Allowable Space Temperature Variation 1

[VerneE]

We have a lab area with many fume hoods, so make-up air is sized for maximum of all. However usually only a few are running so supply air is much more than needed for room cooling requirements. We are getting high temperature fluctuations 70F +/- 5F over the course of the day, sometimes within an hour's time period, depending on outdoor conditions and activities in the space. Can this be considered an acceptable temperature variation?

The client never gave us any specifications for allowable temerature variation, they are nor sure what they require. This is not causing a problem with their processes, but with the perceptions of the people in the space - i.e. it is "too hot" or "too cold".

What is a typical, non-critical space temperature fluctuation amount, +/- 3F or +/-4F or +/- 5F?

 

[KiwiMace]

In lieu of my having a good number here... The building engineer is going to be working to achieve a satisfaction level of 95% (or the squeakiest wheel).

Ultimately, your design needs to satisfy your client. The consultant part of consulting engineering is all about providing the client with budget friendly, code compliant, achievable and efficient space criteria that can meet the (95%) acceptance of the people and processes in the space.

If this is a big room full of people complaining, you can wave ASHRAE data all you like and they will still think you messed up.

 

[DRWeig]

KiwiMace's answer is the best!

It doesn't matter what experience and standards might say, only the people in the space will be able to judge what's acceptable.

Perception rules. A group of rough-n-tough, rugby-playing guys might consider ±5°F to be just fine. A group of elderly folks with thyroid problems might complain if ±1°F happens.

Draftiness, radiation, and the characteristics of the furnishings will make a difference even within the same group.

Good on ya,

Goober Dave

 

[mauricestoker]

I normally look for a 5* spread, plus or minus 2.5*F.

As the air changes are more than sufficient for environment loading, I would recommend looking at the reheats (if you have them).

Even if you meet all requirements for normal conditons, there is no such thing as a temperature that everyone will agree with. A dummy t-stat on the wall may satisfy some, but I don't think you will ever satisfy everyone.

 

[VerneE]

We all agree that perception is a problem, but we do have hard data, i.e. periodic records of what the actual temperature is at 30 minute intervals. while this contradicts what some were saying (its too hot or its too cold), we still have the +/-5F temperature variation. Due to the large volume of are being distributed into the space, when the space temperature drifts below the setpoint so the heating mode is activated (especially on autumn evenings), the unit switches form 70F to 75F supply air, and the space quickly warms up to 75F for a few minutes, until it can switch back to cooling mode.

I just need to know if +/-5F is an acceptable variation or if we need to tweak the controls so the supply air temperature for heating gets reduced to a lower temp. The supply air temp is adjusted based on differential temp of actual space temp minus setpoint. If it is too hot in the space, unit switches to cooling mode, supply temp goes form 70F to 65F depending how far the actual space temp is from set point. Similarly if it is too cold in the space, the unit goes into heating mode, supply temp goes from 70F to 90F depending on the differential.

 

[VerneE]

I should also say that because of the number of fume hoods, the space gets 36 air changes per hour, and the make-up air is sized for that, much oversized for actual space air conditioning requirements, but necessary to meet fume hood exhaust requirements.

Don't ask, recovery wheels and such are not feasible for this space, all we can do is supply conditioned air to match that which is exhausted; operating cost, capital cost, space, etc. were all considered, the only realistic option was to supply conditioned outside air to match the exhausted amount.

 

[sprinkler1000]

Temperature variations should be kept to minimun.

The way to do this is to have variable air volume both in exhaust and in supply (pressure differential transmiters in the exhaust duct and VFDs in both fans).

We have done some labs with capture hoods and they work well.

 

[mauricestoker]

ASHRAE 55 would be one basis to consider comfort.

Have you considered controlling discharge temperature based on outside air? With constant flow and set temperature, outside air is your variable. Room or duct discharge global temperature would control dead band (heating or cooling does not kick in between +/- 2.5%) then heating controlled by outside air temp until room temperature is restored to the high end of dead band (72.5* F)or low end during cooling season.

 

[KiwiMace]

I agree that control, and particularly dead band is your problem here.

If you are going to control the temperature in the space to a tighter tolerance, the control of the incoming air needs to more closely follow the variations in flow and temp. You can either match the temperature requirement more closely or match the air volume more closely, or both.

From your language, you appear to have a single constant volume MUA unit supporting a large number of discrete hoods.
If this is the case, I think it would be more cost effective to change the MUA to VAV based on # of hoods running.