GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted
Guest Blogs

Replacing a Furnace or Boiler

A 15-minute way to calculate design heating load when sizing replacement equipment

Image 1 of 3
Time for a new boiler. There comes a day when the service life of the equipment is up. But even old equipment is useful as a measurement tool.
Image Credit: Image #1: GBA
Time for a new boiler. There comes a day when the service life of the equipment is up. But even old equipment is useful as a measurement tool.
Image Credit: Image #1: GBA
Base 65°F calculations are usually more accurate with 2x4 framed houses.
Image Credit: Images #2 and #3: Dana Dorsett
Base 60°F calculations are usually more accurate with 2x6 framed houses.

Your old furnace or boiler is gasping its last breath and it’s time to pull the trigger on something newer, more reliable, and more efficient. How do you quickly size the new equipment?

If you leave the sizing calculations to HVAC contractors, most would replace the old furnace with equipment that has a comparable output rating. That would guarantee that you wouldn’t get cold, but at least 19 times out of 20, that would be a mistake.

Furnaces are routinely oversized

Most of the installed heating equipment in the U.S. is oversized. In fact, most equipment has a heat output that is between 2 and 4 times the heating load.

There are valid reasons to oversize the equipment a bit, but not by 2 to 4 times the load. Oversizing on that scale results in lower comfort and lower efficiency. Most people want enough capacity to cover somewhat colder temperatures “just in case” there is a cold snap or record low temperatures, or if they need to keep the house at 78°F for a frail elderly parent.

With the average installed equipment being 3 times oversized, that means you’re covered. But what are the consequences of this type of equipment oversizing?

What’s the 99% outdoor design temperature?

For a particular location, the “99% outdoor design temperature” is the temperature which is exceeded for 99% of the hours in an average year. In other words, only 1% of the hours in a year are colder than the 99% outdoor design temperature.

Heating appliances can be sized to meet a building’s heat load at the 99% outdoor design temperature or at the 99.6% outdoor design temperature. Building codes in the U.S. stipulate that every room be capable of being automatically heated to a minimum of 68°F at the 99% temperature bin for that location, making the 99% approach more relevant than the 99.6% approach. So it’s useful to know the “99% outdoor design temperature” for your location.

Three times larger than necessary? What does that mean?

A building’s heat load grows (approximately) linearly with the difference between indoor and outdoor temperatures (otherwise known as the delta-T). According to building code, a heating appliance is correctly sized when it is sufficient to cover the difference between 68°F and the outdoor design temperature.

For example, consider a house in Washington, D.C. (Climate Zone 4), where the outdoor design temperature is 20°F. If your furnace is oversized by a factor of 3, you could heat the house in a location with a delta-T that was three times larger than your actual delta-T of 48 F° — in other words, in a location with a delta-T of 144 F°. With that much capacity, the heating system won’t lose ground until the outside temperature drops below -76°F — an outdoor temperature not seen in Washington, D.C. since the last ice age!

That’s ridiculous, of course — but oversizing a furnace by a factor of 3 is the norm rather than the exception.

Oversizing a little is OK

The AFUE testing protocol (used to determine furnace efficiency) presumes an oversizing factor of 1.7 times, which still gives a large margin for colder weather — more than covering the absolute record low temperatures in most locations. When a furnace is oversized by a factor of 1.7, it isn’t so oversized that it impacts efficiency, but that much oversizing is really too much for multi-stage furnaces. A typical two-stage condensing gas furnace has a turn-down ratio of less than 2:1. With most of these furnaces, the low-fire output is still 60% or more of the high-fire output.

When a furnace with a low-fire rating of 60% is oversized by a factor of 1.7, you could cover 99% of the building’s heating needs at low fire. You might as well hard-wire the furnace so that it never steps up to high-fire mode.

For comfort and efficiency, ASHRAE recommends that heating equipment be sized at 1.4 times the design heat load.

At 1.4 times oversizing, the house in the example above would have its heating load fully covered at a temperature difference of (1.4 x 48 F°) = 67 F°. With a delta-T of 67 F°, the heating system is adequate when the outdoor temperature drops to (68°F – 67 F°) = 1°F, which is 19 F° colder than the 99% outside design temperature. But that’s an outdoor temperature that may actually occur a few times over the 15-to-25 year lifecycle of the heating equipment (but not every year). When that happens it’s only for brief periods of time — short enough that the thermal mass of the house keeps it from losing much ground. So there is usually no comfort problem.

Methods for calculating a building’s heat load

That’s the target for sizing equipment. But to get to this target, it’s important to come up with a reasonably accurate design heat load number.

You could measure up the windows and walls, estimate the U-factors of different building assemblies, and run an I=B=R load calculation on the whole house, or you could even run a Manual J calculation. But those methods take time, and it’s easy to make errors when estimating the U-factors of components of an older house. It’s human nature to err to the high side when in doubt, which is also a mistake.

Fortunately, if you have access to historical fuel purchase history, you don’t have to guess.

You can calculate a building’s heat load in 15 minutes

You have instrumentation already in the house that is measuring the heat load: namely, the existing heating equipment. The way to use it for measurement purposes is:

  • Take a mid- to late-winter fuel bill, and note the exact dates covered by the bill — the fill-up dates or the meter-reading dates.
  • Look for a specification label on your furnace or boiler that includes the input BTU/h rating and the output BTU/h rating for your equipment.
  • Download base 65°F or base 60°F heating degree-day spreadsheets covering those dates for a nearby weather station from a website called
  • Look up the 99% outside design temperature (sometimes called the “heating 99% dry bulb temperature”) for your location from a website — for example, from an online document called Manual J Outdoor Design Conditions for Residential Load Calculation.

Now you have enough information to estimate your building’s heat load with reasonable accuracy, independent of the house construction details.

For example, assume that a house in Washington, D.C. (where the outdoor design temperature is 20°F) used 182 therms of natural gas between January 6 and February 8.

If the gas furnace nameplate shows an input rating of 110,000 BTU/h and an output rating of 88,000 BTU/h, you can use those numbers to determine the furnace’s thermal efficiency — in this case, 80%.

Multiply the input fuel amount by the efficiency of the equipment to determine how much heat was delivered to the building.


Natural gas: 1,000 BTU/cu. ft.Propane: 91,333 to 93,000 BTU/gallonFuel oil: 138,700 to 140,000 BTU/gallonKerosene: 120,000 to 135,000 BTU/gallon

To calculate the net amount of heat that was delivered into the ducts (or into the heating pipes if we are talking about a boiler), take the number of therms indicated on your fuel bill and multiply it by the equipment efficiency:

182 therms x (88,000/111,000) = 145.6 therms

Then multiply therms by 100,000 (the number of BTU per therm) to convert therms to BTU:

145.6 therms (x 100,000 BTU/therm) = 14.56 million BTU (MMBTU).

Next, download and sum up the daily base 65°F heating degree days (HDD) for the nearest weather station — in this case, from station KDCA: Washington National Airport, Virginia — from the web site for the period of January 6 through February 7. (Include only one of the meter-reading dates, not both.) In this example, the sum comes to 937.7 HDD-65°F. (See Image #2, below.)

Next, download and sum the date for base 60°F. The result is 772.9 HDD-60°F. (See Image #3, below.)

14.56 MMBTU / 937.7 HDD is 15,527 BTU per degree-day. With 24 hours in a day, that’s an average of 647 BTU per degree-hour at a balance point of 65°F.

14.56 MMBTU / 772.9 HDD is 18,838 BTU per degree-day, and with 24 hours in a day that’s an average of 785 BTU per degree-hour at a balance point of 60°F.

A balance point of 65°F with design temp of 20°F is a difference of 45 F° degrees, and the implied heat load is then 45 F° x 647 BTU/F-hr = ~29,115 BTU/hr.

At a balance point of 60°F there are only 40 F° heating degrees, and the implied load is 45 F° x 785 BTU/F-hr = ~31,400 BTU/hr.

That’s a range of about 8% between the calculation based on 65°F heating degree days and the calculation based on 60°F heating degree days. Which is closest to reality?

It depends. Most 2×4 framed houses will have a balance point close to 65°F, most 2×6 framed houses will balance closer to 60°F. But unless it’s a superinsulated house, it’s likely balance point is somewhere in that range.

Comparing 65°F HDD calculations with 60°F HDD calculations

At this point, you may be thinking, “Why would the calculated heating load for a house with 2×4 walls (29,155 BTU/h) be lower than the calculated heating load for a house with 2×6 walls (31,400 BTU/h)?”

The short answer is, “Both calculations assume that you’ve used the same amount of fuel over the average outdoor temperatures during the period in question, which yields a higher BTU per degree-hour constant for the house with 2×6 walls.”

Put another way, if the better-insulated house used the same amount of fuel during the same weather conditions, its load is going to be higher when it’s really cold out. If two identical houses were built, one with 2×4 walls and the other with 2×6 walls, the 2×6 house should have used less fuel at the average outdoor temperature over the period, not the same amount of fuel. But if different 2×4 and 2×6 houses use the same amount of fuel, the incremental heat requirement of the 2×6 house per degree will be bigger. When you then use that bigger load per-degree constant to predict the load at the outside design temperature, the calculation results in a bigger number.

What about thermostat settings?

If the average indoor temperature was kept substantially below 68°F, you can account for that fact by dropping the degree-day base.

For example, if you normally keep the thermostat at 62°F rather than 68°F, subtract 6 F° from the temperature bases to get the BTU/degree-hour constant, but add 6 F° to the total heating degrees when you run the final number to be sure it meets code when sizing the equipment.

Error factors

The heat load calculated from the difference in temperature from the balance point isn’t a perfectly linear BTU/degree-hour constant as implied by this calculation method. There is an offset related to the internal heat sources like electrical plug loads and warm bodies. But the error from the difference in slope between the linear approximation from a presumptive balance point method shown here and other methods — for example, an I=B=R linear model (based on the indoor temperature) or a more nuanced Manual-J calculation — doesn’t induce a large error when wintertime data are used.

If the same heating fuel is also used for domestic hot water, this calculation method exaggerates the implied load numbers, since some of that fuel was used by the water heater and sent down the drain. But some of the space heating came via solar gains that would reduce the implied load numbers. These errors tend to balance each other out to a greater or lesser degree.

If you spent 10 days on the beach in Belize during that period, with your home thermostat set to 50°F, use a different billing period.

If an auxiliary heating appliance was being used on a regular basis (say, a wood stove or a ductless minisplit), this calculation method will be too far from reality to be useful. If that’s the case, go back to I=B=R or Manual-J.

In some cases, your heating equipment may be old, decrepit, and not performing very near its original name plate efficiency. That would skew the calculated number to something higher than reality, but it would have to be pretty far off to make a meaningful difference. If that’s the case, calculate it using some lower efficiency. Even a 100-year-old steam boiler is usually still delivering at least 55% efficiency, and often 65-70%.

Equipment sizing

Unless there is an obvious large error factor that skews the result badly, move on:

For sizing the equipment, use the ASHRAE 1.4x sizing factor:

1.4 x 29,115 BTU/hr = 40,761 BTU/hr (with a 65°F balance point assumption)

1.4 x 31,400 BTU/hr = 43,960 BTU/hr (with a 60°F balance point assumption)

If reality happens to be the 60°F balance point — the 31,400 BTU/h implied load number — then using the 1.4x multiplier on the lower 65°F implied load of 29,115 BTU/h yields about 40,761 BTU/h, in which case you’re even covered for the higher implied load with ample margin. Since older equipment probably isn’t fully as efficient as it was when it was new, equipment rated at 40,000 BTU/h should be good enough.

But if you got nervous and sized it at 50,000 BTU/h of output, it would still be only ~1.7x oversized for the lower 29,115 BTU/h estimate, which means it would hit its AFUE efficiency number (even though it would be bigger than ideal). From a practical point of view, any heating appliance with an output between 40,000-50,000 BTU/h will be fine.

The highest comfort occurs when it’s cold out, when the equipment is actually running and delivering steady heat — rather than running for a while and overshooting the thermostat, with a long cooling off period between cycles. If the new equipment is multi-stage or modulating, it’s best if the lowest stage output is well under the 29,000 BTU/h load, so that the firing range is meaningful.

With boilers, use only the DOE output rating for the equipment; ignore the net I=B=R numbers. The fuel use calculation has the distribution and idling losses included — they can’t be separated out. (There are other factors that come into play when dealing with modulating condensing boilers, but that’s a topic for another day.) If the replacement equipment will be a heat pump, consult the extended temperature range tables for its output at the 99% design temperature.

Whatever the equipment type, have the load numbers and minimum / maximum output numbers in hand before talking to an HVAC contractor.

Expect pushback from contractors

HVAC contractors have become accustomed to installing oversized equipment, and may even think that equipment really needs to be that big. But you don’t have to follow them down the rabbit hole.

Have confidence in your fuel use numbers. This calculation method is better than an estimate; it’s a measurement.

If you push back, some contractors will balk or refuse to bid equipment that small. (Good riddance!) Others will want you to sign a waiver. (OK — but really?!)

Still others will understand that a lipstick-on-mirror fuel-use calculation is sufficiently close to reality that they’ll just go with it if you direct them to.

Typical arguments heard from contractors are rules of thumb such as: “It needs to be at least 25 BTU/h per square foot of living space. Your house is 2,400 square feet, so that’s 60,000. Let’s bump it to 75,000 just in case it gets cold out.”

Which reliably oversized most houses by at least 2x. Or: “It needs to be at least 90,000 BTU/h or it’ll take forever to return from overnight setback.”

Which is almost never true.

Recent feedback from a contractor insisting on a 100,000 BTU/h condensing boiler for a house with a design heat load under 30,000 BTU/h (based on fuel use calculations and later verified by Manual-J) went, “It needs to be at least 100,000 BTU/h or it’ll take forever to bring the house up to temperature after you’ve been out of power for a few days.”

Out of power for days? How often does that happen each winter (or decade)?

Every day, contractors come up with new creative reasons for oversizing. But with the load calculation in your back pocket, you don’t have to accept these arguments.


Dana Dorsett has lifelong interests in energy policy, building science, and home efficiency. He is currently an electrical engineer in Massachusetts.


  1. user-723121 | | #1

    Keep it simple
    If you have a furnace of known efficiency you can also do this. On a still and cloudy day you can time the hourly furnace run times, take this fraction times the furnace output for an hourly Btu usage. Divide this by the Delta T for the hourly Btu heat loss per degree F. Now with this information you find the design temperature heat loss. If you do a deep nightly temperature setback you will find a furnace oversizing by a factor of 2 is just about right.

  2. Beideck | | #2

    Efficiency loss
    What is a typical efficiency loss for an over-sized system? and how does that loss relate to how badly the system is over-sized?

  3. GBA Editor
    Martin Holladay | | #3

    Response to Daniel Beideck
    For the most part, the idea that oversized equipment results in a big efficiency hit is a myth.

    I addressed the issue in one of my articles, Saving Energy With Manual J and Manual D. In that article, I wrote:

    "There are strong arguments against routine oversizing of HVAC equipment. The best argument is simple: oversized equipment usually costs more than right-sized equipment.

    "Oversized equipment suffers from short cycling. For example, an oversized furnace brings a home up to temperature quickly, and then shuts off. A few minutes later, it comes on again, only to shut off quickly. Many homeowners find the see-saw sound of a short-cycling furnace to be annoying. ...

    "Increasing evidence shows that energy experts have exaggerated the negative effects of equipment oversizing, however. Studies have confirmed that oversized furnaces don't use any more energy than right-sized furnaces. Moreover, newer modulating or two-speed furnaces operate efficiently under part-load conditions, solving any possible problems from furnace oversizing.

    "Although there are ample reasons to believe that oversized air conditioners are less effective than right-sized equipment at dehumidification, at least one field study was unable to measure any performance improvements or energy savings after replacing an existing oversized air conditioner with a new right-sized unit."

    * * * * *

    For more discussion of these issues, see the comments section below that article.

  4. user-4053553 | | #4

    @ Daniel
    One thing oversizing does is cause rapid cycling as Martin stated, which leads to faster component wear, igniters, motors, sensors last longer when they are run for longer periods and shut down/restarted less frequently.

  5. Reid Baldwin | | #5

    I am attempting to digest this article and several others, such as the one Martin references in #3 and some posts on Allison Bailes blog, to determine which facts there is broad agreement on and which facts are disputed. Would others agree with the following summary:

    For gas furnaces, unlike air conditioners and heat pumps, efficiency does not vary by much when the equipment operates at partial capacity. It appears that there is broad agreement that oversizing up to about 1.5 is good and that oversizing by more than about 2.0 is bad. (Those values would shift higher for equipment with wide modulation ranges.) The disagreement seems to be over how bad it is. Some seem to feel that there is an oversizing epidemic that we need to combat. Others seem to feel that this is pretty far down the list of priorities.

    The advantages of oversizing, which are fully obtained with only modest oversizing, are:
    i) handling especially severe weather
    ii) faster recovery from a thermostat setback
    iii) for two-stage or modulating heat pumps and air conditioners, improved efficiency due to more frequent operation at partial capacity

    The disadvantages of oversizing are:
    i) equipment costs
    ii) requires either larger ducting or operation at higher static pressures
    iii) larger temperature swings due to short bursts of hot or cool air or due to minimum run times
    iv) for air conditioners, reduced ability to remove humidity
    v) reduced equipment life due to frequent cycling
    The disagreements seem to be over the magnitude of these problems. All seem to agree that two-stage or modulating equipment mitigates these problems (except cost), although many would argue that they don't solve these problems.

  6. user-723121 | | #6

    Thermostat settings
    One way to prevent short cycling of heating equipment is to lower the thermostat cycles per hour. For our 2 stage, 95% furnace I like 3 cycles per hour. Once the house is up to temperature in the morning the furnace will just run on low fire and the DC fan motor on low speed, very quiet.

  7. josh_in_mn | | #7

    An analogous method for estimating cooling load?

    Very nice article. I just used this method to confirm that the method my supply house recommended 12 years ago did indeed result in a 2.5x oversized boiler. I'm trying to size some mini-splits for this house now, primarily for cooling, but also for backup heat in the event of a boiler failure, and I'm wondering if you have a similar method for that? We cool now using window units, which are relatively new and have an eer of 10-11.

  8. user-4053553 | | #8

    @ Reid
    What i don't like about two stage is the dumb logic, my furnace decides on its own when to go into high fire, and somehow it always decides to go to second stage then the thermostat shuts it down less then 30 secs later (sometimes less then 5 secs later). I assume this is causing faster component wear. I can't make any adjustments to this, thermostat does not have a fire set number of times an hour option, the furnace has no high fire adjustments except on or off (i can disable the high fire completely). My house load is calculated as 45k, the furnace is 40k (i did undersize it but am planning on further insulation/air sealing to bring load to 30k in the future) and the stages are 25/39k out.
    It is a new thermostat with the furnace, programmable (which was more trouble then it was worth since i am on varying schedules) but does not have the option to select low/high fire. Perhaps there are replacement thermostats with more features when i can afford to replace it.

  9. Reid Baldwin | | #9

    @ Alan B
    There are others on this forum that know way more about HVAC equipment than I do. My knowledge is based on what I have read here and looking through the manuals for equipment I am considering for my house.

    There are single stage thermostats and two stage thermostats. When you use a two stage furnace with a single stage thermostat, the only information that the furnace has is whether or not the thermostat is calling for heat. It assumes that it needs to go to high heat if the current heat call has continued for more than a threshold time. That threshold might be adjustable - your manual would say. The furnace has no idea how much longer the heat call will likely continue. A two stage thermostat has more information available - the current measured temperature. Therefore, it would at least be feasible for it to use more intelligent logic. You can read the manuals for many thermostats online to find out what logic they use.

    We are going to use a zoning system that has logic to intervene between the several thermostats and the furnace. Various algorithms are possible there. The one I expect we will use changes to second stage based on the number of zones that are calling for heat. For each alternative algorithm, there is some scenario in which it does something suboptimal.

  10. GBA Editor
    Martin Holladay | | #10

    Link to Allison Bailes's article
    Here is a link to the article by Allison Bailes that Dana mentioned -- the one where Allison explains how to time the cycles of your air conditioner on a hot day to see if your air conditioner is correctly sized: How to Tell If Your Air Conditioner Is Oversized.

  11. Expert Member
    Dana Dorsett | | #11

    Oversizing costs
    The efficiency hit varies with equipment type, but is much more pronounces with zoned systems than single zone systems. Hot air furnaces suffer very little efficiency loss even at 5x oversizing, but there's a very real comfort disadvantage to that. The fairly flat efficiency response is partly due to the very low thermal mass of hot air systems, but there is still higher distribution losses due to larger duct surface area, and slightly more power use (that doesn't show up an a fuel use analysis) due to the higher air handler power required.

    Most 2-stage hot air systems use a dumb timer to decide when to step up to the second stage. One of the people in my office lives in a town house development with identical 2- stage ~80,000 BTU/hr gas hot air furnaces, yet the design temp heat load of even the largest units is under 40,000 BTU/hr. With retrofit air sealing and new window his is now about 15,000 BTU/hr. Those furnaces always step up to high-fire whenever the call for heat has been longer than ~10 minutes, so when using deep overnight setbacks they always finish at high fire. Annoyed with the hot-blast and lower efficiency he dug into the service operation and like Reid figured out how to hard-wire the controls to never step up to high fire, but it was not a vendor approved hack, and thus voids the equipment warranty.

    Hydronic systems with mid to high-mass boilers experience very dramatic efficiency losses with oversizing due to standby losses from both the boiler jacket and distribution system. If multi-zoned these losses are magnified, since the boiler losses are the same whether serving one small zone or the whole house. Take a look a the regression curves for the different boiler system types in this bit of boiler testing done at Brookhaven National Labs about a decade back:

    If the boiler is 3x oversized, even at the 99% design condition the efficiency will be at the 33% mark on the curve, and the average seasonal load will be below the 15% tick. With heat purging controls the shoulder of the curve moves to the left, which helps (a lot, of only 3x oversized). Some mid-mass boilers have smarter heat purging controls, most don't. If 1.4x oversized per ASHRAE it's running at about the 70% of max output level at the 99% outside design temp, and the average load is around the 33% mark, which isn't terrible, and not over the efficiency cliff. The tabulated results at 3x & 2x oversizing are found in Table 3, p.9.

    It's easy to naively think that modulating condensing boiler will solve this problem, but oversizing modulating boilers present a more complex set of problems that I intend to follow up with in a separate blog article. There are many modulating condensing boilers installed that neither modulate nor condense due to their oversizing factors, and surprisingly few optimally sized for the loads, particularly on multi-zoned systems.

    There isn't a similarly simple way to calculate air conditioner oversizing, but if you're home during peak cooling hours you can time the duty cycling to come up with reasonable estimates. Unlike peak heating loads, the the peak cooling hours aren't well correlated with peak outdoor temperature, and far more correlated with peak solar gain. Houses with a lot of west facing windows will usually see peak cooling loads hours after the peak outdoor temperature. I believe Allison Bailes III had a blog piece that detailed how he measured the duty cycle on the AC to come up with the oversizing factor a year or two ago.

  12. charlie_sullivan | | #12

    Reid's Summary
    I want to add to Reid's summary an additional disadvantage:

    *Noisier operation, and more noticeable noise because of the cycling.

    If we are keeping track Dana also added two items:

    *Higher duct heat losses due to larger duct surface area

    *Issues are a little different and penalties higher for hydronic.

    Dana, thanks for writing this. It's good to have it written up where people can refer to it.

  13. user-723121 | | #13

    Alan B

    Some 2 stage furnaces have a switch on them as to when the furnace goes from low fire to high fire. Our Lennox furnace came from the factory set at 10 minutes, I changed it to the 15 minute setting. It was a small yellow dip switch called (2nd stage delay) on the main circuit board. There are thermostats with adjustable temperature differential settings allowing the furnace to run longer less often, even the most basic Honeywell digital thermostats have a (cycles per hour setting).

  14. leonmeyers | | #14

    Sizing Replacement HVAC
    Though this article deals exclusively with replacement heating equipment, it would be worth a mention that replacing combined heating and cooling equipment requires a different approach.

  15. Expert Member
    Dana Dorsett | | #15

    Measure that reduce comfort need not apply.
    Heating systems are most comfortable when they're actively running. Limiting cycles per hour or expanding the differential temperature swings may improve as-used efficiency, but degrades comfort. At the end of the day the measures that increase the temperature swings are the opposite of what you're seeking with heating system. When you right-size the system the duty cycles are longer the colder it gets, and you can keep the differential temperature band reasonably narrow, which increases.

    If a furnace is less then 1.4x oversized for the 99% outside design temp the run times would always be reasonably long, and you wouldn't need to limit the cycles per hour or mess around with second-stage delays or otherwise open up the temperature differential. Having more flexibility on programming for 2 stage equipment would be useful though.

  16. Expert Member
    Dana Dorsett | | #16

    The method only defines the heating load, true (@ Leon Meyers )
    But knowing the heating load is at least a start, and would be an important piece to know when looking at combined solutions.

    Heat pump solutions aren't always in a reasonable range for both heating and cooling, often oversized for one or the other (and all too often, oversized for both) nor are many pre-packaged gas + cooling coil solutions.

  17. ilovebatz | | #17

    Excellent Article
    I wish I had read your article five years ago before having new heating and A/C installed. I contacted about five contractors and none of them would do Manual J, or anything else. I tried to do a sort of Manual J using an on-line program. I would have felt a lot more confident when talking to contractors if I had used your calculations to see how well they agreed with my manual J calculations.

  18. HarryVoorhees | | #18

    Others sources of degree-day data?
    Dana, Do you know of any no-cost databases for accessing degree-day data (or, I suppose, hourly historical temperature data from which degree-days could be computed)? I was thinking of writing an app that would do the calculation that you describe. The degree-days web interface is free of course, but their API access requires a substantial subscription fee.

  19. Expert Member
    Dana Dorsett | | #19

    Sorry, don't know of any freebies.
    I'm not sure if there is NWS weather station data or other that would be any cheaper than the / , or that would have the same very-local coverage.

    I haven't tried out's linear regression tool (currently in beta testing) for determining the degree base with the best fit, but if you were going to write an app that would be a useful for getting higher precision on the load estimates than simply bracketing it between presumptive 60F & 65F base temperatures. (Not that higher precision is actually needed in most cases.)

  20. Expert Member
    MALCOLM TAYLOR | | #20

    Finally had time to read this in full. What a simple and elegant method. Another good blog Dana.

  21. ROBERT OPALUCH | | #21

    Useful analysis
    I tried Dana’s heating system sizing method on an oil furnace (used for both hot water and space heating) at my girlfriend’s 1,400 SQFT circa 1979 ranch. I used the six oil deliveries (since she’s owned the place) to calculate five analyses of oil usage.
    • I looked up BTU’s per gallon of #2 heating oil (138,000), since Dana’s calculations used gas (100,000 BTUs/therm).
    • The heating (and hot water heating) boiler on my girlfriend’s home is oversized by about 3.6 times, the range Dana notes is typical oversizing.
    • Four of five results were based on the home with about R-20 ceiling insulation, 2x4 walls, R-11 batts under the floor, and single-pane windows with storm windows. The fifth analysis was after ceiling insulation was increased to about R-50, plus ceiling and door air-sealing. The energy-saving improvements cut space heating costs by about 30%. That’s more than I expected.
    • One analysis covers the Fall/Summer/Spring seasons, and could be used to estimate hot water heating. It appears that hot water accounts for about one fifth of the oil usage, seems reasonable.
    • The BTUs per Heating Degree Day for the three other winter periods were within a plus or minus 12% range, without taking into account the hot water heating and solar gain variables.

    Now we can do some homework in advance to be prepared when the old dinosaur boiler does suddenly fail. Rather than duplicate the 110,000 BTU/hr output, we could substitute a 30,000 BTU/hr boiler. I’d prefer to keep the hot water heating system separate for a number of reasons.

    Thanks Dana for this useful article!

  22. Expert Member
    Dana Dorsett | | #22

    I'm glad it worked for you!
    The energy content of alternate fuels is in the side-bar mid-way through the article, if you didn't catch it.

    The hard part is finding a 30,000 BTU/hr output oil boiler. The smallest jets on oil-burners in the US are typically 0.5gph (about 68-70,000 BTU/hr in), and those often gum up & clog on the fuels available in the US. That would still be ~2x oversized for you space heating load. To use a boiler at that ovesizing factor efficiently requires using heat-purging controls. Sometimes it works more efficiently if rather than running the hot water separately, an indirect hot water heater is used to take advantage of it's thermal mass. Boilers such as the System 2000 EK1 Frontier can be jetted as low as 0.68 gph, and the contrlos use an indirect hot water heater as the heat dump for purging heat from the boiler at the end of a burn, which turns out to be quite effective:

    The smallest Burnham MPO-IQ 84 also has smart heat purging controls, and can be jetted at 0.60 gph. It also has internal self-protection from low return water temperatures I(down to 110F, IIRC) which can be an issue when down-sizing oil boilers.

    Measuring up the radiation to estimate the return water temperature is important when down sizing an oil boiler. At entering water temps temps (EWT) below 140F many oil boilers will be destroyed by acidic exhaust condensation on the heat exchangers, and even at 140F EWT there can be substantial flue condensation to manage, much more so than with older-80-83% efficiency oil boilers.

    If the existing boiler has a lot of life left to it, at 3x oversizing a heat purging retrofit economizer such as the Intellicon 3250/HW+ can make a measurable difference in fuel use:

    Of course, knowing the heat load also allows you to figure out right-sized alternatives, such as high-efficiency heat pumps. A ~30K heat load is within the output range of 2-2.5 tons of cold climate mini-split/multi-split in most US locations. Whether or not that's currently a higher or lower carbon solution than another oil boiler depends quite bit on your local grid's carbon foot print, and how you expect it to evolve over the next 15-25 years.

  23. Jon_R | | #23

    Note that the fuel use method assumes average wind - high winds occur and can add considerable load. Hopefully this is covered by the 40% margin.

    While cycles cause wear on equipment, so does the number of operating hours - which is lower with larger equipment.

    My somewhat over-sized gas furnace allows me to use thermostat setback - which saves money and is more comfortable.

  24. joshdurston | | #24

    Just did this, was amazed at how consistent the calc was month to month (once I corrected for non NG consumption). My heating bill conveniently has the heating degree days on it for each billing period.
    My furnace is a 70kbtu 90% efficient single stage. Calculation came out to 27-29kbtu depending on assumed balance point. Interesting point of interest is the popular slantfin heat loss calculator gave me a heatloss of around 44kbtu. Looks like it has that 1.4 oversize built into the calcs.

    I put a wood stove in last year, it dropped the calculated numbers to around 12kbtu. I run the wood stove from November to March without letting it ever go out.

  25. djcasassa | | #25

    For the record, there's a typo in this sentence:

    "At a balance point of 60°F there are only 40 F° heating degrees, and the implied load is 45 F° x 785 BTU/F-hr = ~31,400 BTU/hr."

    Change "45 F°" to "40F°". The product is already correct for "40F°".

  26. Jon_R | | #26

    Be careful with balance points. There is one figure appropriate for long term average fuel usage calculations and a quite different one appropriate for "how many btu/hr do I need to heat the house at 3am".

    For the latter, my *measured* base value is about 1F (= ~700 btu/hr) below the thermostat setting.

    Using the example's 785 BTU/F-hr, a 60F base and 70F inside: at 3am, 7850 btu/hr won't be available from electricity use or people and assuming that would cause under-sizing.

    Also be careful with "design conditions". You will occasionally see much colder temperatures, sometimes for extended periods.

  27. cold_feet | | #27

    How would you go about doing these calculations for an all-electric, heat pump system (measured in tons as opposed to BTUs)? Or is that as simple as just using an online converter (ex, as the last step?

  28. salmonbuilt | | #28

    Doesn't this method assume that the house is being held at temp 24hrs/day? Or am I missing something. When I did the calcs, I got a load of 17,566 BTU/hr when dividing the daily amount by 24. When I take into account that the heat is on only in the morning and evening typically, I divide by 9 for a load of 46,843 BTU/hr. My house is about 2,200 sq ft in Portland, Oregon, so the latter seems more reasonable. The furnace is sized at 65,000 BTU/hr.

    1. salmonbuilt | | #29

      I also did the calculator that I googled at: and got an answer of 11,450 BTUh load which also didn't have and input for heating hours/day. I don't believe that at all.

Log in or create an account to post a comment.



Recent Questions and Replies

  • |
  • |
  • |
  • |