Furnace Sizing Professionalism

October 11, 2017

Now that the fall season has rolled around, the cooler temperatures are a good reminder to brush up on all things related to furnaces. This article by Gord Cooke, which was originally published in Mechanical Business, March 2017, outlines how "we as an industry, particularly contractors, salespeople and designers, need to recalibrate the way we think about furnace sizing."

One of the people I work with was recently shopping around for a replacement furnace in his older home and had a local HVAC contractor come to his house to quote the job. After walking around with a tape measure and calculator for 5 minutes the salesman announced that the house would require at least a 60,000 btu unit. He acknowledged that a 50,000 BTU/hr unit, like the one that was being replaced, would probably do the trick, but seeing as the contractors brand of choice only came only in 40, 60 or 80 increments, he defaulted up to 60,000 BTU/hr. My colleague raised concerns about this size recommendation, as the 50,000 BTU/hr mid-efficiency unit being replaced had always seemed to be short-cycling. The salesman assured him that he would be 'rolling the dice' going with a smaller unit, reassuring him that he'd been sizing and selling furnaces this way for almost 20 years… etc., etc.

Now, the house in question was indeed quite old, but had been recently proudly renovated by my colleague with new low E, argon gas windows, additional insulation in the attic and basement and significant efforts to cut the tested air leakage rate in half. However, none of that made its way into the salesman's calculations, nor did the homeowner's observation that his existing furnace, which was both smaller and considerably less efficient than the proposed unit, already seemed to be oversized.

This better too much heat than not enough mentality would seem reasonable at first glance, and it seems 'good enough' to satisfy most homeowners. After all, to a homeowner, how much of a problem can too large of a furnace really be? It'll be fast to warm the house up, and it gives homeowners peace of mind knowing that their furnace can handle even the coldest winter night. However, I think we can do better than that. Comfort and system performance need not be brushed aside in favor of security and power.

This rule-of-thumb 'sizing technique' outlined above must be in the minds of many contractors, and not just those installing HVAC equipment in older homes. We have seen a troubling rise in discrepancies in sizing recommendations in the common scenario where a buyer moves in to their newly built home and calls up an HVAC contractor to install an air conditioner. The contractor shows up and informs the homeowner that their brand new home has been fitted with a furnace and ductwork that is too small. You can imagine how frustrated the builder and original HVAC contractor are when a disgruntled homeowner calls the builder to complain they think their system is inadequate. After all, the builder employed a qualified HVAC designer, who used a complete set of plans and specifications and the latest CAN/CSA F280-12 Standard Determining the Required Capacity of Residential Space Heating and Cooling Appliances, submitted the plans for permit and had a qualified HVAC installer select and install the equipment. All to be totally discredited in minutes by someone using incomplete information and 20 year old sizing perceptions. In the past few months four of our builders have fallen victim to this frustration. All of them have voluntarily chosen to build ENERGY STAR qualified homes that are 20% more energy efficient than base code levels and employ third party evaluators to confirm air tightness, proper window coatings and insulation levels. In only one of these cases did the after market contractor provide any back-up information for their recommendation. Unfortunately in that instance the contractor did produce a heat loss, heat gain produced on a very common industry software but used an older version and used only default levels for common building envelope components that included single glazed windows, R20 attic insulation, no basement insulation and 1980's air tightness levels with no heat recovery ventilator. No surprise the after market recommendation was a furnace twice as big as was installed and an AC unit 1.0 Tons bigger than was originally specified for the home. In fact, in all four cases the design heat loss approved by building permit was under 40,000 BTUs/hr, heat gains ranged from 2 to 2.5 tons. In all four after market cases contractors recommended 75,000 to 80,000 BTUs/hr furnaces and 3 to 3.5 ton air conditioners.

I am not worried about the impact on comfort and performance of grossly over sized systems, I have written about that before. I am worried that this sort of thing hurts the integrity and credibility of the HVAC industry specifically and the building industry as a whole. We have enough reality show professionals telling homeowners that we don't know what we as an industry are doing, or worse yet aren't to be trusted, without propagating that impression by immediately calling into question the integrity of our colleagues every time we visit a new installation. We don't make ourselves look smarter / better / more trustworthy by suggesting the previous contractor wasn't. There is the well-vetted, successful sales philosophy that says don't speak ill of the competition, adapted from the far more compelling and historical rule of do unto others … that we should all follow. Let's first point out all the things the previous contractor did right, before calling into question the competence or integrity of our colleagues.

Though I should hope this isn't news to most of you, I have to say it… Rule-of-thumb approaches have no place in equipment sizing, or in the HVAC industry at all for that matter. The square footage-based approach has only ever resulted in an appropriate size recommendation by dumb luck and the fact that 30 years ago homeowners expectations of their heating system were very simple, and is now inexcusable given our current level of knowledge and the technology available to us. HVAC modifications impact the whole house system and must consider a great many more variables than just square footage or bedroom counts.

This method could be compared to a dating site that matches you with potential partners based solely on one vaguely relevant input like favourite food, while ignoring dozens of other vitally important characteristics and preferences, such as family plans, political views, age, location, values and so on. Since we are all familiar with these inputs and their implications it is clear to anyone (homeowners included) how ill-suited such a single input method of matching would be, and how irrelevant the online dating business would be viewed if these were their practices.

In the case of installing an AC system, or when commenting on the HVAC system in general, remember this; The compelling code changes, plus the fine tuning of the F280 Sizing standard, has meant that the design heat loss of homes across Canada has dropped in half since homes built as recently as 2005. The design cooling load has dropped by at least 30% primarily because of better window selections. So lets ensure we have updated or recalibrated that old rule-of-thumb with the following ideas.

  1. Purchase and use updated software, software that is F280-12 compliant and populate it with modern insulation levels and more realistic air tightness levels
  2. Remember that in most municipalities if you're dealing with a house built in the last 5-10 years, it is likely that the builder will have a heat loss/gain study available upon requested. This should remove any guesswork from the equation and you should take advantage of this to improve service and reduce your risk.
  3. If they can't get the design calculations, have them pull out the "schedule A" from their original purchase agreement. It will show the insulation levels and window types installed in the home. At the very least educate your sales staff on the implications of better envelope characteristics can have on furnace sizing. This would allow you to compliment people, like my colleague, on the efforts that have taken to improve their home and reward them with smaller, more effective systems.
  4. Invest in some simple measurement tools and use them to help homeowners make better choices. For example:
  • In as much as it is the window glazing that determine cooling loads, be aware that for about $500 you can buy a simple hand held device that will detect low E coatings and indicate the solar heat gain coefficient of the windows. Most ENERGY STAR evaluators carry them and I feel confident in suggesting that professional HVAC contractors who are truly concerned about proper sizing should have access to one as well. Especially if you want to justify a recommendation for a bigger furnace and all new duct work in a brand new home.
  • If you have concerns about the size of the ductwork, start using your digital pressure gauge, that every technician carries, to measure the static pressure across the air handler.
  • Similarly, if you don't think there is enough airflow capacity in the air handler to deliver the required cooling, measure the flow at the furnace using either a simple temperature rise method or purchase one of the air flow measuring devices such as the TruFlow from the Energy Conservatory - it slides quickly into the filter slot at the air handler.
  • Start selling "smart" thermostats on all your service calls, so when it comes time to add or replace a system you have accurate run time data from the existing equipment.

The bottom line is that the science is established and the technology (in measurement devices and sizing software) is widely available. There is no reason why HVAC contractors should be arriving at widely different equipment size selections and making homeowners question the competency of their builders, earlier contractors or our industry as a whole.

As more and more contractors become aware, educate themselves and update their practices, this problem should take care of itself. I think that we as an industry, particularly contractors, salespeople and designers, need to recalibrate the way we think about furnace sizing. It's time we all got on the same page so that we're not contradicting one another, discrediting each other's work and misleading our clients; it makes our entire industry look bad.

The first step is awareness, so please pass this article on to any fellow contractors you know using outdated or uninformed sizing practices - particularly the ones causing my phone to ring!