air handler's ratio of outside air to supply air 1

[ash121]

In designing a split system i have been informed that the percentage of the outside air in the supply air should be less than 22%. I am assuming that this allows the system to perform accordingly without overloading its cooling and heatingm, but thats just an assumption. I don't have any concrete reasons for this method of designing a split system. Does anyone have an answer?

 

[imok2]

Going by a percentage of OSA in my estimation is a bad way to determine the correct amount of ventalation. I prefer to let the OSA air "float". That is to say CO2 controlled ventilation which varies the outdoor air supply in response to CO2 which is used as an indicator of occupancy. CO2 controls may be useful for reducing energy use for general meeting rooms, studios, theaters, educational facilities, etc., where occupancy is highly variable, and irregular.

A typical system will increase outdoor air when CO2 levels rise to 600-800 PPM to insure that maximum levels do not exceed 1,000 PPM. The system should incorporate a minimum outside air setting to dilute building related contaminants during low occupancy periods. A drawback is that CO2 sensors must be calibrated periodically and setpoints may need to be adjusted based on outdoor CO2 levels around the building.

 

[atlas06]

I've heard of coil freezing problems of DX units when using a high percentage of outside air. Heat pumps (if so) do not have the capability for large OSA.

 

[willard3]

I will confirm coil freezing problems with high OA percentage on DX coils.

You can get very expensive controls and equipment that will minimize but not eliminate coil freezing problems.

 

[AbbyNormal]

Free problems like in the winter, horfrost?

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[BronYrAur]

I haven't heard any such restriction on OA just because a system is split. I know that typical "off the shelf" rooftop units for example can only handle about 20% OA (at least in Chicago). This is primarily due to the high latent heat load in th summer. Rooftops generally have "minimal" coils that can't handle low ST Ratios.

For those who cautioned about coil freezing, please elaborate. Are you saying that the ambient (i.e. the entering air) temp could be so low that suction pressure will fall and cause the coil to freeze?

 

[AbbyNormal]

I have heard of limits on the small packaged roof top equipment with gas heat, due to HX condensation problems, but not on an air handler.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[ChrisConley]

Having more than 30% Outside air in very cold climates can cause problems with a standard gas heat exchanger (furnances, rooftop units, etc) made with aluminized steel, we deal with that by specifying a stainless steel heat exchanger in any rooftop unit with high O/A percentages.

 

[AbbyNormal]

I think stainless HX's on the regular packaged stuff, tubular or clamshell be challenged with the cold prairie air Chris.

Stainless clamshell on 100% outside air is a waste of money.

My days of 40 below and 4 feet of snow ended 8 years ago

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[ChrisConley]

Stainless steel works just fine with cold.... what else could we use?

I've seen premature failures with aluminzed steel HX and high O/A loads.

It is currently a balmy -26C (-15F) and we're expecting another couple inches of snow...

Everytime I read your warm weather posts Abby I think: sure I could move, but what would I do with all this cold weather design experience? And then the weather drops another few degrees and I think: I wonder if Abby needs a partner?

 

[AbbyNormal]

Lol, you can sit here and argue with those still up north, with the advantage of experience with the extreme cold and the hot and humid

Vapour barriers, condensation are all the opposite of the north.

I have been recruitting by the way.

The comment was on the heat exchanger design of the cheap packaged unit HX or even a stainless steel ICES clam shell. Compare it to something by EngA, drum, tubes, less thermal stress and a condensate drain.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

Mind you I was not posting much when we were hit by a Category 5 storm

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[cjw81]

Regardless of design factors, New York state Department of Health code requires a minimum of 20% OA under all (heating and cooling) conditions.

 

[BronYrAur]

On December 14 at 2:00 PM Eastern Time, HPAC Engineering will be hosting a free webcast on outside air handlers. For details, visit the HPAC Engineering site:

http://www.hpac.com

 

[BronYrAur]

here is a direct link for registration:

http://w.on24.com/r.htm?e=27723&s=1&k=C99BD413A82A5907E9497B9418E79E59&partnerref=reminder

 

[AbbyNormal]

Thanks for the link Bron

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[cabalen]

Check put ASHRAE Std 62 for ventilation requirements and apply to load calculations. Compare with manufacturers equipment selection for maximum allowed outdoor air. - Thanks