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Adequately sized return ductwork is critically important to the system's performance. Undersized ducts will "starve" the air handler and force it to look for air elsewhere, i.e., cracks and voids. ...

Hi, even better than tieing into the return is tieing into the supply. This way, the dehumidifier, and the air handler can work in parallel, maximizing each of their dehumidification capabilities...

Hi, to technically size the dehumidifier, I would calculate the load and compare that information with your systems' capacity. However, dehumidifier manufacturers often rate their dehumidifiers by...

We created an online Duct Size Calculator that quickly rose to our most popular calculator by a long shot. I am also working on a Diffuser Size Calculator which I find even more useful! If you have...

Hi, HVAC-Talk Community. I have been developing "HVAC Widgets" for repetitive calculations for years. And now, each month, over eight thousand users from all over the world use these calculators.
...

You can avoid mechanical ventilation if the builiding meets the natural ventilation requirements. And a 400 unit multi family would use 62.1, no?

I didn't see it in the posts - do you have a Load Calculation? At this point, I would ask your mechanical contractor to provide one. And if you still have negative pressure, it's usually a problem...

In cases when the ventilation air causes the latent load to exceed the equipment's latent capacity, we add a dehumidifier. The recommended installation is to return air to the dehumidifier from a...

Maybe I should have said "relatively" cheap compared with installing the incorrectly sized equipment. I know that companies are specializing in these calcs and churning them out.

Adequately sized returns are so critical to maintaining indoor air quality. I don't think wall returns are allowed by the most recent IMC or FMC, but I could be wrong. And you may be using a...

Hi Terry,

A Manual J is a load calculation; a Manual S is equipment selection based on the load calculation. It's usually pretty cheap to have these analyses done, and they ensure you have...

I have started adding Demonstration Videos for these calculators if you're interested :putergreet:

Also, if you find yourself doing the same calculation over and over again. Let me know - maybe I...

Hi, there are plenty of companies who offer Manual S. You will also need a Manual J. Is this for a residential project? And what state?

If a load calculation shows you are oversized, you can duct a small outside air duct to the return with a ventilation air damper. No need for additional equipment if your existing equipment can...

Yes, you are thinking right:

Four 6" ducts each carrying 100 CFM would give a velocity of 509 ft/min

One 12" duct carrying 400 CFM would give a velocity of 509 ft/min

where:...

Good morning, You will need to calculate the Equivalent Length (I have not used this calculator, but it looks good). Then you can use this Duct Size Calculator for the static pressure. The static...

Good morning, If you oversize equipment, you risk short cycling and not fully dehumidifying your space - dehumidification is a longer process than cooling, so if you cool too quickly, you won't...

Systems are designed to have a filter at the unit OR filters at the return grilles, usually not both. I would not recommend adding a return grille filter. Also, if you think about it, the return is...

Hi, HVAC-Talk Community. For years, I have been developing "HVAC Widgets" to perform repetitive calculations. I put them on my webpage because it was easy for me to access them anywhere, but I...

Any time I hear negative pressure, I immediately assume insufficient return is the culprit. Your air handler must receive (have returned to it) the same amount of air it supplies. If it cannot get...

Hi Noms23,

Here is a link: http://ashrae-meteo.info/v2.0/index.php Make sure you have 2017 selected. I find it easiest to search the map to find a weather station closest to the address I am...

I really appreciate all your help :)

The mixed air temperature on design day will be 68/58. So by your math, return temp of 50 to 46 deg. So the lowest LAT would be 36. .... but, when it's...

Yes

Hi, yes, DX cooling. The room needs to be maintained at 62oF (approx 50% RH) while in operation; so in order to meet that humidity (grains/#) requirement, the temperature needs to be 45. I said 42...

Hi! thanks for responding. I am not sure what you mean - this is not really my area of expertise. Can you explain a little more?

A little more info: I want the LAT of the evaporator coil to be...

Thanks for your response :)

I want the LAT of the evaporator coil to be 42 (occupied) or 62 (unoccupied).

I looked up a Honeywell DAT controller and it looks like it cannot be installed...

Looking to set/control the coil leaving air temperature on a straight cool, split AC system. Is this possible?? Thanks in advance :)

Hi there, here are my thoughts:

Given:

Area = 25 ft^2
Height = 10' (estimated)
Volume = 250 ft ^3
ACH = 20/hr
CFM = 83.33 cfm (using this calculator)
DP = 25 pa = 0.10 in w.c.

I can't think of any issues. It is dry conditioned air you are returning. So yes, I don't think it matters where they tap in the return.

I recommend getting a load calculation. Ask the person doing the load calculation to let you know the volume of ventilation air your system can handle.


If you have a bit of extra capacity in...

Definitely into the return duct.

If you exhaust into the attic, you are creating a negative pressure situation in your home, which you never want to do.

Also, I assume you have supply air...

Hey there, there are too many unknowns to know for sure what size equipment you need, but we often use residential equipment in small commercial projects. I like multiple smaller units because this...

Even if you solve the water heater issue, it's still important to make sure you have the building under positive pressure.

(And yikes, 100% OA for an RTU here in Florida would be a nightmare...

Completely agree that it depends on the person doing the calculations. That's why chose my words carefully saying "Inputs" not software.

I would say the accuracy depends on the inputs. Do your calculators take the following into consideration:

Wall Construction & Insulation
Building orientation
Window and door characteristics...

Your friend is correct in his logic, but the most correct thing to do is have a load calculation performed. They are not very expensive it will tell you the correct size. Good luck!

Wayne3298, I have had contractors come to me with suggestions that I rejected, but I just see those conversations as teaching opportunities. Other conversations will be learning opportunities.

I am an engineer and always appreciate a call from the mechanical contractor if they have questions. The best result for everyone is the installation of the correct equipment. If I made a mistake...

Without seeing the load calculation, it is not possible to discern if you are getting enough air to the bedroom. That being said, making assumptions of design cfm based on duct sizes, nothing jumps...

Also, the term PXI (peak exterior irradiance) is dependent on latitude, so this further points to the location as being a factor in determining which window is more efficient.

Hi @drose,

ASHRAE's equation for Fenestrataion Cooling Load,q, is: q/A= U(Δt – 0.46DR) + PXI SHGC × IAC × FFs

I suppose if your Δt (change in temperature) is relatively high compared to your...

Based on the default values given in the 2017 Florida Building Code - Energy Conservation (which is based on the International Building Code):


Non-metal, double pane, clear windows:...

Have you read ASHRAE's recommendation for Reopening of Schools and Universities?

You will definitely need a load calculation. 350 cfm/sf is a good "sanity" check to see if you are in the ball park, but never a good idea to size equipment to this. Imagine if the west wall of...

I have never heard of a year to dry out a building. That sounds like kicking the can down the road.

It is possible that you have a negative pressure in the building caused by more exhaust air...

Edit: for Heating I use 99% DB

Hi @TimhvacHI, you can play around with Mitsubishi's Diamond System Builder Software. It is free to download and even if you do not use their product line, you can gain an understanding of how...

I agree with @pecmsg not to exceed 0.5" w.g. sp.

Mitsubishi has a free software package: Diamond System Builder to determine equipment capacity at design conditions. You can use this in...

I recommend doing a load calculation and use ASHRAE 62.2 to determine the OA volume (7.5 cfm/ person + 0.01 cfm/sf). This will determine if you need additional equipment for dehumidification.

I...

I agree that there are too many unknowns to help answer your question. Without knowing your design conditions, it is unclear if it is advisable to load your equipment with 100% outdoor air.

With...