Fan/Blower Coils with DOAS

[Antifed]

Hello,

I searched through existing posts and did not find a post regarding my question (though that doesn't mean that I searched very well) so please forgive me if this question has been asked and answered. If it has, please direct me to the thread.

We designed a a building in climate zone 4A, part of which is a two storey instruction facility. The downstairs portion of the facility consists of classroom spaces and support spaces (e.g. bathrooms). The upstairs spaces consists of some private offices, open floor offices, a conference room, and support spaces. The upstairs and downstairs each have a dedicated outside air system that provides dehumidified air (48-deg F dew point).

Both DOASes are partial recirculating type, have an enthalpy wheel, heating section, DX cooling coil, and hot gas reheat to temper the supply air. The downstairs DOAS supplies air to classroom chilled beams (the CB design has been reviewed by my peers as well as the chief engineer from Titus and it is considered sound) plus a couple of blower coils (where beams wouldn't be appropriate). The upstairs DOAS supplies dehumidified, room neutral air directly to the zones. The air volumes are based on providing ASHRAE 62.1 compliant ventilation airflow rates plus additional air to provide dehumidification of each space/zone (e.g. an office may only need 20-cfm of outside air, but may need 60-cfm of 48-degF dew point air to account for the latent load).

The facility will have an air barrier and non-operable windows. The space design conditions are 78-degF DB & 45%RH. A design infiltration rate of 0.1-cfm/sf of wall was used (based on ASHRAE Fundamentals for typical new construction) so the amount of air supplied to spaces for dehumidification takes into account any latent load from infiltration.

All that being said, the design did not include condensate piping for the upstairs' fan and blower coils based on the DOAS dehumidification. The building is under construction and the contractor has submitted an RFI regarding condensate piping (i.e. "it's not there, what do you want me to do?"). This is my first major design where I did not include the condensate piping based on having confidence in the design. But now that the question is being raised, I am having second thoughts. Our commissioning specialist doesn't have the warm and fuzzies and would feel better having the condensate pipe installed via a change order. I would certainly have the warm and fuzzies if we installed the condensate piping, but then what would have been the point of decoupling the latent and cooling loads with the DOAS (I ask rhetorically)?

Have any of you all designed a system in this manner where you haven't seen any condensation issues? I had planned to incorporate a safety feature into the controls to close off the cooling valve in case condensate is sensed in the drain pan of a fan coil unit which is similar to condensation prevention with the chilled beams (only they are controlled such that when an abnormal space dp is sensed that the beam chill valve would close off).

What would you recommend?

Thanks!

Antifed

 

[BronYrAur]

Will the blower coils receive the same temperature water as the chilled beams? By design, the chilled beam water temp should be above dew point, because you have no drains on those, correct? So, as long as you are not introducing any additional unconditioned outside air(that may have a higher dew point) into the blower coils, there should be no issues. That said, there is always the chance that something (or someone) will happen and water that is too cold will be sent to the beams. Then you will have a mess.

Those are my thoughts, but I don't have direct experience with it.

 

[Antifed]

Hello,

Thanks for your reply. The beams are supplied with 57-degF water, but the fan coils and blower coils are supplied with 44-F water. The fan/blower coil spaces are provided with dehumidified air that is designed to account for the latent load. After subsequent discussions with my colleagues, I am going to have them install the condensate piping just to be safe.

 

[BronYrAur]

Yes, I agree. Small price to pay now. I think you will still see some condensation.

 

[willard3]

As there's always going to be someone who will inadvertently introduce humid air to the space, put in a condensate drain on the fan coils.

 

[ProcessHVAC]

I can't think of a reason why you wouldn't put condensate piping in this arrangement. Aren't you dehumidifying the mixed air? What are your internal latent loads? Also DOAS is dedicated outdoor air, if you recirc air then it is not DOAS, perhaps I did not understand that well?

 

[GT-EGR]

It boils down to the fact that the fan coil units have CHW temperatures that CAN cause condensation. And we can design for scenarios and have everything set up correctly, but once the spaces start getting used in odd ways, there could be some days where moisture shows up on those coils and has nowhere to go. The chilled beams get exempt from this because they have water temp and safeties in place to keep a gap between dewpoint and their surface temp - although they still have had their moments where this safety failed.

This is an argument also made sometimes for IT room cooling, where the load is all sensible and the drain seems pointless and probably is. But it usually takes so much convincing of so many people that you start to realize maybe it is a good added safety to just put the piping in.

That being said, you could push for a moisture sensor in the condensate pan as your safety, but over time these may fail, and also could become a regular nuisance shutting down cooling to spaces just because a drop of moisture shows up.

 

[Antifed]

ProcessHVAC, the unit would be more accurately described as a dedicated ventilation and dehumidification unit. I call it DOAS because the unit is the dedicated source of fresh air for the space. The recirculated air provides the extra volume needed to dehumidify the zones. I ran into an issue with a previous project where I had to change the name of a similar unit from DOAS to VADU (ventilation and dehumidification unit).

GT-EGR, thank you very much. I thought about the chilled water temperatures the other day, that I would have had to supply the F/BCUs with the 57-F water to help prevent condensation from occurring in the units.

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