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What exactly is an ASHRAE design temperature?

aunsafe2015 | Posted in General Questions on

I understand that Manual J calculations are performed based on summer and winter ASHRAE design temperatures.  HVAC contractors have told me that in my area (the Raleigh NC area), summer design temp is 92 F (apparently it used to be 90 F).

What exactly does a design temp of 92 F mean?  Is that supposed to be the top 1% for daily high temperature?  Or is it something else like a time average of temperature above a certain point?

I ask because, in the one-month period from June 24, 2019 through July 23, 2019, the average high temperature in my area was 97.0 F.  In the entire 30 day period, there were only TWO days where the high was below 92.  For 25 of the days, the high was 95 or higher.  It was frequently already 90 degrees by 10 am, and did not drop back below 90 until 9 pm or so.  These temperature measurements were taken with a weather station located in the shade on my property, but all of the weather stations that I can view on Weather Underground had comparable readings through this 30-day period.

I have not lived in this area that long, but my understanding is that this was a rather unusual heat wave in that it lasted so long with no respite, but that it is not at all uncommon to have many different several day periods of 95 + high temperatures over the course of an average summer (just not 30 days straight…).

Anyways, if an HVAC system were sized such that it could maintain an indoor temperature of 75 degrees on days that the outside high is 92, you would have been quite toasty–in your own house–for an entire month.

So I guess what I’m asking is how can the ASHRAE design temp for my area be only 92 degrees when we have brutally hot 30-day periods like the one that ended July 23?  Is this just the result of global warming?  Or am I misunderstanding what an ASHRAE design temperature is supposed to represent?

Thanks for any input.

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  1. joshdurston | | #1

    The switch from 90f to 92f was probably going from 2% to 1% design temperature.

    The design temperature is the extreme hot or cold temperature that includes everything up to or below a certain percentage of hours in the year. So a 1% design cooling temperature will be higher than a 2%, but lower than a .4%. There are several percentages used so you need to include the percentage when talking about a design temperature.

    1% cooling, means 1% of the time it has been hotter historically (over something like the last 25-50 years). There are 8760 hours in a year, so for 87 hours it will typically exceed the cooling design temperature each year.

    Luckily for cooling, the peak loads are often heavily influenced by solar which doesn't vary much depending on the temperature. So even if the OA temp is a couple degrees higher, it doesn't necessarily follow that your cooling load went up proportionally.

    Also, you house has thermal inertia so it would have to heat soak for much longer than an afternoon to get away on you too much. But luckily the sun goes down, and temps drop a couple degrees overnight giving you time to catch up.

    All this to say you are better to safely under size slightly, than to oversize out of fear and guarantee discomfort. Also going with a multi stage or modulating unit (AC and airhandler) can help comfort and dehum substantially.

  2. joshdurston | | #2

    Ashrae knows about the Weather patterns and their recommendations are based off real climate data. So I wouldn't be too quick to oversize based on a max temperature that might occur for a couple hours each year.
    You can see the 5/10/20/50 year extreme temps are 101.1F/10.2.9F/104.7F/106.9F respectively from Ashrae's own data.

  3. Jon_R | | #3

    You can size AC above design day load by 15% to cover the issue you mention. This will increase comfort (not cause discomfort).

    > Also going with a multi stage or modulating unit (AC and airhandler) can help comfort and dehum substantially.

    Let's see some data regarding the substantial improvement in dehumidifcation (it often does the opposite).

    1. joshdurston | | #5

      I think you're referring to the mini split capacity versus fan speed discussions that have happened here in the past. I'll concede there is some dehum concerns there. But with conventional split units and air handlers the compressor typically can't go low enough for that to be a problem. (I've personally conceded that I wish I would've went with a 1ton mini split instead of a .75 for dehum). By safely undersize what I mean is relative to a manual J which has some oversizing built in. So a slight undersized like I'm proposing where you go with say a 2ton unit instead of a 3 ton, even though your load came in at 28,000btu/hr. It's probably still a slight oversize in the real world, unless you were really precise and aggressive with your inputs. Also, you air handler needs to have the ability to match the air flow to your selected AC units capacity. This is often an issue in Ontario, where an oversized furnace necessitates a oversized AC unit. Pushing 2-3 tons of air thru a 1.5 ton evaporator is not going to dehumidify.

      For conventional North American split setups though, I've personally witness marked improvements in comfort by going from a single stage to a 2 stg AC unit even when 1st stage is 66%. The upstairs and bedrooms were noticeably more comfortable. The longer run times really reduced the temperature differences between rooms that you get with a oversized unit and short run times.
      My office boardroom has a grossly oversized RTU serving it, so that even when at design occupancy and OA temps it barely cycles long than a couple minutes and lets a lot of humid OA into the room between cycles making it feel like a cave. The offices around it have tightly sized LG VRF heads that control humidity very well.

      I'll agree that for dehum the most important things are probably:
      - not running constant fan (you want the codensate to drain not evaporate between run cycles)
      -having a reasonable CFM per ton, or even better an ECM fan air handler with a Dehum input to reduce the CFM per ton under higher humidity conditions. (this can be a negative to mini splits).

      Rounding up (15% to even 50% oversizing) to the obvious size based on equipment availability is totally different than the more common putting a 4-5ton unit when when a 1.5 to 2 ton will do (see 100%+ over sizing all the time). The extra fan noise alone is a comfort issue IMHO. I guess I'm a little sensitive to gross oversizing, rather than informed equipment selection that results in a slight oversize.

      Once the sizing is within a reasonable range over sizing becomes less important, the thermal inertia of a house can over some sins with regard to over sizing and make it not a big deal. Personally I'd rather have a critically sized unit the possibly never stops (on a design+ day) and maybe an interior that gains 1-3deg, than a unit that barely runs and short cycles the rest of the time. But, putting a 2 ton unit in instead of a 1.5 is unlikely to cause comfort issues assuming proper duct work and air handler and can carry you thru to 0.4% temperatures if you're really concerned about your house gaining a degree or two.

      1. Jon_R | | #7

        We agree on limiting AC sizing to 15-50% over design load and using good (as in *decreasing* CFM/ton values at lower loads) modulating equipment when possible.

        People often spend lots of time on getting sensible load exactly right and then either ignore latent load or only consider it at the rarely seen design load.

  4. MattJF | | #4

    Manual J is generally very conservative and has historically oversized equipment even when used perfectly. It doesn't account for any thermal mass, which has a big effect. The oversizing gets worse when contractors don't use correct inputs. I don't think I've ever heard of anyone doing a manual J and later determining the equipment was undersized.

    The equipment also likely won't be sized to exactly to the load. Due to what is size increments available, you will end up with additional extra capacity. After choosing equipment, you can go back to the manual J and see what input temp maxes out the equipment.

  5. Expert Member
    Dana Dorsett | | #6

    >"Anyways, if an HVAC system were sized such that it could maintain an indoor temperature of 75 degrees on days that the outside high is 92, you would have been quite toasty–in your own house–for an entire month."

    Not really. The daily air temperature highs are well above the average daily temperature- the AC can always catch up. And, the thermal mass of the house and it's contents limits the temperature overshoots for those few hours per day that it overshoots 92F outside.

    Also, the dependency relationship of cooling load to outdoor temperature isn't nearly as strong with cooling loads as it is with heating load. Solar gains vary with glazing orientation, and are only rarely perfectly synchronized with peak outdoor air temperature. High latent loads are usually associated with more haze/lower solar intensity too, which is also less correlated with peak outdoor temperatures.

    With all of the moving factors a system sized EXACTLY for the 1% load MIGHT end up feeling "...quite toasty–in your own house..." for a FEW HOURS on SOME days when outdoor temps overshoot the design temp every day for a month, but there is no way that you'd be "...quite toasty–in your own house–for an entire month." In fact those "...quite toasty..." HOURS would be pretty rare, in most cases.

    I believe ASHRAE's published design temps have for a long time been the 0.4% & 1% & 2.0% and 97.5% & 99% & 99.6%, temperature bins based on the most recent 25 year hourly temperature data for the location, updated every ten(?) years. And yes, those bins have been trending warmer for most US locations over the past few decades. The temperatures are all binned and hours counted for the full 25 years- the 1% design temperature is the temperature at which only 1% of all hours for the 25 year period have been warmer than that number. There is a lot of variance year to year, but in a perfectly typical year that would be only 1% of the 24 hours x 365 days = 8760 hours in a year, or 87.6 hours for that year that would be warmer than the design temperature.

  6. aunsafe2015 | | #8

    Thanks for all the replies. I understand it better now.

    Btw, I have a 2-ton Trane XV18. Manual J was about 18k btu using the 92 design temp. It was able to maintain 75 even on 100+ degree days during this heat wave but my indoor temperature did slowly rise. Another hour or two and it probably would have gotten to 76+ but by then it was 7-8 pm and things had cooled down enough that my indoor temperature stopped going up. And by 9 pm it was easily able to achieve my nighttime setpoint of 73.

    1. Expert Member
      BILL WICHERS | | #9

      >"It was able to maintain 75 even on 100+ degree days during this heat wave but my indoor temperature did slowly rise."

      That is what I like to call "losing the battle". It was NOT maintaining the indoor temperature if the indoor temperature was "slowly rising", so that means the heat gain of your house was exceeding the ability of the AC to remove it. That may not be a problem if it only happens on the occasional unusually hot day, and you don't get too uncomfortable before your system can recover over night when things naturally cool off outside.

      At some customer sites with drycoolers we will mist them with water using little garden sprinklers (we jokingly call this "afterburners" :-), which gets about 10 degrees of performance boost. The tradeoff is the coils rot out faster. You can do this as an emergency measure if you need to eek out a bit more performance on an uncomfortably hot day. You're essentially using evaporative cooling for the outdoor condenser unit this way. I wouldn't do this all the time though.


      1. aunsafe2015 | | #11

        Yeah, I suppose it is more accurate to say I was losing the battle. That said, the few days we that we did see a slow creep upward, the low temperature the night before had been 77-78, and the high temperature the days we were losing the battle was 100+. And even on those days, I don't think the indoor temp ever exceeded about 75.5. So I feel pretty comfortable that we'd probably stay pretty comfortable unless the outdoor temps got up in the 105+ range, which according to the ASHRAE data posted above is about a once per 50 year event. Maybe if that happens we would try the afterburners idea... hah... or, we would just live with a few days where the inside temperature was 77-78 instead of 75 :)

        1. Expert Member
          Dana Dorsett | | #14

          >"...we'd probably stay pretty comfortable unless the outdoor temps got up in the 105+ range, which according to the ASHRAE data posted above is about a once per 50 year event."

          With the accelerated global warming that may end up being a once every 5 or 10 years by the end of the equipment's lifecycle, but I think you can probably still handle that level of discomfort, eh? (Less clothing and cool drinks can fix lots of heat-comfort issues- just don't let the dog sit on your lap. ;-) )

    2. Jon_R | | #10

      Sounds like your ~33% over design load sizing (and variable speed) is about right for optimal comfort.

      1. aunsafe2015 | | #13

        Yeah, and 2-ton is the smallest XV18 model available, so we couldn't have gone smaller unless we went with something like a ducted mini split (which we considered). The variable speed is great for dehumidification. According to Trane's product data, it maintains about 25% latent capacity even at its lowest operating capacity, which is around 8500 btu at 85 F outdoor temperature. So the "oversizing" is not a problem at all, and it probably helped us out a bit in this recent heat wave.

  7. 300TTto545 | | #12

    I live in Raleigh (in the area for 22 years). It has been hot but not that hot. I think the data is wrong - obviously that is my opinion and I have to admit to being out of town from July 2-11. You should know that June 20-July 20 is probably the hottest 30 days anyway. August can be surprisingly cool - I remember a year or two ago we didn't hit 90 in the month of August.

    The other thing is that I thought our design temp was 95. We typically have 5 days a year above 95. Everyone is shocked by that because they think it is hotter here. We are above 90 for over 30 days (somewhere in the 30-40 range). So I always thought 95 was just right.

    Manual J is garbage in my opinion when it comes to cooling in our climate (probably garbage in most circumstances based on garbage in). My near correct size a/c can keep my upstairs at 70 running 2 hours a day when it is 95+ out.

    As others have said solar gain is the big issue. Last time I ran a manual J - it treated east and west window orientation equally. Well you never get peak temperature drive in the first half of the day and you don't tend to get wind either. So west is more important. Also, almost no where in Raleigh do you have zero shading. Manual J probably assumes you are on a flat field with no trees. Most people I know in the city can't do solar panels because of shading - from trees. Completely destroying the manual J solar's calcs.

  8. Hugh_Will | | #15

    ASs ASHRAE 55 recognizes, what a homeowner is probably really after is comfort, not any particular thermometer reading. I too am in NC, which is considered to have a "hot/humid" summer climate. For us, humidity control is just as important as indoor temperature, for "thermal" comfort. Increasingly, (although slowly) HVAC pros are beginning to recognize that oversizing the a/c to handle "worst case" conditions is a poor solution.

    I find my home is comfortable at 77 degrees, if I keep the RH below 44%. (Courtesy of the previous owner, I have ceiling fans in every room, although I rarely turn them on).
    RH is controlled by deliberately NOT oversizing the (two-stage) heat pump, trimming the blower speed a bit, and supplementing with dehumidifier. (Which itself is a significant consumer of electricity and a heat source). But there's even more to it than T and % RH.

    Controlling radiant (insolation) is another key: Mine is a (1950's) ranch-style house, 2800 sf conditioned space (2000 main level, 800 conditioned basement), brick veneer, long axis oriented NE/SW, with a typical amount of windows. So the "straight-on" late afternoon sun gets spread across the long NW facing side, with only the smaller SW end of the house fully exposed to the hot afternoon sun. Bedrooms are at the SW end of the house, and they do get warm during the daytime. The northeast end (family room, kitchen, etc - where we spend our days) tends to stay cooler.
    Also important is the attic and exterior walls - Insulating the stud bays inside of those hot brick walls and bringing the attic insulation up to R-60 with "wind-wash resistant" blown-in cellulose, plus soffit/ridge passive attic roof cooling - significantly reduced the living space's mean radiant temperature, and thus occupant comfort.

    An older (2014) article "Thermal Comfort: More Than Just Air Temperature" by Dave Moser is excellent, and still available. I've tried to take its lessons to heart.
    Besides dry-bulb air temperature, factors affecting thermal comfort include: Mean Radiant Temperature. Insulation. Humidity. Air Velocity. Metabolic rate. Clothing.

    P.S., Several years ago, when we had a several day super-hot heat wave, and realizing my system was deliberately not designed to cope, I rigged up a drip hose on my a/c condenser coil. Dropping the coil's discharge air by nearly 25 degrees, it increased the cooling output enough that the house maintained temp - and the a/c even cycled off and on throughout the 112 degree afternoon. I don't recommend it as a routine fix, but in a heat "emergency" situation, it works.

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