Reuben's Home Inspection Blog

An Over-sized Furnace Will Not Heat Your House Better

By In Oversized furnace On October 22, 2013


Big TowerAt my house, we say “Go Big or Go Home.”  Bigger is better when it comes to a lot of stuff: snow forts, leaf piles, power tools, computer monitors, towers made out of Duplos… but not furnaces.  When it comes to sizing a furnace, there are a lot of reasons to not go bigger.

Earlier this year in a blog post warning consumers about cheap AC tune-ups, I mentioned how a hack service tech / salesman tried to convince me that a bigger furnace would be good for my house because it would “heat my house better”.  Now that we’ve officially reached the ‘heating season’ in Minnesota, it’s time to take a closer look at over-sized furnaces.

A perfectly sized furnace will probably run almost all day long on the coldest day of the year, and it might not get the house up to 70 degrees about 1% of the time during the winter.  No joke.  That would be a perfect system.  For 1% of the time during winter, you might need to put on an extra sweatshirt.  No big deal.  For us in the Twin Cities metro area, that would be any time the temperature drops below -11.  Old-school folks will say -20, but that’s the old number.

An undersized furnace will work fine for the majority of the time, but every once in a while it won’t get the house quite as warm as desired.  How bad would this really be?  You’d need to wear an extra sweatshirt sometimes.  It’s coldest at night, and people that have a programmable thermostat already turn the temperature down at night, so would this really be a big deal?  No, probably not.

An over-sized furnace will keep the house warm no matter how cold it gets outside, but it does so at a cost.  First, it will probably make the house less comfortable.  When the furnace kicks on, some areas may warm up very quickly, so much so that they get uncomfortably warm before the thermostat has even been satisfied.  In poorly insulated, drafty houses, this heat can be quickly dissipated, causing the furnace to turn on and off frequently.  Many people run the furnace fan 24/7 to help even out the heat.

An over-sized furnace will also be less efficient.  A furnace is least efficient when it first fires up, and doesn’t reach its peak efficiency until the temperature of the air coming out has reached a steady value.  When a furnace is over-sized, it spends a large portions of its running hours in the ‘warming up’ phase.  Once it reaches a steady temp, the thermostat has been satisfied and the furnace shuts back off.

When a furnace constantly cycles on and off, the life of the furnace is dramatically reduced.  The best analogy I can think of is a car with all city miles and no highway miles.  City miles involve a lot of starting and stopping, which is what puts a lot of wear on a vehicle.  Heat exchangers on furnaces fail from the metal heating and cooling repeatedly; when a furnace is over-sized, the furnace turns on and off constantly, putting all ‘city miles’ on the furnace.  This is not a good thing, and will lead to premature failure of the furnace.  Guaranteed.

Besides premature failure, an over-sized furnace will be prone to short cycling, which is a term that refers to a furnace shutting down before the thermostat has been satisfied.  Furnaces are equipped with a safety feature that prevents them from overheating.  When a furnace is over-sized, it will run hotter than it should, which puts it closer to the temperature at which it will shut itself off.   Throw in a dirty furnace filter, and it will probably be enough to push the furnace over the edge.  When a modern furnace short cycles too many times in a row, it will go into shut-down mode to help prevent further damage.  This is most likely to happen when it’s extremely cold outside, which is the time when you really don’t want your furnace to quit working.

What about boilers?

The same stuff applies.  Most boilers are oversized, and it’s not a big deal if a boiler is slightly undersized.  An oversized boiler will make the house less comfortable, will be less efficient, and the constant turning on and off will shorten the life of the boiler and probably cause unexpected breakdowns.

How common is this?

From my own experiences and from everything I’ve heard from everyone in the know, a large portion of the older furnaces out there are over-sized.  HVAC contractors who are passionate about their trade love to talk about this stuff, and when they do, I listen.  I’ve learned a lot about this topic from a couple of local, passionate HVAC contractors; one being Enviroworld USA, and another being Chris Jirak of Neighborhood Plumbing Heating and Air.

Chris estimates that at least 75% of the furnaces that he replaces are over-sized.  As furnaces get more efficient, the trend should be to install furnaces with lower BTUs, not higher.  Many HVAC contractors are reluctant to go smaller because they don’t want to deal with homeowner complaints about the house not heating enough, but just a little bit of client education is really all it takes to prevent this.  A furnace that takes a long time to heat a house is probably sized properly.

When a furnace is replaced, the HVAC contractor should perform a Manual J calculation (or the equivalent) to help them determine what size furnace the home really needs.  This is a calculation that requires several of a homes variables to help determine the proper heating and cooling requirements.  If a calculation isn’t done, the contractor is just making a guess at the size of the new furnace, which means they’ll probably go big, then go home.

Author: Reuben Saltzman, Structure Tech Home Inspections

          


About the Author

Reuben

Reuben is a second generation home inspector with a passion for his work. He grew up remodeling homes and learning about carpentry since he was old enough to hold a hammer. Reuben has worked for Structure Tech since it was purchased by Neil in 1997, and is now co-owner of the company. Reuben’s favorite customers are the ones who have a lot of questions; he grew up thinking he was going to be a school teacher because he enjoyed teaching others so much. In a sense, that’s a lot of what home inspections are about, so Reuben truly does what he loves. Reuben has an A.A. degree in liberal arts and has attended most of the Building Inspection Technology classes at North Hennepin Community College. Reuben and his wife are the proud parents of two young childen, Cy Alexander and Lucy Nicole, and have a German Shepherd named Stanley. With two young children Reuben doesn’t have much free time, but he still tries to play disc golf as often as possible during the summer. Reuben lives in Maple Grove, MN. Professional Qualifications / Memberships: *ASHI Certified Inspector *President, ASHI Heartland Chapter *Member, Minnesota Society of Housing Inspectors (MSHI) *Licensed Minneapolis Truth-in-Sale of Housing Evaluator *Licensed Saint Paul Truth-in-Sale of Housing Evaluator *Licensed Maplewood Truth-in-Sale of Housing Evaluator *Licensed Hopkins Truth-in-Housing Evaluator *Licensed Robbinsdale Point of Sale Evaluator *Affiliate Member, Southern Twin Cities Association of Realtors

6 Comments

  • John 12 MONTHS AGO

    Hi Reuben, You mentioned that some people leave their blower fan on 24/7. I'm in this category, but the only reason I do this is because my father told me to years ago. What's your take on this. Better yet, what do you do at your house? Aside from increased electricity usage, I've been told that it helps keep air circulating so that basements don't get musty, but I was wondering if this is true, or if it's an old wives' tale. Thanks!

    • Reuben Saltzman 12 MONTHS AGO

      @John - I'm inconsistent about running my furnace fan. I don't know if it really makes any difference or not. If it does, I don't notice it. If I had a two-stage furnace, I'd probably run the fan during the winter, because they cost less to operate. As for musty basements, I've never heard that.

  • Karl 12 MONTHS AGO

    How does this translate to boilers?

    • Reuben Saltzman 12 MONTHS AGO

      @Karl - I just modified this post, adding a paragraph about boilers. It's all the same stuff, for the most part.

  • Susan Waites 12 MONTHS AGO

    Good article, but please attend to your grammatical errors in paragraph 6. Its is possessive and its use was required instead of it's, the contraction for it is. When unsure, read your sentences out loud and substitute it is every place you have written it's. If it makes sense, fine. If not, you need to use its and not it's.

    • Reuben Saltzman 12 MONTHS AGO

      @Susan - doh! I know the difference well, but it's one of those things I have a hard time getting right unless I stop to think about it. Thank you!