Dehumidification issue - REPOST

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Hello all,

I posted this awhile back and received no response. Testing my luck again and hoping someone will have some insight!

Hello list,

I thought the dehumidification calculation would be a relatively straightforward calculation for QUEST, however, I have some results that are a bit puzzling.

I have a simple all-electric packaged VAV system that serves living quarters of an in-patient clinic. The system has 50% OA and uses an economizer. The system is designed to heat and dehumidify air only (no cooling, none of my zones have cooling set point schedules). My dehumidification set point is 60% RH. The building is located in Chibougamau, Quebec (quite north) where cooling and dehumidification requirements are relatively small, but still exist. According to the SS-N report, the maximum RH is never surpassed for my system.

This is all fine and great until you start digging a bit deeper into the hourly results of the system in question. In my hourly data report I have the following variables : Total central heating/cooling coil outputs, dehumidification reheat heat output, return air humidity ratio, mix air humidity ratio, humidity ratio of air leaving cool coil, Moisture added or removed from air for (de)humidification, and maximum humidity set point. The strange thing I noticed is that my return air humidity ratio and mix air humidity ratio NEVER surpass my maximum humidity set point, yet I still have loads on my cooling coil during the summer, and moisture removed from the airstream. Furthermore, I noticed that there are 815 hours during which there is a load on the cooling coil, but no humidity is removed from the airstream whatsoever (mix air humidity ratio = humidity ratio of air leaving cool coil). My team here has experienced these results in more than 1 simulation that have a system with only dehumidification requirements on the cooling side.

So my questions are:

1. How does eQUEST control the maximum humidity allowed in the supply airstream?

2. How could one possibly explain a cooling load/consumption without any cooling set point schedules in any of the zones supplied by my system and zero moisture removed from the air?

Any insights would be much appreciated!

Many thanks.

[Bouthillette Parizeau]

Jason Ng_ing. M.Sc.A.
jng at | | t: 5143833747x2813

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via Equest-users's picture
Joined: 2016-07-15
Reputation: 400

Hi Jason,

A couple of thoughts which may or may not be helpful:

The DOE2 dictionary states that the humidity is controlled by measuring the relative humidity in the return air stream. You have an hourly report of the humidity ratio. Have you checked how this translates to a relative humidity in the return air stream?

Also, does the VAV system you are modelling have a central heating coil? i.e. have you entered a ?Hot Deck Max Leaving Temp? under the Heating Coil Cap / Control tab? Without an input for this entry eQUEST does not create a central heating coil. Without a central heating coil the system may not be able to cool and reheat to dehumidify the air which may prevent the system from turning on dehumidification in the absence of a cooling load.

VAV systems are controlled by default to supply cool air at 55?F at the central system level and then reheat as necessary to meet zone set points. It?s possible that the system could be cooling the supply air even in the absence of zone cooling loads simply to meet its supply air temperature control requirements.

Hope that helps, please feel free to share your input files if you?d like someone to have a closer look.

All the best,


Daniel Knapp PhD, P Phys, LEED AP
danielk at

Energy Efficiency Expert
Arborus Consulting
2-43 Eccles Street
Ottawa ON K1R 6S3
(613) 234-7178

via Equest-users's picture
Joined: 2016-07-15
Reputation: 400