Bizarre VAV problem 2

[DouginMB]

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.

 

[DouginMB]

I am the engineer and I didn't design it for that temperature drop. That was the temperature drop that was submitted and i suppose that was required to meet the other design parameters.

 

[Rhonda2]

Imok2,

Coils only set the delta T at design flow. In all other cases, control valves set the delta T. This coil is clean.

When you read DouginMB's specs for the coil, you will see that its design parameters are 52.2 deg F delta T on the hot water with 160 deg F EWT.

Actual measurements show that the control valve is overflowing so that only 23.6 deg F delta T is achieved while delivering about 19 MBH (70% of design).

Delta T should rise, not fall, at part load for the same air flow. I've now got projects delivering 80 to 100 delta T across the hot water coils designed for 50 deg F.

 

[RatMan2]

Is the coil piped correctly? Is the flow thru the Griswold in the correct direction?

 

[imok2]

Rhonda Misunderstanding! I was talking about the control valve that controls water flow to the coil! not the griswolds. A thought: are you saying that the only control on these coils is a flow control and not a 2 way?

 

[Rhonda2]

Imok2. I'm talking about a single pressure independent modulating 2-way control valve installed on the return side of the hot water coil. No balancing valve required.

 

[DouginMB]

RatMan2: observation of entering and leaving pipe temperatures indicate VAV is piped properly. Sticker on valve body indicates gpm, but how do you verify if correct cartridge is in correct body? I don't think you can.

Rhonda is almost correct. There's a modulating three-way control valve on the discharge. When I took the above readings, the valve was manually forced open to allow full flow through the coil.

 

[imok2]

So Rhonda do you have a solution for the man?

 

[RatMan2]

I have seen griswolds installed backwards (flow opposite the directional arrow on the valve body), even when the coil is piped correctly. This will affect the volume that goes thru the coil.

I believe that there may be markings on the cartridge that can be cross referenced by griswold.

 

[Rhonda2]

Throw out the 3-way valve. Buy a pressure-independent modulating 2-way control valve. Control with the coil leaving air temperature. Observe high delta T performance with no additional fan energy. You may have to see it to believe it.

 

[imok2]

Ratman ..your right!
Rhonda did I miss something? I didn't see anyone say that there was a 3-way valve on this coil? I always assumed it was a 2-way!. Oh I believe I have that stragity at least once in the pass few years or is this something new.Just kidding!

 

[Yeldud]

Back to the air temperature readings, could it be the air coming off your heating coil was not well mixed, where your temperature probe was located, and when you measured 83F, that is not representative of a fully mixed condition. The further away from the coil, the more thorough the mixing. Typical estimates of temperature drops through ductwork range from 0.5 to a few degrees max, depending upon environment (ambient temp surrounding duct, duct temp, insulation value, duct length etc.) so your air temperature measurements don't make sense.

 

[imok2]

Dougen, I was thinking about your problem and I was wondering ..Have you noticed if the supply hot water was piped counter flow to the air flow, and as Ratman said is the valve piped in according to direction? Let us Know what you come up with!

 

[DouginMB]

Yeldud, we took averages about 5' downstream of VAV discharge.

imok,

Piping connections appear to be made per manufacturer's recomendation. Griswold is piped correctly. There's an arrow cast into the valve body that indicates direction of flow.

 

[wilg]

A 7 deg F drop from the box to the diffuser
in a 15 foot run ????
Is the duct ripped open to the ceiling plenum
somewhere on the duct run? Im assuming
this is a flex duct not hard ducted
to the diffuser. Try hard ducting the
15 foot run from the box to the diffuser.

By the way, isn't Griswold the family
name that they used on the National Lampoon
Comedy series with Chevy Chase ?

I'll bet the valve manufacturer didn't
appreciate that.

 

[DouginMB]

The duct is well sealed.

I appreciate everyone's responses. I'm getting the VAV box manufacturer involved and will take more field measurements. I'll keep you posted on what I find.

 

[DouginMB]

Well I solved half of the mystery. The VAV box manufacturer provided 1-row coils where they were supposed to provide 2-row coils. That takes care of the capacity problem.

The other half of the problem is determining why the air temperature is dropping so quickly from the VAV dischage to the grilles. I found out that the building perimeter has a 2.5" gap where the structural steel rests on the framing beams. The architect didn't show any draftstopping of insulation on his plan to cover this gap so, in essence, the plenum is open to the outdoors. I imagine this is inducing a convective draft that is causing heat to be drawn off the ductwork. Insulated or not, the heat loss would be significant.

 

[Rhonda2]

DouginMB,

I glad you have sorted out a portion of the issues. Sometimes the most obvious things are right there in front of us.

I'd encourage you to address the low delta T and capacity issues head on and consider all the options. There are an awful lot of small fixes proposed out there that will only treat the systems and not the root cause of these issues.

This was a stimulating thread. Thanks.

 

[imok2]

I believe in one of my posts that I believed the coil was too small for the application no matter what the mfg said. I'm glad you have it under control and if there's anything else your having a problem with please let this forum know, we love a good problem and Ha, it's free!!

 

[DouginMB]

Rhonda,

Apparently 1-row coils were installed in every VAV box in the system (18 boxes in all). The submittals called for 2 and in some cases 3 row coils. That's the reason for the low delta-T. There's not enough coil surface area to get a good heat exchange and drop the coil water temperatures. I can't imagine low delta-T being a problem in a hot water system.

imok,

Yes, you did say that. That's one reason I got the manufacturer involved. Thanks for your input.

Once again I appreciate everyone's input. I've been in the business 10 years and have seen alot, but this was a new one.