts and plenum. Have you gotten any other quotes?

  3. ctkanes | | #3

    Darryl, we had an operating cost estimate done, which included heating and cooling requirements for our home. I wasn't satisfied, however, as this estimate has us using 1,804 therms for heating, when our historical data has us using around 900 therms per year. If the estimate is off by this much, then the heat pump makes sense, as we'd use fewer kWh than if we needed twice as much energy for heating.

    Rick, we've actually had 4 different quotes, and the ones above are the lowest. Cost includes the fabrication of fittings to connect with the plenum and ducts and setting up a crushed stone pad outside for new heat pump/AC condenser. Wires must also be run for the thermostat for any Greenspeed technology that is installed. No major reworking is necessary. The highest prices include the cost of removing the old furnace and putting in high efficiency furnace and AC, as well as venting directly outside. Perhaps labor costs are higher than your state, this being CT (one of the most expensive states for labor)?

  4. GBA Editor
    Martin Holladay | | #4

    It usually costs less to heat a home with natural gas than it does with an air-source heat pump.

    It will probably cost less to cool your home in the summer with a few ductless minisplit units than to adopt any of the approaches suggested by your contractors.

    Most contractors don't know how to perform an accurate heating load or cooling load calculation, and therefore end up specifying oversized equipment. You may want to hire a good energy consultant to perform accurate heating load and cooling load calculations.

  5. user-945061 | | #5

    Without knowing your rates for natural gas and electricity, I can't really say for certain, but the answer is probably that it's not worth installing a heat pump. I would guess the point at which the heat pump operates at the same cost as the furnace is between a COP of 4.5-5. The GreenSpeed will hit this at somewhere over 50F, which means that it could (economically) cover only a small part of your heating demand.

    Heat pumps look really good when we use site energy as a metric for savings, so consequently there's a lot of confusion about when to install them. The GreenSpeed is >300% efficient, and your furnace might be 98% efficient, yet the furnace will be less expensive to operate, owing to the low cost of gas and the high cost of electricity.

  6. Expert Member
    Dana Dorsett | | #6

    The R49 attic is good, but how good is the insulation elsewhere (including the foundation), and what type of windows?

    A typical IRC 2012 code-min 3500' house in CT will have a heat load above the output of a 3 ton (IIRC the biggest in the series) GreenSpeed at typical CT outside design temps, which means you would be using significant amounts of auxilliary power when it's cold out. If your house is significantly better than code, it might cut it. But you really need a careful heat load calculation using Manual-J methods, or at the very least a fuel-use against heating degree-day data of what it takes to heat the house with your current system. If you have a recent couple of months gas bills with the EXACT meter reading dates and fuel use during those intervals, with a ZIP code we can look up the weather data for the period on, and work backwards from the fuel use to put an upper bound on the heat load at your 99% outside design temp. (That's simple enough to do in a web forum like this.) Short of that, hire an energy nerd to run the load calculations- somebody who has nothing to sell you other than the accuracy of their numbers, since typical HVAC contractor calculations are all over the place, as Martin has noted.

    Residential rate electricity in CT is among the highest in the lower 48, but if you have access to the sun and favorable roof angles you can beat the utility rates with rooftop solar by about half (on a lifecycle basis, including the financing costs), in which case a modulating ducted heat pump like the GreenSpeed may eventually "pay back". But at grid-retail rates in 2015 for both electricity & gas in CT gas will have a somewhat lower operating cost.

    Odds are pretty good that the 5 ton AC system is at least 1.5x oversized for your actual cooling load, and a 3- ton GreenSpeed would almost certainly cover it.

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