Replacing oversized equipment? 2

[apollowolfe]

Hello, I have not been in the industry very long and wanted some feedback on a design that took place at work.

We had about a 9,000 SF single floor office building in Tennessee with no major loads. The original equipment was a 24,000 CFM AHU and a 60-ton air-cooled chiller. The client wanted to switch to a DX system while leaving all the current ductwork and VAVs. The load I ran for the building yielded about 23 tons of cooling. The lead engineer of the project ended up just getting a new 24,000 CFM AHU and a 60-ton VRF system. Would you consider this approach acceptable, and if not, what would be a better solution?

Thank you for the Input,

 

[HVAC-Novice]

Without knowing the details of each load calculation, it is hard ot say who is right or close. In the old days they had less efficient lighting, no ERV, and maybe just assume someone infiltration. They also may not have accounted for diversity of VAV and just treated it as CAV system.

Only a trial by combat between you and the original engineer will bring clarity.

 

[MintJulep]

Why not ask the lead engineer to teach you?

Seems like between the two of you, you have all the information, and we have none.

 

[apollowolfe]

My lead does not has much experience, and I am not confident with some of the stuff he says. On this instance he said he ran a calculation and it was very low so he just wanted to defer to what was existing.

It seems like installing equipment more than double the necessary load does not follow the IECC sizing requirements.

 

[MintJulep]

...you have all the information, and we have none.

 

[apollowolfe]

I am not concerned with getting a solution to this older project, but was using it as an example.

My question is generally what would you do in a situation where you have to replace an existing VAV AHU system with over twice the required capacity?

Is it acceptable to replace with same capacity equipment? Or would a complete system redesign be required?

 

[MintJulep]

There is the notorious "400 cfm per ton" rule of thumb, and 24,000/60 = 400.

So it's likely that the original system wasn't engineer at all. Perhaps the original HVAC designer somehow came up with the cfm, and applied the rule of thumb to get the chiller size, or the other way around.

You might engage with the owner, telling them that the original equipment seems oversized, and that they could potentially save a lot of money on equipment and operating cost by more carefully sizing the system.

Is the payback there?

 

[HVAC-Novice]

Before installing a significantly smaller system, I'd very carefully review all assumptions of enclosure (inc. thermal bridging), infiltration etc. What is your peak of all coils load vs block load? What software have you used? What safety factors? Accounted for the equipment heat gain, like fan heat? Duct and pipe heat gains in attic?
Did they have an IT server room at some point? what outdoor air conditions percentile did they assume?

60 tons for a 9,000 ft2 office seems very high. But we don't know if you are in Saudi Arabia or Alaska and if that is a big glass house.

Edit: and since you say VRF was used... does that also heat? if so, it may be sized on heating load. Either way, us internet Randoms can't be the judge in the " who's design is right" contest.

 

[LittleInch]

Maybe 2 x 30 ton units?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[apollowolfe]

Ultimately, I do not see this project as up for debate because my lead stamped it so that is his issue. I am just trying to use this as an example to learn best practices.

I utilized Trane Trace for my load calcs, and I was generous with the internal load allotments. One of the less than 100SF offices may be a server room because from pictures I notices a wall mounted AC unit, and I accounted for that in my calculations.

The existing system was an air-cooled chiller. We ended up doing a set of two cooling only 30-ton VRF systems piped together to an AHU. The heating is supplied by a separate boiler.

My concern with oversizing the system so much you will be inefficient, have humidity issues, and does not meet code? Are these valid concerns or does utilizing a VRF systems handle this gross oversizing appropriately?

Thank you for all the feedback so far.

 

[HVAC-Novice]

VRF can modulate, but you still have over twice the size. Humidity is a concern and upfront cost as well.
How is outside air treated? Is that a separate DOAS and even more cooling capacity? What climate is this?

I'm not sure it is a code concern since that is just matching the previously approved system.

I sometimes replace boilers. I typically match the existing or maybe get one step smaller if we know from past experience they don't run full fire. For boilers a new system usually is sized to have each boiler provide 2/3 of the load for redundancy and still good part load. But I don't know what the original designer did. Maybe they sized each to carry 100% of the load, maybe not. I once attempted to re-calculate the heating load. But I realized depending on infiltration I ASSUME, the new boilers could be smaller, or larger. This is even harder for cooling. You now use less lighting power, but more for computers. Maybe in 2 years they all use two 43" monitors and put twice the people in the space.

You say "One of the less than 100SF offices may be a server room because from pictures I notices a wall mounted AC unit, and I accounted for that in my calculations." . So did you actually visit the site and talk to owners about the building use and future plans?

So your boss designed the replacement? Let me tell you the best career advice... boss is always right. Especially if the system is already designed and installed.

 

[apollowolfe]

I never went on site the lead went and took a few pictures that did not allow me much to reference. This site is in Tennessee.

I do have a lot of field experience because I did building inspections for military hospitals across the US for three years. So, I have a good grasp on what to expect on various types of building. I just do not have much of the actual design and calculations experience.

Most of the time I agree to just defer to the lead, but I have had to push back on some things. Like he tried to get me to add natural gas unit heaters to a mechanical room that was functioning as a plenum. I had to explain why you cannot do that, but he only relented after I dug up a code reference. Also, I do not think he has any hydronic experience because he was trying to remove the air separators and expansion tanks without replacing them on the boiler loop.

I have voiced my concerns to our department head, and he agreed and said he would seek a senior engineer next time he hires.

 

[BronYrAur]

Ultimately, I think that your lead engineer was lazy. So easy to just replace something with the exact same size because "it worked." In fact, it may not have "worked" worth a crap all those years. May have short-cycled or had other operational issues.

You both pulled loads and came up significantly less than the existing equipment. It should have warranted further investigation. In the future I would go further in your design.

I can't tell you the number of times I have heard from building operations people that, "it's always been that way," even though it is dead wrong. People are lazy. They won't correct something that is wrong so long as it is "working."

 

[FacEngrPE]

Try to stay away from finger pointing. I have a past HVAC job that was carefully calculated, using the lighting to provide reheat for humidity control. Shortly after the job was finished the electrical team reduced the lighting electrical load by 2/3 (new fixtures, essentially the same lumens). This resulted in a impossible to solve humidity problem as the budget to replace the reheat requirement never materialized.

Sometimes "design bugs" are unavoidable.

Best practice will calculate design conditions, and off design conditions.