Variable Refrigerant Flow (VRF) Presentation

I recently had the opportunity to speak at Buckeye Association of School Administrators (www.basa-ohio.org) and talk about Variable Refrigerant Systems for schools.  I want to thank all who were in attendance and I hope that you learned a few things about this system.  Below you will find a link to the presentation, if you want to download the file simply right-click and select the "save link as..." option to download it to your local hard drive.

Variable Refrigerant Systems Presentation

 Over the next week or so I hope to have the presentation posted with integrated audio for those of you who wish to hear and see what I was saying as opposed to just looking at the bullet points on the static slides.

Oh and a Happy St. Patrick's Day to all!  Go Green!

HVAC Freezestats - A Coil's Best Friend

A freezestat is a safety device that protects a water coil from freezing (DX coils do not require them).  Another name for a freezestat is a low-temperature cut-out, limit, or detector, take your pick.  In my years of inspecting these simple devices, the installations have been like snowflakes, no two were alike.  This seems odd because they all come with installation instructions that are pretty clear, to me at least. Here's how they should be laid out.

1. Mount the body of the unit outside of the ductwork and near the top.  The rule of thumb is that each sensor is about 20 feet long (exact lengths may vary) so at a ratio of 1 linear foot of element per 1 square foot of coil, one sensor should cover about 20 square feet.  And at a standard coil velocity of 500 feet per minute, one sensor should cover the coil area for a unit supplying approximately 10,000 cfm.  Now this doesn't mean that if you have a 11,000 cfm unit you should use two sensors to cover a coil, but if you have a bigger unit, realize you may have multiple sensors (I saw one the other day with five).

2. Where the sensing element enters the case you should have some sort of rubber grommet or bushing.  Airflows within the unit's casing can cause the element to vibrate and damage itself.

3. Don't crimp, pinch or crush the sensing element.  This capillary tube is delicate and care should be taken to install in properly with the approved turning and mounting devices.  Failure to do so may render the element ineffective. Resulting in a burst coil.

4. Install the element mostly in the horizontal plane and in a downward direction from the body of the device.  You should first layout how you are going to install it on the coil.  Then starting at the bottom, you should work your way back to the body of the unit.  A lot of times I have seen the opposite type of installation where you work from the body to the end of the tail.  Often this results in ending up with a foot or so of extra element and the tendency is to just wrap the tubing back on itself.  This is not correct.

5. The bottom of the coil will freeze first.  The row with the end of the sensing element should be six inches from the bottom of the coil with the next row 12 inches above the last.  The elements cover six inches above and below, that's where you get the one linear foot to one square foot ratio.  Also, if the bottom is most likely to freeze and the top is the least likely to freeze if you don't cover the very top of the coil its really not that critical (unless the designer says so).

6. Freezestats should be installed on the downstream side of the heating coil.  This assumes that you have a heating coil or that you heating coil is in the pre-heat position (i.e. before the cooling coil as seen by the flow of outside air). If you cooling coil is the only coil or if you have a heating coil in the re-heat position (after the cooling coil) then I would say put in on the downstream side of the cooling coil.

Once installation is complete, the operation of the freezestat is pretty simple.  If a portion of the element (typically 12-18 inches depending on the manufacturer) falls below the temperature set on the sensor body (which typically is adjustable from 15-55 deg F) then two sets of contacts will open.  The first line voltage contact shuts the unit down and should return all components to the unoccupied conditions (i.e. outside air damper 100% closed, etc) and the second low-voltage contact will send the alarm to the building automation system.  Once the freezestat is tripped it requires a manual reset from the building staff.  This ensures that someone visually inspects the unit prior to returning it into service.  This reset button also typically doubles as the test button to make sure that the freezestat is working properly.

This manual reset is really important, but it can also be the bane of the building operators.  Because if the freezestat is not properly installed it can cause nuisance tripping that may cause the operators to start a series of corrective measures that could result in poor system performance or safety violations. So it all starts with proper installation.

Time Tip of the Day - Use the Phone

Yesterday I committed the oldest mistake in the book, going to a meeting without verifying that it was being held!  It didn't help that this meeting water a couple hours a way and that the day before I saw a e-mail that said that the meeting was on, but what I missed was the 7pm email from the organizer saying that the meeting was cancelled because of snow! Aaargghh!

So here's my time tip of the day - Use the phone!  I always tell the people who work under me that you should never assume that a meeting or jobsite activity is actually going to happen.  I have seen it too many times that you show up only to be alone.  So when you are dealing with contractors or maintenance staff, always coordinate directly with the person you are meeting with and call them before you get in your car because things change out in the field at the drop of a hat.  Maintenance people get pulled away, contractors get pulled off a job for a day, and Vendors don't have the proper paperwork together.  In the age of cellular phones, taking 30 seconds to call and confirm that you are still good to meet will save you a lot of heartache.  On the flip side if you have set up a meeting and things get called off for one thing or another, don't rely on e-mail to cancel the meetings (especially the night before) call everyone up. This assumes that you have everyone's cell number but I you don't typically get this information maybe now is the time to start.

On a similar note, never assume that contractors, or whoever for that matter, will call you back.  It's not going to happen, it's human nature.  I've had people stew for days that so-and-so never called them back.  Take some action, pick up the phone, and force the issue.  All that time stewing could be put to better use elsewhere.

Okay, I'm going to call it good for now. The soapbox is yours.

Contractors, Drains, and Backflow Preventers, Oh my!

I was at a meeting today and there was a quick discussion about double check backflow preventers and reduced-pressure backflow preventers.  These devices prevent water from being sucked out of a piping system and contaminating it.  This is most commonly a concern when you're talking about connecting a potable (i.e. drinkable) water source to a device that will mix chemicals with that water (say a film developinig machine).  A comment was made by the contractor that only the reduced-pressure needs a floor drain.  This is partially true, as the reduced-pressure backflow preventer will 'spit' out of the bottom during operation and will need to be connected to a floor drain (typically via an air-gap fitting), but as an engineer I would want a floor drain under the double check as well for future maintenance.

A word of caution.  This was an experienced contractor with a lot of knowledge, but as a younger engineer I found it difficult to be able to tell whether I could take what a contractor would say for the truth.  My advice to you is to do your homework.  Take what you have been told and use it as a starting point to research your answer.  And of course for the young contractors the converse will also be true, the only difference lies in the responsibility of being right. 

Now my whole conversation centered around whether or not a drain had been shown on the plans and this is a great question.  Often times drains will not be shown or not shown in the correct locations for various reasons. Also drains may be located in places that will cause condensate or drain pipiing to cross a walking path, thus causing a tripping hazzard.  Or I have seen drains located half under a concrete house keeping pad.  So my advice to you is know what equipment needs a drain and review the location prior to bidding and again early in construction.