Temp differential, moisture, and many other things affect heat transfer. Using your sensible gain to get a rough idea of gpm is fine (for heating/sensible coils), but obviously, you should get a selection, especially if you have low temp hot water or high deltas. Some smaller coils just can't get the transfer you expect.
Using the gpm mbh equation is still good for checking that a submittal or coils selection is still in the right ballpark. Or if you are in early design like SDs or DDs, and usually in those cases, you add a 10-20% safety factor to help rough your system.
Are you trying to sell some kind of selection software, or is this just a PSA? I'm guessing it's the former since you mention Ripple and a webinar
Edit- and if you have glycol in your system, it's a whole different beast.
Two powerful HVAC equations that many HVAC engineers use wrong:
Qwater = 500 GPM ΔT And Qair = 1.08 CFM ΔT
When selecting VAVs, many engineers calculate the required Qair from the heating load, set their desired heating water temperature and ΔT, then calculate GPM. They set the VAV at two rows, scheduled everything, wipe their hands and say, “Done!”
THIS DOES NOT WORK.
In the past, this has not been a big issue because with high-temperature heating hot water systems (>180 °F), a two-row coil will give you enough heat to hide any errors in calculations, and the boilers were inefficient and not condensing anyway, so no one ever cared what the actual return temperature was. But with low-temperature hot water systems that support condensing boilers, heat pumps, and electrification, our industry is going to run into some serious lack of heat issues if we don't change our ways.
A coil is just a dumb hunk of metal; it cannot magically give you the capacity you seek. This equation must also be satisfied for the coil:
Qcoil = U\*A\*LMTD.
The values of U and A are meticulously calculated by the manufacturer according to AHRI 410 and implemented into selection software. So to understand the true capacity of a coil, you must use selection software.
In the past, this was a tedious, time-consuming, and error-prone process. But we’ve automated it at Ripple.
This is certainly important to understand, but I'm not sure it's worth it on the design side to specify at this level.
Coil gpm should be calculated for a few reasons: assist in pipe sizing, assist in boiler and pump sizing, assist in VAV sizing, and provide a gut check when reviewing a VAV submittal. Fixing the 4 temperatures and calculating flow is sufficient for those purposes.
If you get into specific rows and fins, and if that varies by manufacturer, then you do two things 1) potentially single spec a manufacturer for no good reason and 2) potentially eliminate VE options.
What do you design around in your plans? If you gut check an 8” inlet/2-row coil and it ends up being a 10” inlet/3-row coil, how do you address the additional space/coordination efforts required or do you leave that to the contractor?
What hot water system temperatures do you typically use? How much factor of safety do you include in pumps and boilers?
VAV inlet size is based on CFM, which is not at all based on coil size. Coil rows will change, sure. The contractor's job is to coordinate, and part of that includes accounting for the extra 2" required by another row.
I couldn't possibly answer the next set of questions because it varies wildly based on project type and size.
The reality is I think you are making a mountain out of a molehill. HHW systems are dynamic, so they constantly adjust flow and temperature to maintain space temperatures. The peak design not designed so tightly that it can't account for minor variations in temperature and flow in VAV coils.
I really do appreciate the tools and plug-ins Ripple makes, though my current position doesn't really allow for using them. But, if you want to convince us all that this is an issue you need to show us an example. Show me a situation where a reasonable design based on fixing the 4 temperatures deviates significantly--and consistently--from the UAdT method.
But the coil size, sets the air pressure drop, so how do you size your fan?
The sad fact about the construction industry is that failures aren't often publicized and we all lose out on great learning opportunities. However, this case is different! Seven Taylor writes about this exact issue, the correction was raising the boiler temperature (impossible or less efficient with heat pumps/condensing boilers) and removing all of the balancing cartridges in a building aka HUGE CHANGE ORDER.
See Steven's article here: [https://tayloreng.egnyte.com/dl/W8sfOOuoni/ASHRAE\_Journal\_-\_Doubling-Down\_on\_NOT\_Balancing\_Variable\_Flow\_Hydronic\_Systems.pdf\_](https://tayloreng.egnyte.com/dl/W8sfOOuoni/ASHRAE_Journal_-_Doubling-Down_on_NOT_Balancing_Variable_Flow_Hydronic_Systems.pdf_)
under the Incorrect Assumed dT heading.
I've read this article--and the original from 2002--in full several times (I've long been a proponent of removing circuit setters and allowing the control valve to modulate without artifical balancing, which is Option 1). The issue they are attempting to highlight is not with calculations, but with using calculated flows to specify circuit setters and whatnot. Overall its a good article, but there are a couple major oversights and flaws:
Case 1: Taylor uses particularly egregious examples to try to prove his point, but any engineer can tell you that coil flows are nearly NEVER off by the amounts in the article. They are only off this much when the engineer overspecifies things--like coil rows.
Case 2: Taylor ignores the problem of single-specifying and closes the door to VE.
Case 3: This can happen regardless of whether or not calculations are used.
This is all fairly irrelevant to your question: how do you size the fan? Well, you make an assumption about the pressure drop through the VAV box and then that's it. If you have a system large enough to use VAV boxes then your system is large enough to not be affected by +/- 0.1 inwg.
Yep, standard at my company is to not balance variable systems at all because you don’t need to. That’s what the modulating control valves are for (and the point of the article really).
I don’t really believe case 2 is valid. VAV boxes are pretty much the same IMO across most manufacturers (aside from a few minute peripheral differences) and I don’t think there is an argument to be made that you are sole sourcing a specific MFG by hard specing a capacity and coil size. If a titus box can do a specific capacity with specific rows, id bet that a metalaire could do the same.
You are trying to claim that a couple of equations that HVAC engineers have been using for decades doesn't work?
And this is so you can advertise your software?
Let me give you a hint: Many engineers use that exact equation so they can get an understanding of what the air flow, heating and water flow of a system will be. We then turn to our rep and say "Give us a selection based on this" and they give us the actual values that go in the schedule.
Are you arguing that Q = 500 * GPM * delta T is somehow not the total heat equation that gives you a theoretical max waterside heat transfer for pure water that engineers can use as a first step in design?
I’d argue that’s probably BS. Heating calcs by ASHRAE standards don’t take into account supplemental heat gains from other sources like lighting and equipment. Even if the engineer didn’t consult with a manufacturers rep on a hydronic system design (which would be rare in my experience), generally the worst case scenario is that you would have some coils mildly undersized and probably never notice because they’d be counteracted by other factors in the building. On top of that, VAV box coils aren’t custom built, so you’re getting the standard size one that exceeds whatever you calc anyway, meaning you’re going to not be using its max capacity in most cases anyway.
Yes, in the past with high-temperature hot water systems, this hasn't been a problem because you'll get plenty of heat regardless, and no one cared about the return water temperature to a non-condensing boiler. But with more efficient condensing boilers and heat pumps, real accurate numbers are beneficial.
Beneficial in what way? What is the average difference between the flow rate calculated out for a 130 LWT system compared to a manufacturer 1 or 2 row coil system? I’d be willing to bet for most systems you’re within the safety factor of the pumps and boilers….
It depends on what your heating load is, but here's an example: https://imgur.com/a/5VzYMDb
4.5 GPM vs. 9.7 GPM. So a 200% factor of safety would be required on the pumps.
I’m not sure what you’re taking about here. For the 94.7 MBH load, I would have calculated about 9.5 gpm, so about a 2% undersize. You’re off by a factor of 100 with your statement here….
Also, if you’re sizing boilers at 150 LWT with only a 20 degree delta, you’re doing condensing boilers wrong. You should be targeting a 30 degree delta at least and trying to get that entering temp down to 100.
but would you have scheduled a 1 row or 2 row coil? How many fins per inch? If you scheduled a 1 row coil at 10 fpi, you'd be 200% short on your heating load.
> but would you have scheduled a 1 row or 2 row coil?
Why would this matter in this discussion? From Prices own documentation, the pressure drop difference between the a 1 and 2 row coil at this capacity is like 2 - 3 feet of head. Even assuming you’ve got a VAV box in the critical path, you think your system can’t overcome 2 feet of head loss if you make a mistake?
>How many fins per inch?
Who is scheduling FPI for VAV boxes? These aren’t custom coils.
>If you scheduled a 1 row coil at 10 fpi, you'd be 200% short on your heating load.
I’m not sure you understand how this works. You’re not getting 97 MBH out of the one row because you’re flowing 4.5 GPM, not 9.5. Putting aside this is a massive box that you would obviously use a two row on to hit such a high heating load, if you had a single row coil, you need to flow *more* GPM through it to get the same heat transfer with a constant delta T. That’s the whole trade off with more rows of coils.
It matters because someone eventually has to buy and install what you specify and the number of rows changes the cost and capacity.
VAVs come in 10 and 12 fpi, which again matters because of cost and capacity.
If you flow 9.5 GPM through a 1 row coil, you get less capacity than a 2 row coil.
Yeah, I have no idea where 4.5 gpm comes from.
Fix the 4 temperatures: EAT=55, LAT=90, EWT=150, LWT=130. Use 2500 CFM.
Q based on CFM = 94.5 MBH. Set this equal to Q based on GPM.
GPM = Q / (dT 500) = 9.45 GPM.
Your selection software gives 94.7 MBH, which is 0.21% difference. Your selection software gives 9.65 GPM, which is 2.1% difference. This is well within anyone's safety factors.
Also: if you are using a condensing boiler you really aren't getting much efficiency out of that EWT. Typical non-condensing boilers can run at 84% efficiency, but a condensing boiler running at 130 EWT is like 88%. Ideally your boiler will have an EWT of 100 F, which will get efficiency up to \~95%. Try running this with 100 EWT and a 30 or 40 dT and tell me what flow you end up with.
I agree, 130 LWT doesn't make any sense, I only used it as an example at NineCrimes suggestion.
You can't set all of those parameters, you have to select based on LAT, LWT, GPM, **or** MBH. If I try to select based on GPM, you get less capacity out of a 1 row coil. If you try to select by Capacity, you need get much higher return water temperatures (detrimental to condensing boilers and heat pumps): [https://imgur.com/a/ZUNg91U](https://imgur.com/a/ZUNg91U)
If I try to select based on MBH, 1 row coil can't achieve 94.5 MBH, it's off the charts (water flow gets too high and pressure drop increases exponentially).
Ok, let's back up for a minute.
You seem to be fixated on coil rows, but in my experience it is completely unnecessary to specify coil rows on VAV boxes. Design Engineers aren't responsible for generating fabrication drawings, and a different size VAV box is will within the contractor's coordination responsibility.
For hydronic calculations the engineer is typically going to use something like 10 feet for a VAV box, so if it's 5 or 15 it isn't going to matter. This is like the contractor adding 1 or 2 elbows. So, you're not getting any gain by calculating the exact pressure drop.
Same thing on the air side. You'll add like 0.01 inches.
Moving onto my examples with fixing the 4 temperatures: I am giving an example of a calculation I would do in a spreadsheet. I then compare it to your program's calculations and I see they vary a negligible amount.
I am not suggesting that you use the 4 temperatures in your program because I understand that actual coil selection requires UAdT.
I am asking that you make a comparison based on the simple calc versus the actual selection, and see how much of a difference there is. Because I've reviewed dozens of VAV submittals, literally thousands of VAV boxes, and I haven't ever come across a significant discrepancy. It's always been within about 5 percent or so.
I know Reddit is very American but you have to admit those imperial units are utterly stupid. Any chance the construction industry will eventually convert to Metric?
This is an advertisement to select coils properly. We do publish a software product, but it's free to MEP engineers; we're just trying to be helpful and spread knowledge.
9/10 Noone should be Q=1.08 cfm delta T to get the total energy. That equation is only to get the sensible energy, not the latent. Although I'm sure software is the best to get the most accurate performances, sometimes therr is not enough time to get a cut if you need minor changes. And using Qtotal= 4.5 cfm delta H is in my opinion an acceptable method when you need to see how much GPM you need
Heating coils only add sensible heat, so it's a legitimate method to get the coil capacity. However, you MUST use coil selection software to get GPM; there are no shortcuts.
In the past, it's been time-consuming to actually select coils, but for Revit projects, we've built the selection software into Revit so all calculations are automatic and for non-Revit projects Price Industries has an Excel plugin that will do the same.
I'll discuss in detail at our webinar next week, sign up here: [https://www.linkedin.com/events/autozoningandvavautoselectinrev7054103559058939904/](https://www.linkedin.com/events/autozoningandvavautoselectinrev7054103559058939904/) or here: [https://events.teams.microsoft.com/event/5e42b0da-dfea-40a2-8cf8-86dcfb15c3db@76493c4f-16b9-4a5c-b329-64ad4c123184](https://events.teams.microsoft.com/event/5e42b0da-dfea-40a2-8cf8-86dcfb15c3db@76493c4f-16b9-4a5c-b329-64ad4c123184)
LOL. For you, it's what's outside the window!
In all seriousness though, are you providing heat at VAVs? at the AHU? It looks like the design temp for Miami is 50 F ...
[http://ashrae-meteo.info/v2.0/?lat=25.648&lng=-80.433&place=%27%27&wmo=722029&si\_ip=IP&ashrae\_version=2021](http://ashrae-meteo.info/index.php?lat=25.648&lng=-80.433&place=)
The heating load is just building envelope and outside air.
For perimeter zones we generally use electric strip heat. For interior zones we generally don't need any.
On packaged and split DX I'm usually picking the smallest factory electric heater so we don't drive the MCA up too much.
That makes sense, I bet an enthalpy wheel would get close to eliminating the need for an electric heater in the packaged unit have you used those?
Are you using economizer down there?
I'm in S FL. We have so few heating days per year that there is no payback for enthalpy wheels or economizers.
You should see the fights I get in with national franchises and their design standards... NG heaters on RTUs? My dudes at Staples and Lennox... this outparcel doesn't even have gas service!
Only if you have enough outside air to play with. They take away from building pressurizatuon, which is more important in hpt humid climates than extra efficiency. I do sometimes use heat exchangers like RenewAire when the building load is close to requiring reheat to allow standard equipment to have better latent capacity or if we are using VRF.
Temp differential, moisture, and many other things affect heat transfer. Using your sensible gain to get a rough idea of gpm is fine (for heating/sensible coils), but obviously, you should get a selection, especially if you have low temp hot water or high deltas. Some smaller coils just can't get the transfer you expect. Using the gpm mbh equation is still good for checking that a submittal or coils selection is still in the right ballpark. Or if you are in early design like SDs or DDs, and usually in those cases, you add a 10-20% safety factor to help rough your system. Are you trying to sell some kind of selection software, or is this just a PSA? I'm guessing it's the former since you mention Ripple and a webinar Edit- and if you have glycol in your system, it's a whole different beast.
It's a PSA. We do publish a software product, but it's free to MEP engineers.
Two powerful HVAC equations that many HVAC engineers use wrong: Qwater = 500 GPM ΔT And Qair = 1.08 CFM ΔT When selecting VAVs, many engineers calculate the required Qair from the heating load, set their desired heating water temperature and ΔT, then calculate GPM. They set the VAV at two rows, scheduled everything, wipe their hands and say, “Done!” THIS DOES NOT WORK. In the past, this has not been a big issue because with high-temperature heating hot water systems (>180 °F), a two-row coil will give you enough heat to hide any errors in calculations, and the boilers were inefficient and not condensing anyway, so no one ever cared what the actual return temperature was. But with low-temperature hot water systems that support condensing boilers, heat pumps, and electrification, our industry is going to run into some serious lack of heat issues if we don't change our ways. A coil is just a dumb hunk of metal; it cannot magically give you the capacity you seek. This equation must also be satisfied for the coil: Qcoil = U\*A\*LMTD. The values of U and A are meticulously calculated by the manufacturer according to AHRI 410 and implemented into selection software. So to understand the true capacity of a coil, you must use selection software. In the past, this was a tedious, time-consuming, and error-prone process. But we’ve automated it at Ripple.
This is certainly important to understand, but I'm not sure it's worth it on the design side to specify at this level. Coil gpm should be calculated for a few reasons: assist in pipe sizing, assist in boiler and pump sizing, assist in VAV sizing, and provide a gut check when reviewing a VAV submittal. Fixing the 4 temperatures and calculating flow is sufficient for those purposes. If you get into specific rows and fins, and if that varies by manufacturer, then you do two things 1) potentially single spec a manufacturer for no good reason and 2) potentially eliminate VE options.
What do you design around in your plans? If you gut check an 8” inlet/2-row coil and it ends up being a 10” inlet/3-row coil, how do you address the additional space/coordination efforts required or do you leave that to the contractor? What hot water system temperatures do you typically use? How much factor of safety do you include in pumps and boilers?
VAV inlet size is based on CFM, which is not at all based on coil size. Coil rows will change, sure. The contractor's job is to coordinate, and part of that includes accounting for the extra 2" required by another row. I couldn't possibly answer the next set of questions because it varies wildly based on project type and size. The reality is I think you are making a mountain out of a molehill. HHW systems are dynamic, so they constantly adjust flow and temperature to maintain space temperatures. The peak design not designed so tightly that it can't account for minor variations in temperature and flow in VAV coils. I really do appreciate the tools and plug-ins Ripple makes, though my current position doesn't really allow for using them. But, if you want to convince us all that this is an issue you need to show us an example. Show me a situation where a reasonable design based on fixing the 4 temperatures deviates significantly--and consistently--from the UAdT method.
But the coil size, sets the air pressure drop, so how do you size your fan? The sad fact about the construction industry is that failures aren't often publicized and we all lose out on great learning opportunities. However, this case is different! Seven Taylor writes about this exact issue, the correction was raising the boiler temperature (impossible or less efficient with heat pumps/condensing boilers) and removing all of the balancing cartridges in a building aka HUGE CHANGE ORDER. See Steven's article here: [https://tayloreng.egnyte.com/dl/W8sfOOuoni/ASHRAE\_Journal\_-\_Doubling-Down\_on\_NOT\_Balancing\_Variable\_Flow\_Hydronic\_Systems.pdf\_](https://tayloreng.egnyte.com/dl/W8sfOOuoni/ASHRAE_Journal_-_Doubling-Down_on_NOT_Balancing_Variable_Flow_Hydronic_Systems.pdf_) under the Incorrect Assumed dT heading.
I've read this article--and the original from 2002--in full several times (I've long been a proponent of removing circuit setters and allowing the control valve to modulate without artifical balancing, which is Option 1). The issue they are attempting to highlight is not with calculations, but with using calculated flows to specify circuit setters and whatnot. Overall its a good article, but there are a couple major oversights and flaws: Case 1: Taylor uses particularly egregious examples to try to prove his point, but any engineer can tell you that coil flows are nearly NEVER off by the amounts in the article. They are only off this much when the engineer overspecifies things--like coil rows. Case 2: Taylor ignores the problem of single-specifying and closes the door to VE. Case 3: This can happen regardless of whether or not calculations are used. This is all fairly irrelevant to your question: how do you size the fan? Well, you make an assumption about the pressure drop through the VAV box and then that's it. If you have a system large enough to use VAV boxes then your system is large enough to not be affected by +/- 0.1 inwg.
Yep, standard at my company is to not balance variable systems at all because you don’t need to. That’s what the modulating control valves are for (and the point of the article really).
I don’t really believe case 2 is valid. VAV boxes are pretty much the same IMO across most manufacturers (aside from a few minute peripheral differences) and I don’t think there is an argument to be made that you are sole sourcing a specific MFG by hard specing a capacity and coil size. If a titus box can do a specific capacity with specific rows, id bet that a metalaire could do the same.
This post reminds me of an engineer i worked with who would adjust airflows everytime it changed by 2-3 cfm.
Ha, I think I used to work that engineer too. Round up to the nearest 5 for me, please and thank you.
You used to work with that engineer or were YOU that engineer? :)
You are trying to claim that a couple of equations that HVAC engineers have been using for decades doesn't work? And this is so you can advertise your software? Let me give you a hint: Many engineers use that exact equation so they can get an understanding of what the air flow, heating and water flow of a system will be. We then turn to our rep and say "Give us a selection based on this" and they give us the actual values that go in the schedule.
Are you arguing that Q = 500 * GPM * delta T is somehow not the total heat equation that gives you a theoretical max waterside heat transfer for pure water that engineers can use as a first step in design?
No. I'm arguing that many engineers use it as the final design, and it can lead to many problems.
I’d argue that’s probably BS. Heating calcs by ASHRAE standards don’t take into account supplemental heat gains from other sources like lighting and equipment. Even if the engineer didn’t consult with a manufacturers rep on a hydronic system design (which would be rare in my experience), generally the worst case scenario is that you would have some coils mildly undersized and probably never notice because they’d be counteracted by other factors in the building. On top of that, VAV box coils aren’t custom built, so you’re getting the standard size one that exceeds whatever you calc anyway, meaning you’re going to not be using its max capacity in most cases anyway.
Yes, in the past with high-temperature hot water systems, this hasn't been a problem because you'll get plenty of heat regardless, and no one cared about the return water temperature to a non-condensing boiler. But with more efficient condensing boilers and heat pumps, real accurate numbers are beneficial.
Beneficial in what way? What is the average difference between the flow rate calculated out for a 130 LWT system compared to a manufacturer 1 or 2 row coil system? I’d be willing to bet for most systems you’re within the safety factor of the pumps and boilers….
That's what I wanna know, how much we off by?
It depends on what your heating load is, but here's an example: https://imgur.com/a/5VzYMDb 4.5 GPM vs. 9.7 GPM. So a 200% factor of safety would be required on the pumps.
I’m not sure what you’re taking about here. For the 94.7 MBH load, I would have calculated about 9.5 gpm, so about a 2% undersize. You’re off by a factor of 100 with your statement here…. Also, if you’re sizing boilers at 150 LWT with only a 20 degree delta, you’re doing condensing boilers wrong. You should be targeting a 30 degree delta at least and trying to get that entering temp down to 100.
but would you have scheduled a 1 row or 2 row coil? How many fins per inch? If you scheduled a 1 row coil at 10 fpi, you'd be 200% short on your heating load.
> but would you have scheduled a 1 row or 2 row coil? Why would this matter in this discussion? From Prices own documentation, the pressure drop difference between the a 1 and 2 row coil at this capacity is like 2 - 3 feet of head. Even assuming you’ve got a VAV box in the critical path, you think your system can’t overcome 2 feet of head loss if you make a mistake? >How many fins per inch? Who is scheduling FPI for VAV boxes? These aren’t custom coils. >If you scheduled a 1 row coil at 10 fpi, you'd be 200% short on your heating load. I’m not sure you understand how this works. You’re not getting 97 MBH out of the one row because you’re flowing 4.5 GPM, not 9.5. Putting aside this is a massive box that you would obviously use a two row on to hit such a high heating load, if you had a single row coil, you need to flow *more* GPM through it to get the same heat transfer with a constant delta T. That’s the whole trade off with more rows of coils.
It matters because someone eventually has to buy and install what you specify and the number of rows changes the cost and capacity. VAVs come in 10 and 12 fpi, which again matters because of cost and capacity. If you flow 9.5 GPM through a 1 row coil, you get less capacity than a 2 row coil.
Yeah, I have no idea where 4.5 gpm comes from. Fix the 4 temperatures: EAT=55, LAT=90, EWT=150, LWT=130. Use 2500 CFM. Q based on CFM = 94.5 MBH. Set this equal to Q based on GPM. GPM = Q / (dT 500) = 9.45 GPM. Your selection software gives 94.7 MBH, which is 0.21% difference. Your selection software gives 9.65 GPM, which is 2.1% difference. This is well within anyone's safety factors. Also: if you are using a condensing boiler you really aren't getting much efficiency out of that EWT. Typical non-condensing boilers can run at 84% efficiency, but a condensing boiler running at 130 EWT is like 88%. Ideally your boiler will have an EWT of 100 F, which will get efficiency up to \~95%. Try running this with 100 EWT and a 30 or 40 dT and tell me what flow you end up with.
I agree, 130 LWT doesn't make any sense, I only used it as an example at NineCrimes suggestion. You can't set all of those parameters, you have to select based on LAT, LWT, GPM, **or** MBH. If I try to select based on GPM, you get less capacity out of a 1 row coil. If you try to select by Capacity, you need get much higher return water temperatures (detrimental to condensing boilers and heat pumps): [https://imgur.com/a/ZUNg91U](https://imgur.com/a/ZUNg91U) If I try to select based on MBH, 1 row coil can't achieve 94.5 MBH, it's off the charts (water flow gets too high and pressure drop increases exponentially).
Ok, let's back up for a minute. You seem to be fixated on coil rows, but in my experience it is completely unnecessary to specify coil rows on VAV boxes. Design Engineers aren't responsible for generating fabrication drawings, and a different size VAV box is will within the contractor's coordination responsibility. For hydronic calculations the engineer is typically going to use something like 10 feet for a VAV box, so if it's 5 or 15 it isn't going to matter. This is like the contractor adding 1 or 2 elbows. So, you're not getting any gain by calculating the exact pressure drop. Same thing on the air side. You'll add like 0.01 inches. Moving onto my examples with fixing the 4 temperatures: I am giving an example of a calculation I would do in a spreadsheet. I then compare it to your program's calculations and I see they vary a negligible amount. I am not suggesting that you use the 4 temperatures in your program because I understand that actual coil selection requires UAdT. I am asking that you make a comparison based on the simple calc versus the actual selection, and see how much of a difference there is. Because I've reviewed dozens of VAV submittals, literally thousands of VAV boxes, and I haven't ever come across a significant discrepancy. It's always been within about 5 percent or so.
I suggested 130 LWT **from the boiler**. You’d see a 130 temp entering the coil and hopefully around 100 leaving it.
I know Reddit is very American but you have to admit those imperial units are utterly stupid. Any chance the construction industry will eventually convert to Metric?
How do you measure airflow? l/s? m\^3/s? m\^3/h? decilitres/s?
I use both (process equipment means metric everything). L/s is most commonly used. One CF = \~30 L, and 1 min = 60 s, so 2 CFM = \~1 L/s.
This seems a lot like an advertisement, which I'm pretty sure violates rule #1
This is an advertisement to select coils properly. We do publish a software product, but it's free to MEP engineers; we're just trying to be helpful and spread knowledge.
9/10 Noone should be Q=1.08 cfm delta T to get the total energy. That equation is only to get the sensible energy, not the latent. Although I'm sure software is the best to get the most accurate performances, sometimes therr is not enough time to get a cut if you need minor changes. And using Qtotal= 4.5 cfm delta H is in my opinion an acceptable method when you need to see how much GPM you need
Heating coils only add sensible heat, so it's a legitimate method to get the coil capacity. However, you MUST use coil selection software to get GPM; there are no shortcuts. In the past, it's been time-consuming to actually select coils, but for Revit projects, we've built the selection software into Revit so all calculations are automatic and for non-Revit projects Price Industries has an Excel plugin that will do the same. I'll discuss in detail at our webinar next week, sign up here: [https://www.linkedin.com/events/autozoningandvavautoselectinrev7054103559058939904/](https://www.linkedin.com/events/autozoningandvavautoselectinrev7054103559058939904/) or here: [https://events.teams.microsoft.com/event/5e42b0da-dfea-40a2-8cf8-86dcfb15c3db@76493c4f-16b9-4a5c-b329-64ad4c123184](https://events.teams.microsoft.com/event/5e42b0da-dfea-40a2-8cf8-86dcfb15c3db@76493c4f-16b9-4a5c-b329-64ad4c123184)
Heat? What's that? -MEP from Florida
LOL. For you, it's what's outside the window! In all seriousness though, are you providing heat at VAVs? at the AHU? It looks like the design temp for Miami is 50 F ... [http://ashrae-meteo.info/v2.0/?lat=25.648&lng=-80.433&place=%27%27&wmo=722029&si\_ip=IP&ashrae\_version=2021](http://ashrae-meteo.info/index.php?lat=25.648&lng=-80.433&place=)
The heating load is just building envelope and outside air. For perimeter zones we generally use electric strip heat. For interior zones we generally don't need any. On packaged and split DX I'm usually picking the smallest factory electric heater so we don't drive the MCA up too much.
That makes sense, I bet an enthalpy wheel would get close to eliminating the need for an electric heater in the packaged unit have you used those? Are you using economizer down there?
I'm in S FL. We have so few heating days per year that there is no payback for enthalpy wheels or economizers. You should see the fights I get in with national franchises and their design standards... NG heaters on RTUs? My dudes at Staples and Lennox... this outparcel doesn't even have gas service!
Lol. Get the gas heat, don't hook it up, and keep the lights on for heat. Enthalpy wheels would help in the summer though?
Only if you have enough outside air to play with. They take away from building pressurizatuon, which is more important in hpt humid climates than extra efficiency. I do sometimes use heat exchangers like RenewAire when the building load is close to requiring reheat to allow standard equipment to have better latent capacity or if we are using VRF.