reheat coil problem

Tom_78

Thanks for all the suggestions. We've discovered that the valves on the system only allow flow when they're more than 85% open (full closed to full open is from 85% to 100%). That was what was screwing with the feedback loop in the controller. Coupled with a longer-than-normal pipe run and we ended up with the wild swings. I'd like to go to a different valve. The current valves are two-wire and it looks like replacements (such as Belimo) are three-wire. I'm not eager to pull new wire in a very crowed building. (But on the other hand, I do need to keep the secretaries comfy.)

Thanks again,
Tom

Tom_73

reheat coil problem

We are having problems with one of the reheat coils in a lab/office building. The control valve for the coil is cycling from 0% to 100% to maintain a (fairly uniform) space temperature. The result for the occupants is blasts of hot/cold air.

The building has a DDC system and the software seems to be uniform between all the coils in the building. We have replaced the control valve and actuator. There is a functional bypass with a circuit setter on the coil. The strainer is clear. There is differential pressure from supply to return. The coil has been purged of air.

The problem is that the control valve slowly opens until it is about 85% open before the coil gets warm. The valve continues to open until the coil is hot. The room overshoots on temperature, and the valve closes (0 to 25% or so) before beginning the cycle over. The office in question is the secretarial space (very important!).

How do I smooth out those blasts of hot/cold air to make the secretaries happier?

Thanks,

Tom

Al Letellier_9

re-heat coil

A lot depends on the set up you have, Tom. Is this an on-going problem or is it something new? If ongoing, sounds like you have uneven temp in the heat source or your valve has the wrong actuator. In this case a fully modualting actuator is best to eliminate those wild temp swings and it should be a three way valve to bypass when heat is not needed. Also the fan should run continuously during occupied periods to even out the air flow. Properly set up, a modulating valve controlled by a sensor in the air stream, should deliver constant discharge temp out of that coil. And if any outside air is being introduced, you need the correct control set-up to keep discharge air temp constant.

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Brad White_9

Discharge Sensor versus Room Sensor?

What this seems like is classic "hunting".

Is there a discharge air sensor? Sometimes these are installed for monitoring the coil performance. On occassion this may be wired to act as the room stat so will react accordingly.

If not, some other thoughts:

There seems to be an interuption in the feedback portion of the control loop or it may be that the space sensor is located too close to the air stream or a heat source.

Normally the setpoint is maintained with very little modulation of the valve.

Things I would check:

1. Valve size and Cv relative to load.

2. Sensor location in the space relative to the supply air stream or heat sources. Sensor over a computer/monitor? Hang on for a ride.

3. Control PID Loop tuning. Control feedback may not be allowing enough of a pause before reacting, in short.

4. Discharge sensor versus Room Sensor readings.

Just a few thoughts.

The fact you gave that this is happening in one room leads me away from other thoughts such as fluctuating HW temperatures or supply air temperatures.

Tom_73

This is an on-going problem. The control valve is a diverting-type 3-way valve. The sensor for that space is wall-mounted. It has a filing cabinet under it, no heat-producing equipment. This coil is one of 14 coils. All have the same valve. From observation, it looks like we need a three pound differential to get flow through that coil. This coil was added under a change order during construction. It does have longer supply/return piping (with more fittings) than the other coils. Could that be it?

Thanks for the help!

Brad White_9

More thoughts

It would only be an issue if it is the furthest coil or most restrictive run by a great deal. Not too concerned, how far away could it be? My point here is, if restricted and say your flow is 50% of design gpm, your output would still be about 90%. But you seem to be getting plenty of heat. Do check the balancing reports, but I still think this is a control issue.

If 3-way valves are used, at least when I specify them (I am an engineer not a tradesman, full disclosure here), I make sure that the bypass port has a balancing valve such that the DP through the bypass port is the same as the DP through the valve and coil in series, if that makes sense.

Without a balancing valve, when in full or even partial bypass mode, your DP at the valve bypass port is huge (full DP of supply versus return, enhanced by the fact that there is a coil and modulated valve straight ahead). Valve cracks 20% and 90% of the water heads south to use discussion numbers.

Is the valve a Belimo? Some of their actuators (love them) have a reverse-acting switch so you can set them up NO/NC or last position. Just thinking it might be reversed.

How long a cycle from hot to cold? Continuous full valve stroke?

You say all coils have the same valve; are the coils all about the same size/capacity and how relative to the valve size (not type, but size) are they?

Just probing here. Let me know.

Brad

seabee570

valve problem

I think that it may be a control problem as brad said...with the ddc system,you should be able to tune or adjust the valve to open quicker,consult your ddc supplier/contractor.Another thing is that in this space you may need to install more than 1 sensor,example;3 sensors in series would give you an 'average' temp for that space,giving a truer reading.The last thing you could do is install a sensor in the return air and have the valve open by looking at the space temp versus the return air temp,creating a reset off of the return temp,this would be done by adding a sensor,and ddc programming changes

EBEBRATT-Ed

Try closing the circuit setter somewhat and see what happens. You may need to slow the valve action down.

Ed

larry_9

Wild reheat coil

Tom

One of the things you siad was that it was a diverting valve not a mixing? On most reheat coils they are mixing valves if it is deverting valve you can get wilded swings. Is the supply being feed into the A port and the return of the coil into the B port with the return to the system form the AB port?

Brad White_9

?

Personally I have never used a 3-way mixing valve on a reheat coil (unless it was a larger one in an air handling unit with a circulator and separate circuit).

I think the 3-way diverting type ("one in, two out") is fine so long as the bypass port is balanced to the same PD as the coil port, but mostly I use 2-way modulating valves. (Assumes a DP bypass somewhere of course.)

How is a mixing valve properly applied in the way you describe, Tom? I am trying to picture a mixing application ("two in, one out") when all on one side of the pump. I usually associcate a mixing application being on the suction side of a pump only. Maybe I am missing something, just trying to get my head around it. Who knows, maybe I have mis-applied them my entire career!

Thanks