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.
Regards,
Santiago.
Energy efficieny: VRF + DOAS vs VRF + unconditioned Ventilation system
Jul 21, 2015
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
size).
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!
cheers,
Andrew
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?
Santiago Velez
Santiago,
Another benefit is that the VRF fan is delivering air against a very small
pressure drop - it has no duct to speak of and is commonly multi or
variable speed. It also often uses a higher efficiency ECM motor
An air handler probably has 5-10 times the pressure drop with standard AC
motor.
Hmm. Not sure but it seems likely that a wet coil could affect the
efficiency performance of the VRF FCU.
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
system.
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!).
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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
modelling.
Best regards,
Santiago Velez
It?s also worth noting that VRF FCU typically don?t have a lot of latent capacity, so they can only handle a limited amount of OA within a certain envelope. There are VRF DOAS FCU that are designed to handle OA though. These can be used if you want to take advantage of heat recovery from the ventilation load.
Cory Duggin, PE, LEED AP BD+C
Associate/Energy Engineer
TLC Engineering for Architecture
direct:
615-346-1939