AC units without Roof Curb

[sowhatso]

Dear Friend ,

I am a Mechanical engineer that newly graduated , I am working in a project at which we have to install 5 Carrier AC units with models : 50TFQ012 , 50JS048 ,50TFQ008, and 50JZ036. My question is : Can I install the first three models of AC units without Roof curbs , and what is the disadvantages ?

Thanks for all yout inputs

 

[gepman]

Being on the design (rather than the install side) I am certain that some of the installers (and people with a lot of field install experience like imok2) will have different comments.

You can access the Carrier manual for this unit (which you should have if you are going to design a system with it) at

http://www.commercial.carrier.com/c...ation, Start-Up, Service, Export and Domestic

I assume that you DON'T have a horizontal unit. The manual (for the 50TFQ units) states that you can install it without the roof curb but it appears that you will have to essentially build what amounts to a roof curb anyway. Where I have worked we always have specified the factory roof curb (which appears to be shipped with these units) for several reasons:

1) It will be the correct size which prevents a mistake building one from scratch in the field.
2) The general contractor can build the roof curb into his roofing system. This is usually done by the general contractor and NOT the HVAC contractor. This helps prevent contractor coordination problems.
3) The labor cost will be much lower with a factory built part than a field built part.
4) It allows the engineer to use a standard mounting detail which references the factory built roof curb.
5) You don't have to analyze any strength requirements for a custom roof curb.
6) Standard vibration isolators (which is in our standard detail) will fit.
7) It lifts the unit slightly above the roof to prevent leakage problems with the roof, allow easier condensate connections (with trap) etc.
8) It gives you something to attach the ductwork to.

I am certain that there are a lot more reasons. I noticed that this is a standard efficiency unit. You may want to consider 50HJQ.

I have attached the relevant pages of the installation manual obtained from the Carrier site.

 

[sowhatso]

Dear Gepman ,

Thanks alot for your input .

I looked through the manulas to see if there is any information about the standard vibration isolators and I'm didn't see anything. can you help me in showing the location for installing the vibration isolators and any data about them ?

Again , super thanks for all your help

 

[imok2]

I agree with Gepman and install roof curbs with new installs because it makes the job easier to install, cleaner and simpler to lay out with regards to system duct install either downflow or horizintal. Consider a roof curb as a template for installers and frammers when setting the unit.

 

[RossABQ]

I can't remember any job with a field-constructed curb that went well. Another advantage to add to Gepman's good list is that a factory curb can be erected well in advance of the unit arriving on-site, so the roofing can proceed. This is a big advantage on larger or custom units that can be 24 weeks out.

 

[gepman]

The "standard" vibration isolators were our engineering office "standard". If you get burned once or twice with a noisy installation due to vibration from the rooftop unit then you put in vibration isolation in your mounting detail. It isn't as important on small units like you have especially with a scroll compressor. You could omit it or use a neoprene pad instead of a spring isolator. We specified Mason Industries (

http://www.mason-industries.com/masonind/home/index.cfm)

but I am sure that there are many others.

This vibration isolator only helps with unit vibration due to the compressor or the fan. It doesn't help due to vibration from the duct itself.

 

[BobbaLoui]

As someone who's has been installed for 20+ years, gepman nailed it. It makes life easier for everyone.

 

[stanlsimon]

Keep in mind that you can have ponding on the roof due to heavy rainfall. With this comes wind and even waves. The roof curb should be 12" above the surface of the roof. Some roofs may have variable 2-6" of insulation if tapered. You may be located in the thick insulation. In that case the the curb needs to be another 6" higher. If you are in an area where snow drifting is a problem, you may need to go even higher to get the intake up out of the snow. Improper roof curbs and flashing are a major cause of roof leaks and mold damage, lawsuits, etc. An attempt to install rtu's on too short curbs is like playing with a loaded gun.

Field fabricated curbs are possible for small tonnage units if you can get a carpenter who can read the RTU shop drawings and get the outside dimensions and supply and return duct drop openings straight. It would be nasty to have the crane waiting while somebody is trying to modify the curb to make the unit fit.

 

[EddyC]

I would also suggest, if you haven't already, to have a civil/structural engineer verify that the roof can take the weight of your proposed equipment. While smaller equipment can sit directly upon a roof curb, larger equipment needs to sit on steel dunnage framework, posted up from the building structural skeleton.

 

[CountOlaf]

EddyC: you're correct that it's always a good idea to consult with a structural engineer regarding any roof-mounted equipment; however, I've seen and designed some fairly large rooftop units with manufacturer-furnished curbs. If it's a downflow unit, I pretty much always want a curb. The structural work is then below the roof level with new structural memebers usually running between existing members along the outline of the curb above, as well as some additional members added to frame the opening for the supply and return duct downcomers. Reinforcing of existing members may be required as well of course. I have had the "posted up" grillage type designed for predominantly horizontal discharge type applications, although I suppose it could be done for downflow configurations.

gepman: clarify this whole vibration isolation discourse for me. The RTU manufacturers usually recommmend the neoprene or gasketed sealing of the unit to the curb, but the large components such as the compressors and fans have (or can be specified with) "internal" isolation like the spring isolators from Mason you cite above, which are connected to the RTU frame and don't have any real involvement with the curb. Am I missing something? Why would standard vibration isolators not work if the manufacturer's curb was not employed? They would be "inside" of the unit envelope, right?

 

[gepman]

CountOlaf
You can, as you say, specify vibration isolation between the fans and/or compressors and the RTU frame however sometimes the owner may have already ordered it without them, the manufacturer does not offer them (or only at long lead times), the cost charged by the manufacturer is prohibitive, etc. Therefore you can put the isolator between the RTU frame and the curb. I have attached a typical detail and a detail from Mason which will actually make the curbs if you want a custom one. I did not mean to say (if I did) that standard vibration isolators would not work if the manufacturer's curb was not employed, I meant to say that one of our standard details included the vibration isolators and the manufacturer's curve.

This discussion does bring up another good reason to use the manufacturer's curb and that is if you don't use the manufacturer's standard curb you will have to (at least you should) detail the custom curb for each individual RTU. This would take a lot of time and money which will either cost the client and/or the engineer.

 

[CountOlaf]

Gepman: Thanks for the reply/details. I get it now. I didn't realize there was a way to incorporate vibration springs right into the curb. I guess most of my "industrial" clients don't complain as much about the noise as long as it does the job of heating & cooling.