Building Chatter

Now it may be of some surprise to designers and engineers that HVAC rooftop equipment needs to be serviced, so I have come up with the solution: genetically-enhanced maintenance people that are 20 feet high.  Alright, so my solution has some flaws, like door handles being broken off the AC units by those giant hands, but seriously now, why such a drastic idea.  Two reasons and both reared their ugly heads recently on my projects, namely roof and equipment access.

Let’s talk about roof access first.  I know it’s not the engineer’s job to specify and layout roof access, but it should be at least reviewed by them.  Try to put yourself in the shoes of the maintenance staff when they go to service the equipment or when something goes wrong.  Typically this does not happen on a nice sunny Tuesday afternoon, it will usually occur in the middle of the night in a downpour or a blizzard.  And to add insult to injury, work has to be performed in those conditions on roofs that are made from that white, plastic, slicker-than-a-greased-pig roofing material.  All these conditions can quickly turn a simple situation into a worker’s compensation claim, with the maintenance or repair still left undone.

ACTION ITEM #1: In design, check all roof surfaces that have roof drains or HVAC equipment and make sure that you can reach all areas without carrying a ladder around.  Remember even a three foot elevation change can be dangerous in some weather conditions.  This check can be effectively done by checking the architectural roof plan and making sure that they have permanent ladders noted where you would require them.  Also, in addition to checking for changes in elevation look for access to the roof itself.  Man-doors are preferred, but a hatch and a ladder also work.

Second is the HVAC equipment access.  In the age of high-efficiency systems, I have seen my fair share of the heat-wheel type energy recovery units; these are my number one access offenders.  These AC units can be as tall as a double-decker bus, with a fan, motor, filters and possibly other components located on the top deck, and all of which require maintenance.  Add in that white roof and the inclement weather and your recipe for disaster shows up again!

ACTION ITEM #2:  Also in design, check the height on the rooftop HVAC air-handlers (cooling towers, chillers and anything else for that matter). If they require a ladder for proper service then consider adding a permanent catwalk system.  If that isn’t possible because of budget reasons, try to get good walk-way pads all around the unit that aren’t slippery and add some permanent ladder tie-off points to the equipment casing.  Other ideas may include removable or moveable scaffolding, but a lot depends on what can be easily moved to the roof and the available storage.  I would also check out what OSHA has to say about this as part of your due diligence.

These simple things are easy enough to gloss over in your efforts to get you designs out the door but if you want your design to stand the test of time, with consistently low energy consumption, it has to be maintained and for that to happen you have to make it easy for the people working on you equipment to properly access to it.



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!

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.