Can someone weigh in on what's involved to change out an 80 percent furnace (or worse) to a 90 percent (or better)?  In terms of both work scope AND cost?  Take out the furnace, and what's the cost for all ducting, labor, etc.?  Even a rough number for my reference is fine.

Here in Colorado, we have required "hi / low venting" to allow makeup air for <90 percent furnaces.  I know it's important for the furnace to have that, but MAN, I hate 'em.  They always add a significant amount of air leakage to a home when we run a blower door.  I'm so glad to see that go away as people upgrade furnaces.

Also, is it possible to reuse the old aperture in the wall for hi / low venting for (PVC) exhausting and makeup air venting?  In other words, the holes are already cut in the wall.  Can they be repurposed and sealed around the vent pipes?


Melissa B.

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Hi Melissa,

I'm sure others will weigh in here, but I always find it interesting to test this change with current and improved energy efficiency.  Often the saving for the change is difficult to justify once the home becomes very well insulated and sealed.  Yes, new appliances can be good, but fixed improvements can often be better.  Another part of this decision is, how old is the old furnace?  If it is close to replacement, then a significant portion of the cost was coming due anyway.  But I still like insulation and air sealing first.


Cost? - throw a dart at a board which you will see why below (not including all the options available for units & price differences from 90% to better)



Manual J & D - to see if everything is properly sized & what's needed

Assuming ducts are fine... Duct Test - are you slapping a Ferrari's engine in a Yugo where it won't perform?

New Vent - what do you have to drill through

Make up air?

What are you doing with old vent, is this going to cause issues with an orphaned item

Permits, final duct test? 

As for the rest & probably a few missed steps - hopefully an HVAC specialist will jump in

I can't speak for Colorado, but in Michigan we see a wide range of prices for a lot of the reasons that Sean Lintow mentioned. From my perspective, we also see a lot of homeowners concerned about indoor air quality and adding 4" filters or other air cleaning devices which also inflates the overall installed cost.

At the bottom end, (smaller heat output, simple installation) costs run at about $4000. At the top end, (complex install, higher heat output) projects can cost around $8000 or more in extreme cases. 

You should also be aware that time of year will affect the price of a new furnace. Since we're coming out of the heating season, there are a lot of HVAC shops that want to unload inventory, so they may be offering substantial discounts on heating equipment through the spring. 

I hope that helps answer your question!


You've gotten some good feedback so far, and can see that there are A LOT of variables involved in answering your question. Unfortunately, even as an HVAC contractor, I can't give you the short answer you are looking for either. The long answer is that is completely  dependent on application. In converting an 80% furnace to a 90+, you have to consider the following:

1. Venting - Based on your question, you want to bring in outside air for combustion (recommended anyway), but this means venting 2 pipes. The cost of this is variable based on length of runs, diameter of pipe, where venting is penetrating, the venting route (through finished space, across a garage or basement, in an chase, etc), and whether the furnace is above or below grade. All the while, the venting has to retain proper slope back to the furnace for management of condensate.

2. Drainage - Unlike an 80% furnace, 90+ models condense water. We have to drain that condensate to an approved interior drain (preferred) or run it outside to a french-drain. The location of that drain, and potential need for a pump all effect the cost of doing this.

3. Orphaned water heaters (or other appliances) - If your old furnace was vented in combination with another gas appliance (typically a water heater), you have to make sure that you can maintain proper draft with the single appliance. This often requires re-venting that appliance, or replacing it. In your case, if you are looking to eliminate the holes to the exterior of the home for combustion and make-up air, you will have to make sure that ALL appliances are direct vent.

4. Renovation costs - in doing all of this work, there is often the need to fir-in venting or drain pipes, or to repair holes cut into walls or ceilings to access chase ways. Repair of those finished surfaces has to be factored into your costs - and they can be high, especially if you are dealing with lead-safe practices or asbestos.

5. Costs will vary by market and quality of the contractor - often substantially.

6. I would counter the duct work costs brought up by by saying that duct work needs to be addressed regardless of what AFUE furnace is being installed. Duct testing should be done, design checked against manual D, and ducts renovated as indicated. What is true is that if you are properly sizing ANY furnace, and care about efficiency and comfort, then the ducts will need to be addressed. But there is no real increase in costs for duct work based on the efficiency of the furnace.

All that being said, the closest I can give you to the SHORT ANSWER you want is as follows:

In our experience, if we have easy and short venting routes available in unfinished space, orphaned appliances are not an issue, and we can gravity drain to an interior drain without a pump or having to install a french-drain system, then the cost difference in installation (including equipment costs for similarly featured models) is around $1000 - $2000. But costs go up quickly from there based on the installation.

To answer your other question - You might be able to re-use the old holes that were cut in to provide combustion/make-up air, but in our experience this is a rare exception. They usually don't allow for proper slope of venting, or can be in areas where it is against code to have an appliance venting (too close to windows, doors, eaves, etc.).

Finally, I will leave you with a suggestion - If your primary concern is to reduce air leakage, and the furnace does not need to be replaced, then take the mechanical room and move it outside the conditioned space if at all possible. I don't mean to move the mechanical systems (that is insanely expensive). I mean put in an insulated door with self-closing hinges, and install door seals so the room is effectively moved outside of the conditioned space. Then follow Bud's suggestion and improve the envelope first, as the CAZ is no longer an issue, and that is where you will see the biggest payback. Just pay close attention to ventilation, as you will have made the home significantly tighter.

I hope this was helpful. If you need more assistance, I would suggest finding a good HVAC contractor in your area, and working with them on a case-by case basis. You can find a list of ACCA member contractors who specialize in Home Performance at Just check the Home Performance box when making your selection.

Adam Gloss - Bel Red Energy Solutions

I am talking about this at ACI next week.  Most furnace I see are over sized by 2-4 times needed.  The 90% to 98% take more air a lot more air.  When I started HVAC we had a temp rise of 90'to105'  now 20' to 40'  also total SP was .15 total now  .8 with coil and air filter BUT most are using the same friction rate as 60 yr ago.   You can not just take out a 60% box and put in 98% box with out finding available Static Pressure,  I down size the furnace to meet the building,   Then Add return to make the filter work.   ACCA say up to 40% larger than size of building  I am seeing 400% day after day,  WHY do we let this happen?  Gerrr!  Its just a lack of knowing what to do or feeling safe with sealing the home up.

 I take out the high low when I can, the air does not care and shows no respect to code or what is talked about, If a water line is bare will frease up  over night 30' away.

Get some bids from your area not 1000 miles away  you have to derate in Colorado some times 30%  mostly 10%.

I like the PVC vented supply to heat up the return for fluing I keep the no heat call down when gets to sub 0

"Most furnace I see are over sized by 2-4 times needed."

Exactly why furnaces never actually see their AFUE in the real world. Same with AC units, few ever see their rated SEER. A correctly sized 80% will burn less gas over a winter than a 90% that's 2-4 times it's size.

This one is pretty good: 

At the bottom end, (smaller heat output, simple installation) costs run at about $4000. At the top end, (complex install, higher heat output) projects can cost around $8000 or more in extreme cases.

Size and equipment quality may have serious impact on price if the contractor uses a flat markup pricing strategy, and not so much if they use a labor burdened pricing approach.  

For example, jumping a size may add $100 to the furnace cost and nothing to the effort to install, but it may add $200 to a flat mark up contractor price vs $120 to the labor burdened contractor price.  This swing can get big if you jump to a nice communicating furnace and communicating stat, which might add $1000 to the wholesale cost over a crappy 90%er and plain Jane stat. 

Personally I feel the best equipment is really well worth it for a huge list of reasons, so I'd try to find a contractor using labor burdened so you are carrying the overhead burden of the contractors installation of crappy furnaces.  

But if you don't plan to stay long, get the flat rate contractor as he's selling the crappy stuff cheap and most home buyers have no way to differentiate and recognize value, so you won't the incremental cost back. 

A) air flow is the biggest thing.  I can put in a funace with out return and just burns it up in side and runs on high limit size it right and add lots of filter space. get it down to .5 total SP and WOW lots of air flow.  Long life very little serveic calls 81% do not have the return or fiter space

B) flue it right make the flue gas go up the stack or out the PVC  41% are not flued right

C) get reide of condesate water not in the motor or controls,  38% have under sized piping for condestate 

D) match BTU need to BTU supplyed  79% of time the heat is 2X needed 

I have to disagree. Top end equipment should be bought for comfort reasons, not energy savings. If a house is reasonably tight, furnaces over 80% don't have a reasonable payback time in the south. If a HO spends $500 per winter in NG costs, it would take 20 years to pay for a 90% furnace in energy savings.

2 stage AC's will never pay back in energy savings vs. 14-16 single stage units unless energy costs are extremely high.

Bob, could you expand on your opinion?  Specifically:

Top end equipment should be bought for comfort reasons, not energy savings.

Why do you think people buy furnaces, simply to keep pipes from freezing?  Seems you think comfort and efficiency are two distinct and possibly opposed things.  

furnaces over 80% don't have a reasonable payback time in the south.

Conversations of "Payback" are fundamentally opposed to good thorough analysis of value. Has anybody ever helped you understand how to perform analysis where multiple benefits exist?  Home Performance is not selling solar panels, there are other important benefits to consider besides the dollars theoretically attributed to increasing "rated" efficiency.  

If a HO spends $500 per winter in NG costs, it would take 20 years to pay for a 90% furnace in energy savings.

Look, I get it.  You are attempting to justify the fact you've sold a lot of crappy furnaces.  What you did with good intentions is not something anyone can hold against you.  But to continue after you know better IS cause for questioning your ethics.  Harm done KNOWINGLY is a whole different ball game.

If PAYBACK is something you obsess over, What is the payback on replacing an old 80% furnace with a new 80% furnace.  If there is none, what do you tell people?  Are you selling furnaces to people with perfectly good furnaces, or to people who need new equipment?  

If a HO spends $500 per winter in NG costs, it would take 20 years to pay for a 90% furnace in energy savings.

Again, are you able to factor in problems that need solving, or only able to look at equipment replacement the way one might look at replacing a toaster?  

So I best understand really "stinkin thinkin" where it used to be how I thought.  If your thought process defaults to "payback" you might want to consider thinking about incremental cost instead.  Payback is a really un-sophisticated and un-thorough way to make decisions, and if you are teaching or encouraging your clients to think that way you do them harm.  

They say "God is in the details," and really good equipment can solve problems that otherwise lead to enclosure, health, safety, and behavioral problems that cost unnecessary time, money and energy.  These "costs" don't show up on "payback" analysis.  

At the end of the day, if you think of efficiency as the opponent of comfort, you will soon be perceived as a dinosaur.  The job of Comprehensive Home Performance is to deliver comfort, and do it efficiently.  

Oh, and do no harm.  I always recommend the best.  If people want to buy less that's OK.  Free country.  But when a specific issue presents itself that my design would have avoided, the fact I recommended a total solution puts responsibility for the harm they experience firmly on their shoulders. If I don't, I leave myself exposed to: "You mean for another $3 a month we could have AVOIDED ALL THIS!!!"

Look, cheaper is cheaper.  You can look for shortcuts if you like, but if your orientation is always what corners to cut, your work will always be cheap.  There is a place in the market for cheap.  Often it is ultimately more expensive than good.  Just don't ever call it comprehensive.  Cheapest upfront and comprehensive are not synonymous. 

Don't get me wrong, if the customer wants the better equipment then sell it. Just don't push the sale on high end gear promising the customer unrealistic energy savings, I'll leave that to the fly by night window salesmen. My point is sell high end equipment based on COMFORT, with the energy savings being an added bonus. 2 stage and variable speed equipment does offer some definite comfort advantages. Sizing correctly and good air distribution are the biggest comfort factors when it comes to equipment replacement.

Another thing to consider it replacement parts costs for higher end equipment. A board or motor that may cost $100 on a standard unit goes to $300+ on a high end unit AND it isn't stocked at most local distributors.

Selling ductwork upgrades/repairs which would benefit most customers even more than higher AFUE/SEER equipment (both comfort and energy costs) seems to be the tough sell when other contractors don't want to mess with ductwork. Customers tend to get stuck on the AFUE/SEER on the box and think they will automatically get that.

So, very small adjustments to orientation can cause huge variation in destination.  

What I'm suggesting is YOU are the expert to the homeowner.  They don't tell the surgeon what operation they want.  You are the taxi driver helping them get to the destination they are looking for, but don't have a clue how to get to. 

I mean, you put together a full comprehensive package without financial bias, then tailor back based upon the budget they gave you and suggest "sacrifices" to get to the sweet spot of best project to solve problems, meet their needs, objectives, budget, and energy efficiency opportunity, right? 
If we shortcut process, we become no better than the hvac guy trying to get a contract signed as quickly as possible and move on to the next project... 


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