100% redundancy question for HVAC system 2

[qman5]

Hello all,

If there is a specification requirement for a "100% HVAC redundancy requirement", what would you consider in terms of redundancy? Let me explain further.

For our particular situation for a small building that contains 2 rooms, each with AHUs capable of handling 100% of the load for that room, but they are all controlled from a central PLC. This PLC is not redundant, in the sense that if it fails, the system itself will fail.

The customer specified that "the HVAC system shall be 100% redundant".

From your position, how would you see this in terms of system design? We have interpreted it in the sense that the AHUs are designed such that if 1 of the 2 units fails in the room, the other unit can pick up the heating/cooling requirements. The customer interprets this as needing an additional PLC.

My argument is that where does redundancy stop if they go by that definition? Do we provide multiple AC/DC feeds to the units, multiple controller control cable paths, etc. I think the specification is poorly written and there are no grounds to enforce "100% redundancy" without a solid definition of exactly which components must be redundant.

Any ideas? This is coming from an electrical guy, so I would like to hear feedback from the community's experts in the field.

Thank you!

 

[itsmoked]

A single point failure in the controls or the hardware certainly tanks the 'redundancy'.

But, why is a PLC needed when a setback thermostat can do the job?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[lukaiENG]

If I were given a requirement for 100% redundancy (I often am in my facet of the industry) then I would interpret this to be removing all single points of failure (SPOFs). This would include: redundant equipment, redundant air/water distribution (if there are any SPOFs), redundant controls, redundant power supplies.
It sounds like your customer knows what they want, I would suggest talking with them and getting a firm understanding of the drivers behind why they are after "100% redundancy". This should inform the final solution.

 

[Drazen]

Each AHU should have its basic controller able to function even with total failure of central controller, that is basic setup. I am not sure why would you use PLC, but that's another issue.

As regards to electrical redundancy, you simply need to look at the specs - does AHU system spec included power supply or not. If not, than electrical supply redundancy is not covered within AHU redundancy requirement.

 

[trashcanman]

Here is a novel idea: Ask the Owner what he/she means by 100% redundancy and stop guessing.

 

[mk3223]

IMO, confirm with the Client/Owner for the actual intent of the system design for the redundacy requirement. It will be nice to have a double system (or even triple) in order for a safe and reliable operation, which, however, will come down on what's the priority and cost.

 

[nglty]

The data center industry has what? 4 tiers of redundancy?
N+1, N+N, and whatever else.
The power supply to the building is more likely a single power source. AHU's are connected to the same electrical panel, etc.
If owner means zero interruption, then we are talking emergency generator for AHU and heat removal source (chillers, Condensing unit, controls) on emergency power.
Standard understanding in industry is Two AHU's, each with dedicated control panel.