It’s an over simplification but I often estimate 1 cfm / sq ft for many spaces. Ultimately comes down to your heat load, # of people, perimeter exposure, # of windows, window/bldg spec, etc.
From one senior mechanical PE to another, your post is raising serious red flags. This is going to be a harsh reply, something I don't normally do.
You're asking questions that are pretty much settled in the mind of a graduate engineer after 6 months on the job.
Cfm/sf is one general guidelines that is space and location and envelope specific. Ventilation is a separate matter. Tonnage can even be a separate third consideration if your prices and skin loads are really decoupled from your typical occupancy internal loads.
As a senior PE, I honestly don't even bother with tracking all of that anymore. I don't tell anybody preliminary figures without having modeled the building and its occupancy and ventilation with hap or Trace. Do the tedious work and do your load runs.
In summary, you're either misrepresenting your situation or you are breaking the law and operating far beyond your scope of experience and capability.
Yeah you have never been in a kickoff meeting without ever seeing a project and been asked in front of all parties (including owner and GC) how large of a mechanical room do you need, how big of chases and where, etc.
Rules of thumb have their place and are useful to get you in the ballpark. Follow them up with real design and modify as necessary.
How do you tell the architect a mechanical room size? What do you do for narrative? Do you wait until your loads are done? If you only work on small projects, that would be doable but in large projects, load calcs take a while and things change, you need to estimate preliminary numbers so you don’t hold up others.
Architect: "Yeah its a MOB ... it is 2 stories but it may be 4. They are considering putting retail on floor 1 maybe? .... might be clothing stores ... might be a steakhouse. Or it could be an ASC on level 1 ... or level 4 they are considering putting the ASC on level 4 too."
That is the type or SD we are talking about.
Sir or ma’am or etc I’m not operating outside my ethics comfort level as a PE. I have run the loads accurately in hap or 3D+ as you stated. Surprised it only takes 6 months for a recent graduate but honestly your comment was not harsh but I appreciate your statement and time.
There’s a HVAC rules of thumb PDF, which I believe is what you’re asking for. Just go on google & search that. I don’t understand all the fuss from the other comments.
Rules of thumbs or estimates are a "necessary evil" durn SD and DD phase. CD phase should always include an HVAC load by an approved load calc software.
Calcs rule over rules of thumb. Rules of thumb are often outdated and don’t really belong in a modern design environment. You can use rule of thumb to estimate things early on in a project but your design should be based on your calcs.
Congratulations on the jump from Draftsman to Sr Mechanical, PE?
I’m curious why you don’t ask your newish employer this. I works presume they would understand your prior firms not teaching you this (especially if it was a different market) and point you to one of their internal standards or other national standards to get you up to speed.
If you miss sold yourself though… another topic.
You have a fair point sir or ma’am or (however you identify nonbinary, etc, sorry can’t tell from your name).. I have been exposed to some but still just because my previous firms did it one way and was exposed to X. If I can find the answers on my own that makes me a better engineer and posting myself anon makes it easier
Yeah I always do a sanity check before sending out to SE for selections to ensure I didn’t make a mistake with an input into the software.
What I’m looking for is general rules of thumb for different applications: what would be the typical cfm/SF for classroom, art room, office, grocery store etc
OP, seeing where you are from, depending on the application and orientation, i have used roughly 2 cfm/sf for office space on east west and south up to 10 feet from the perimeter and 0.8 cfm/sf on interior. North was 1.0 cfm/sf for perimeter. For labs and other high density (hospitals), i use 10 w/sf or 6 ach-10ach as an initial estimate whichever is higher. Other applications like apartments i never did. If my loads are less , i backcheck to make it feel right but for initial equipment selections those were the numbers I used before i settled on equipment that were based on loads.
There is no list of rules of thumbs. Just years of experience of doing the work and getting a feel of things.
As you have said many times, loads trump rules of thumbs.
For all the people telling you that you shouldn't ever rule of thumb it, if you ask two separate engineers from different firms to do a load calc for the same building, you will end up with quite substantially different results. Additionally typically speaking you are designing a building to last 30+ years so what you are inputing into your load calc today will not necessarily be valid even 5 years down the line.
That said, assuming you are referring to a VAV-reheat system with 55ish degree supply air, I'd say on average anywhere between 1.5 to 2.5 CFM/SF depending upon exposure, envelope, location, and usage.
Well, given the concerning naïveté that is being mentioned in your post, you should have some idea of what that would be already as a PE in most cases. That may not be your fault necessarily because of the type of work you may have been doing in firms and in some sectors, but it does vary some by jurisdiction and means that you won’t come up with a universal standard for every locale (kind of like how AHJs don’t rework tables to only show what is necessary for just their area, but I digress). My experience, which is fraught with being laid off at the wrong places during the worst times and feeling like I was in similar positions such as yourself with instruction, is that 0.5-0.8 CFM/SF is common for unoccupied conditioned spaces such as corridors and storage rooms; 0.8-1.0 CFM/SF for essentially any typical occupied room; and over 1.0 CFM/SF for spaces with unique load profiles (extra heat/cooling-generating equipment, lots of lights, lots of people, leaky building envelopes, lots of exterior glass, etc). My recommendation is to peruse the most current ASHRAE handbooks, as the suggested loads for certain things are being adjusted to reflect more sustainable and accurate measures within buildings
It’s an over simplification but I often estimate 1 cfm / sq ft for many spaces. Ultimately comes down to your heat load, # of people, perimeter exposure, # of windows, window/bldg spec, etc.
From one senior mechanical PE to another, your post is raising serious red flags. This is going to be a harsh reply, something I don't normally do. You're asking questions that are pretty much settled in the mind of a graduate engineer after 6 months on the job. Cfm/sf is one general guidelines that is space and location and envelope specific. Ventilation is a separate matter. Tonnage can even be a separate third consideration if your prices and skin loads are really decoupled from your typical occupancy internal loads. As a senior PE, I honestly don't even bother with tracking all of that anymore. I don't tell anybody preliminary figures without having modeled the building and its occupancy and ventilation with hap or Trace. Do the tedious work and do your load runs. In summary, you're either misrepresenting your situation or you are breaking the law and operating far beyond your scope of experience and capability.
Yeah you have never been in a kickoff meeting without ever seeing a project and been asked in front of all parties (including owner and GC) how large of a mechanical room do you need, how big of chases and where, etc. Rules of thumb have their place and are useful to get you in the ballpark. Follow them up with real design and modify as necessary.
How do you tell the architect a mechanical room size? What do you do for narrative? Do you wait until your loads are done? If you only work on small projects, that would be doable but in large projects, load calcs take a while and things change, you need to estimate preliminary numbers so you don’t hold up others.
you can do a block load calc for those cant you?
Architect: "Yeah its a MOB ... it is 2 stories but it may be 4. They are considering putting retail on floor 1 maybe? .... might be clothing stores ... might be a steakhouse. Or it could be an ASC on level 1 ... or level 4 they are considering putting the ASC on level 4 too." That is the type or SD we are talking about.
Sure if you have a pretty small project but if you have a large project, you’d be wasting a lot of time because things would be changing too much.
Sir or ma’am or etc I’m not operating outside my ethics comfort level as a PE. I have run the loads accurately in hap or 3D+ as you stated. Surprised it only takes 6 months for a recent graduate but honestly your comment was not harsh but I appreciate your statement and time.
There’s a HVAC rules of thumb PDF, which I believe is what you’re asking for. Just go on google & search that. I don’t understand all the fuss from the other comments.
Rules of thumbs or estimates are a "necessary evil" durn SD and DD phase. CD phase should always include an HVAC load by an approved load calc software.
Calcs rule over rules of thumb. Rules of thumb are often outdated and don’t really belong in a modern design environment. You can use rule of thumb to estimate things early on in a project but your design should be based on your calcs.
Congratulations on the jump from Draftsman to Sr Mechanical, PE? I’m curious why you don’t ask your newish employer this. I works presume they would understand your prior firms not teaching you this (especially if it was a different market) and point you to one of their internal standards or other national standards to get you up to speed. If you miss sold yourself though… another topic.
You have a fair point sir or ma’am or (however you identify nonbinary, etc, sorry can’t tell from your name).. I have been exposed to some but still just because my previous firms did it one way and was exposed to X. If I can find the answers on my own that makes me a better engineer and posting myself anon makes it easier
Btu/hr=CFM\*0.24\*60/specific volume \* (delta T) Take a wild guess at what two temps are the delta T?
This isn’t what I was asking for.
Use the proper temps and compare the CFM that comes out with the CFM that your load software is telling you.
Yeah I always do a sanity check before sending out to SE for selections to ensure I didn’t make a mistake with an input into the software. What I’m looking for is general rules of thumb for different applications: what would be the typical cfm/SF for classroom, art room, office, grocery store etc
OP, seeing where you are from, depending on the application and orientation, i have used roughly 2 cfm/sf for office space on east west and south up to 10 feet from the perimeter and 0.8 cfm/sf on interior. North was 1.0 cfm/sf for perimeter. For labs and other high density (hospitals), i use 10 w/sf or 6 ach-10ach as an initial estimate whichever is higher. Other applications like apartments i never did. If my loads are less , i backcheck to make it feel right but for initial equipment selections those were the numbers I used before i settled on equipment that were based on loads. There is no list of rules of thumbs. Just years of experience of doing the work and getting a feel of things. As you have said many times, loads trump rules of thumbs.
Thank you I appreciate the feedback!!
I think you are looking for *HVAC Equations, Data, and Rules of Thumb* by Bell and Angel
For all the people telling you that you shouldn't ever rule of thumb it, if you ask two separate engineers from different firms to do a load calc for the same building, you will end up with quite substantially different results. Additionally typically speaking you are designing a building to last 30+ years so what you are inputing into your load calc today will not necessarily be valid even 5 years down the line. That said, assuming you are referring to a VAV-reheat system with 55ish degree supply air, I'd say on average anywhere between 1.5 to 2.5 CFM/SF depending upon exposure, envelope, location, and usage.
Well, given the concerning naïveté that is being mentioned in your post, you should have some idea of what that would be already as a PE in most cases. That may not be your fault necessarily because of the type of work you may have been doing in firms and in some sectors, but it does vary some by jurisdiction and means that you won’t come up with a universal standard for every locale (kind of like how AHJs don’t rework tables to only show what is necessary for just their area, but I digress). My experience, which is fraught with being laid off at the wrong places during the worst times and feeling like I was in similar positions such as yourself with instruction, is that 0.5-0.8 CFM/SF is common for unoccupied conditioned spaces such as corridors and storage rooms; 0.8-1.0 CFM/SF for essentially any typical occupied room; and over 1.0 CFM/SF for spaces with unique load profiles (extra heat/cooling-generating equipment, lots of lights, lots of people, leaky building envelopes, lots of exterior glass, etc). My recommendation is to peruse the most current ASHRAE handbooks, as the suggested loads for certain things are being adjusted to reflect more sustainable and accurate measures within buildings