How Can Replacing a Furnace Make You Less Comfortable?

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hvac furnace atmospheric combustion oversizing comfort

Let's say your trusty old furnace is at the end of its life. You've got to buy a new one, so you call your HVAC company and they rush over to make sure you don't freeze during the next cold snap. They go and take a look at your furnace and find its capacity. They come back and tell you that you have a furnace rated for 60,000 BTU per hour and then talk to you about some of the options.

You could replace the old one with one that has the same efficiency, 80 AFUE (Annual Fuel Utilization Efficiency, essentially the percent efficiency over the whole heating season). Then they show you the price of their high efficiency furnace, which is 95 AFUE. It's only $400 more and they convince you that since you're going to have the furnace for another 15 or 20 years, you'll quickly get that money back in savings on your gas bill.

So you tell them to go ahead and install the 95% furnace. Your old one, remember, was rated at 60,000 BTU/hour of input capacity. They look in their product catalog and find that the high efficiency model comes in a 46,000 BTU/hr model or a 69,000 BTU/hr model. There's not a 60,000 BTU/hr model, and they don't want you to freeze, so they install the 69,000 BTU/hr model.

Looking at the wrong capacity

Hmmm. See a problem here? Many HVAC sales people and techs think in terms of input capacity for furnaces. To them, the 69k model is a little bigger than the old 60k model they're replacing, but not too much bigger.

Actually it's worse than that.

The relevant capacity of the furnace isn't the rate of BTU input. It's how quickly those BTUs go into your home, the output capacity. Let's look at what happened to those numbers when you replaced the old furnace. You started with 60,000 BTU/hr at 80% efficiency, so your old furnace supplied 60,000 x 0.8 = 48,000 BTU/hr. The new furnace has an output capacity of 69,000 x 0.95 = 65,550 BTU/hr. To summarize the numbers:

Input:    60,000 (old furnace) ==> 69,000 BTU/hr (new furnace)  —  15% increase

Output:  48,000 (old furnace) ==> 65,550 BTU/hr (new furnace)  —  37% increase

So, your new furnace is 37% larger in terms of its ability to provide heat to your home. If you had started with an older furnace, perhaps only 60% or 68% efficient, the amount of oversizing would be even worse.

Why it matters

How does that affect you? Your furnace will be on for shorter periods of time.

How does that affect you? If you live in a high performance home with a well-insulated and air-sealed building enclosure, you may not notice much of a difference. Well, let me rephrase that. You won't notice much difference most of the time...when the furnace is off. But when it comes on, you'll feel a blast of Sahara Desert heat making you long for the cool on the other side of the pillow. (RIP, Stuart Scott.)

If you live in an older home with air leakage and insulation problems, you may notice that you're less comfortable. The furnace comes on and blasts you for a few minutes, possibly making you uncomfortably warm in some parts of your home. Then it goes off, and you feel the cold walls and the drafts.

What can you do?

If you're getting a new furnace, ask the HVAC companies you get bids from how they're planning to size the new system. There are a lot of ways to do it, and many of them are wrong. Some acceptable ways would be to do a Manual J heating and cooling load calculation, use manufacturers' sizing software, or monitor the amount of runtime of your current system, especially at design conditions.

Probably the least reliable way to do it would be just to look at what size you have now and install that size or larger. To make it worse, just look at input capacity and ignore the effect of installing a new system with higher efficiency.

Bigger isn't always better. When it comes to heating and cooling your home, it's often worse.


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Stephen Fink
Dec 13 2012 - 10:35am

While I agree with everything you said in your article, there are even more reasons to do a load calculation then the reasons that you gave. Most furnaces being replaced are over twenty years old, and during those twenty, or more, years, ther have been changes to the structure. Additional insulation, new windows, air sealing, etc. We just replaced a 110,000 btuh (input,) furnace with a 60,000 btuh input furnace. Our actual calculated load was 40,000 btuh. This was in a condo that has three systems. The Bedroom, living area, and basement, each had a 110,000 btuh furnace. All of them were grossly over sized. Any contractor that doesn't do a load analysis, isn't doing his job.

Dec 13 2012 - 10:56am

A homeowner who has lived thru a winter, will be able to tell whether the furnace cycled during their cold spell. The degree of cycling indicates how much oversizing. 
To make the math simple consider a case where the furnace ran 50% of the time... a furnace with half the output would run 100% of the time but still satisfy that heat demand. Plus it would do it with better comfort, efficiency, and equipment life due to more continuous operation. 
The amount of math is well below what is required by the ACCA Manual J alternative. 
As you pointed out, one must measure the furnace output, not input.  
The sizing issue becomes less important when you install equipment with variable capacity, which can better approximate demand on a given hour.

Gordon Kay
Dec 13 2012 - 11:05am

That 46000 Btuh high efficiency furnace could very possibly work if you consider that you are using exterior combustion air with a two pipe system and do some insulation improvements to the attic. Manufacturers are also making furnaces with optional two stage heat and variable drive motors for even greater comfort. Heat loss & heat gain calculations are your best option.

David Butler
Dec 13 2012 - 3:32pm

As M.Johnson mentioned, cycle timing is the best way to size replacement furnaces. If a major efficiency and/or duct upgrade is being done at the same time and the existing system is still operational, it's best to do the hvac replacement after the other upgrades have been completed so the new system can be sized based on the home's new performance level. It's not necessary to wait for a design day to do cycle timing. As long as the delta-t is reasonably close to design, extrapolation will get you close enough. 
Nearly all of the homes I work on have small heat loads, so when specifying a furnace, I usually specify single stage with an ECM (not variable) blower, such as the Lennox ML80E045 or the Carrier 58PHA045. If the furnace is inside the envelope, I specify the Lennox EL195045E or the Carrier 59SP5A040E. Two-stage models in the 40/45 range are hard to find. If a 70k two-stage furnace is installed in a home with a load of less than 40k, it will never operate on high stage, thus wasting the additional cost of multi-stage.

Chuck Rose
Dec 14 2012 - 7:29am

I appreciate the way you write your articles. They are always simple to understand yet full of good information. I deal with a lot of customers who don't know a lot of the technical stuff but they want to understand what we're doing. I can always refer them to your blog.

James Ferguson
Dec 15 2012 - 9:06pm

Very good observation.  
But an even better method of getting sizing right is to use smart metering history in conjunction with weather data which means you can have your sizing calculated for design conditions at any time and not hope that the old one gives up when you need it most. 
These values will become available more and more as smart-metering is adopted and can save a fortune on excess plant costs. 
Several related issues have been covered on our blog (as well as a whole load of cynicism and opinion) -its for the reader to decide what they like :)

Dec 16 2012 - 9:45am

There are a few really good uses for smart meters, and I do think we can look forward to using them the way James F. said. 
You do require an occupied home to do this, whereas ACCA Manual J does not care whether the house is occupied or not. Trade-offs abound.

John Proctor
Dec 16 2012 - 2:45pm

While Manual J doesn't require an occupied home, it is based on a number of assumptions that very well not be true for the home in question. Percent of time on in the coldest days/hours is the best indicator. 
Remember God did not write Manual J, People did.

Dec 20 2012 - 2:40am

Furnace systems should be given regular maintenance and services or else they will cause number of typical problems, which causes a furnace of 1 year old itself to be replaced. Choosing a appropriate furnace systems based on location, available place and duration to be used should be kept in mind before buying a furnace system, which will reduce the number problems been caused.

Allison Bailes
Dec 20 2012 - 12:01pm

Irv haller: Ah, you're one of those guys! Perhaps you ought to catch up with 20th century HVAC knowledge, Irv. I'm sorry for all of your poor customers, who are most likely paying too much for systems that don't serve their needs well. But thanks for dropping by and giving me a chuckle this morning!

ted kidd
Dec 20 2012 - 4:37pm

C'mon Allison,  
GOTCHA! There's nobody named Irv Haller. That was me. I was just teasing you.  
You know I'd put a 2 stage 3 ton heat pump in there and call it a day.  
The homeowner indicated the old furnace never ran more than 15 minutes an hour, which confirms my calc of 45,000 is still grossly over sized.  
Great article.

james ferguson
Dec 20 2012 - 8:19pm

Ted / Allison (Even Irv) 
So pleased this was a wind up. I was so incensed by the stupidity of Irv that I could not comment - I knew I would regret my "phraseology". So I was giving myself a day to calm down! 
Not knowing US norms - I am guessing that at < 20kW this is an atmospheric boiler - rather than blown gas. 
If we were talking a bigger numbers I would add the pre-post purge costs to Teds safety margin and say it was at least double oversized! 
The point being the larger the installation (in general) the greater the complexity of heating services and the greater the crazily inbuilt tolerances. But then I guess Irv would argue - you just can't have too much redundancy :)