ERV in Ottawa

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I've been trying to understand how ERV works because I am finding the energy savings to be somewhat less than I had expected. Ottawa has a very cold climate in the winter and a warm, humid climate in the summer and I would expect enthalpy wheels to be a very good energy saving strategy. Instead, I am finding that in many cases the building uses more energy and in the best case scenario (OA exhaust DH, mixed air reset, modulate HX) on a VAV system saves only 0.15%. I have built a simple box model with one system in a two-storey office building and tried a number of different control strategies (see attached tables for results). I have tried a VAV and a SZ system. The ERV is able to save marginally more energy in the SZ system, but in neither case are the results what I was expecting.

My question is this: are these results to be taken at face value and believed or is there a better way to model ERVs in a Canadian climate? It seems very unlikely to me that ERVs cannot be controlled in such a way as to save significant energy in a climate of extremes. (.inp file attached as well).

With thanks and best wishes,

Daniel Knapp, PhD, LEED? AP O+M

Daniel Knapp's picture
Joined: 2011-09-30
Reputation: 0

Could it be added fan power due to higher pressure drops across the wheel and added ductwork? We've been warned this might happen.

Good luck!


Mark Darrall's picture
Joined: 2011-09-30
Reputation: 0

A couple of thoughts -- from your energy end-use breakdown the consumption
in the heating season far outweighs the cooling season, so based on your
particular situation, the extremes seem to be weighed towards heating.

Also your percentage of outside air in the office building is important. If
the percentage of outside air isn't very high, most of the heating for the
desired MAT comes from the return air. Try using the variable reporting to
see how much load is on the preheat coil in your baseline case.

Lastly, if your building has major envelope losses, the ERV won't affect the
use of your perimeter heating system if one is present, which may be
carrying a lot of your heating load.

Hope this helps!

*David S. Eldridge, Jr.**, P**.**E**.**, LEED AP BD+C, BEMP, HBDP*

David S Eldridge's picture
Joined: 2011-09-30
Reputation: 2000

To add to David's excellent comments, the operation and control of the ERV is also very important.
In that kind of climate there may be a lot of periods when the main AHU is trying to maintain a 74 degF interior space temperature with 55 degF supply air, and at the same time the outdoor air temp is in the range of 55degF-70degF. In this scenario, the ERV is heating the incoming outdoor air before it has to cool it to 55degF, so the net effect is an increase in energy consumption during the moderate seasons. An energy wheel should be shut off or idled during this time, (when OA enthalpy < RA enthalpy and the unit is in cooling mode), if it is a fixed plate ERV then there should be a way to bypass the ERV in the design.

Joe Fleming

Fleming, Joe's picture
Joined: 2011-09-30
Reputation: 0

Hi Daniel,

I haven't looked at your model yet -- will try and take a look this

One thing to consider is the outdoor air fraction and the supply air
temperature setpoint. If you have a low OA fraction, say 15% and a low
supply air temperature, say 55F -- then often you will be mixing to
satisfy that SAT and the heat recovery will only come into play for very
cold temperatures. Meanwhile, you will be adding static pressure on
your system and seeing notable increased fan energy.

On the other hand, if you have a dedicated outdoor air system then you
will see dramatic savings with an ERV.

I saw that you were playing with the ERV control -- FLOAT is the default
and maximizes heat recovery always -- this would be typical of a heat
pipe where you can't modulate the heat recovery. Trim economizer is
non-typical -- bringing in more OA to compensate for excess heat
recovery. I usually use MA reset. Have a look at the ERV report in the
.sim report to see actual heat recovered (plus the added fan energy
which is NOT reported on the SV-A report and defaults at 1" for both
supply and return).


bfountain's picture
Joined: 2011-09-30
Reputation: 201

I have come across this a lot in eQuest, and the solution for me is usually to only run the ERV in cooling mode, because I am usually in a mostly cooling climate.

I am not sure exactly how eQuest handles the ERV controls under "Operating Mode:", but it does say something in the help file that leads me to believe choosing heat/cool is a bad idea for an operating mode...

"OA-HEAT/COOL the default for enthalpy ERVs, indicates that the ERV will operate whenever the absolute value of the differential between the outdoor air and the exhaust exceeds the specified differential."

[cid:image001.png at 01CB97B2.90A7F3D0]

The difference between OA cooling only or OA heating only and OA heat/cool is the term "absolute value". If the conditions are such that the absolute value between the exhaust and outdoor airflow is less than 5 degrees the ERV is off. If it is greater than 5 degrees the ERV comes on. So the entire period where you are in cooling mode but it is less than 70 degrees outside, (assuming a 75 degree space temp, abs|69-75| = +6, the ERV is adding to your cooling load and is hurting your cause, because we chose OA heat/cool.

If there were a way in eQuest to use an ERV in heat/cool mode but eliminate the absolute value part and make it +5 for cooling and -5 for heating, instead of Del_T > abs|OA-EA| then that would allow the ERV's to operate perfectly and operate in the same fashion as the standard ERV sequence in the field.

Does anyone know if this is correct, or if eQuest treats the control sequence different from what is described above?

Joe Fleming, E.I., LEED AP BD+C, BEMP

Fleming, Joe's picture
Joined: 2011-09-30
Reputation: 0

I find that ERVs use more energy when the Make-up Air Temp Ctrl is specified as "Float". In other words, there is no temperature control of the air leaving the ERV. For an enthalpy wheel, choose "Modulate HX". This allows the rotational speed of the wheel to vary in order to maintain a particular leaving air temperature.

Kelsey Van Tassel

Kelsey VanTassel's picture
Joined: 2011-09-30
Reputation: 0

Thanks Kelsey, that's good to know.
For cooling savings float works fine for just cooling mode, but not heat/cool. With the fixed set-point option it looks like the heat/cool mode works much better, it works the same as float in cooling only mode when you set the cooling temp down to 30.
But the ERV still operates outside of a typical control sequence and still undercuts the theoretical energy savings.
The heating load increases considerably in fixed setpoint mode, over float.
A constant wheel speed, an off mode, and a floating temperature is standard for the price devs will pay down here.
The optimal savings happens for heating in float and for cooling in float + cooling only. The float heat/cool should be achievable, unless it butts heads with other system operations in eQuest.

Standard temperature control
The unit can be configured with normal air flows and
controls but still have the benefit of a large amount of
makeup air, better humidity control and lower
operating cost than a unit without a heat wheel. The
energy recovery unit operates in four (4) basic modes;
fan only; economizer; cooling and heating. Each of
these modes has specific functions as defined below.
Fan only mode: When the unit supply fan is started,
and there is no call for cooling or heating, the unit
economizer moves to its minimum position, the
heatwheel is activated and the heatwheel fan is
started. If the unit is equipped with heatwheel bypass
dampers, these are closed.
Economizer mode: With the unit supply fan in
operation and a call for cooling is made, if the outdoor
air temperature and humidity are below the enthalpy
setpoint, the heatwheel exhaust fan is activated, the
heatwheel is deactivated and the economizer
modulates to maintain the mixed air setpoint. If the unit
is equipped with heatwheel bypass dampers, these
are opened to accommodate the increase in outside
air volume.
Cooling mode: With the unit supply fan in operation
and a call for cooling is made, if the outdoor air
temperature and humidity are above the enthalpy
setpoint, the economizer moves to its minimum
position and mechanical cooling is activated. The
heatwheel is activated and the heatwheel exhaust fan
is started. If the unit is equipped with heatwheel
bypass dampers, these are closed.
Heating mode: Upon a call for heat, the heating
function is activated, the supply fan is activated and
the economizer moves to its minimum position. The
heatwheel is activated and the heatwheel exhaust fan
is started. If the unit is equipped with heatwheel
bypass dampers, these are closed.
Notice that in all four (4) basic above modes, the
operation of the heatwheel is determined by the
position of the economizer. With the exception of unit
shutdown or a night setback mode, the heatwheel
exhaust fan is in operation.
When control systems are "by others", all of the above
modes of operation must be considered.

As for Daniel's project, he is getting the short end of the stick on his heating savings when he switches to fixed setpoint and a shorter end when in float.
Maybe the weather file does not have enough high enthalpy days for the ERV to make a difference in cooling mode, (the enthalpy exchangers do make a big difference in cooling mode using a Miami weather file).

Joe Fleming E.I., LEED AP BD+C, BEMP

Fleming, Joe's picture
Joined: 2011-09-30
Reputation: 0

Hi Brian,

Your point about OA fraction and supply air setpoint is well taken, but I have also tried modelling this with 50% OA (25,000 cfm) in a single zone system at higher supply air setpoints of 95?F without seeing much of a return. Maybe my expectations have been set unreasonably high by EE4's crude method of cutting OA volumes to model ERV, but I still tend to think of OA heating as a fairly high fraction of the peak heating load in winter. (ERV peak heating is nearly double the building peak loads (LS-C report), but is still a fraction of the HVAC peak load (SS-D report, which is partly so high because it's at building warmup)).

Is there a clear way to separate out the outdoor air heating load in the SIM file?

Thank you everyone for your thoughtful and helpful replies!


Daniel Knapp, PhD, LEED? AP O+M

Daniel Knapp's picture
Joined: 2011-09-30
Reputation: 0

I agree that heat recovery makes a huge difference on the heating load
and have seen that in many models. Try changing your cooling setpoint
in the VAV model from "warmest" or "fixed" to "OA-Reset".

Regarding separating out outdoor air heating loads -- the quick,
imprecise method is to look at the difference between the LS-C and the
SS-D peak heating loads -- which you are already doing. The primary
difference between these two is ventilation. (but peaks could occur at
different times etc.)

The more precise method is to run your model with and without outdoor
air (which is a pain).

Note that EE4 only does the reduction of OA for zonal system types --
for other systems it is using heat recovery routines in DOE-2.1e.

(Still haven't had a peek at your model -- hopefully tonight.)

bfountain's picture
Joined: 2011-09-30
Reputation: 201


When modeling ERVs be sure to examine all of the input possibilities and
select the mode, operation, control, etc., that best meets your situation.
Never accept defaults at face value; they are often not the best choice.



cmg750's picture
Joined: 2010-10-05
Reputation: 0