Parallel Roof Top Units
Parallel Roof Top Units
(OP)
Hello All,
I am contemplating a ventilation and cooling system retrofit where two (2) 20 ton roof top packaged units are connected in parallel.
i.e. The discharge and return ducts are interconnected so that a total cooling capacity of 40 ton is available.
The rationale is that in the event of failure of one unit, some amount of cooling can still be provided to some areas.
I believe that the external static pressure and CFM for both units should be the same.
What are the other issues/factors that should be considered when connecting two packaged units in this way?
Does anyone know of any literature that can provide guidelines that should be followed in order to ensure the best performance?
Thank you.
I am contemplating a ventilation and cooling system retrofit where two (2) 20 ton roof top packaged units are connected in parallel.
i.e. The discharge and return ducts are interconnected so that a total cooling capacity of 40 ton is available.
The rationale is that in the event of failure of one unit, some amount of cooling can still be provided to some areas.
I believe that the external static pressure and CFM for both units should be the same.
What are the other issues/factors that should be considered when connecting two packaged units in this way?
Does anyone know of any literature that can provide guidelines that should be followed in order to ensure the best performance?
Thank you.





RE: Parallel Roof Top Units
2. How are the units controlled? 1 thermostat? Is it connected to both units? 2 thermostats, 1 to each unit? How is capacity controlled and by which thermostat? You will also need motorized dampers for supply and return air ducts.
3. It does not matter what you believe about the e.s.p. and cfm, Calculate the answers.
Check with ASHRAE Handbooks of Fundamentals and Equipment.
RE: Parallel Roof Top Units
There should be no major technical hurdles in getting to what you want. Paralleling equipment for partial or full redundancy is done all the time. You will also get better turn down and possibly improved efficiency at low loads.
If the system is vav downstream, the flow and temperature control is straightforward. A constant flow system will be improved with a balanced main duct arrangement.
Probably the most foreseeable headache is maintaining the code required physical separation distance between your outdoor air intake and exhaust openings at roof level.
The roof waterproofing system should integrate with the unit curbs, or put equipment on steelwork to gain access underneath. Coordinate early with the structural engineer.
RE: Parallel Roof Top Units
With CAV system and constant speed fan I am not sure how that could work in case of failure of one unit. Both volume flow and pressure change and terminal units would mostly not be able to cope with that, cold air drop or drafts could easily occur somewhere in the building.
RE: Parallel Roof Top Units
knowledge is power
RE: Parallel Roof Top Units
RE: Parallel Roof Top Units
CV, half the building will starve for air, as the first diffusers will take all the air flow and no air will reach the last diffusers.
So regadrdless, parrallel or not, for CV, half the building will be without cooling, might as well go with 2 RTU's, each serving it's area and keep it simple.
If CV - this is a cheap design.
RE: Parallel Roof Top Units
KiwiMAce : what did you mean by" balanced main duct arrangement."
do you mean a bypass duct as VVT systems or something else?
RE: Parallel Roof Top Units
RE: Parallel Roof Top Units
I encountered a parallel ahu system once, long ago, that would become unstable at times. The two fans seemed to be trading the load at a regular interval. The current on one fan would rise as the other fell, then back again -- much the same as an LC resonant circuit. The fans were about 15 meters apart, blowing into a large supply plenum.
I wasn't around to hear what the solution was. Is there a way to prevent or predict this sort of resonance? Or did I just see a freak event?
Best to you,
Goober Dave
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RE: Parallel Roof Top Units
- What if we connect a 15 tons unit with a 15 tons unit
- What if we connect a 20 tons unit with a 10 tons unit
- What if we connect three 10 tons of each unit.
RE: Parallel Roof Top Units
RE: Parallel Roof Top Units
Best to you,
Goober Dave
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RE: Parallel Roof Top Units
Additionally, units need to match speeds (or volume vanes, or VAV bypass control damper) commands when operating in parallel. Otherwise one unit will tend to handle the load while the other idles. I’ve seen “alpha” units at full speed and parallel units at minimum, actually being back-fed. You will need a building controls programmer that understands the goal and a BACnet or Lon interface that can receive inputs and provide outputs to the packaged units.
RE: Parallel Roof Top Units
RE: Parallel Roof Top Units
My query sparked some very good discussions. I have learnt that vac design is not just about determining the heating or cooling load and selecting the equipment. It has a lot to do with devising the best strategy and balancing acquisition cost, energy consumption, system reliability etc.
Thank you all for your reply.
Trashcanman: Your query caused me to give a lot more thought to the controls. These days control systems offer a lot more flexibility to allow programming an elaborate control scheme. I will give considerations to the operational logic.
Cry22: The system will be VAV. We intend to pay a lot of attention to testing and balancing so I expect that starving parts of the building of air will not be an issue.
All: There would have to be some amount of programming logics. In the event of failure of one RTU, the VAV boxes in the non-priority areas would be driven to minimum position (partially closed): still allowing some air flow. The main dampers isolating the failed unit would close and the priority area would continue to receive cooling.
Given the potential cost for engineering programming time, I have decided to explore other alternatives as well. Thank you all for your feedback.