Fresh-Air.

[fridg]

How pedantic are you guys/gals with Fresh-Air intake on AC Systems. Is it mandatory by regs or standards in your parts.

Here in OZ it is a requirement to drag in Freshair.

[DPinst]

Fresh air requirements are based on a number of factors . Classification of occupancy is a main factor in determining FA

requirements. This then determines if you base the FA on occupants or square footage. Next you consider if the space has

exhaust fans ,and also consider air changes per hour . So to answer your question for AC fresh air is required and there are

regulations for its application. These are set out by ASHRAE.

[teddy bear]

Our contractor fear fresh air because of the inability to control humidity. What do you do to control %RH?
I suggest supplemental dehumidification.
Regards Teddy Bear

[dlove]

In commercial, Mechanical engineers will size the equipment and duct work with outside air requirements in their calculations.

residential 99% dont

[DPinst]

Measuring indoor air quality ie CO2 and goes a long way in reducing operating costs in relation to fresh air. As teddy bear

mentioned pre conditioning the air helps in this process. It all depends on your local. For example teddy bears concern

regarding humidity are completely different to say somebody who lives in Arizona.I live on the wet coast so concerns about

temperature and humidity and fresh are totally different.

[smsmechanical]

Why afraid of fresh air at units?
No fresh air and building could be negative. Then unfiltered fresh air comes through the doors when opened. Better to go through unit.

[timmy2734]

I have a couple of 30,000 cfm AHU's. Minimum OA is a tad over 50%... We're basically exhausting all of the cool or warm return air outside. So when you do the math, it can be quite expensive to run large amounts of OA.

And again, as TeddyBear mentioned, controlling humidity in the summer is extremely difficult. On paper, the engineers designed it perfectly....in the real world, not enough reheat capabilities within the AHU and no VAV reheats.

[smsmechanical]

Everything looks great on paper!

I don't quite follow. The oa should bring fresh air in. A power exhaust would send it out. Not being a smart ass just trying to understand. A heat recovery wheel would help that situation.

[dlove]

I love fresh air, when i can use it. But when its VERY hot or VERY cold, I will shut down my oa to help with the unit. The only problem with that (like i said before) engineers size the units with oa. when you shut down oa, the fan will draw in air from where it can get it, around the curb, through the doors and in most cases its unfiltered. So in some cases closing oa will help Humidity and temps but in others it wont. If you draw in and its not pulled over the coil, you can make things worse with higher humidity and temps.

I have been trying to get new build outs not using economizers on RTU and having a separate HRV's instead to meet oa requirements but its a battle to change the norm.

I love to purge a building in the morning, smells like victory

[mrkelly]

I can see a 5-10% on Commercial,but you 'figure in' more and unless your in a 'temperate zone' is a very heavy beast of burden,imo,costing way more than the lack of.
So,how do you 'compromise' a bldg or resi in Houston,TX? Or,anyplace South?
Yeah,I get the early a.m.,fresh air gig,but come 9:00a.m,all 'fresh' is a loss. from May till Nov.
Yeah,I know....Blah,Blah,Blah.

[hvacker]

The problem with depending on the heat/cool equipment for ventilation is for fresh air to be introduced the fan needs to run. Except for larger commercial equipment most don't run a constant fan.

I know there are some work arounds for this but usually $$ Then there are people complaining about being uncomfortable.

[fxb80]

The State of Arkansas has adopted the International Mechanical Code. So by law there must be ventilation air in non-residential buildings.
Could be a dedicated OA system. More usually the HVAC systems are intended to bring it in. The local engineers are good about following
Code but contractors and technicians seem to only care if ventilation air is specified on plans. Inspectors don't seem to care much.
I see new systems that don't have ventilation air pass inspection, and there is no other ventilation air in the building. And many residential
thermostats are installed in non-residential buildings.

According to the International Mechanical Code:
“Mechanical ventilation systems shall be provided with manual or automatic controls that will operate such systems whenever the spaces are occupied.
Air-conditioning systems that supply required ventilation air shall be provided with controls designed to automatically maintain the required outdoor air
supply rate during occupancy.”

The only way I see to satisfy "designed to automatically maintain... during occupancy" is with a programmable fan function
(On during Occupied, Auto during Unoccupied). If the thermostat doesn't have a programmable fan you would have to depend on the occupant
keeping the Fan in the On position, which is not an automatic control. Or jumper the controls to make the fan run all the time.

[mrkelly]

Inspectors don't seem to care much.

As for 'Inspected' jobs,
Guess you haven't ran into 'Richard' lately?
Staties may be 'different,but most Major munisip's are BIG on this game.

[fxb80]

I don't know who 'Richard' is. Must be in the central part of the state?
I'm NorthWest. None of the cities here seem to take ventilation seriously.
Never seen or heard of a job being turned down for lack of it.

[teddy bear]

If the home owner don't demand fresh air ventilation,air filtering, and humidity control, allow involved figure out a way around indoor air quality.
The uninformed assume, the home will be perfectly healthy and the home will be comfortable with standard heating/cooling equipment.
Not so, sooner or later, mold and dust mites will build up to the point where sensitive occupants will be unhealthy and uncomfortable.
It is slowly changing. In 5 years all codes will require it.
Till then, good luck.
Only the well informed home owner will demand it. Or the innovative will offer/explain the need for healthy, comfortable home.
We at Ultra-Aire did our first ventilated, air filtered, humidity controlled home in 1992 for an allergy doctor/sick patient. Now there are probably +100,000 homes a year built with solid fresh air and humidity control. The codes are changing. Most of the national space conditioning companies offer a whole house ventilating system.
This in an opportunity homeowners and hvac contractors.
This year at ASHRAE, contractors and a/c designers actually look us up and said, "I have been putting this off too loan, a customer actually demanded the supplemental dehumidification and fresh aire. Imagine my supprise!
Getting excited here!
Regard Teddy Bear

[Rich057]

A lot of the newer units I take care of are using a heat wheel and a indirect evaporator on the exaust/return for temperature control, and DX with a reheat coil(hot gas) for humidity control. With OA requirements between 30-70%.

[WAYNE3298]

Fresh air in commercial buildings as some responding to this post have stated is required. The quantity of outside air in most commercial buildings is not properly set. The reasons for this vary but one big one is that the design engineer doesn't provide an acceptable place in the duct system to accurately measure the outside air CFM. The velocity grid at the outside air intake won't do because it is worthless in the wind and requires a correction factor which can only be obtained by pitot traverse of the duct. For minimum outside air quantities HRV'S don't help unless there is a waste exhaust air stream to recover heat from. This is usual from the general exhaust which includes toilet exhaust if there is enough available. The design minimum quantity of outside air is usually only enough to slightly pressure the building. Most commercial buildings that have VAV boxes are not designed to maintain minimum outside air at less than full load.
Most operators close the outside air damper in severe hot or cold weather because the AHU will not adequately condition the air. While true the building goes negative and pulls outside air in from any source available it is still the lesser of the evils. A properly designed and balanced system will not only introduce the design quantity of outside air but will condition it. Sometimes the engineer has no fault because of sloppy dampers, balance or control problems but all too often they share the blame.
I run heat balance calcs on all commercial jobs I balance and on occasion find screw ups. I pointed out on one job that the heat quantity in the AHU would only heat the air to 46 F. I was informed that value engineering had determined the unit could and should be downsized and there was no problem. It got to zero that winter and the value engineering didn't help heat the office.

[pecmsg]

Minimum requirements for commercial buildings 20% OA.
Hospitals and others can require up to 100% OA at all times.

[fridg]

Minimum requirements for commercial buildings 20% OA.
Hospitals and others can require up to 100% OA at all times.

Very close to our regs 20litres per person in commercial & 100% Hospitals.

[maka]

10 L/sec per person is the manditary code compliance for commercial applications for a building certification. Adds heat to design ac loads, if you want to wing it and not allow fresh air, that is your call, toilet / kitchen areas extract will do the extract rates that may be close if continuous operation, and will still add heat loads.