Adding Air Conditioning to an Existing Furnace

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retdinsb
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Adding Air Conditioning to an Existing Furnace

Post by retdinsb »

Our house currently has a natural gas forced air furnace that we've been thinking about adding central air conditioning. The current installation was built in 1968 but the furnace was updated approx. 10 years ago and runs fine. The current system is a single stage 80% AFUE updraft furnace located in a small closet in a central hallway. Our heating bills are typically about $75 per month for additional gas and electric. There are typically about two months out of the year where air temperature in the house gets over 80 F during the day and we use fans to cool down the house at night to sleep. Air conditioning would be a luxury but as I get older it would be nice.

We've had three contractors come by to evaluate our system, and two of them have suggested complete re-designs where the system would be upgraded to a newer high efficiency furnace combined with a new air conditioner and be re-located to the attic. This would of course involve additional structural, ducting, electrical and gas piping work. I haven't heard back from the third.

It seems to me that based on the amount of time I'd be running the the air-conditioning, I should able able to simple add it to the exiting system and ductwork. It may not be the most efficient system but for my limited needs I'd think it would be just fine.

Question is has anyone ever added air-conditioning to an existing heating system without a complete re-design and re-install of all the supporting systems and ducting ?
Last edited by retdinsb on Fri Jun 10, 2016 9:11 pm, edited 1 time in total.
RudyS
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Re: Adding Air Conditioning to an Existing Furnace

Post by RudyS »

One really has to see the current installation. Theoretically, an A-frame coil can be added to the plenum chamber (ductwork) of the existing furnace. But is there room? Will the blower be able to pump out enough air (a/c requires a stronger blower than just heating)? Might a window unit be good enough for you?
icecreamma
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Re: Adding Air Conditioning to an Existing Furnace

Post by icecreamma »

We had an ac unit added to our new furnace install in the same location as the previous furnace 10 years ago. This was in a utility room in our basement with plenty of space. I am guessing you don't have the room in your closet to fit the ac portion of the unit.
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fetch5482
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Re: Adding Air Conditioning to an Existing Furnace

Post by fetch5482 »

Do you need central ac? Would split air conditioner work?
We are thinking if the latter option in a similar situation (1971 town home with furnace).
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mhalley
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Re: Adding Air Conditioning to an Existing Furnace

Post by mhalley »

Many years ago we had a natural gas furnace and separate ac unit. The gas line developed a leak and would have required many thousands of dollars to repair. We elected instead to have a heat pump installed that used the same duct work as the Of course it is more expensive to heat than with a gas furnace. I don't know the current rates, but I think it is roughly 3x the cost of gas heating.
FandangoDave5010
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Re: Adding Air Conditioning to an Existing Furnace

Post by FandangoDave5010 »

Our house was built in 2001 with a Bryant propane furnace. We added a Bryant 5 ton air conditioner in 2007 using the existing heating ductwork. It has operated without problem except that we replaced the squirrel-cage fan and humidifier in the ductwork last year. The house was designed to incorporate central air conditioning at a future date.

My mother's house built in 1959 with a forced air heating system also added central air conditioning without a problem.

HOWEVER, when we lived in New Jersey and everyone burned oil and had hot water baseboard for heat there were no ductwork for AC so that we had to installed window units. BUT, if you have forced air ductwork in place like my mother did, I don't see why you can't add AC to the existing furnace. Talk to a heating/AC contractor, not necessarily a salesperson at a big box store.
mrc
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Re: Adding Air Conditioning to an Existing Furnace

Post by mrc »

You'll also need to drain condensate -- not sure if the 80% AFUE unit needs one already, and not sure furnace and AC drain/pumps can be combined. If the new system would be a major league compromise because of the limited space, you may be happier with a completely new attic system in the end. If space is too tight for any A-coil condenser you don't have much choice. The split system is an alternative too. Much depends on the home layout.
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retdinsb
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Re: Adding Air Conditioning to an Existing Furnace

Post by retdinsb »

Thanks to everyone for the responses.

There is room for an A coil on top of the existing system. We considered a split type of system but didn't care for the way they looked on the walls and a window air conditioner wouldn't work in the main living space without some type of major alteration to one of our french doors.

Most of the suggestions from the contractors are directed to getting a higher efficiency system but I can't justify the additional cost. The new system would never pay for itself over a projected 20 year lifetime based on my current energy cost for heating the house. Their reason for moving into the attic would be to reduce noise which isn't much of an issue, provide for a gravity fed condensate drain which can be handled by a pump and allow for a ceiling return which would improve the air conditioning performance but since the system is used primarily for heating I would prefer the current return location which draws cooler air lower in the hallway.

My only concern is the existing ductwork and whether its adequate but again would be willing to sacrifice a little performance based on the minimal use expected for the air conditioning. The current ducting is metal with fiberglass insulation and is in pretty good shape. Condensate may be a concern since the current system probably wasn't designed for Air Conditioning but I'm not sure how to assess that, and how to determine if I need sometime of vapor barrier.
mrc
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Re: Adding Air Conditioning to an Existing Furnace

Post by mrc »

Cold air is heavier than warm air, so verify the ductwork capacity for sure before moving on. That's another benefit of an attic unit (easier to push heavy cold air down than push it up). The condensate from A/C or an ultra high efficiency furnace is removed via a small pump, and will need a line to daylight. Can understand not wanting to replace a 10-year old 80% system, the payback period is probably too long. You mention not using the AC much. Is that because you live in a northern or drier climate? Or do just plan not to use it much?
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4nursebee
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Re: Adding Air Conditioning to an Existing Furnace

Post by 4nursebee »

My vote would be for a single zone ductless minisplit in the bedroom.
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retdinsb
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Re: Adding Air Conditioning to an Existing Furnace

Post by retdinsb »

We've thought about a ductless mini split system but we don't like the way it would look in the main living area.

We only have about two months out of the year where air conditioning would be nice but it's definitely not required. It's a luxury that I'm not sure I'm willing to pay that much for. Upgrading the furnace by itself in the current location would cost on the order of $2500 for a newer high efficiency system. The first bid I received for re-positioning the system into the attic and adding air conditioning to a new furnace with new ducting was $8500. I can't justify that cost.
4nursebee
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Re: Adding Air Conditioning to an Existing Furnace

Post by 4nursebee »

retdinsb,
There are many styles of ductless units, did you not like the way that ALL of them looked?
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RudyS
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Re: Adding Air Conditioning to an Existing Furnace

Post by RudyS »

How about just a window unit in the bedroom? Can you tolerate the warmer air in the main living spaces? Seems like the lowest cost option. Have done that myself in our "starter" home.
daveatca
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Not easy to "add"

Post by daveatca »

Today, 92+% efficient furnaces and AC are packages.
It is all about the "plumbing".

Just adding AC to an 80% furnace is going to be very difficult.
em2006
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Re: Adding Air Conditioning to an Existing Furnace

Post by em2006 »

We have a heat only forced air furnace and use window units for the few months that we want to sleep with cold air. I had my 20 year old furnace checked out a few years ago and the tech said if we wanted to add AC at some point, we would need bigger ducts to handle the cooling load. Natural gas furnace heats well so I'm not replacing until I have to. Long Island NY
mrc
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Re: Adding Air Conditioning to an Existing Furnace

Post by mrc »

No way I would ever replace a natural gas furnace with a heat pump either! Sure, it's cheaper to move heat around than to create it. But we were never so uncomfortable as the winter we spent in a townhouse with a heat pump. Current house had a heat pump with oil backup. I always used "emergency" (oil) heat in the winter, and the heat pump in AC mode in the summer. When the heat pump died, we installed a dedicated AC unit. No more heat pump shmeat pump.
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