SA Heating Setpoint

[jtc]

I live in a very warm area so heating is seldomly used but what's a typical supply air heating setpoint for a variable air volume system w/ vavs?

[xarralu]

I live in a very warm area so heating is seldomly used but what's a typical supply air heating setpoint for a variable air volume system w/ vavs?

Good place to start is 120° (at least in Texas.)

[control_man_az]

Sorry to disagree but I believe that this is way too warm for a warm climate. In AZ we use somewhere between 65 and 75 as the air temperature for heating. If we are talking about morning warmup, this is a different story.

[crab master]

Do your VAV's have reheats?

The answer to that is key, but typically VAV systems are designed to provide cooler air and then allow the VAV box to manipulate that. Granted there are plenty of cases of morning warmup/central plant heat. Almost all my VAV systems are designed to look at the zone with the highest cooling demand and adjust the supply air temp to satisfy that zone. I agree with control man on the temps as 120 deg F supply air temp short of morning warmup and likely even then you are going to overheat too many zones.

[osiyo]

I live in a very warm area so heating is seldomly used but what's a typical supply air heating setpoint for a variable air volume system w/ vavs?

Hmmm. You don't provide much info. Do the VAV's have reheats? Are there other supplemental heating methods such as fin tube radiation? Etc.

I live in Minnesota, and it does get a little cool here from time to time. Typically here, VAV systems will include VAV's with reheat. So we'll set the system up so as to control AHU discharge to 55'F. With an allowable reset that would boost that up to 65'F, based upon either the coldest room served, or the average of all rooms served dropping below a selectable set point. Depends, in some cases one strategy works best, in other cases the other works better.

In any event, after controlling discharge temp thus, we let the VAV reheats and/or FTR or whatever handle individual instances of a room being too cool.

Of course, buildings vary as to their age, state of proper maintenance, how well they are insulated, how tight the building envelope is, etc. So we might adjust the numbers I gave accordingly. But between 55 and 65'F, with full reset applied is the average most commonly used numbers which work well. Here. Granting some supplemental form of space heating such as a reheat, FTR, etc.

I don't remember ever seeing a building here that has cooling only VAV's and nothing else, no reheat or FTR or whatever. Except for some interior rooms that are unlikely to ever get cold.

For systems that involve solely a constant volume AHU, with the only source of heat coming from the AHU discharge. The strategy isn't much different. We'll poll one or more space temp sensors. Have the AHU controlling to 55'F discharge, but allow a reset upwards of up to 30 or maybe even 40'F (for a resulting possible high of 85 to 95'F). Never had to use that high number except in some older, drafty, poorly insulated buildings.

I've never seen the 120'F discharge that one fellow mentioned except for some constant volume units in some pretty old, run down buildings. The exception being cases where a dedicated AHU/FCU/whatever might be serving an interior loading dock area or something similar where large garage doors routinely cycle open or closed to admit trucks or whatnot. In such cases it is sometimes necessary to dump a lot of heat in such a space in a hurry.

But, generally speaking, occupants of normal building spaces don't much like 120'F air dumping over them.

[DreamCar]

The main question is - are the VAV's change over? Do they recognize (or do you tell them) when there's hot air in the duct vs cold air in the duct.

A true VAV system is cooling air only. One that is change over like that found in most "zoning packages" is what we call VVT (Variable volume, variable temperature - yes there's 3 v's here, but its still VVT ).

We typically use 95 to 100 deg F for heating temperatures.

[viceman]

hey jtc.

there are some very knowledgeable people here trying to help.

where are you.

[xarralu]

We typically use 95 to 100 deg F for heating temperatures.


Huh really, you don't say? Maybe even start out the morning with 120°?

[ccut]

I'm surprised to see such high temps being used for discharge temps. Don't you run the risk of over heating? I like the post calling for 55 degree. that way the different zones have some options. If they want cooling, 55 degree air is there, if they want heat the damper closes and draws plenum air into the space.

[orion242]

>damper closes and draws plenum air into the space.

Only an option on fan powered boxes.

Here in the north we use 100+F temp for morning warmup all the time.

The VAVs I use are programmable, when they reach set point I have them close completely during warmup. Once occupancy hits, they operate as a normal VAV with Min CFM.

The hardline 55F DAT from the AHU is old school and a huge waste of energy. If every space is satisfied there is no need to run 55F. I add up deviance from setpoint and use that to reset DAT.

You can't make everyone happy all the time, your better off making most of the people happy all of the time.

[ccut]

>damper closes and draws plenum air into the space.

Only an option on fan powered boxes.

Here in the north we use 100+F temp for morning warmup all the time.

The VAVs I use are programmable, when they reach set point I have them close completely during warmup. Once occupancy hits, they operate as a normal VAV with Min CFM.

The hardline 55F DAT from the AHU is old school and a huge waste of energy. If every space is satisfied there is no need to run 55F. I add up deviance from setpoint and use that to reset DAT.

You can't make everyone happy all the time, your better off making most of the people happy all of the time.
good info, thanks

[printer2]

I live in Minnesota, and it does get a little cool here from time to time.

Bit of an understatement don't you think?

[Cagey57]

With cooling only VAV boxes I usually set the Heating DA Setpoint 10-15 Degrees above the desired Occupied Heating temp (68-72), so that would end up being somewhere between 78 and 87.

If you have Fan Powered Boxes with electric reheat going much over 85 starts to cause problems with the saftey limits on the electric coils because thay are almost always manual reset.

I still stick to the old rule that you should not discharge air below 55 or above 110 into an occupied space, of course there are exceptions.

It can be even more confusing if the Fan Powered Boxes are Series rather that paralell.

Resetting the DA Setpoint based on Warmest/Coldest zone is usually the most reliable.

osiyo was being scracastic, right ?

[osiyo]

Bit of an understatement don't you think?

That it does get a little cool in Minnesota from time to time?

Seems like a reasonable statement to me.

Especially these days when there are people from all over the world who read and participate in HVAC-Talk. Some of whom may live in places where it gets colder, more often, than is the case with Minnesota.

Besides, choosing to describe a particular outdoor temperature as either "cool" or "cold" is more a matter of opinion and personal perception than fact, isn't it?

Additionally, for any one individual his or her opinion about what temperature constitutes "cool" versus "cold" may vary from time to time.

i.e. Back in Oct-Nov I saw many folks who were shivering and complaining about the "cold" weather when temperatures were in the low 30's - hi 20's (Fahrenheit). Now, some months later and after a few spats of weather where temps dropped into the sub-zero realm for days on end ... I see em step outside for a while to enjoy a little fresh air when temps are in the 20's, let out a little sigh of pleasure, and comment something like, "Now this is a nice day, just cool enough to be refreshing without actually being cold."