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10 Tons of AC for 9000 sq ft basement??

JUGHNE Member Posts: 11,108
I was asked by a local committee to review their preliminary drawings and specs for a new 9440 sq foot, infant and day care building.

I would not have a dog in the fight but another HVAC contractor called me and said that there are ridiculous specs involved. So the local GC asked me to look at the papers.

They state that 28-32 tons of AC would be needed for 5 "busy" rooms and admin, kitchen and conference rm etc. And an estimated 400 MBH for heating.

This would house about 65 children max, from infant to 5-6 years old.

They did point out that a detailed load analysis will be completed.
These estimated capacities are based on expected loads per sq ft for buildings of this nature.

I am curious to see their final calculations.

I pointed out to the board the tonnage for several public buildings.
For instance our 67 seat movie theater has a total of 8 tons of cooling...people are wearing sweaters or jackets.

OK, so the load should be less that stated. I will give them that.

However they say to consider two 5-ton systems for future finish of the basement area.

This basement is completely below grade with no windows. The exterior walls are 12" thick Insulated Concrete Forms, I believe this gives you 4" of Styrofoam wall insulation....below grade.

To make this statement in their specs gives me doubts of their creditability and knowledge.

I did point out that they have a structural floor beam drawn thru the footprint of the elevator shaft.....should I offend an architect??

These companies throw something together to impress the committee.
Yes, they might notice the beam for the final drawings, but not necessarily so.

Then after the final prints they have to issue a change order, they would imply there would be no additional cost, but someone has to pay for the new drawing.

These professionals seldom do anything for free.


  • Matt_67
    Matt_67 Member Posts: 295
    I’m curious what they are using for ventilation and how much. Engineers are good at calculating worst case latent loads and sizing systems to that. Are their kitchens or other areas that might have high exhaust rates?
    EBEBRATT-Ed Member Posts: 15,839
    The only way to know is to calculate it.

    But at a glance it looks very wrong to me,

    Insulated block, below grade with a ceiling above not exposed to the outdoors.

    Ventilation air is an issue with all those kids. I would suggest using Co 2 sensors to control the OA you will end up with the correct amount of OA that way.

    Looks like they are using the 250 square feet/ton that Ashrae lists a a guestimate for commercial.

    below grade the load is small very little sun exposure
  • GGross
    GGross Member Posts: 1,105
    couple tons just in human heat at least, not taking into account the employees

    then you have a kitchen that is feeding 65 children? they will size the load based on all that equipment running more than likely.

    There will be a lot of ventilation, most likely they are conditioning the fresh air coming in, perhaps they sized the A/C to cool the building even if the fresh air systems condenser is down?

    Looks like they suggested 2, 5-ton units, my guess is that they are putting the load closer to the 5-ton mark (reasonable considering the above) and have planned the system for redundancy. one condenser goes down and the system will still work
    JUGHNE Member Posts: 11,108
    There is a carry in and reheat with microwave kitchen.

    ERV's will be installed for fresh air.

    We are climate zone of 5A, design temp of -15 winter an 94 summer.

    Regardless of design sizing, I predict overcooling which is quite standard with (over) engineered jobs.

    They mention 6 zones. Typically what happens is the zone control wiring is barely understood by the installer and within about 5 years nobody knows how it is supposed to work.

    I read another HVAC web site in addition to this one and 2 issues frequently pop up; one is zoning and their controls and the other is T-stats....especially the word Nest comes up often.

    The basement will most likely never be used for other than storage and storm shelter.

    The elevator is "proposed", probably never installed.

    Another item in specs calls for master back flow preventer on the water service. Here they are only required on UG lawn sprinkler systems and boilers. Supposed to be tested annually, but seldom are.

    This is just my rant about over design/building.

    Thanks for listening! :)

  • The Steam Whisperer
    The Steam Whisperer Member Posts: 1,219
    Right there with you on oversized equipment by engineers and other screw ups. On a recent job for a highrise residential condo in Chicago, with north, west and east exposures, and finished space above and below, the engineers speced out 10 tons of cooling for the 2800 sq ft space. A more recent job, there was no heating provided in several exposed spaces, in others not even close to enough radiation capacity, and for others loads calculated so high that there was no way to fit new radiators to replace existing convectors isnstalled at some time in the original radiator alcoves. This is going around with everyone running around saying we need to cut down on pollution....what a waste these grossly oversized new systems are.
    To learn more about this professional, click here to visit their ad in Find A Contractor.
    JUGHNE Member Posts: 11,108
    The architect reviewed my notes on his preliminary drawing/specs and he agreed with them.

    The GC said he was amused by some comments, especially the structural beam thru the elevator shaft.

    Another question is he mentions ERV's with electric pre-heating.....is this commonly done??

    The building will have NG cond furnaces with split AC's.
  • GGross
    GGross Member Posts: 1,105

    Maybe he is talking about a dedicated outdoor air system? (referred to as DOAS on prints) I do see these on most new construction medium-large size commercial projects. Engineers love them because they allow for complete control of the fresh air. Mostly when you see these they are quite large.
    JUGHNE Member Posts: 11,108
    Thank you, is that similar to preheated make up air as you might have for comm kitchen hood?

    I don't think he has the EE and ME on board yet as this is very preliminary.

    When they pull the trigger for final prints then the big clipboards will come out.
  • GGross
    GGross Member Posts: 1,105

    I believe the way they work is similar, outdoor air is pre-conditioned before going into the building. the DOAS units get pretty large, and can be setup quite complicated in some jobs, I see them on a lot of schools and similar buildings. If the CFM required is lower they may specify more of a standard makeup air unit