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Question DetailsAsked on 1/28/2017

Is it more money to have 2 furnaces in a 3000 square foot home?

Its a brand new home, 9 foot ceilings on the first floor. Its a 2 story home, no cathedral ceilings. It have a full basement and 2 new carrier units in the home. One for upstairs and one for downstairs. Is that going to cost a lot more to have 2 units?

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Certainly more money up-front, to install two furnaces - by at least a couple to few thousand $ probably. For that size home, unless a very spread-out ranch or hacienda/mission where duct runs through unheated spaces (crawlspace or unheated basement) would be very long to the other end of the house so would result in unacceptably cooler air (though proper insulation can mitigate that), two furnaces is pretty rare. Except that in some very cold winter areas that practice is more common because in the event of one furnace failing you still have at least some heat in the home - presuming the failure is not due to a gas supply or electrical supply failure, the latter of which is actually the most common cause probably, due to storms.

Here is a link to a previous question with a lot of responses on the selection of furnaces and one versus two issue FYI -

But it sounds like you have two units already installed in a home you have already bought - so perhaps you are asking about operating costs. Yes it will normally use more energy, because while the overall heat needed by the house is the same, each unit has some basic heat losses which are greater (combined) in two units than in one larger one - especially if the furnaces are located in spaces where their heat losses from the furnace itself do not contribute to the home heating - like in unheated, outdoor air ventilated crawlspaces / utility closets / basements / attics. Depending on the specific units, electrical usage due to running two fans versus one, standby electric usage on two units, etc might or might not be noticeable but probably less than $2-50/year difference. If they have a standing pilot light, that might add $2-10/month in gas cost for the second furnace in normal gas cost areas.

The total heating energy usage theoretically would be the same because they are meeting the same heating demand. If the hot air ducts are all internal to the house (not in outside walls) having a separate upstairs furnace might be slightly more efficient in that respect, versus having a main duct running up through an outside wall - though any good design would try to avoid that and keep at least the hot ducts more centered in the house.

Theoretically, (but maybe not noticeable in the bills) running two smaller furnaces (one for each floor, presumably) should be a bit more efficient than running one larger one when only one area is perhaps demanding heat, because you are not pushing the air so far (hence less fan electric cost) and would not be possibly heating an area which does not need it just because the other area is demanding heat. Certainly, one unit for each floor should be a bit more efficient than a single unit with fixed ducting and a single thermstat service both floors, and should give a heating performance more closely matched to your desired temp than a single fixed duct system - at least if there is a door between upstairs and downstairs so the two furnaces do not fight against each other by having hot air from below rise to the upper level and cooler upstairs sinking down to the downstairs.

Whether having two versus one would be more efficient compared to a single larger furnace with individual thermostats on the two levels and electrically controlled duct damper limiting the heat to the area (upstairs or downstairs) which is demanding it I would doubt - that is probably the more normal way to handle two-story heating, with one furnace.

Probably the biggest wastage would be if one or both (or their ducting) are in unheated areas or poorly insulated outside walls - so the "waste heat" coming off the unit itself does not go to heating the house. For instance, my tuck-under garage is, until it gets down well below freezing outside, totally heated by the waste heat from the boiler and water heater (though granted boilers have more waste heat than most furnaces because it is constantly hot). In the summer I have to vent the excess heat when working in there or it gets way too hot.

Assuming the second unit is not in a location where its waste heat is "wasted", and obviously without any specifics on the house or the units, I would guess you are looking at maybe -10% to +10% or so difference in energy demand for two versus one unit depending on all the factors above - unless carefully designed for two units and with a door closing the upstairs from the downstairs, almsot certainly on the +, or somewhat less efficient, side. An additional 10% fuel usage might typically run into $100-125/year for a normal house using natural gas - up to a couple hundred/year for propane or fuel oil or in an area with very cold winters or very high fuel prices.

Where cost will be significantly more is of course initial (and hopefully not till many years down the road, replacement) installed cost, periodic servicing (probably 50% more to do two than one unit on a single service call), and of course maintenance - because two units can be expected to have about twice the number of repairs, so you are probably looking at a good few hundred to maybe well into the mid hundreds per year average annual cost increase with two versus one unit. One good thing on that - Carrier is my preferred brand for moderate maintenance, low factory defect occurrence, and very long life, so you lucked out or made a good choice there, in my opinion.

So - bottom line - if I had to throw out a ballpark number, I would say probably around $150-300/year additional amortized cost (depending on your time value of money - likjely in the $300 range assuming you financed most of the house cost) and maybe $500/year rough ballpark range as a typical number for additional annual fuel, electric, cleaning and maintenance costs.

Answered 3 years ago by LCD

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