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Design Day Conditions

in the 2 ton camp. Be careful mixing air handler sizes also. I sometimes run low fan speeds (350 cfm per ton) to help with moisture removal. If you use a larger AH you will increase sensible removal while sacrificing moisture removal. I like the 2 speed idea also. I use them almost always when the customer is smart enough to listen to me. WW

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  • Constantin
    Constantin Member Posts: 3,796
    Design Day Conundrums...

    ...Prof. Silberstein, I hope that you can help me with sizing the AC system for my home. Both my contractor and I have put together manual-J compliant heat gain calculations - I used HVAC-Calc, he used Wrightsoft.

    Remarkably, both results are very close whenever the design conditions are similar. Trouble is, HVAC-Calc tells me that outdoor design conditions in Boston, MA are 88 degrees F with 88 grains of moisture, while my contractors design conditions are 96 degrees F with 56 grains/lb difference between the indoor and outdoors.

    Is there a hard-and-fast ASHRAE design-day summer temperature/grain number for Boston that I can hang a hat on? I would be happy with either the 1% or the 0.4% conditions, seeing that the thermal mass and insulation of the structure should probably carry us through the hottest spells just fine.
  • D lux_2
    D lux_2 Member Posts: 230
    whats the tonage

    difference ?
  • Constantin
    Constantin Member Posts: 3,796

    ...my contractor wants to install 3 tons of condenser capacity for the air handler in the attic due to his design conditions.

    However, I doubt that I'll ever want design conditions of 70°F on the inside, with an indoor RH of 50% - it'll be freezing! My feeling is that 74°F with that RH is as low as I'd want to go.

    What confuses me are the different design conditions that are being thrown about for Boston. Anything from 88°F to 91°F seems to be fair game as far as the 1% design condition is concerned, i.e. the maximum temperature at the tail end of the 99th percentile range. Granted, those could be some hot 87 hours on average per year, but nothing insurmountable.

    Anyway, when I plug in 91°F outside with 88 grains of moisture and 74°F/ 50%RH on the inside, I get a heat gain for the condenser/AH combo on the order of 22kBTU sensible/2kBTU latent. Meanwhile, due the design conditions he used, his calculation shows about 30.6kBTU of total gain, with a similar sensible/latent heat ratio.

    At issue is that the 2ton condenser /3ton air handler combination has a total capacity of about 27kBTU, along with a 90/10 sensible/latent ratio. Thus, by my calculation, the two-ton condenser should be OK, by his calculation we would be short by a couple thousand BTUs.

    The condenser series we want to choose from comes only in single-ton increments. Thus, our condenser would be significantly oversized much of the time if we were to chose a 3 ton unit.

    Any thoughts?
  • D lux_2
    D lux_2 Member Posts: 230
    change manufactur of condenser to 2.5 ton

    or a 2 speed comperssor
  • don_144
    don_144 Member Posts: 27
    I would

    think being you only reach peak demand only a few times of the year that your choice of a two ton system would be just

    I mean after all the ac is running at part load 97% of the
    time anyway right?

    Also keep in mind that with homes getting tighter the sensible load will come down and the latent load will go up.

    That two ton looking better all the time to me.
  • Constantin
    Constantin Member Posts: 3,796
    Thanks Don & D Lux

    My wife and I feel that an AC system is a bit of a luxury as it is. Thus, we're OK with the inside of the home getting a bit warmer on the hottest of days. Plus, the longer the AC runs, the better the moisture removal, which is really what my wife is after.

    I'm currently still in the two-ton boat. Whether that two-ton has a 2-speed compressor or not seems less relevant to me, as the 1st speed is fixed at 66% of capacity. By my reckoning, the second stage of a 3-ton condenser would never light off except in 0.4% conditions, so why bother?

    Anyway, does anyone know what the official 1% ASHRAE exterior design conditions are for Boston? That seems like the most relevant question right now as far as convincing the contractor to adjust his load caclulations.
  • bob_44
    bob_44 Member Posts: 112
    ASHRAE Boston AP

    Design Dry-Bulb and Mean Coincident Wet-Bulb 1%=91/73 2.5%=88/71 5%=85/70 Mean Daily Range 16 Design Wet-Bulb 1%=75 2.5%=74 5%=72.
  • Eugene Silberstein 3
    Eugene Silberstein 3 Member Posts: 1,380
    Books in the office

    Sorry I didn't respond sooner, but I just got back from a quick trip to Puerto Rico. My ASHRAE books are in the office, but I just read the last post and it contains the ASHRAE numbers. I am assuming that they are correct, but I will check on them when I get into the office tomorrow.

    As far as the capacity issue goes, mismatching air handlers and condensers (Manufacturers and capacities) was a common practice for many years. Nowadays, given the higher EERs and varying operating characteristics of different manufacturers equipment, one should tread very carefully. It's always best to match the capacities of both the condensing unit and the air handler.

    In your case, as the other eluded to, the 2-ton system will most likely meet your needs.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
    70° @ 50% RH

    Since you say you find A/C mainly a luxury and don't expect to use it a lot, believe that 70°F @ 50% RH would be quite uncomfortable unless you became accustomend to such and then you'd need to run the A/C a LOT more.

    Your indoor humidity estimate of 50% sounds a bit high to me. We've just had over a week of some of the most miserably humid weather in years (hurricane Dennis remnants) and with an indoor temp of about 77° my indoor RH only topped 50% during the two days of constant rainfall with outdoor temps nearly identical. A/C still ran a LOT however. Basement (with just a mid-size dehumidifier and no A/C) has hovered a touch below 60% RH only spiking during that rain period. Lowest dewpoint I've seen for over a week was 72°. Right now we're getting rain showers at 76°.

    Would imagine that your indoor RH will be significantly lower than 50% making 70° feel even cooler... Again, if you get accustomed to those conditions, you'll be running the A/C a LOT and you'll HATE the outdoors.

    3 tons for the attic only? 2nd floor and attic? 2nd floor only?

    Our homes are similar in size and age but yours has significantly better infiltration control and insulation. Can't recall if you used a HRV or ERV--if an ERV you won't be getting much humidity gain from the outdoors. Have a two-ton here in my attic. On the hottest days unit is undersized as temp will rise--particularly if t-stat "set-up" is used. Will rise to 78°-82° depending on how the t-stat has been programmed, but thankfully humidity is reasonably low. Since this is a significantly warmer and more humid climate would suggest that 2 tons would be fine for the attic alone.

  • Eugene Silberstein 3
    Eugene Silberstein 3 Member Posts: 1,380
    Relative Humidity

    50% relative humidity is the parameter accepted by ARI and ASHRAE. Although this numbe may seem high to some, it is actually a middle-of-the-road number to prevent excessive dryness and also to keep mold growth to a minimum.

    It is estimated that, at relative humidity levels below 40%, skin dryness, furniture cracking and other dryness-related issues increase in occurrance and intensity. At relative humidity levels over 60%, the rate of mold growth is significantly increased. Since 40% is too low and 60% is too high, 50% seems to be the magic number.

  • jerry scharf_3
    jerry scharf_3 Member Posts: 419
    I'm right in with Mike T


    A 70F design day goal is great for people wanting to waer business suits in your house in the middle of summer. Somehow, this doesn't strike me as your style. :)

    Walking into a 70F house is quite a shock after being outside on a hot day for any amount of time. Though not perfectly comfortable, I have no problem being at 76 on design day, and running higher than that on extra hot days.

    Also, I have a sense with what you did with your insulation. :) With either of these programs, they dod not correctly cover the superinsulation case well, and overstate the gain and loss. I've gone through days in the high 80s without turning on the A/C (we have drier air) with good comfort. It's a tad unfair, because occupancy activity was well below normal, but it gets the point. Corbond generally recommends that you reduce the A/C capacity by 1/3 beyond what the manual J says.

    Finally, given my research on molds, I would also stick with a lower target RH than 50%. I'm in an extreme situation, but my target is 40-45% and capping at 50%. Robert Bean's site covers this in great detail and he recommends moisture ranges like I am after. If you're serious about molds, I would also recommend a UV-C lamp in the expansion coil cabinet running 7x24 during the months that the A/C is run. They don't kill spores, but they will stop mold growth. Moist, dark, lot's of skin flakes and dirt for mold food.

    You're on the right track. Tell the contractor that you'll take the risk of being slightly hot on peak summer afternoons and go with the smaller unit.

  • jeff miller_2
    jeff miller_2 Member Posts: 4

    ASHRAE Design Data:

    0.4% - 91DB / 73WB.
    1% - 87DB / 71WB
    2% - 84 DB / 70 WB

    You can get the grains/lb from the psych chart or McQuay's software (free on their web site).
  • Eugene Silberstein 3
    Eugene Silberstein 3 Member Posts: 1,380
    Thanks Jeff

    Thanks for the info.
  • Joe Grosso
    Joe Grosso Member Posts: 307
    Thanks Bob!

    I guess HVAC-Calc uses the 2.5% design condition, while some other resources I've seen prefer the 1% condition. Either way, it's not anywhere close to the 96 degree condition that my contractor was aiming for. Many thanks again for helping me out.
  • Joe Grosso
    Joe Grosso Member Posts: 307

    ... double post, sorry ...
  • Joe Grosso
    Joe Grosso Member Posts: 307
    Well, here is my thought process...

    ... the idea all along was to button up the house, reduce infiltration, and control the incoming and outgoing air as much as possible. Then perform a load calculation to figure out just what the sensible/latent load mix is given local conditions, anticipated occupancy levels, etc. For example, all bedrooms are expected to have 2 people at rest on the inside.

    Because of all the wood inside, my sensitive nose (it likes to bleed), etc. the name of the game then was to hit the latent/sensible mix as best as possible. That way, I could prevent the house from becoming too dry in the summer (if the latent side was oversized) while the humidifier provides comfort in the winter.

    The three-ton attic AH deals with the upper levels of the (typically square) Mansard core of the building. That means it handles 4 bedrooms, two bathrooms, a stairwell, and the study. It's about 1,600 sq ft. total. While the 2nd floor windows are largely unchanged (except for the addition of Harvey Tru-Channel storms), the third floor has nothing but Marvin Ultimate Double-Hungs, i.e. very efficient windows that should minimize solar uptake.

    The HRV/ERV that feeds that part of the house is a 195TRV from Lifebreath/Nutech, i.e. functionally similar to the DCS series but with one HRV and one ERV core instead of two HRV cores.

    The basement AH is a 4-ton unit that feeds the rest of the house. Even though the total load came out to about 27kBTU/hr, I still decided to leave some headroom for hot muggy nights with a lot of guests in the formal spaces and traffic in and out of the house. With a 3-ton 2-speed condenser, I have about 38kBTU of capacity on hand.

    The two evap/condenser combinations are taken straight from Lennox' design manuals, i.e. nothing that is not expressly approved by the manufacturer nor tested with ARI ratings to match. I'd like to think that the resultant performance will be energy-efficient, comfortable, and quiet. Time will tell.
  • Joe Grosso
    Joe Grosso Member Posts: 307
    Thanks Jerry!

    Many thanks for your insights regarding the higher efficiency of foam-based insulation materials. I didn't dare increase the performance measures a great deal as there is plenty of old wood in the walls to short-circuit the thermal paths into the structure.

    I also look forward to comparing my calculated energy needs vs. reality. Like you, I expect to do better than the programs predict but the proof will be in the pudding. Due to my sensitive nose (sniff!), I will stick to slightly higher RH targets, as the red cross is no longer interested in my blood (I've spent too much time in old europe. Evidently, I should start tottering from an BSE infection soon).

    We will live with the AC system as is for now and then see what improvements make the most sense. For example, we may spring for a TFP in addition to the Apirlaire filters, or the UV-C sanitization system. Some experience in the new place will tell us what we need. The fewer gizmos I have to babysit, the less complicated my life, the happier I am.

    The postscript to this issue is that the subcontractor re-ran the numbers using a 91 deg. F design day with 74 deg. F indoor conditions and came to the conclusion that my sizing was accurate and that the 2/3ton combo was adequate. Thus, that is what we'll be installing in the near future. Whenever the two Lennox XC21's are in place, I'll upload some pictures.
  • Joe Grosso
    Joe Grosso Member Posts: 307

    I appreciate the insights, Prof. Silberstein.

    Given your worries about mismatching the tonnage sizes of condensers and evaporators using 410A as a refrigerant, is this something I should worry a great deal about considering that Lennox published these combinations in their ARI specification tables and the detailed design condition manuals for the XC21?
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928

    Do understand 50% RH as the "magic" number, but I have a hard time believing that such is actually achievable in any but the most controlled, sophisticated and maintenance-intensive large-scale commercial systems.

    Must say I've been a bit surprised at summer indoor RH typicaly around 43% average (like temp, humidity reflects the solar cycle, but unlike temp it also reflects the outdoor humidity, especially rain) for a couple of years. Multiple dataloggers and while humidity is the least accurate measure in the system, that range between 40% - 60% is the "sweet" zone.

    In my admittedly poor understanding of indoor RH as related to outdoor RH as modified by an "air conditioning" system, it would seem that a relatively oversized A/C system will remove a greater portion of sensible heat while a relatively undersized will remove a greater portion of latent.

    Both undersized and oversized can be required for that 50% level depending on conditions--both indoor and outdoor.

    That Rawal valve would [seem] to be the only current means of approximating this changing requirement.
  • Eugene Silberstein 3
    Eugene Silberstein 3 Member Posts: 1,380
    The Rawal Valve

    The Rawal valve, as mentioned in an earlier thread, is a device that maintains a constant suction line temperature. Contrary to popular belief, the valve is not a metering or expansion devoce, but a modified/glorified hot gas bypass.

    You are corect regarding oversized and undersized systems and it pays to repeat.

    Occupied spaces that have oversized ac equipment often encounter high humidity problems since the system constantly short cycles, reducing the de-humidification effect.
  • Mike T., Swampeast MO
    Mike T., Swampeast MO Member Posts: 6,928
    Hot Gas Bypass Producing Costant Suction

    But isn't suction considered chaos in a simple A/C system in a changing environment under digital control?

    "Constant suction" would [seem] to imply order from chaos.

  • Eugene Silberstein 3
    Eugene Silberstein 3 Member Posts: 1,380
    Go Figure

    Very interesting device though. I invited the designer of the RAWAL valve being referemced to join the conversation about the valve's operation, etc. but he (Davis) has yet to join in the fun.

    I had a lengthy conversation with him about two weeks back.
This discussion has been closed.