Fresh air for residential building.

[fineartsnyc]

Hi all, I’m working on HVAC system for luxury residential building.
Unit #1 two floors. Four bathrooms 75 cfm exhaust each. Total exhaust is 300 cfm.
Unit# 2 one floor. Two bathrooms 75 cfm exhaust each. Total exhaust is 150 cfm
Unit#3 one floor. Two bathrooms 75 cfm exhaust each. Total exhaust is 150 cfm
Unit#4 two floors. Four bathroom s 75 cfm exhaust each. Total exhaust is 300 cfm.

The exhaust fan will be controlled by local bathroom light switch.

I’m planning to bring a fresh air to each air handler unit equal to the amount exhausted from the toilet.
Thru a common duct to the roof with a gooseneck, each air handler fresh air takes-off from the main fresh air duct.

My Question is.
How do I treat the outside air to protect the coil from freezing? Should I install electric duct heater slip in type with a booster fan?
If so, how the control would work. Please advice. Thanks!!

 

[317069]

Leave toilet exhaust alone, it doesn't work all the time.
You can use an HRV for each air handler

 

[sprinkler1000]

It depends on the system you are installing and the minimum outside air temperature expected.

You may have not only a coil freezing problem but a heating load higher than the unit can handle.

If your option is a VRF system they have indoor units to treat outside air or even Heat Recovery Units to exchange heat with exhaust.

 

[fineartsnyc]

in fact I'm using VRF system (Mitsubishi system) . 95 DB, 74 WB summer condition and 15 F winter Temp.
20 cfm/person X 5 = 100 cfm min. outside air.

 

[fineartsnyc]

since the toilet exhaust fan is not continuously we cant use ERV. Also its only 100 cfm O/A required.

 

[sprinkler1000]

Check with Mitsubishi. I am more familiar with Daikin, but they should also have a dedicated outside air unit.

The 15°F may be a little low, but it will work most of the year.

 

[fineartsnyc]

i don't like the idea of using a dedicated outside air unit for just 100 cfm. is there any other solution?

 

[sprinkler1000]

The idea was that you supply the total amount of fresh air (900 cfm) with the outside air unit.

I thought you had a common duct.

 

[fineartsnyc]

Thanks. No we can't use a common hvac unit however the ductwork can be shared. We have to use a separate unit since the unit owner has to pay the electric bills and maintenance.

 

[Waramanga]

what is your mixed air temp onto the coil? Give the mitsubishi engineers your parameters and see what they come back with, your luxury units are not likely to have small FCUs so I think you will be OK. Otherwise it is heat recovery, there are quite a few small ones avaiable now.

 

[fineartsnyc]

Thanks Waramanga, since the toilet exhaust fan is not continuously we cant use ERV. Also the 100% outside air unit from Mitsubishi is 1200 cfm, therefore I cant use it.

I'm thinking of using a small electric duct heater with inline fan to maintain the duct heater velocity in each fresh air take-off to the air handler. (The building will share a common fresh air duct work but the electric coil and fan has to be operated, maintained and serviced by the apartment owner from the take-off)

 

[ProcessHVAC]

"I’m planning to bring a fresh air to each air handler unit equal to the amount exhausted from the toilet" this is 900 CFM not 100 CFM OA as you said after, very confusing...

 

[chicopee]

If this residential structure is a condominium with annual maintenance fees, I would have a duct or a chase, dedicated for all the toilets, with a roof mounted exhaust fan whose cost for replacement and electricity would be included in the annual fees.

 

[fineartsnyc]

Initially the design was to equal the fresh air intake and toilet exhaust, since the toilet exhaust fan energize by light switch therefore the fresh air reduced to 160 cfm FAI to each duplex condominium and 100 cfm FAI for the single unit.

yes the owner concern about the condominium maintenance fees. each unit will have own exhaust system and fresh air intake.

I came up with an idea to use a small inline fan/electric duct heater with temperature sensor for each unit.

Any comments?