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Upgrading a crappy HVAC setup

Matt Culik | Posted in Energy Efficiency and Durability on

My wife and I are in the process of purchasing a new home in Northern NJ. The home was built in 1986 with forced hot air heat and withOUT air conditioning. Air conditioning was added later.

As a result, the house has two sets of ducts. One set feeds heat to both floors from the basement. The other feeds conditioned air to both floors from the attic – but there are too few ducts on the first floor for the A/C to work well for that floor.

The furnace uses oil and is likely nearing the end of its life. Also, the house has a pellet stove.

My thought is to replace the oil-fired furnace with a heat pump setup. The heat pump will provide heat in the winter and can be supplemented by the pellet stove on the coldest days, and will also provide A/C in the summer to complement the attic-based system. This setup also allows me to use the existing ducts.

What do you think of this approach? Heat pumps a bad idea in Northern NJ? What about utility costs vs. oil?

If you don’t like this idea, what would you suggest?

Thanks!

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Replies

  1. D Dorsett | | #1

    Heat pumps have improved a lot since 1986, and if you size it right will give you very good service, and will be cheaper to operate than an oil furnace. If the AC ducts for the upper floor are in the attic above the attic you may want to cap them off and use only the original ducts, or go with a modulating high efficiency ductless system (mini-split / multi-split) rather than using the same ducts, which is also easier to zone efficiently.

    Sizing it correctly is key for both efficiency and comfort so getting an accurate read on the heating & cooling loads is important. The "right" way to do it would be to hire an independent competent engineer to run a "Manual-J" load calculation based on the house construction, location, site-factors, etc., but that is almost never done correctly by HVAC contractors. In my area that costs several hundred USD, using a RESNET rater or a professional engineer. YMMV.

    Some HVAC contractors will run some sort of calculation, but it almost always trends toward oversizing by a significant factor- they don't want the 5AM call from a shivering irate customer on the coldest day of the year, so they tend to err to the conservative rather than aggressive on any unknowns or assumptions. When using an independent professional, instruct them to be aggressive rather than conservative, and use only the 99th percentile temperature bin for the design temperature (not the 99.6th percentile or colder), and a code-min 68F for the interior temperature, even if you normally keep it 74F indoors.

    The reason for being aggressive rather than conservative is that being conservative usually puts it at least 30%-50% higher than reality, but the step sizing of the equipment is pretty chunky, and you would then opt for the next size up from the calculated number making it 75-100% oversized, which has comfort & efficiency downsides. Upsizing it 25-50% from an aggressive estimate will always leave you some margin for weather colder than the 99% outside temperature bin.

    Short of a Manual-J you can get accurate fuel use information from the prior owner it's possible to estimate the size of the heat pump needed to meet your heating loads. If they were on a regular fill-up service they may have fill up slips with a "K-factor" stamped on the end. Even one mid or late winter fill up with a K-factor would be good enough to size a ducted system, but more are better. If they were not on a regular service, at least 2 and preferably 3 fill up quantities with the EXACT fill up dates would be needed to get a good handle on it. A ZIP code would also be necessary to look up weather data between those dates, and to estimate the 99% outside design temperature. For the basic methodology, see:

    https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/out-old-new

    and

    https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/should-we-promote-heat-pumps-save-energy

  2. GBA Editor
    Martin Holladay | | #2

    Matt,
    If your attic is a vented unconditioned attic -- one that is cold in winter and hot in summer -- then you don't want to have any ducts up there.

    Step number one, as Dana correctly advised, is to perform a Manual J calculation. Step number two is to evaluate your duct systems and figure out how you will be running your ducts. It's possible that you will decide that some areas of your house are best served by ductless equipment.

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