Preheat Coil Use

[pwn01]

Is the only use for an AHU preheat coil to prevent freezing air from entering the AHU in the event that closing the OSA damper to minimum position will not prevent it?

[just_opinion]

It to prevent the cold air freezing the chilled water coil. It also help heating the outside air if needed since the reheat coil is to design for reheating the return air only.

[pwn01]

The freezestat in our AHU is in the mixing box on the upstream side of the preheat coil. If the freezestat trips, it closes the OSA damper to minimum position. It seems to me that this would make the preheat coil almost unnecessary unless freezing air were still able to make it past the mixing box with the OSA damper in minimum position (which seems unlikely).

[just_opinion]

The KEY WORD on you 2nd post is "minimum position", therefore fresh air still is introduced into the envelop. Thus, it needs preheat coil.

I don't know what application you are in to have the freeze stat closes the fresh air down when OD temp is cold. Please elaborate it a little bit more. I am just interesting to know

[viceman]

usually used on systems that have large outdoor air requirements.

[just_opinion]

if that how you think Iceman, then why does the controller shut the OD air off when it is cold outside like the original poster said? Maybe he has an unusual application

[pwn01]

This is how the DDC is currently set to respond to a tripped freezestat. If the freezestat trips, the only way to stop the freezing air from going across the freezestat and into the rest of the AHU is to close the OSA damper. The other option I guess is the have it open the preheat coil HW valve, but it seems to me that this could be triggered using the mixed air temperature which would cause the freezestat to be unneeded. The freezestat requires a manual reset and so anything triggered by it is irreversible without human intervention. Since the mixed air temperature sensor is in virtually the same place as the freezestat, it seems to me that the sensible thing to do is to use the mixed air temperature to control any response to freezing air. Should I bypass the freezestat and always turn on the preheat coil when the mixed air temperature falls below freezing? Since the filters are in the mixing box before the preheat coil, is there a risk of stopping the filters up with ice if the OSA damper is not closed to minimum when freezing air is being drawn in?

This may seem a little beside the question in the initial post, but these are thoughts that prompted the first post.

[just_opinion]

I think I know what the original engineer wanted.

DDC must control the preheat valve to maintain the outside air introducing into the A/H at the design temperature (say like at least 55 degrees). NOW, if the freeze stat senses that the air is cold (say like 32 degrees), then it trips to shut down the fresh air damper to prevent busting pipes KNOWING THAT YOUR hot water valve is not function. It also send the signal to alarm the operator the fault condition.

I follow you now

[pwn01]

What puzzles me is that the freezestat is not on the downstream side of the preheat coil, it is on the upstream side. It only detects mixed air before it passes through the preheat coil. Because of this, it seems that OSA could never be used once it dropped below freezing if the freezestat is in control of the OSA damper.

[just_opinion]

look at the original blue print. MAYBE the installer installed it in the wrong place all this years.
OR MAYBE someone relocate the sensor without knowing its purpose. MAYBE you and I smoke cheap KRAK and say wierd stuff without looking at the blue print.

LOOK UP BLUE PRINT

[maxster]

the freeze stat is there to protect the chilled water coil,as long as the fan is running the return will mix with the outside air the cooling setpoint will be made.when the freeze stat trips the fan will shut off and the heat should go to 100% open and OA shuts 100% until reset.that OA will open as much as it has to to make setpoint for that cooling and that return at 70f will mix.return air dampers and the OA dampers should be 100% opposite each other.the longer the mixing section is the beter for that freeze stat and coil,if thier say 2' away from the filter rack that is tight and could cause trips

[pwn01]

It does seem to be a close fit. The OSA damper is maybe three or four feet from the filters and the filters are another three feet from the preheat coil. This puts the freezestat six or seven feet from the OSA damper. The OSA damper has total dimensions of about 3'x7'.

The picture that I get is that the preheat coil only serves to bring the air up to the supply air temperature setpoint, but is not used for freeze protection. If the OSA cannot be brought up above freezing by mixing in return air even with the OSA damper at minimum position, the system should shut down because ice will probably form on the filters even if the preheat coil keeps the freezing air from getting past it into the rest of the AHU.

[just_opinion]

PWN01,

Are we planning to redesign to understand the ORIGINAL purpose. Like I said - LOOK AT THE BLUE PRINT. I am sure the building like that should have one.

[pwn01]

I cannot put my hands on the blueprint at the present time. No modifications have been done to the AHU to my knowledge. I am just trying to verify that everything is set up correctly. The preheat coil is not in use at the time because of a faulty HW valve. When I replace it, I want to verify that the DDC is set up to respond properly.

Read my profile and other threads that I have started. Maybe that will help with the perspective a bit.

Thanks.

[maxster]

forget about the ICE forming on the filter rack it's never going to happen as long as that fan is pulling RA that heating coil is to temper the mixed air to setpoint.in the "winter mode" on that unit you might be doing(controlling) from the mixed air section so the OA and the heat are the balancers for whatever setpoint your looking for..big question?where is the air is delievered to is there baseboard heat or other reheats on the duct work.you basically have 3 running temps to condition your selected setpoint...OA...RA...and wet coil.if your MA sensor is before the filters and pre-heat it has no control action too the heating,your DA sensor would do that...the MA would be damper action only if AO is suitable to cool say 55F in the winter would modulate.if the DA is set for 75F the pre heat would make up the difference

[just_opinion]

PWN001

NO blue print now. But you can get it later? RIGHT?

Other wise these guys in here start to shoot blank all over the places. Then you be lost totally.

The key thing you need to remember is that you Freeze stat is MANUALLY reset type. Thus is very important. Don't go srewing it up if you don't know that plan was.

Go to the blue print and stop goning to here.

[pwn01]

I've looked for the blue-print, but cannot find it. This was a donated unit and no modification was done on it to my knowledge.

. . .

The freezestat has been set up to shut the AHU down. Because it is on the upstream side of the preheat coil, it is clear (as maxster indicated) that the preheat coil is only intended to temper the OSA if it drops below the supply air setpoint. If freezing air is still making it to the freezestat after the OSA damper is at minimum position, then the system needs to shut down because that indicates a problem. The supply air should always be able to bring the OSA up above freezing under normal circumstances(?)

. . .

There are reheats in the ductwork.

[maxster]

that you have reheats in the supply ducts into the space makes the pre-heat within the AHU more defined in just maintaining a range close to setpoint prior to discharging it into the space.you probably have rooms(interior)that require cooling in the winter so the reheats would work the (perimeter)air up to warm the space.you don't want the DA setpoint to be to high(preheat)out of the AHU because it would kill the cooling required in the interior spaces.once you get the HV fixed try a 55F DA setpoint and work the space to see who is cold and or hot(always note that OA temp)for reference of how the unit/space reacts.

[pwn01]

Thanks for the help. The supply air setpoint is reset on the OSA temperature. It swings from about 58 to 55 deg F over the OSA range of 33 to 80 deg F. So far in central NC, the OSA temp has not gotten low enough that the mixed air temperature could not be controlled just with return air and OSA.

The reason that this question has come up is at the end of last winter I discovered that the enthalpy control in the DDC had never been sequenced to the OSA damper and so it was not operating in a true economizing mode. In addition, the OSA temperature lockout on the OSA damper was set up to close it to minimum automatically if the OSA went below freezing. Both of these situations pretty much made saving money by economizing a chance event within a window of a few months. Now that I am using the enthalpy control to work the OSA damper I may get rid of the lockout so that I can still get free cooling when OSA temp is below freezing, but only if I can ensure that I won't freeze up my equipment.

I've also had some (what appear to be) nuisance trips of the freezestat on occasion (not in the dead of winter) and so I want to ensure that everything is up to par and triggering only when necessary.

[maxster]

fan vibration can trip the freeze if mounted on the same frame if you didn't have the OA that cold think about relocating it off the unit.

[bulldogker lu74]

First off a pre-heat coil is used to maintain a certain temp being supplied to the sapce when there is an excessive amount of outside air being introduced to the system (for instance the customer uses 100% outside air the preheat will warm the cold air entering so the re-heat coils can bring the air the rest of the way up to temp. If you bring in 30 degree air that is not a comfortable air temp for cooling so a pre-heat coil will bring that up to 55 which is your average cooling discharge temp. on the other hand if you are bringing in 30 degree air and trying to heat a building your re-heat coils cannot handle that load so you use a pre-heat to help them out. And whatever you do relocate the freezestat where it should be and do not disconnect it.

[TitleLess]

I usually see preheat coils in air handlers that need to bring in large amounts of OA, the reason is that one large heating coil installed in the air handler to give the full temp rise needed will usually put you into a bigger cabinet or at minimum a bigger blower, or as with a lot of manufacturers they just will not have it, or be able to fit it into their units, so they basically just supply two coils with two coil sections,

Often I will see it with a face and bypass damper setup, so the coil will have constant flow through it, and less chance of tripping a freeze stat,

You just get a lot better control with minimum hunting and better air mixture when you use a preheat coil in combo with a reheat.

[pwn01]

Considering the fact that the preheat coil has the power to bring the air above freezing, what is the reason for putting the freezestat before the preheat coil? There is nothing positioned before the preheat coil that would be in danger of freezing. So it seems to me that if the preheat coil malfunctioned and quit working, if the freezestat were on the downstream face of the preheat coil, it would still trip before anything had a chance to freeze and would therefore still do its job. It just seems to me that with the freezestat on the upstream face of the preheat coil, it keeps the system from using OA when it should still be able to use it.

Is there an engineer in the house?

. . .

I have no blueprints. I think that I said before that this was a used unit donated to us (we are non-profit). It was just dropped in as-is. It was set up by some HVAC folks that handle commercial installs, but they didn't take time to refine the install. I've found a few things that I've been able to improve to make the system run more economically. I hope that getting this freezestat/preheat coil situation straight will help to ensure that this segment of the system is running most efficiently.

[TitleLess]

There is no reason to have a freeze stat upstream of a preheat coil. At least I have never seen it or seen it asked for, it just doesn't make sense to have one there.

[drivewizard]

Originally posted by TitleLess
There is no reason to have a freeze stat upstream of a preheat coil. At least I have never seen it or seen it asked for, it just doesn't make sense to have one there.

I agree, although I have seen them their and they do not work properly.(nusiance trips)
I usually end up relocating. especially if you have a short mixing box, even at 30 deg OA at minimum position it could trip. Move it just down stream and make sure it's hard wired to fan S/S and your BAS will close OA dampers if it trips. Also make sure it is an averaging element.

Oh, and one more thing, you don't need an Engineer, unless you really have a lot of time and money on your hands. It was probably an engineer who spec'd it on the upstream side to begin with. (note: not all Engineers are bad, some of my best friends are Engineers.)
Actually, the Engineer probably did not specify a location, but left it up to the installing control contractor. Who, for whatever reason put it in a bad place.

[Edited by drivewizard on 02-04-2006 at 12:13 AM]

[TitleLess]

Originally posted by drivewizard

Originally posted by TitleLess
There is no reason to have a freeze stat upstream of a preheat coil. At least I have never seen it or seen it asked for, it just doesn't make sense to have one there.

Actually, the Engineer probably did not specify a location, but left it up to the installing control contractor. Who, for whatever reason put it in a bad place.

[Edited by drivewizard on 02-04-2006 at 12:13 AM]

It has always been up to the installer, in my experience,

[atmosphere]

The engineers that designed the Penn/Johnson FZT's recommend that the bulb is mounted downstream of the coil intended for freeze protection.
It'd be a good idea to make sure your OA Dampers have spring return to close on power outages-ice storms,etc.
The prints, or at least engineers notes , or at least a sequence of operation, would be the last word on this, but lately they have been a distant luxury for me.In which case I go to the control tech sheets and fly by the seat of the britches.
I recently inherited a plant operation that had been worked on, and worked over, by various mechanics- of three 500K btuh HW coils, that froze yearly, all three had the FZT bulbs upstream. They have been changed to downstream, and fail safe damper motors, and so far so good.
One more thing..you may want to make sure ALL the water in the presently unused HW coil has been drained out.Even a little water in there is enough to burst a tube under the right conditions.
And Best to Ya.

[pwn01]

. . .make sure it is an averaging element

The element zig-zags across the upstream face of the preheat coil so I think it qualifies as what you have described as an "averaging element" although I didn't know it by that name.


. . .if you have a short mixing box

The mixing box is six or seven feet long with the filters right in the middle. This seems relatively short, considering the velocity of the air and the fact that air coming in at the bottom of the OA damper is almost equal distance from the freezestat as from the RA damper.

[crab master]

As others have stated the freeze stat should be located downstream of the preheat coil. Putting it upstream of the coil would be equivelant to putting a freeze stat upstream of the outside air dampers.
The preheat coil is there because your building cooling demand is high enough that in cooler temperatures - below 50 deg F - you still need cooling for your building. If the outside air dampers backed off enough to maintain a 50-55 deg mixed air temp then in colder weather say below 45 degrees your outside air dampers would begin to close down (to keep the mixed air temp 50-55 deg) and not allow for enough cool air to come into the building as it would be slowly mixing with the return air and not allow for enough cooling effect to satify building cooling demand. By opening the outside air dampers 100% and then preheating the outside air you can get more of a cooling effect and exhaust more of your warm return air. If you start suppling 40 deg air to the space you are definitely going to get complaints of cold air blowing in. In the winter we reset many of our outside air systems as high as 62 deg if it can still satify building demand. We have others where we have to run it at 50 degrees.
Flow of outside air should go as follows -
1. outside air comes in through spring return normally closed dampers.
2. outside air comes through preheat coil.
3. preheated outside air comes across freeze stat.
4. preheated outside air comes into mixing box.
5. preheated outside air goes through mixing box and is supplied to the space via supply fan.

Now if your freeze stat trips due to your preheat coil not working - no heat from preheat - then your outside air dampers shall close (break power to dampers as they should be spring return closed) and the AHU with the tripped freeze stat should completely shut down - no fans running at all. There is some other thing I would do here as well if this unit is tied in to a building control system but as long as the above happens you should be safe. I would set the freeze stat at around 40 degrees for starters and test it on a cooler day - force the outside air dampers open and turn off preheat - and make sure it trips. Also have a temp probe in near the freeze stat sensing bulb to verify where it trips. This is not something you just want to set and walk away from - make sure it does its job.

Also maybe you said in an earlier post but is this a hot water/steam or electric preheat coil? Also if it is hot water does you system have glycol?

[pwn01]

Its a hot water preheat coil. It does not have glycol.

Interesting, the placement you put on the mixing box. I'm calling the mixing box the area in the AHU in which the outside air is mixed with the return air. In this AHU it happens first thing, then the mixed air goes through the filters, the freezestat, the preheat coil, the coiling coil, the fan.

[gonefishing]

Normal pre-heat begins when outside air temp falls below a given set point. That set point is nominal and depends on system design. This is generally below 35 degrees F.

Pre-heat is designed to help load the return air with fresh air to meet minimum fresh air requirements when the OAT falls below acceptable temps for mixed air to keep up with demand. Typically, mixed air is set to 55 degrees F.

The Pre-heat coil is supplied with constant water/steam flow at a predetermined temperature to avoid freeze up and allows fresh air introduced into the mixed air chamber to maintain a reasonable temperature so as not to overload the facility HVAC mechanical system. The only protection the pre-heat coil has is constant circulation or an anti-freeze solution.

The pre-heat coil is determined to be sacrificial in a failure. This is why it is normally placed in an area that will not be contaminated by a freeze and rupture. It is the primary function of this coil to maintain a given quantity of outside supply of air at an acceptable temperature.

The freeze-stat is in place to protect the remainder of the facility environmental system. If the freeze-stat is tripped, all air devices in that zone will shut down, all R.A. dampers go to full open, and all O.A. dampers go to full closed.

Just a reference image below.

Regards.........



http://www.geocities.com/gonefishing48/freezestatlocation.jpg

[atmosphere]

Pwn-
Not to worry about your mixing box-
It is oversized to give you clearance for bag, angular (zig-zag) or cartridge filters.Odds are you have inlet louvers upstream of the OA dampers, and between the two they do an adequate job of diffusing the air before it ever reaches the pre heat coil,so the turbulence has been averaged out.

[pwn01]

The position of the coil in question is at the HW coil on the diagram. My system uses reheat coils in the ducts for heat rather than a HW coil in the AHU. But the coil in question was piped-in in spite of this fact which is why is appears to serve as a preheat coil. If it is indeed a HW coil to serve the same function as the in-duct reheat coils, the freezestat is in the correct place. . . . But, if the OA damper has a minimum position stop which is manually adjusted to maintaining minimum fresh air under any circumstances, why would you ever put a preheat coil directly in the OA stream? "Preheat" would only be necessary if the mixed air temperature were still too low with the OA damper in minimum position. In that case, the "preheat" coil should go after the mixing box.

[gonefishing]

I guess, one other thing that could be addressed here is
facility pressure. Most of my work on this application
was in medical facilities where positive building pressure
was required. This would be one reason to place a pre-heat
coil in that area.


Regards.......

[TitleLess]

How can it be constant flow without face and bypass in the steam app. ?

[crab master]

gonefishing - nice pic to reference looks like ALC.

pwn01 - if your saying the preheat coil is where the hot water coil is according to gonefishings pic then you don't have a preheat coil. Also the mixing box is where the outside air and the return air meet.
With a preheat coil you would have two heating coils - the pre-heat coil and the heating coil. If this is a hot water system without glycol I would be cautious and make sure your freeze stat trips and the sequence of events as I described does happen. I've got an application like what you are describing - roof top unit with dx cooling and a hot water heating coil and a economizer, where I am using Indoor Air Quality and a CO2 sensor and using the economizer to maintain proper IAQ and CO2 settings. My freeze stat is at the mixed air temp location - granted I could probably get away with putting it downstream of the heating coil but in order to do that it would also be downstream of the cooling coil and I don't want to have nusiance trips and the location wasn't ideal to do it. I am just modulating the outside air dampers and then modulating the hot water heating valve to maintain a couple of supply air reset schedules. One schedule for the a call for cooling that keeps the supply air temp above 53 degrees and the other I call standby when the IAQ/CO2 needs improved where I maintain a more neutral setting 67-72 deg supply air depending on Outdoor air temp. 72 deg supply air at 30 deg or less outside and 67 at 60 deg outside. In my application even at 10 degrees outside my mixed air temp with the dampers at a minimum position was still above 50 deg. Also in my application if the outside air gets below 40 degrees the IAQ and CO2 settings are ignored as the system isn't designed to warm up the outside air adequate enough to help with heating demand. If I had a preheat coil I could get away with continued control below the 40 deg mark. Anyway kind of long winded but I hope this helps explain a bit more for you.

[atmosphere]

gonefishing
Ditto on the graphics and thorough explanation.

[pwn01]

Looking at the diagram, it looks like the coil in question is indeed a HW coil and not a preheat coil as I had the impression it was from other conversations at other times. I am not sure why it was piped in considering what I stated before that I heat with reheat coils in the ductwork. My system is identical to the one in the diagram other than the fact that mine has no fan in the return duct. All of the air in the system is circulated with the AHU fan. Other than that, the OA damper, the RA damper, the filters, the freezestat, the HW coil, the CW coil, the fan are in the exact locations indicated. The RA damper and OA damper are locked together to open and close opposite to one another.

The problem I've run into is the "nuisance" trips of the freezestat which may or may not have been nuisance trips but they do not occur often, maybe once in six months or a year. Also, when I inherited this system, the OA damper was locked out below 34' F, and the "preheat" coil was piped, but the valve was not working so the coil was non-functional. I thought that I could eliminate the freezestat trips and get rid of the OA damper lockout if I fixed the "preheat" coil and made sure that the freezestat was in the correct location.

But, again, from the diagram, the "preheat" coil is actually a HW coil and the freezestat is in the correct location. Also, from crab master's post, it looks like I may be able to get rid of the OA damper lockout with out any worry of tripping the freezestat.

So, my conclusion from all of this is that I should 1) drop the OA damper lockout 2)make sure that the freezestat is operating properly and, 3) drain the HW coil and forget about fixing the valve on it.

Would there be any benefit in moving the freezestat to the downstream face of the HW coil and using the HW coil as a preheat coil?

[Edited by pwn01 on 02-06-2006 at 11:01 AM]

[crab master]

"But, again, from the diagram, the "preheat" coil is actually a HW coil and the freezestat is in the correct location. Also, from crab master's post, it looks like I may be able to get rid of the OA damper lockout with out any worry of tripping the freezestat."
You may be able to but it depends on your climate and how much OA you have coming in at minimum position.

"So, my conclusion from all of this is that I should 1) drop the OA damper lockout 2)make sure that the freezestat is operating properly and, 3) drain the HW coil and forget about fixing the valve on it."
If you drain the HW coil completely then you don't have to worry about a busted coil and if you really don't use it anyway it may be OK - beings your valve is busted.

"Would there be any benefit in moving the freezestat to the downstream face of the HW coil and using the HW coil as a preheat coil?"
Yes if you need the extra cooling in the cooler outdoor temps, but here you have to have control logic in place to make all this work properly. Also you would need to fix your heating valve - a given - and make sure you using a modulating valve so you can control the temp just downstream of the heating coil. Here you could use supply air temp to modulate the heating valve and outside air dampers but without having glycol in your system I would be very careful on operation of the system below 35 degrees. Make sure all is working properly and test it, especially your freeze stat and shutdown. I would set the freeze stat no lower than 38 degrees and try to get away with setting it at 42 degrees initially. I would only set it lower if you get nusiance trips. I would make sure you have averaging sensors on your freeze stat and for your mixed air and supply air temperature control.

The other thing with your nusiance trips when do they happen? We have one building when it gets below 15 degrees we have to run the indoor blower continuously due the outside air damper leakage even when they are closed. The cold air falls down the duct and causes the freeze stat to trip when the system is off. We looked at putting in new seals on the dampers but the problem is not at the "long" edge of the damper it is at the sides where the damper shaft goes through the housing - we've got as much as a 3/8" gap on each side - old system and they don't want to pay for new dampers. Anyway you might want to double check your seals.

[crab master]

One thing I just found on the last set of specs I looked at - dated 1966 - for a control upgrade. They call the 40.0 KW heater downstream of the airhandler the "Electric pre-heating Coil." This system then goes to a bunch of electric duct heaters for each zone. Anyway different terminology than what I am used to - to me a preheat coil is prior to the ahu and mixing box - anything after that is a heating coil. Also just read an article that may help you out some more with understanding this subject. "AHU Operation in Low-Temperature Enviroments" by John Clark, P.E. It is in the Vol. 23 No. 1 Jan. 2006 magazine.

Actually here's the link
http://www.esmagazine.com/CDA/Archives/3fb00d9e19ea8010VgnVCM100000f932a8c0____

[pwn01]

The weather in central NC is fairly mild. It sometimes gets down into the teens at night during mid-February, but not usually. This winter has been very mild so far. Within the past two weeks, it was in the lower sixties during the day and the upper forties at night. Thats is very unusual for this area. I don't think that converting that HW coil to preheat the air is a real urgent matter. I'll just verify that the freezestat operates properly.

Thanks for all of the help. I'll read that article.

[gonefishing]

Nuisance trips are caused by a faulty mixed air sensor, or the wrong set point. The air temp in the mixed air plenum should be regulated to 55* F.

A lo-ambient lock-out at 34*F is not unusual in this type system, unless you are needing the OA dampers open to meet positive pressure requirements of the facility, or minimum fresh air requirements.

We need to clarify Mixing Box. If you have small fan coil type units with dampers, placed through out the facility with the ability to recirculate or blend attic or return air, that is the proper term for mixing box.

If we are referring to the area where RA and OA meet, in the primary central AHU, then that is simply a plenum.

Re-heat in mixing boxes, supplied with air from a central station AHU are in place to maintain a given space, or “zone” temperature when the primary system is more in cooling mode or economizer, than heating mode, ie., more of the facility needing cooling than heating.

This supply air temperature is usually controlled by a re-set controller that is looking at OAT vs SAT, with the supply air temperature at the AHU being re-set either up or down based on OAT.

The re-heat allows cooler zones to be tempered when the primary air supply is below a temperature that would allow it to achieve desired setpoint.

Normally, these mixing boxes are placed in the building perimeter offices due to more heat loss. Mixing boxes, or zones that are interior, seldom have re-heat capabilities. Again, depending on original, or facility up-grade scope of operation.

During colder weather, building heat is provided thru the central AHU from a more economical source, the HW or steam coil. You can dis-able this coil, but if your facility is designed as above, interior zones will be dependent in internal heat load to satisfy heat demands. This can be a tricky balance point.

I would suggest looking at the total scope of facility requirements before I made a decision to define the HW coil as pre-heat. This might be a good up-grade to the facility, but it may also come back to haunt you.
Changing one part of a central station AHU without looking at the total system, as well as total facility requirements, is not a good idea.

Without knowing the total system configuration it is hard to say what is best in your application.


Regards.........

[pwn01]

The air temp in the mixed air plenum should be regulated to 55* F.

The OA damper maintains setpoint on the supply air for this system. The mixed air temp is monitored, but I do not maintain it at a particular temperature. I am looking at the data log for today and my supply air setpoint drifted from just below 58' F this morning to just above 56' F this evening. I was able to do all of my cooling with outside air in this building, the mechanical cooling stayed off. The mixed air temperature (that is the air in the RA/OA plenum) stayed 5-7' F below the supply air temperature so it went from about 52' F to 50' F from this morning to this evening.


We need to clarify Mixing Box. If you have small fan coil type units with dampers, placed through out the facility with the ability to recirculate or blend attic or return air, that is the proper term for mixing box

This system uses Carnes VAV boxes with reheat coils in them. There is no inlet to mix RA in at that point in the ductwork. The rooms get pure supply air from the airhandler and the reheat coils heat the air if the SA itself is too cool to maintain setpoint.


This supply air temperature is usually controlled by a re-set controller that is looking at OAT vs SAT, with the supply air temperature at the AHU being re-set either up or down based on OAT

The setpoints on both the OA damper and the mixing valve for mechanical cooling are reset on the OA. I have the setpoint on the mixing valve about two degrees higher than the setpoint on the OA damper in order that if the SA setpoint can be maintained with OA, it will automatically be below the mixing valve setpoint and so the mixing valve will stay shut eliminating the use of mechanical cooling.


Normally, these mixing boxes are placed in the building perimeter offices due to more heat loss. Mixing boxes, or zones that are interior, seldom have re-heat capabilities. Again, depending on original, or facility up-grade scope of operation

This particular building has no truly "interior" rooms. They all have at least one wall exposed to the outside so they all have reheat coils. This is a large church building in which the largest room, the sanctuary, is not used a number of days out of the week, but there are large classrooms at the north and south ends of the buildings as well as offices which are used the whole week.


During colder weather, building heat is provided thru the central AHU from a more economical source, the HW or steam coil. You can dis-able this coil, but if your facility is designed as above, interior zones will be dependent in internal heat load to satisfy heat demands. This can be a tricky balance point

Unfortunately, the use of outside air to provide cooling for the entire day such as occurred today is a rare occasion. Up until now, it has been only for part of the day. Due to this, nearly every day I am using my CW coil to maintain my supply air setpoint. Being that the HW coil in the AHU is before my CW coil in the airstream, this eliminates the possibility of using the HW coil for heating the building. I think that in the way the system is configured, if I cannot use the HW coil for preheat, it will have to remain out of service. The way it looks, I may not even need preheat if the system can keep the mixed air at 50' F even when the OA is around 20' F by mixing in RA.