Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

HVAC Oversized?

Osborn Member Posts: 31
Hello - I'm a building owner working with an architect to design a heating system for a 2600 square foot commercial space in Detroit. The archiect's heating designer concluded that I need a 10 Ton 224BTU 3 phase RTU for the space. He says this will help with the required conditioned ventilation requirements. This was bad news for me because I prefer hot water heating and I find it massively oversized, leading to lifelong waste of energy and $.

My envelope is: ceilings R40; walls R20; infiltration average; slab on grade floor. The construction is careful. I calculate a heat loss of 75K to 100K btus on design day through on line calculators. I have used this method before in sizing a boiler for a 7K sq/ft building with success.

I would like to use a condensing boiler/radiant floor heat. The architect then told me I would have to spec the boiler for the outside air intake (which apparently by code is necessary - even at night and when empty - this seems off.) But again I'm new to commercial hvac requirements...

Can anyone offer me some advice on how I can move the designer towards the radiant system? Or: could his calcs possibly be right, or is it clear that he's calculating heat loss just by volume?

Any thoughts, appreciated!


  • Zman
    Zman Member Posts: 7,573
    The goods news is, you are the owner. You pay the bills and you should get what you want.

    The answer to your question is in the Uniform Mechanical Code.
    You start with the use of the space and the number of occupants.

    He is correct that you need air changes. You won't need it active when unoccupied. Sometimes you just need operable windows.

    Post some more details and let's see what you have. There should be a table in your mech design with the design assumptions.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    EBEBRATT-Ed Member Posts: 15,614
    You do not need to bring in outdoor air when the space is unoccupied. That's a HUGE waste of energy.

    If you don't want a RTU use a heat recovery ventilator (HRV) this recovers exhausted hot or cooled air and uses that energy for incoming air...must more efficient.

    Sounds like you need a different designer
  • Zman
    Zman Member Posts: 7,573
    RTU's are popular with architects because they are simple to design and don't take up space in the building. All the designers need to do is send there plans to their favorite sales engineer and they will (over) design it for them for "free" and then mark up the product on the back side.

    In some situations, like commercial kitchens an other high air change applications, RTU's are the way to go. As Ed mentioned, for many smaller applications, an energy recovery ventilator, with occupancy controls is a much more efficient.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    JUGHNE Member Posts: 11,075
    Here is just a story of sizing and Mech engineering.

    We have a 1971 nursing home with attached assisted living.
    One RTU was a 2 stage 15 ton Lennox from 1971.
    Another RTU was a 2 stage 7 ton Rheem from 1984.
    These were scheduled for change out in the 2012 renovations.(somewhat overdue)

    I contacted the architect whom I had know for 30+ years and suggested both these units could be down sized.
    He in turn called ME who said no, they should almost be sized up. The ME's numbers were based only on area of the buildings.

    My reply was that each unit has had a compressor failure and had been cooling on one stage only.......for the last 5 years......most of the women working there were cool enough most of the time. Residents had sweaters and lap quilts year around.

    ME replied that is impossible, maybe the compressors were not equally sized and only the small one failed, insinuating that me as a local yocal was not aware of that. Or that they were still operating and I was unaware of it.

    I replied:
    I know these compressors were equal tonnage as over the last 30 years I worked on these and physically carried the replacements to the roof. Installed the 1984 unit myself. One compressor had a burn out and was unwired. One was mechanically locked up and I unwired it also.

    Plus the kitchen load was dropped as it had it's own 5 ton AC unit.
    All resident rooms had mini split HP/AC's or PTAC's. (9000 btuh each inside head)
    The TV room had it's own 2 ton mini.
    The hobby/crafts room had a 9000 btuh mini.
    The 15 ton RTU had inadequate return air ducting and was subject to coil freeze up. (this duct work was not to be changed)
    The had thermal envelope had improved over the years with insulated rubber roof and new windows.

    Despite all I mentioned above the ME was adamant that it needed the original tonnage, perhaps more.
    I discussed all this with the architect, I was surprised that he actually came to a compromise and over rode the ME advice.
    This seemed to be a rare Act of God.

    One thing is guaranteed is that if you have a "professional" ME (over) size your system, it will never be too small for either heating or cooling. They will never be liable for under sized equipment of any type.....tanks, piping, mixing valves, etc.
    Few owners will realize the consequence of over sizing.

    You sir, are the rare exception who sees this in the planning stage.
  • hvacfreak2
    hvacfreak2 Member Posts: 500
    Just fyi , Trane has some new offerings in their smaller RTU's that might fit your situation. Single Zone VAV and True VAV and available with Demand Controlled Ventilation with variable power exhaust. These replace typical change over bypass arrangements and are also available with energy recovery options. Something like this should work well with your hydronic efforts as well.

    Mechanical Enthusiast

    Burnham MST 396 , 60 oz gauge , Tigerloop , Firomatic Check Valve , Mcdonnell Miller 67 lwco , Danfoss RA2k TRV's

    Easyio FG20 Controller