[Equest-users] Heat Recovery Increases Cooling load

Bishop, Bill bbishop at pathfinder-ea.com
Fri Dec 18 08:56:48 PST 2015

Good advice from Dan and Brian!
Lastly, you can model the option where the system fans serve the ERV, and the ERV has a bypass section or ductwork around the coil/wheel:
- Set ERV fans to "HVAC Supply/Return" in the second heat recovery tab.
- Enter the static pressure loss for the coil/wheel in the second heat recovery tab.
- Subtract this static pressure loss from the fan power entered at the system level.
- Verify that total fan power is correct by adding SV-A fan power to the fan power shown on the ERV report(s).
The extra static pressure of the ERV section will only add to system fan power for hours that the ERV is controlled to operate.

Remember - Fear leads to anger. Anger leads to hate. Hate leads to suffering.
~Bill "not a Jedi yet" Bishop

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer
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T: (585) 698-1956                        F: (585) 325-6005
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-----Original Message-----
From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Daniel Knapp
Sent: Friday, December 18, 2015 9:54 AM
To: Nufal, Maye <Maye.Nufal at stantec.com>
Cc: equest-users <equest-users at lists.onebuilding.org>
Subject: Re: [Equest-users] Heat Recovery Increases Cooling load

Here are some of my observations on modelling ERV in eQUEST with a VAV system: 

The main problem with adding heat recovery to a system in eQUEST is that reliance on the default settings causes a large increase in the fan energy, and also leads to over heating of the mixed air stream during periods of low space heating and space cooling demand. Over-heating of the mixed airstream leads to activation of the cooling coils when they would otherwise not be needed (e.g. during economizer operation) and to higher space cooling energy overall. Taken together, this increase in both fan energy and in space cooling energy can overwhelm the energy savings from heat recovery.

As Brian notes, in the second heat recovery tab, the static pressure defaults are high and adding 1” of static to the supply/return fans increases the supply and return fan power on the full supply air flow rate during ERV operation and overestimates the fan energy. It is better to use the self-contained option and set the fan and motor efficiency parameters to match the ERV fan power specs. In this case, the increased pressure will only be applied to the flow through the ERV itself and only when the ERV is operating. It may also be the case that the supply and return static pressure entered at the system level already accounts for the extra static pressure from the ERV (it often is if you are using cut sheets from the manufacturer and the ERV is built into the air handler). In this case, you should set the ERV fan power to zero by setting the static pressure to zero. Note that fan power from the ERV is not reported with the system fan reports in the SIM file, it is only reported in the ERV reports.  

Also, as Brian notes, it important to allow the ERV to be turned on and off as needed. Control is usually achieved through the use of bypass dampers or in the case of an energy wheel through modulation of the heat recovery by changing the speed of the wheel. In cold climates, it is important to model frost control which is set in the second ERV tab. I usually set this to match the control method specified in the first ERV tab. 

Hope that helps,

Daniel Knapp, PhD, PPhys, LEED® AP O+M
danielk at arborus.ca

Arborus Consulting
Energy Strategies for the Built Environment www.arborus.ca
76 Chamberlain Avenue
Ottawa, ON, K1S 1V9
Phone: (613) 234-7178 ext. 113
Fax: (613) 234-0740

> On Dec 18, 2015, at 9:03 AM, Brian Fountain <bfountain at greensim.com> wrote:
> The default settings for heat recovery control is'float' which always maximizes heat recovery, even if it reduces free cooling. Changing this to 'mixed air reset' will help.
> Also, on the 2nd heat recovery tab, the default is 1" of static added to the supply fan (on top of whatever you have on the fan tab) to account for the added fan power to push the air through the heat recovery device.  
> From: Nufal, Maye
> Sent: Friday, December 18, 2015 8:58 AM
> To: equest-users at lists.onebuilding.org
> Subject: [Equest-users] Heat Recovery Increases Cooling load
> Hi,
> We’ve been facing the same issue in multiple projects ;  using heat recovery somehow increases our cooling load.
> This was evident in a VAV system as well as Central Plant Systems .
> We’ve tried using both Sensible HX/Enthalpy wheel with same results.
> Has anyone been able to tackle this issue ?
> Help/workaround ideas is appreciated.
> MN
> Maye Nufal , LEED BD+C
> Sustainable Buildings Consultant
> Stantec
> 100-401 Wellington Street West Toronto ON M5V 1E7
> Phone: (416) 598-7157
> Fax: (416) 596-7892
> Maye.Nufal at stantec.com
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