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Makeup Air Temperature

S

Samxxu

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Mar 4, 2021
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4
Location
Phoenixville
508.1.1 Makeup Air Temperature

The temperature differential between makeup air and the air in the conditioned space shall not exceed 10°F (6°C) except where the added heating and cooling loads of the makeup air do not exceed the capacity of the HVAC system.

Hi, I'm looking for some help w/ this requirement. How can I prove that the existing a/c system is capable of handling the added commercial hood kitchen ventilation? We're installing a small hood 4'x8' in a little more than 2,000 sf convenient store of a gas station. So the exhaust is 3,200 cfm, and the make-up air is 2,800 cfm. Two 5-ton roof top a/c units is capable of handling this added load, but I'm having trouble of proving it. If anybody has the answer or good suggestion, please let me know. Thanks a lot.
 
Welcome

Mechanical engineer? Write some of that engineer talk

Air balance test???
 
Not an ME but do have some mech experience.
This could turn into a lot of work.

I would perform a complete heat loss and heat gain calcs for the building, that would include the additional ventilation loads to see if the new system will be able to provide the neccessary heating & cooling. If you have a copy of previous drawings/documents that would have included that information, then you can easily analyze the incremental change of the new make up air introduced.
 
You could likely reduce your make up air by installing a UL 710 compliant hood. The Mechanical Code defines the rating of the equipment and the International Energy Conservation Code, Table C403.7.5, (2018) gives maximum maximum exhaust flow rates. Most of the hoods I look at use way too much exhaust cfm. 3200 cfm on an 8' long hood seems excessive. Reducing the exhaust cfm could make your job easier. Ok, I don't know the equipment under the hood and I'm assuming your not installing an open flame, solid fuel cooking appliance under the hood.
 
Samxxu said:
2,000 sf convenient store of a gas station
Click to expand...
The door in the store will be opening and closing how many times per hour? How much make up air will be coming through that 18 sq ft opening every time it opens and closes?

508.1 Makeup air.
Makeup air shall be supplied during the operation of commercial kitchen exhaust systems that are provided for commercial cooking appliances. The amount of makeup air supplied to the building from all sources shall be approximately equal to the amount of exhaust air for all exhaust systems for the building. The makeup air shall not reduce the effectiveness of the exhaust system. Makeup air shall be provided by gravity or mechanical means or both. Mechanical makeup air systems shall be automatically controlled to start and operate simultaneously with the exhaust system.
 
Agree with mp25, you need to have someone do heat/cooling load calcs. Not sure about your location, but two 5 ton rooftop units wouldn't come close to handling either the cooling or heating load of adding 2800 CFM of untempered makeup air from the outside here.
 
jeffc said:
You could likely reduce your make up air by installing a UL 710 compliant hood. The Mechanical Code defines the rating of the equipment and the International Energy Conservation Code, Table C403.7.5, (2018) gives maximum maximum exhaust flow rates. Most of the hoods I look at use way too much exhaust cfm. 3200 cfm on an 8' long hood seems excessive. Reducing the exhaust cfm could make your job easier. Ok, I don't know the equipment under the hood and I'm assuming your not installing an open flame, solid fuel cooking appliance under the hood.
Click to expand...
Hey jeffc, I did a quick search of the UL710 hood and found they are limited in size... like 3' wide. The 3200 cfm was calculated 8'wide x 4' deep of the hood. We're installing two 21" deep gas fryers and a 36" electric pizza oven. I find the 3200 cfm exhaust do exceed the amount in the 2018 IECC Table C403.7.5. Then, I talked to the owner of the store w/ the same size hood of a different store we designed several years ago and was told the exhaust was good and no issue w/ over exhaust. I also talked the the contractor who installs commercial hood regularly and learned that there were no complains about oversizing issues for the hoods we designed he installed for many year. Some amount less 3200 cfm for the 8' hood is probably ok, but we're gonna change it for now. Thanks for your input and help me to learn more in this field.
 
mp25 said:
Not an ME but do have some mech experience.
This could turn into a lot of work.

I would perform a complete heat loss and heat gain calcs for the building, that would include the additional ventilation loads to see if the new system will be able to provide the neccessary heating & cooling. If you have a copy of previous drawings/documents that would have included that information, then you can easily analyze the incremental change of the new make up air introduced.
Click to expand...
Hi mp25, I've done some research and talked to colleagues in the same field and decided to make some small changes before the more draw-out calculations. The make-up air was designed at 87.5% of the exhaust, and I suspected this was the problem. So I'm increasing the make-up to 100% so the system is balanced. Since the exhaust and make-up kick in at the same time, there is minimum heat or cool air lost in the operation, I'm gonna say full calculation is not needed. I'm gonna to see if this is acceptable and let you know. Thanks for your reply.
 
Samxxu said:
Hey jeffc, I did a quick search of the UL710 hood and found they are limited in size... like 3' wide. The 3200 cfm was calculated 8'wide x 4' deep of the hood. We're installing two 21" deep gas fryers and a 36" electric pizza oven. I find the 3200 cfm exhaust do exceed the amount in the 2018 IECC Table C403.7.5. Then, I talked to the owner of the store w/ the same size hood of a different store we designed several years ago and was told the exhaust was good and no issue w/ over exhaust. I also talked the the contractor who installs commercial hood regularly and learned that there were no complains about oversizing issues for the hoods we designed he installed for many year. Some amount less 3200 cfm for the 8' hood is probably ok, but we're gonna change it for now. Thanks for your input and help me to learn more in this field.
Click to expand...
You might want to do a little more looking into UL710 listed hoods. Almost every hood I see installed is listed to UL710, and most of them are 4' deep canopy hoods. They typically allow airflow rates to be half or even less of the rates required by the IMC for unlisted hoods and usually meet the IECC requirements. Most new hoods on the market today are listed to UL710.
 
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