Stages with standard rooftop unit (York 15t)

[matjr01]

Hi, i am from montreal canada,
My needs: 5000 cfm VAV, cooling 120 MBH, heating natural gas 126 MBH

I have to go with a standard York rooftop and since the 12.5t can not give me the ext static pressure i need i have to go with the 15t ( 130 MBH cooling and 240 MBH heating).

I am worried about how my unit will operate under part load conditions. I asked for 4 stages on cooling but i can only go with two stage gas heat.

Since the first stage on heating is at 60% it gives me no modulation at all for my needs. Anyone has experience the same problem with a standard unit?

Mat

 

[chicopee]

Get hold of the York people and ask them that you are interested in modulating your firing rate to a four to one turn down ratio. Let us know what happened.

 

[cdxx139]

And ask the same question to the other big vendors Trane, Carrier McQuay and compare products. Reznor also has a very good gas heat product line.

Also, can you over size the ductwork to lower your ESP and get the right size unit vs one size up?

knowledge is power

 

[317069]

- Why do you want to modulate on heating.?

 

[cdxx139]

Better controllability; therefore better occupant comfort.

Why wouldnt you want to modulate heat?

You have a 5000 cfm VAV AHU, with a gas burner twice the size required.

knowledge is power

 

[317069]

he has two heating stages unit, his load is satisfied with one stage only, and he want to modulat this stage to smaller stages for heating,why?
let make question this way: why york produce 130MBH cooling/ 240MBH heating unit with four stages for cooling and two stages for heating

 

[317069]

5000 cfm is small amount for VAV system

 

[cdxx139]

York makes the product to compete in the industry. They all make a no frills product so they can compete, when the owner wants the cheapest possible unit.

The example has a 240 MBH burner for a 120 MBH load, (thats his first problem), so there is one stage for his load, which is fine for design day, but horrible for part load.

And its big enough for VAV. If code requires a 100% OA unit of that size to be energy recovery, then its big enough for some fan savings with a VAV system. Plus how much is a VFD add these days? I would use a VFD just for balancing, minimal fan savings and future potential.

knowledge is power

 

[317069]

- you said "The example has a 240 MBH burner for a 120 MBH load, (thats his first problem), so there is one stage for his load, which is fine for design day, but horrible for part load."
1- two stages unit has more than one burner.
2- doea it mean that every installed constant voleum unit is horrible

"York makes the product to compete in the industry"
1-they can compete with 4 stages unit too, it is not a logic reason

"If code requires a 100% OA"
When do you think code will ask for 100% OA in winter.

 

[Drazen]

rooftop unit makes little sense with vav anyhow, i doubt you have pressure-controlled fan, even if they advertise it as vav, they are likely "modulating" air flow by additional bypass.

i don't know how market things go in your place. here, where i work, you can specify custom made unit for comparable cost. the only thing is that roof penetration for ducts will have to be made by roofing contractor, but that should not be premium deal. rooftops are convenient only if you can pick model with parameters that are close to design requirements.