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Questions about furnace air returns

davemax21 | Posted in General Questions on

Regarding furnace air returns.
1. Can I get central air installed, have a gas forced air furnace?
2. Would I benefit from fresher air in the home if I had an air exchanger installed?

House built 1920, had coal furnace, converted to gas, had the big octopus in the basement with gravity heat.
Because of that only one duct upstairs in hallway,
None in 2 bedrooms, doors were kept open.
Only one return in living room floor.

Have had forced air furnace for 17 years, live in a electric municipality where electric rates are low, 20-30/month.
House is cape cod style only 880 square feet.
Very warm with heat on 68.
This is central N.Y., cold.
Was told because so small, would not need extra returns.
Thanks, Dave

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Can I get central air installed, and have a gas forced air furnace?"

    A. Yes, you can do that if you want. If you get a split system air conditioner, the HVAC contractor will install a cooling coil in the supply plenum of your furnace.

    Q. "Would I benefit from fresher air in the home if I had an air exchanger installed?"

    A. I'm not sure what you mean by "air exchanger," but I'm guessing that you're talking about a heat-recovery ventilator (HRV) or an energy-recovery ventilator (ERV). It's hard to know whether your house needs an HRV or ERV; the answer depends on how leaky your house is. If you old house is leaky, it may be getting plenty of fresh air already. If it is very tight, the indoor air may be stuffy. To find out how leaky your home is, you would need to conduct a blower-door test.

    Concerning your return-air ductwork: It's not unusual for a house to have a single return-air grille. This type of system isn't ideal, but it is common. It's hard to tell from your description whether your home has adequate return air ductwork. If your HVAC contractor is competent -- not all of them are -- he or she will design your duct system properly for the furnace that is being installed.

    -- Martin Holladay

  2. davemax21 | | #2

    Thank you Martin.
    The HVAC company is very reputable in my area. They stated years ago when they installed the furnace, that with the house being as small as it is, the return in the living room floor was ample for return. The furnace doesn't work harder due to that.
    Question was if any more returns would be need if central air installed?
    I was told that again, with the size of the house, cooling would not be a problem, just possibly the small bedrooms upstairs, with only having the one duct in the hall floor. Doors would be open, but did state that worst case, possibly a window unit might be needed in the main bedroom, if not cool enough. Just concerned with no returns upstairs, or in any other room for that matter. Okay still for central air with only the one duct?

    - regarding the air exchanger, or HRV.. not the tightest house in the world, so maybe it is getting substantial "outside" air .
    Thank you again.

  3. GBA Editor
    Martin Holladay | | #3

    The best system would include supply ducts and return ducts in each bedroom. The second best system would have supply ducts in each bedroom, and jumper ducts for return air. (For more on jumper ducts, see Return-Air Problems.)

    The third best system is what you've got. Whether you want to spend money for a fully ducted system is up to you. In some houses, it's easier to box in new ducts than in others. In a small house, it's often hard to figure out where to put the ducts.

    -- Martin Holladay

  4. davemax21 | | #4

    Thank you.
    I'm glad I didn't hear a firm NO!!!
    Agree, running ducts a bit of a problem, and depends on what someone can or wants to pay, anything can be done for the money.
    Due to the size of the house, and the layout. I may either not install any more ducts, or have the contractor explore the smaller flex return ducts , 1 or 2 additional which might be able to be installed with the least intrusion into walls, etc...(I do understand the jumper ducts).
    Thanks again for responding.

  5. Expert Member
    Dana Dorsett | | #5

    In locations with cheap power rates it may be worth the upcharge to go for a heat pump rather than
    AC-only. A 10 HSPF heat pump or higher will often have marginal heating costs comparable to a condensing gas burner at 15 cent/kwh electricity, and could be quite a bit cheaper if you're on one of the cheaper municipal power companies.

    With a house that size it's difficult or impossible to right-size a ducted central AC system. Odds are pretty good your cooling load is less than 1-ton, and most ducted AC starts at 1.5 or 2 tons. Make no decisions on this without an aggressive Manual-J load calculation conducted by a third party, not an HVAC contractor.

    If the house is one-floor + basement (not a dormered out attic) it may be possible to do both heating & cooling with a 1.5 ton Fujitsu 18RLFCD mini-duct cassette (HSPF 11+), which can deliver over 15,000 BTU/hr @ 0F outside. If you need more heating capacity than that it's possible to right-size a modulating ducted Mitsubishi MVZ series air handler.

    Short of a Manual-J, you can use the existing gas-burner to measure an upper bound for the heat load using the methodology outlined in this bit o' bloggery:

    For either a Manual-J or a fuel-use based load calculation sure to use the 99% outside design temperature and a code-min 68F for an inside design temperature, not the 99.6th or any other temperature. NY state codes require a minimum oversizing factor from the 99% load to cover the extremes, and if you use some colder outdoor temperature or higher indoor temperature and up-size from there you'll almost certainly end up with efficiency & comfort robbing oversizing factors.

  6. davemax21 | | #6

    Thank you.
    I believe I am actually at a 4.5-6 cent/kwh. Although efficiency in the equipment is indeed important, I never really had to consider the operating costs. but thank you for all the information.

  7. davemax21 | | #7

    Correction; I'm more like 8-9 cents/kwh.

  8. Expert Member
    Dana Dorsett | | #8

    Buck a therm gas in a 95% efficiency condensing gas furnace:

    Net heat per therm going into the house: 0.95 x 100,000 BTU/therm= 95,000 BTU/therm. So per milllion net BTU (MMBTU) it takes 1,000,000/95,000 = 10.5 therms At $1/therm that cost $10.5/MMBTU

    And that does NOT include the power used by the air handler.

    A heat pump with an as-used HSPF of 10 delivers 10,000 BTU/kwh, so it takes 1,000,000/10,000= 100 kwh per MMBTU. At 9 cents/kwh that cost $9/MMBTU

    And that DOES include air handler power.

    And, it's possible to right-size a heat pump for a low-load home, but not for gas furnaces.

    And, gas pricing is volatile, whereas electricity is undergoing major deflationary price pressures (particularly but not exclusively in NY.) eg:

    At last year's ~$3.50/watt installed cost for rooftop solar PV, after reducing that to $2.45 for the federal income tax credit the 30 year simple cost of power from the array runs about 7 cents/kwh. By the time the tax credit stops in 2021, assuming the installed price has fallen to $2.50/watt (likely the high side of what will be) that drops to 5 cents. Financing costs will make the full levelized cost somewhat higher than that, but between distibuted solar and wind the deflationary pressure on electricity, is pretty high, whereas natural gas is about as cheap now as it can be- constrainted primarily by pipeline capacity (which is expensive to build), and the comparatively high cost of fracking (which created the current glut). Over the intermediate term gas pricing has nowhere to go but up, whereas electricity pricing should be flat or falling.

  9. davemax21 | | #9

    Also heard air exchangers, can be helpful with keeping radon gas down.
    But that's for another discussion.

  10. Expert Member
    Dana Dorsett | | #10

    Yes, almost any ventilation will help mitigate radon down, and even modest amounts of balanced ventilation like an HRV is likely to cut the radon levels in half. That may or may not be enough to duck under the 4 picocuries/liter actionable level set by the EPA, but it's a good start.

    In Europe they've set it at 6.7 picocuries/liter, and in reality anything in single-digits could be deemed "pretty safe for non-smokers", since it's hard to control for even the passive-smoking or local air pollution risk from the radon risk in the statistics at those levels.) At 40 picocuries/liter the risk is measurable, but still not as extreme as one might think based on it's being 10x the level suggested by the EPA as needing mitigation.

  11. davemax21 | | #11

    Thank you,
    Yes, lots of definitive news on hrv's helping radon levels.
    My basement was tested at a 6-7, unoccupied. 1st fl 2.8 approx.

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