ERV Control

[AmanDeol]

Hello everyone!

I have concern about ERV operation schedule.

ERV supply fresh air to the fan coil units in the building.
ERV will keep on running until all the fan coil units are turn off.
Now the problem arises when all the fan coil units are off, ERV will be off and if this situation remains for several hours during the day, there will not be any fresh air circulation into the building.

If we circulate fresh air in the building if will call the fan coil units to turn on as the O.A temp is low and it will add the heating load in the winter and cooling load in summer.

Any Suggestion.. how it can be controlled to circulate fresh air in to the building when all the fan coil units are off. We need to circulate minimum cfm into the building.

 

[cry22]

And why do you want to that? i.e supply min CFM OA into the building.
your building is not occupied. (All FCu's are OFF). All OA dampers and fans should be OFF.

If your strategy is for occupied mode and the building load is satisfied (FCU's OFF during occupied mode) you should call for FCU fans to run continuously during occupied mode, You filter continuously and you provide ventilation at all occupied times.

You need to control temperature by thermostat modulating CHW/HW control valve instead of cycling the FCU fan ON and OFF. The fans remain ON during occupied mode.

 

[AmanDeol]

But minimum fresh air requirement cannot be met for occupied period by just circulating the air through the fan, since when fan coil units are off .. ERV is off.

Since building is going for LEED certification.. I dont think its a good idea to leave fan running all the time even if the load is met during occupied period.

 

[cry22]

No man, this is standard control practice in the industry to have the fan contnuously filtering the air and moving fresh air.
What makes you think that this violates LEED?
When FCU's are OFF, then you are in unoccupied mode, which makes sense to turn off your ERV then.

your only other choice if you do not feel comfortable with the suggestion, is to supply air directly into the space and not into the FCU, then again, you will require reheat at the ERV, which may get you into trouble for simultaneous cooling and heating in violation of 90.1

Another thing is: if you are in a humid climate, you will need to dehumidify your fresh air at the ERV, using ERV, you mean flat plate HX right? meaning no cooling at ERV of any kind. So if you were to supply the untreated humid air into the space, you may create some mold in your nuilding.

You need to sate your system with a little more details (i.e ERV with or w/o Cooling/heating coil, how large is your system, system choice description, etc.), and indicate your climate condition for folks to give you the right advice.

Keep it simple and stop speculating in your mind about will it or will not be accepted by LEED reviewers. Lay out your strategy and don't worry about others opinions, what if they think this, that? well, then you will give your reasons for this and that.

 

[ChasBean1]

AmanDeol, fan coils probably cycle on space temperature, correct? So we could theoretically be occupied but have no FCUs calling. I agree with your concern. Not just for ventilation but for building pressurization. FCUs can continue to cycle, but your ERV and building exhaust fans, etc. should cycle on a time schedule, not on the FCU poll.

Also agree with cry's reheat/dehumidification concern - hopefully the ERVs can also provide dehumidification and room neutral (65-70°F range) air delivery?

 

[AmanDeol]

Cry22 .. you mentioned to keep the fan running continously for occupied period of time. but how can it help in bring in the fresh air.. cuz the system is integrated such that all the exhaust from the fan coil units is ducted to ERV units( which is standard ERV with Wheel to extract energy from the incoming exhaust using a counter flow between exhaust and supply air.. in addition we have put aa duct heating coil in the supply duct to heat the incoming air to 55 F and then it goes to mixing box of individual fan coil unit .. where it mixes with return air to further heated by FCU to desired temp)

Now if all the load for the building is met during the occupied period the ERV will be turned off.. and its damper will be in closed position.. as it is aa two position damper.. Since damper is closed there will not be any intake of fresh air into the system ... even though the fan for the fan coil unit is running.. it will be just recirculating the room air.

Concern about the LEED is that the energy required to run the fan coil unit fans all the time during occupied period is consisderable and can effect the LEED EA credit ... which is targeted toward reducing the energy cost ..
LEED is my secondary concern .


Project : Canada 40 km North of toronto ..

TWo ERV units used for offices space in arena.. 2500 CFM each ..
Two floors .. 16 Fan coil units ..


CHasBean ...yes FCU cycle on space temperature requirement .. I think thats the only option we are left with to run the ERV on fixed schedule during occupied period .

Now I thinking about the two possible solutions..
1) ERV to follow fixed schedule during the occupied period ..and FCU to turn on as per the space temperature.. and to turn the FCU on only when it is required by the space demand... make the ERV schedule such that it can meet the minumum fresh air requirment as well as maintain building pressure...

2) To cycle ERV with the FCU's .. in addition set them on fixed cyle to turn on after certain period of time.. so that fresh air can be circulated in the building ..

Please suggest

 

[cry22]

so you are in the -40F weather and may be some 80F summer temperature. Do you really need a wheel?
No cooling at ERV from what you describe. is OA cooling load on FCU? I suppose it is.

An option would be to provide VAV box for each FCU and use VFD on ERV supply fan controlled with static pressure. Your system is too small to warrant such an extenvive control scheme.

You need fresh air at all times during occupied mode, you need exhaust at all times during occupied mode. makes sense to have ERV on a time schedule, regardless of FCU operation.

You can dump the fresh directly into the space, but then again, you will need to heat fresh air to 70F, not 55F because when your FCU's are OFF, you do not want to cool the space with ERV and then bring the FCU ON to keep 70F.

You have created a control problem by cycling the FCU fan, and you keep being stuborn about it, the control system will be cheap, simple, and reliable if you only allow FCU fan to run continuously. We are talking about 16 FCU of 1/2 HP each. May be savings of 2 hours of fan operation each per day? say 32-hours of 1/2 HP. Saving of what? $2.00 or $3.00 per day? is it really worth complicating your controls AND make it expensive and Unreliable for such small savings?

You cannot really model the hours the FCU is OFF for your LEED documentation any way, your energy model with show fan power continuous during the entire occupied time.

Incidently, I'd go with a flat plate for such a climate, not a wheel. Un-necessarily expensive, you'll add filter maintenance for the wheel on top of that, you have insignificant OA to eliminate and dehumidify to justify a wheel. Consider face and by-pass on your HX.

down teh mason dixon line we say use the "KISS system". No ofense of course.

 

[sroman]

I agree with cry22. The most imprtant thing to be considering is the min.CFM of OSA requirted by ASHRE.

 

[sroman]

In addition, depending on the design of your structure, (I am assuming it is a high rise) the fans should be running during occupied mode anyway. The positive pressure of <+1.5 in. is needed for comfort.