I am having problems with a VAV terminal box serving a row of offices along the exterior of an office building. The VAV box discharges 83F air. The duct discharges from the box into about a 5 foot run of rectangular duct and into a dovetail wye fitting. The duct then runs each way over the office spaces. A 6 ft. length of round branch duct taps off each duct to a ceiing square cone diffuser for each office. The air discharging from the diffuser is 5 degrees lower than the discharge are from the VAV box. The temperature off the air at the entrance to the wye fitting averaged 81F. The duct is wrapped with 2" blanket insulation and vapor barrier. Branch ducts are insulated as well. The insulation is well sealed. I'm measuring the 5 degree drop out of a diffuser that is only 15 linear feet away from the VAV. The diffusers down the at the end of the duct trunk are discharging 76 degree air. Also, I've had the airflow measured at the diffusers and they match the minimum design settings. I'm absolutely baffled and would appreciate any insight or ideas as to what might be happening.
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Bizarre VAV problem 2
- Thread starter DouginMB
- Start date
SlipMatWax
Mechanical
The diffuser will induce room air into the airsream. This may be your problem depending on how far your measurement is from the actual supply air stream.
Like imok2 said your supply air volume may also be too low in the heating mode
Like imok2 said your supply air volume may also be too low in the heating mode
Have you checked the reheat coil at the box is it operating at capacity? You didn't state whether it is HW reheat or electric but if it is HW then I would also check the control valve and actual water flow at the coil. It wouldn't be the first time I've seen a reheat coil get plugged up with dirt and debris from the system.
Tony
Tony
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1
- Thread starter
- #5
I watched as the T&B contractor checked the airflow at the grilles with a flow hood and it was balanced exactly as designed. Duct temperature was taken 3ft. in front of coil (83F) and 5ft. downstream from coil (81F), and in duct at neck of diffuser (76F).
The hot water flow to the box is metered by a Griswold valve. It's actually getting more water than the coil was designed for. Here are the design parameters:
min. cfm = 755
gpm = 1.06
EAT = 55F
LAT = 88.3F
EWT = 160F
LWT = 107.8
MBH = 27.15
Here are the field measured parameters:
cfm = 755
gpm = 1.5 (Note: auto-flow control valve. Pressure drop across valve was within manufacturers limits)
EAT = 57.8F
LAT = 81.0F
EWT = 166.2
LWT = 142.6
I examined the coil and it was clean. It was also hot from top to bottom.
Also, the ambient temperature around the duct is 71F.
Question: what's a better indicator of coil performance, water temperature drop or air temperature rise?
The hot water flow to the box is metered by a Griswold valve. It's actually getting more water than the coil was designed for. Here are the design parameters:
min. cfm = 755
gpm = 1.06
EAT = 55F
LAT = 88.3F
EWT = 160F
LWT = 107.8
MBH = 27.15
Here are the field measured parameters:
cfm = 755
gpm = 1.5 (Note: auto-flow control valve. Pressure drop across valve was within manufacturers limits)
EAT = 57.8F
LAT = 81.0F
EWT = 166.2
LWT = 142.6
I examined the coil and it was clean. It was also hot from top to bottom.
Also, the ambient temperature around the duct is 71F.
Question: what's a better indicator of coil performance, water temperature drop or air temperature rise?
- Thread starter
- #7
- Thread starter
- #9
i'm not sure I'm follwoing you imok. How does slowing down the velocity of the water help? I'm trying to heat the air, not cool the water. An excess of turbulent hot water isn't a bad thing. It will only replace the heat drawn off the coil faster. And in turn the water temperature drop will decrease.
I agree with IMOK on the water flow, if the design was for 1.06 and you are now at 1.5 then the water flow is too great for the volume of air you are passing over the coil. Are these actually Griswold automatic flow controllers or are these the Griswold manual balance valves? If they are the automatic style what is the actual delta P across the flow controller? Check to make sure that someone installed the correct valve for the box.
Tony
Tony
Tony I believe my thinking may be off! I was stuck on the fact that Dougin was at design CFM but apparently design is not hi enough to satisfy the heat load SOOO, I think he should leave the water flow and raise the CFM. That done the TD on the water will/ should go up. How can we argue with mother nature Q = CFM x TD x 1.08? Dougen..Do the easy thing and raise the CFM (if you can)even though it exceeds design. Oh as far as turbulent water flow is concerned ,your right the more, the better the heat transfer
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- #12
imok2,
I'm surprised to see you recommending additional air flow to raise the delta T. Sure it will work. Throw more air at the coil and the delta T will rise to make up for the poor control valve performance. Pretty typical band-aid for a low delta T issue, and a waste of energy if implemented on a larger scale.
This Griswold valve is clearly passing more flow than it should to address the load. You should ask yourself why this is the case. At EAT 57.8, LAT 81.0, and CFM 755 the load is about 70% of design. With a clean coil, I expect the hot water delta T to be 75 deg F or higher given the EWT and LAT shown.
DouginMB, you may wish to trend a bit of the data so you can see what is happening over time. Check into a pressure independent modulating control valve (Flow Control Industries or Belimo) before settling for additional air flow and low delta T.
In addition, I would take a look at the central plant for the building. If condensing boilers are in use, you want much lower LWT in the system than the 142.6 deg F LWT you are getting at this coil. On the other hand, if conventional boilers are used, this may be about right.
I'm surprised to see you recommending additional air flow to raise the delta T. Sure it will work. Throw more air at the coil and the delta T will rise to make up for the poor control valve performance. Pretty typical band-aid for a low delta T issue, and a waste of energy if implemented on a larger scale.
This Griswold valve is clearly passing more flow than it should to address the load. You should ask yourself why this is the case. At EAT 57.8, LAT 81.0, and CFM 755 the load is about 70% of design. With a clean coil, I expect the hot water delta T to be 75 deg F or higher given the EWT and LAT shown.
DouginMB, you may wish to trend a bit of the data so you can see what is happening over time. Check into a pressure independent modulating control valve (Flow Control Industries or Belimo) before settling for additional air flow and low delta T.
In addition, I would take a look at the central plant for the building. If condensing boilers are in use, you want much lower LWT in the system than the 142.6 deg F LWT you are getting at this coil. On the other hand, if conventional boilers are used, this may be about right.
- Thread starter
- #13
Yes, it's a Griswold and at 1.5 gpm it's passing more water thru than it's supposed to. The design was 1.06 gpm. The pressure drop across the valve is 8.5 psi. But they are rated from 2-40 psi, so flow is okay. There isn't a problem with providing too much water. But there is a problem with pushing too much air over the coil and I think that's where the problem lies.
The real problem I found so strange was how much heat was lost from the discharge of the VAV to the diffusers (5degF). I was hoping someone could give me some insight into that part of it as well.
The real problem I found so strange was how much heat was lost from the discharge of the VAV to the diffusers (5degF). I was hoping someone could give me some insight into that part of it as well.
Note that submitted heating coil performance may not be exactly per design. The coil manufacturer supplies their standard coil with performace as close as posible to the design. The water heat loss should approximately match the heat absorbed by the air. In reality there will be some time lag effect.
The heating of air Btu/hr = CFM x 1.08 x (°F LAT -°F EAT)
The cooling of water = gpm x 500 x (°F LWT - °F EWT)
Accuracy, calibration of instruments & parallax in reading of instrument readout scales would be factors in the difference. The more accurate instrument in general should be more trustworthy.
The heating of air Btu/hr = CFM x 1.08 x (°F LAT -°F EAT)
The cooling of water = gpm x 500 x (°F LWT - °F EWT)
Accuracy, calibration of instruments & parallax in reading of instrument readout scales would be factors in the difference. The more accurate instrument in general should be more trustworthy.
DouginMB,
2 Questions:
1) Is their a larger AHU upstream serving multiple VAV boxes? I am wondering if there is not significant variation in leaving air temperature off the hot water coil.
2) Were the temperatures measured at the same moment in time? Is it possible that the discharge air from the VAV was measured then fell before you measured temperature at the diffuser.
2 Questions:
1) Is their a larger AHU upstream serving multiple VAV boxes? I am wondering if there is not significant variation in leaving air temperature off the hot water coil.
2) Were the temperatures measured at the same moment in time? Is it possible that the discharge air from the VAV was measured then fell before you measured temperature at the diffuser.
That is a large temp drop for the conditions you describe. When you say you measured air temp in the diffuser neck, be sure you are out of the induction zone of the diffuser. Are the 3 air temperatures being read simultaneously using three (at least rough) calibrated measurement devices? It would be easy to introduce this error due to measurement technique, if using more than one thermometer, or if multiple thermometers have not been checked for reading accuracy against each other, or if not reading temps almost simultaneously. The other possible scenario, is that the duct system thermal mass is affecting the readings. That is if the vav box was in a cooling mode, and discharging 55-60 F air, and you manually put the box into a full heating mode, and immediately go take the readings you described, the thermal mass of the duct system will cause the supply air to cool and its temperature to drop,until sufficient time has passed so the bulk temperature of the ductwork has been raised up to near it's equilibrium temperature with the supply air temperature. I'm guessing now because i have not run any calc's but think this transient could easily last 15-20 minutes or more. Any other ideas?
- Thread starter
- #17
Lilliput,
I would expect near steady-state conditions to eliminate any time lag. Which lead to the answer to Rhonda's question....
Yes there is a central air handler serving multiple boxes. The discharge air bounces around a little because the boiler is controlled on return water temperature and fires on and off. The water temperature can vary +/- 10 degrees, but at the time I was measuring, we had probes in the duct, pipe and diffuser neck and the water temperature was remianing fairly contant at design temp. The readings i took were instantaneous and were consistent using the same probes at each point.
Getting back to the water system, the T&B report does demonstrate consistently poor heat transfer across the entire system. And that's really aggravating. I really tried to design as a simple system as you can get: turn on the pumps and its self balancing. Go figure.
I would expect near steady-state conditions to eliminate any time lag. Which lead to the answer to Rhonda's question....
Yes there is a central air handler serving multiple boxes. The discharge air bounces around a little because the boiler is controlled on return water temperature and fires on and off. The water temperature can vary +/- 10 degrees, but at the time I was measuring, we had probes in the duct, pipe and diffuser neck and the water temperature was remianing fairly contant at design temp. The readings i took were instantaneous and were consistent using the same probes at each point.
Getting back to the water system, the T&B report does demonstrate consistently poor heat transfer across the entire system. And that's really aggravating. I really tried to design as a simple system as you can get: turn on the pumps and its self balancing. Go figure.
- Thread starter
- #18
Yeldud,
The VAV box hasn't been out of heating mode since the system was started-up before Christmas. The ambient temperature in the plenum was 71F. If it was considerably lower, I would be be concerned.
On the other hand, at minimum airlfow the duct area is considerably larger for the airflow. But, like I said, the duct is well insulated and sealed.
As far as sensor being out of calibration, we were using digital thermometers with probes. I was pulling the probe out of the discharge duct (83F), turning around on the ladder, and sticking it in the flex duct connection to the diffuser. I watched as the temperature dropped to 78F. The duct distance between the VAV and diffuser was about 12 ft. It was strange.
The VAV box hasn't been out of heating mode since the system was started-up before Christmas. The ambient temperature in the plenum was 71F. If it was considerably lower, I would be be concerned.
On the other hand, at minimum airlfow the duct area is considerably larger for the airflow. But, like I said, the duct is well insulated and sealed.
As far as sensor being out of calibration, we were using digital thermometers with probes. I was pulling the probe out of the discharge duct (83F), turning around on the ladder, and sticking it in the flex duct connection to the diffuser. I watched as the temperature dropped to 78F. The duct distance between the VAV and diffuser was about 12 ft. It was strange.
As ar as the balance between the air and the water,acording to your measurements they are pretty close. next time you measure an outlet take off the difuser as was mentioned you could be getting some induction.
I agree with Rhonda up to a point but for practicle reasons if you need the heat then I still believe increasing the CFM is as good a solution as any for the time being. Rhonda, how did you determine the control valve was performing poorly?
AT this point I beginning to think the coil may be too small for the application even if the manfg says otherwise Sooo it seems to me you only have 2 options
1. increase the CFM or
2. increase the TD
Dougen I hope you did't buy these boxes on the cheap, please say you didn't.
I agree with Rhonda up to a point but for practicle reasons if you need the heat then I still believe increasing the CFM is as good a solution as any for the time being. Rhonda, how did you determine the control valve was performing poorly?
AT this point I beginning to think the coil may be too small for the application even if the manfg says otherwise Sooo it seems to me you only have 2 options
1. increase the CFM or
2. increase the TD
Dougen I hope you did't buy these boxes on the cheap, please say you didn't.
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