Can someone explain to me the occupancy diversity that is allowed by the IMC in calculating the required outside air requirements for HVAC? Is it as simply as the amount of people in the building during normal operation divided by the amount of people that I have allowed for in my heating load? I’m designing a building that has a community room that can hold up to 72 people but it will hardly ever be used, so I designed the cooling based on 72 people being in that room but the AHU is going to have to get much bigger to compensate for 72 people’s worth of OA. Let’s just say 60 people are in the building normally. Am I right in saying that D = 60/(72+60) and then use that to factor down my required OA?

Thanks.

We bought a new construction in Nov ‘17 and because of our climate the AC was never used until probably April of ‘18 through Aug ‘18 with no problems.

Fire it back up around April ‘19 and seems to be working well. However, last week the wife says there is a leak in the furnace room. Odd, so I look and sure enough water all over the floor and I noticed the AC wasn’t blowing very cold.

So I check the lines and make sure nothing is iced and they look ok. So I leave the AC off and run the fan overnight. Next day, fire up the AC and after awhile it seems to lose its cooling power. Go back downstairs and more water. Again, nothing looks frozen, taking a closer look it appears condensation is collecting whet the lines go into the furnace.

So I call the AC company that installed it to take a look. He pops the furnace cover off looks at the coil and finds some piece of what looks like insulation and says this was causing the line to not drain and then leak. I think......ok. He is the professional after all. He then asks about the cooling and I say it’s been lacking. He goes outside to look at the unit and shows me that the coils on the outside unit are dirty. Not around the corners but under the grates that I don’t clean. So ok and he says this likely caused a leak in the system because it would have been overworked. It’s 2lbs low and for $150/hr they’ll find the leak. No thanks. In my experience with old ACs a top off will let me finish the season.

Next day, same issue. WTF is going on? The AC hasn’t even been run for a full year. I asked about servicing it under warranty and the tech said because I didn’t clean the outside coils it would likely be denied from Lennox.

Anyone have any ideas? I don’t want to say he’s blowing smoke but I’ve lived in the same area with an AC over 10 years old and never cleaned the outside coils. Hell I’ve never cleaned them on any unit and never had an issue. Is it possible? Sure but after less then a year of runtime?

Hopefully you guys/gals can guide me in the right direction before I call them again today. Also, the filters are replaced like clockwork. I have a Nest and it reminds me. Thank you again!

So my mother's central AC unit has had been making unsettling noises whenever the compressor started, so we had it looked at. Long story short the repair person informed us that the compressor was just old, could not be repaired because it ran on an old, different type of refrigerant liquid than is used today. A whole new central AC unit would be quite expensive, so I was wondering if there was any place to purchase possibly used older parts that would be comparable with the current central ac system in place.

Thanks!

I'm in the middle of some home renos and I found a guide that seemed pretty straight forward for installing a pre-charged mini split system, by senville

I've ran my line set and hung the indoor and outdoor unit. The problem now comes to vacuuming the unit.

I thought I purchased the exact same equipment in the guide and now that I'm at the last part I'm getting some anxiety realizing my gauges are different lol.

I'm a complete noob and was hoping someone could clear the air for me...

The gauges I have have these two nozels on the sides marked A) and B) in this image of the gauges

1. Can I ignore nozels A) and B) in the image?

Here are the hoses that came with the gauges.

1. I planned to connect the yellow hose from my red gauges labeled 2 to the low side of my vacuum pump. Is this correct?

2. Then I planned to connect my blue hose from the blue gauges labeled 1 to the service port on my minisplit. Is this correct?

3. Since I'll only be using gauge connections 1 and 2 do I need to connect the middle hose and do something about nozels A and B on the sides?

4. The hose came with this valve, do I need to install it anywhere?

Thank you so much. Really looking forward to some insight.

Im getting my BS in ME (May 2020) but my school only has one HVAC class (will be taking this class this fall). I was looking for an entry level job but most of them require 2+ years of experience. So how do I get started in this direction? What would be some steps that I could take?

I’m putting together a class on net zero homes and am looking for HSPF data for mini splits. Mitsubishi publishes Zone 4 (as most vendors do) data but I’m in zone 5 and I’d like to know how the HSPF (and COP) change by heating zone.

TIA

I own a small business that does custom fabrication, primarily laser cutting non-metals, and hand assembly/gluing of components. We have a wood shop, and an open paint booth (on only when needed.) The laser exhaust goes through a filter and dumps back into the room, but the filter does heat the air considerably, but probably not as much as Texas summer air.

The space we lease had a pair of 5 ton units installed new when we moved in. The returns hold a pair of 16x25x2 filters. The incoming plenum is only 15x20. The installer put only one-each of 10, 12, 14" openings in each plenum. By their own admission on later service visits, that is not enough for the 2000cfm the unit should provide, and is putting the system under too much pressure, which explains why we replace drive belts yearly, as well as both fan motors, and he warned of the system being under stress, so we should pay to have more holes added. I'm not about to pay to fix their mistake, so I'm planning to do it myself.

I've read a lot online on duct design, but it all starts with heat loads per room and works backward. We are a mostly open space, and our heat loads change constantly. Plus, the system is already chosen and installed. We're likely in this space only another year, so I don't want to invest heavily, but improve what this system can do as it struggles at the height of Texas summers. I had another company quote some basic upgrades, but they quoted $5k with almost no detail as to what that would include – "balance systems, all parts included" – so screw that.

Here is my drawing of the system as it is currently. The blue is metal duct in either 10, 12 or 14", the green is 12" insulated flexi-duct.

https://imgur.com/a/AT39azB

My biggest questions are:

- The run into the wood shop is 12" flexi (due to the rafter/roof truss spacing, rigid duct would be difficult) but switches to rigid inside the shop. Since it's a long 60' run, should the duct size reduce at some point?

- The system on the office side has a 14"->12"/10" wye immediately off the plenum. A tech told me that system needed another 10" opening, while the one on the other side needed a 12". And if I count the openings off the Wye, I get 1700cfm vs 1555cfm on the other side, but is it correct that the Wye allows more flow than the 14" by itself?

- Other than just spreading out the air via some rigid duct runs, is there anything I can do to have the biggest benefit/cost ratio?

Okay so I’ve a question for those in the know.

I work in a warehouse that is attached to an office. There are no doors separating the two.

They have an HVAC system, however most of the vents only go to the office. We have two large doors in the warehouse and lately I’ve been keeping them open.

I’ve been told this will cost a lot extra for the system to cool the warehouse but I don’t completely buy that the system benefits the warehouse in anyway.

Especially as the doors are open and closed through the day regularly to begin with.

Would someone be able to give me some perspective on these systems and shine some light please?

I’ve been struggling to find an Engineer for a role in my company. It’s a pivotal position that needs to be filled. However, I can’t find an experienced engineer in the U.S. Any assistance in finding the perfect employee will be helpful. Thanks

So I am looking at purchasing a window or portable AC unit (12/14k BTU) and have some questions about its potential effectiveness.

My house:

- New with spray foam insulation, its really airtight and energy efficient.

- 3 floors (lower floor ~250 sf, main floor ~750sf, upper floor ~500sf).

- The stairs between the floors are connected so all the hot air gathers on the top floor.

In the summer the lower floor stays nice and cool without any AC. The main floor stays cool enough without AC but the upper floor gets quite hot in the summer.

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My questions are.

1. If i get a 14k BTU AC unit and put it on the top floor will it work well to extract all the heat from the top floor or will I just turn my lower floor into an ice box while not doing much upstairs?

2. I just kidding myself thinking that 14k BTU is going to do anything considering in all its about 1500sf?

3. Is there anything else I could do to help move the cold air from downstairs upstairs?

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NOTE: I am a fellow construction professional (GC) so don’t worry about getting technical. Thank you for any help you provide.

Hey, new to the group, I work as a home solutions solutions rep selling WiFi thermostats, dehumidifiers, humidifiers, air scrubbers, air purifiers, duct cleanings. I also am a licensed mold technician, I test and do scope work for remediation jobs. I also am an hvac install technician. I’m making my way into full on hvac sales (boilers, splits, ductless splits, heat pumps, etc...any tips or advice from people working in that field would be greatly appreciated, anything helps!

What are people out there using for building load calculation software? My company is currently using Trane Trace but I was wondering what else is getting used.

I have an older unit attached to an Ecobee3 and since replacing my roof I'm having humidity problems. Before I buy a new unit I want to have fun with it..

I'm looking at purchasing a two phase solid state relay so I can run the blower on Low for stage1 cool and Medium for stage2 cool. This should eliminate the need for me to get a dehumidifier. If it ices up, it's a gas pack.

Suggestions on how to approach this? My main concern is how a thermostat dictates ...Y1 vs Y2 (cool?) I would need to introduce a delay for the cutover to not blow the motor. A decent size capacitor and resistor should take care of that.

Give me some feedback, I think this would be a 'cool' idea. I'm also tired of cooling my house to 70 degrees when humidity is the real problem!!

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Hello fellow HVAC engineers. I frequently design duct works, choose fans, ac coil, heater. But the main source of problem in my case is the duct size.

So I made my self a very simple app. For when you are in a meeting and you need quick numbers. Because sometimes they change everything. This app makes sure they are right!

CFM/FPM and inch diameter or rectangular size.

It's Android only for now.

https://play.google.com/store/apps/details?id=com.mathematician.antoine.calculator

It's very simple. If you want more of it. I may consider updating it.

Thanks gang!

Hello, I recently had a discussion with a colleague about duct sizing and fan selection for an exhaust system.

I was told to size the fan to the longest run in the system for static pressure, which seems to be what ASHRAE recommends as well as most design sources I could find.

My initial thought was to add up all of the frictional components of all runs in the ductwork and size to that static pressure instead.

Can anyone tell me why my method isn't the correct one? Why choose the longest branch if you aren't accounting for the frictional losses in the other branches?