[Bldg-sim] Energy efficieny: VRF + DOAS vs VRF + unconditioned Ventilation system
santiagogvelez at gmail.com
Tue Jul 21 10:31:17 PDT 2015
Thanks Andrew Dan and Jim for the insights. Many things I didn't know or
never thought off. Now, to figure out how to model what you just said to
try and inform HVAC design.
I guess as with most things related with building performance, there are a
quite a few variables to consider simultaneously before drawing a
conclusion. Ill get back to the group if I get any new insights after
On 21 July 2015 at 13:03, Dan Nall <dannall at mindspring.com> wrote:
> The major determinant for energy efficiency of this strategy is the dry
> bulb and wet bulb temperature distribution of the local climate, which will
> determine the availability of airside or waterside economizer. This
> strategy would not be appropriate for San Francisco or Seattle where
> free-cooling is available most of the time. I don't know of any VRF fan
> coils that have auxiliary water coils for waterside economizer, and the
> DOAS will not be sized for bring in sufficient cool outdoor air to provide
> airside economizer cooling.
> Doing all of the dehumidification with the DOAS, to reduce the indoor
> de3wpoint temperature below that of the contact surface of the cooling
> coil, resulting in a dry coil, will definitely reduce pressure drop across
> the coil of the fan coil. Dry coils generally have much less than half of
> the pressure drop across the same coil when it is wet. A significant
> energy efficiency strategy with chilled water systems is to use a low
> temperature cooling source (5 DegC) for the DOAS so it can provide air
> with a very low dew point temperature (10DegC or less) to the space, and
> using a separate higher temperature chilled water source (13 DegC) for the
> fan coil. If the coils of the fan coil are dry, then the pressure drop is
> sufficiently low that the puny fan coil motors and fans can force air
> through a much more robust coil, perhaps 8 row, 10 fpi. As a result, the
> fan coil can have a very close approach of the SAT to the entering CHWT
> (Less than 3 DegC)). Energy efficiency is achieved through the reduction
> in transport energy with the fan coil system, and through the increased
> energy efficiency of the chillers making 13 DegC water. I do not know if
> the VRF fan coils can be controlled to operate at a higher saturated
> suction temperature, thus increasing refrigeration efficiency.
> The energy efficiency of this strategy is lost if an all air system could
> be providing cold outdoor air, through much of the year, to do all of the
> cooling with no chiller or compressor at all, Also, obviously, with a
> chilled water system, 13 DegC chilled water can be made for many more hours
> a year by a waterside economizer than can 8 DegC chilled water. Of course,
> nighttime conditions have a lower wet bulb temperature, so the occupancy
> profile of the space should be considered to determine the efficacy of
> chilled water storage with the waterside economizer. Much analysis needed,
> using sophisticated modeling tools that fully capture the operation of the
> equipment. The solutions, as always, are determined by both occupancy and
> climate (along with the preferences of the owner and operator).
> .-----Original Message-----
> From: Andrew Corney
> Sent: Jul 21, 2015 11:32 AM
> To: Santiago Velez
> Cc: "bldg-sim at lists.onebuilding.org"
> Subject: Re: [Bldg-sim] Energy efficieny: VRF + DOAS vs VRF +
> unconditioned Ventilation system
> Hmm. Not sure but it seems likely that a wet coil could affect the
> efficiency performance of the VRF FCU.
> From an energy perspective heat recovery centrally could be made to be
> very efficient. If you bring air in from outside and let the VRF system see
> it then you might not get those benefits and the ventilation load might be
> a lot higher. This could be offset for heating with a zone by zone MVHR
> That said you could potentially have latent heat recovery at a central AHU
> where it would be very hard in an MVHR ventilation unit. In more extreme
> climates this could make a substantial difference. For example this could
> help rehumidify the air in very cold places or dehumidify the air in hot
> humid climates. The heat recovery would then be a direct saving against the
> coil load (although it would increase pressure drop in the DOAS system!).
> On Tue, Jul 21, 2015 at 4:06 PM, Santiago Velez <santiagogvelez at gmail.com>
>> Thanks Andrew, I wonder though if besides first cost or maintenance
>> related issues there are any energy savings at all, and if so why is that
>> the case. I Read about better performance of evaporator running with lower
>> air temperature in a DOAS, but you still need energy to treat ventilation
>> air in the DOAS system, so were are the savings coming from?
>> On 21 July 2015 at 10:39, Andrew Corney <andrew.corney at sefaira.com>
>>> Hi Santiago,
>>> One benefit might be the ability to do the dehumidification remotely via
>>> the DOAS system. This could alleviate the maintenance of drip trays /
>>> condensate drains from your VRF fan coil units. You might also be able to
>>> down-spec the VRF fan coils that you are using as well (possibly reducing
>>> Another benefit would be that it might be cheaper to do a higher
>>> performance style of heat recovery centrally that could help reduce heating
>>> energy in particular.
>>> So there are 2 reasons!
>>> On Tue, Jul 21, 2015 at 2:33 PM, Santiago Velez <
>>> santiagogvelez at gmail.com> wrote:
>>>> Dear Group, Can someone enlighten me about the energy efficiency
>>>> benefits of conditioning the ventilation air in a VRF + DOAS system, versus
>>>> letting the VRF handle all the load (internal loads + Ventilation loads).
>>>> Thanks in advance.
>>>> Bldg-sim mailing list
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>>> Andrew Corney
>>> PE M.CIBSE M.ASHRAE
>>> Product Manager - Sefaira Systems
>>> Mobile: +44 (0)7887 059 722
>>> Office: +44 (0)2037 147 619
>>> Skype: andrew.corney.sefaira
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> Andrew Corney
> PE M.CIBSE M.ASHRAE
> Product Manager - Sefaira Systems
> Mobile: +44 (0)7887 059 722
> Office: +44 (0)2037 147 619
> Skype: andrew.corney.sefaira
> Sefaira - Sustainability. Performance. Design.
> *For company contact and registration information, please
> visit: http://www.sefaira.com/info/contact
> *Check out our latest webinar showcasing the product here
> <https://vimeo.com/130461932>. Subscribe here
> <http://www.sefaira.com/subscribe-from-email/> to our bi-weekly newsletter.*
> Bldg-sim mailing list
> To unsubscribe from this mailing list send a blank message to
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