VAV system max-min air flow 5

[remp]

A typical VAV increases air flow to Max. as the cooling load increases. As the cooling load decreases the box closes off. Then as the temperature drops below set point the reheaters on the box come on at the minimum flow rate.

I have a VAV box at a perimetre office with a hugh heat loss. At the min. air flow the supply air temp needs to be 40 deg C , this is too high. Is it ok to switch the VAV box to max. air again during heating and modulate the control valve, so basically during heating its constant volume and variable temperature????
What do you reckon..please help

 

[imok2]

why don't you set the min air so you have more air EX: instead of say 40% air at min set to 50/60%

 

[AbbyNormal]

Is the problem more temperature rise than the electric heaters are rated for at minimum air flow?

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

 

[willard3]

Why don't you put radiation at the source of the heat loss, especially in a perimeter office.

High air velocities increase latent cooling, a thing you want to avoid with heating in winter.

 

[GMcD]

OR deal with the high heat loss permanently by getting better windows if the project is still in the design phase. If this is an existing building, then I agree with Willard3- just use a standalone basebord heater or ceiling mounted radiant heating panel.

 

[ChrisConley]

If you can't add perimeter heat (which would be the best way) then you can modify the control sequence as you describe. Essentially you'll be creating a VVT system (variable volume and temperature). Rather than pushing the box to MAX though, open it to 'heating' MAX which may be the 40-50% of MAX as imok stated.

One question, why do you view 40C (104F) as too high? I've read guidelines that state that air over 35C (95F) will promote stratification, and it's a good number to aim for, but a higher induction diffuser and the location of your return grille can combat statification. I've seen air at temperatures of 44C (110F) and higher delivered into spaces.

 

[SAK9]

Remp,

Looks like the zone you have is a strong caididate for fixing a Parallel Fan Powered VAV box.When the VAV box turns down to its minimum position,the fan comes on as the stage 1 reheat.It draws the warm air from the ceiling plenum and pumps it back to the space.If that is insufficient,the terminal reheater comes on as stage 2 reheat.

 

[ChasBean1]

I agree with SAK9’s suggestion about FPT boxes. As an alternative, Remp, you’re right. I know the most recent Johnson Controls software allows you to enter a heating VAV max value. Four values are shown via their HVACPro platform: cooling max, cooling min, heating max, and heating min. Typical VAV control would have a cooling max value while the other three (cooling min, heating max, and heating min) would be the same minimum value — usually a quarter to a half of the max. In one application that functioned well I've seen heating max values set the same as cooling max values. This allowed the box to go to minimum flow with reheat to control temperature, but increased airflow in heating mode when necessary.

Another dilemma about VAV control is that diffusers are sized for certain air throw at certain flows. Obviously typical diffusers don't change between modes, so throw characteristics tend to be compromised when the diffusers are sized for max cfm and the boxes go to min flow. This can tend to stratify warm air around the ceiling of a room. Having a heating max value resolves that...

That directly answers your question, but the other suggestions above (heater ratings, radiation heating, and windows) definitely apply.

-CB

 

[SAK9]

Have a read thru the Advanced VAV Design Guide, chapter on VAV box Selection:

http://energy.ca.gov/reports/2003-11-17_500-03-082_A-11.PDF

which recommends Dual Maximum settings( one cooling max and one heating max) then there is a third setting for the minimum flow to comply with local codes.

There is an illustration of CB's point.

 

[remp]

Thanks for your replys everyone. I have got agreement from the client on the perimeter electric heaters for fabric heatloss during the winter.
This leaves me with a slight control issue. I am thinking of ramping the VAV box down to the minimum setting for the first stage of heating. The second stage of heating will be to bring on the perimeter heaters- base board panel electric heaters. The glass fascade is over 20m long in an open plan office and I was thinking of controlling the little baseboard heaters off a thyristor controller (SCR) so as go ramp thier capacity up/down to match the load. On/off control would be pretty crude. Any comments? Would the manufacturers of baseboard heaters have a problem with this sort of control??

 

[AbbyNormal]

I am in a year round cooling environment so any reheat here reheat would be just for humidity control, sized to heat minimum air up to a neutral temperature. Usually though I would not need any reheat in an office occupancy though.

Why not follow the same approach, if you had electric reheat coils on the VAV boxes, just have them temper it to room temperature with an SCR discharge air controller and then the electric baseboards go "on/off" as needed?

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

 

[SAK9]

SCR control is possible with electric heating.Make sure you have the right no of thermostats and correct zoning if the 20 m is distributed over more than one facade.Though I have not done it for electrical heating,weather compensation control(ie varying heater output in proportion to outside temperature) also could be an option.