Vav Sytems setup
I am new to commercial controls and I have a few questions:
(1) I have a VAV box with a series fan and hot water re-heat the current setup is for a 1000 cfm box. Max airflow is set to 600 cfm. They set it up originally in Tracer for the min cooling and min heating the same. My question is how do you know what to set the max airflow, min cooling and min heating. I always thought the min heating was supposed to be lower with hot water reheat. Does the series fan pulling plenum air have something to do with setting the min cooling and heating the same?
Many times a series fan is just to aid in the movement of air to get it where it needs to be. Whenever the space and/or VAV box is in the "occupied mode" (assuming it has this) the fan needs to be running.
It is not uncommon for the min cooling and heating to be the same. When the designer runs the calculation for heating, the CFM is generally no where near the requirements for cooling. You do however have this large duct work that's set up to accommodate the cooling CFM. Since this is the case, they will generally design the overall heating CFM at 50% of the cooling CFM (make sense?). Another thought is to make the min 10% for cooling and a little bit more for heating. You will probably still require more CFM for heating since the heat transfer of the coil will be dependent on the CFM across it. So based on your settings you could go:
600 CFM Max Cooling
60 CFM Min cooling
300 CFM Heating.
A series fan powered VAV Box is designed to deliver a constant volume of air to the space. Depending on the deviation from setpoint in the occupied cooling mode the discharge air temperature will vary depending on whether the VAV is pulling mostly primary conditioned air from the AHU/RTU supply duct or a mix of return / plenum air.
Typically engineers specify a series fan powered box in areas where they require a constant CFM regardless of what the load in the associated space is.
The design cfm of both the fan and the primary air must be the same. Too much primary air will dump out the back of the box. When there is not enough primary airflow the fan will draw in return air mixing it with primary air and cause the discharge temperature to be high.
On a 1000 cfm box I would think you would want nothing less than a 200 cfm minimum for ventilation air.
As far as the settings the maximum cannot obviously be more than the max of the box. In some cases you could have a 1000 cfm terminal unit but only need 850 cfm for max cfm or when the box is in the full cooling mode. The minimum’s are generally selected by the engineer during the design, that’s not to say that they cannot be changed down the road. Simsd’s example is a good one. It used to be 20% of the max was a typical standard, now it not uncommon to see 30%-50%. More of a constant volume variable temperature application.
For hot water reheat I believe a 10% minimum with no less than 300 FPM through the box is the lowest you can go. Not knowing what the design spec for the box heating leaving air temperature was makes its hard to say exactly.
Engineers also prefer series versus parallel because they offer a continuous sound since the fan is not intermittent and in systems where it used with low temperature supply air they have the ability to downsize the ductwork.
TP is correct in his evaluation. In addition to this I think we both want to make sure of something. This is definitely a series fan correct? The fan is inside the box, correct? Parallel fans are on the outside of the box and typically draw return air from the ceiling plenum. In California, the application that I have seen them used for is as the first stage of heating. If that can't maintain the room temp then then reheat valve will kick in.
Is there an issue with the min's and max's?
In the most cases the heating flow is based on the calculated heat loss of a room. The supply airtempature to room may not be higher than 35C /95F.
The reason for this is the warm air will not coming down to the floor so you get hot head cold feet. The series fan is only requierd if you have to high pressure lost in your downstream ductwork. If you do,t know the heating capacity of the heating coil then use a setting of 45 to 50% of Max cooling. If you know the capacity than you can calculate the airflow. The min (cool )airflow setting is based on the minimum requierd fresh air rate. Keep in mind that the airflow to a supply diffusor may not go below 50% of the design flow to prevent draft.
I have made a calculation program but this is only for induction vav and it is in metric.
These are series. I know in parallel that the UCM uses fan first stage, second and third electric or hot water heat.
I am a serviceman and was doing a checkout of our Trane Varitrane boxes. I have seen in the past on some Delta systems where they had the min cooling higher than the min heating. They were just VAV with re-heat no series fan.
These I looked at were at a two year old school. They are series boxes. All of them had a maximum setting less than the maximum box rating, cooling minimum and heating min were the same about 40% of the maximum setting. They have hot water reheat. I havent worked with the programing side to see the seq. of operation. The series fan runs nonstop during the occupied setting.
All of the boxes were 74 cooling and 69 heating and the discharge air varies due to the mixed air working with the air valves. The boxes on saturday (it was 95 here) were in cooling and all the rooms were around 72 with no call for re-heat water. Everything is working I just wanted to know how to set up boxes in a situation where the blueprints from construction are not availible.
Does anyone know of a good book that can help with airflow, fresh air requirements. I know the equipment side but with controls and the airside I am a little green.
whta yoiu are asking Jake has to do with the engineering and design and not controls. But let me give you some ideas.
If you are are using a box on the perimeter of a building, the typical design parameter is 1.5 CFM per square foot as your max. The perimeter is generally defined as the first 15 feet towards the interior from the exterior walls. So if you have a 10 x 12 office on the perimeter, regardless of which way the office is oriented you would use 120 x 1.5 for 180 as the max. Now there are other factors to consider, but this is a starting point.
For the interior space, generally you can use 1 CFM per square foot.
Dutchman is right in his remarks. If you want to get a really good response post your question on this link and you'll a good response.
http://www.eng-tips.com/threadminder.cfm?pid=403 Just sign up and post.
The Trane UCM programming is fairly simple once you have used it. I have done it through a Tracer 100 terminal, Tracer Summit workstation and using Eware.
As far as the airflow requirements I think that will vary considerable. For example single duct cooling only shutoff box, series fan powered, parallel fan powered, fan powered with electric reheat etc..
Are you trying to comply with certain ASHRAE standards, Demand Controlled Ventilation.
On a terminal unit with electric reheat you would need at least 70 CFM per KW across the coil.
The diversity factor will vary by system and by building. Its hard to say what numbers you need. Each job is setup differently. If you try setting your max. CFM to high on a number of boxes, that could effect system static pressure.
Also I have seen customers and service techs change VAV Box airflow setpoints to try and compensate for known problems with the associated RTU or AHU. Such as low system supply duct static pressure or high system discharge air temperature. The base building system and all supply air ductwork need to be in good shape before you would even consider changing original setpoints.
Also if someone has added additional diffusers on a box without doing a calculation to determine if the box has the capacity, you could make the max CFM whatever you want, and it will probably never satisfy the space on a design day.
Hey twisted pair, the link you pasted in your last reply what as it? It did not allow access.
Also, thanks to all who replied. It was very helpful and eliminated me sitting in a class for hours listening to war stories and the one that got away......lol
It is the latest Trane VAV UCM IOM manual for rev 4.2
The link worked for me. If you need it I can email it to you.
Generally speaking the max cooling CFM is sized based on the heat loss of the space.
The reason that minimum CFMs are the same is because of the minimum airflow per perspective occupant of the space. You need to have so much air delivered per occupant. So even if the calculated min heating flow(based on heat load calcs) is lower than the code minimum, the minimum heating flow is set at the code minimum due to the occupant level in the space. Obviously, the cooling minimum would also have a bottom level of the code minimum also.
If you think about it.....The cooling minimum really is not to cool the space. If the space is satisfied, we don't need any cool air or we will over cool. However, we have to keep the code minimum coming into the space due to occupants.
The code minimum is based on your percent of OA in the air stream, and how many occupants are in the space.
zarembad - I think we know each other!
>However, we have to keep the code minimum coming into the space due to occupants.
And that is where the $hit storm starts with vav systems. There is little to no way to guarantee that X% fresh air is getting to each room.
>zarembad - I think we know each other!
LOL, I bet I do also.