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Thread: HVAC Roof top options/cost?
06-14-2007, 10:22 AM #1
I need to add air conditioning to my leased 2500 sf bay/building. A roof top unit is my only option. Two companies have given me estimates, both right at 17k. this includes a 7.5 ton roof top unit and duct work. I was in my local Home Depot and noticed their units didn’t need any ducts…just a hole in the roof. What are these units called and are they going to be cheaper than running ducts?
06-14-2007, 10:31 AM #2
Keep in mind that anything you do to THEIR building, belongs to THEM. and I would assume that includes not only the ducts, but also the Ac unit itself, when you want to leave.
I had a little over 3000 sq ft in the Philly area and it was cooled quite nicely with One HUGE in the wall unit and 2 more small home type window units. I'm sure you'd not spend anything near 17k on a reasonable wall unit install.
The building was concrete block with a steel quansit hut type roof and no insulation.
06-14-2007, 10:50 AM #3
Thanks, I understand the "I bought it, installed it...but it's theirs when I leave. I'm between other bays so while there is a 25' wall at each end I really don't think the land lord is going to allow me to cut open the block wall to install window units.
06-14-2007, 10:59 AM #4
Why not?... Ask..
They can always be closed up...
06-14-2007, 11:10 AM #5
One thing to consider before reviewing the "wall A/C issue" with your landlord is your local neighborhood.
If its industrial, then window A/C units by implication means a nice, person-sized entry hole into your spaces. Getting to and through the roof is somewhat harder, perhaps.
With scallywags ripping A/Cs out of walls to get the copper coils I'm not sure I'd choose that option if the neighborhood is "bad" and/or exterior walls are unprotected by a stout, sharp fence.
06-14-2007, 11:18 AM #6
17k for a 7.5 ton rtu sounds right
the home depot thing might be those mitsubishi 3" hole things
they are quite pricey
7 ton is a bunch of ac
06-14-2007, 11:34 AM #7
Just me, but if I were the landlord, I would rather have a hole in the side of my building than in the roof. I've been around too many roof top units that were not install correctly and they ended up taking them out to fix the roof. One of them leaked so bad that it cost more to fix the roof than the unit was worth.
06-14-2007, 11:49 AM #8
Why don't you try to re-negotiate your lease and have the landlord pay for the AC? Even if it cost you an extra $100+ a month, you'd still be ahead.
06-14-2007, 11:50 AM #9
Cheenist ,The units your refuring to at home depot are called gas pacs. Its a self contained
unit all you do is cut a hole in your roof and lay down some 4x4s around the hole screw them down and coat them with roofing tar and place the unit on top of the 4x4s. Run power to it and hook up a themistat to it .You dont have to run duct work .Its that simple. I would look for a used one there everywere There usualy salavaged before bulidings are torn down. Install it yourself.
Here is a 5 ton unit heat and cool you can buy it on line.It even has a link on how to install it. See how much it costs ,I think all hvac saleman are crooks.
06-14-2007, 01:17 PM #10all you do is cut a hole in your roof and lay down some 4x4s around the hole screw them down and coat them with roofing tar and place the unit on top of the 4x4s
Carl, based on your answer to Gary, it sounds like you have a space that's 25x100. The no-duct device you're asking about is called a concentric diffuser. They'll work fine in s space that's fairly square, but not worth a hoot in a long rectangle like you have.
Ductwork is a major portion of the quotes you've got, and the fact that the ductwork has to be installed in an occupied and working space makes it an even greater part of the overall cost.
Another big cost item is the installed cost of the roof curb for the unit. Last time I had one of similar size installed was about 10 yrs ago. At that time, it cost about $1400 to have it cut in, flashed, and roofed in after we had placed the assembled curb on the roof and marked out the location. Installed cost to the owner was in the neighborhood of $2000.
I'd look at the cost of (2) 5 ton split systems with condensing units on the roof and air handlers hung within the space. These can be spaced sufficiently to eliminate ductwork with the exception of a piece 3-4' long on the discharge of each air handler, with a splitter or some turning vanes in the end. No return ductwork required. Equipment cost to the contractor will be the same or less than the cost of the 7.5T RTU. Each system will require a small roof penetration for passage of refrigerant piping and conduit, such as a pitch pocket or something similar, depending on the type of roof. But, there's no curb involved. Future maintenance like filter changes can be accessed from within the space instead of by getting on the roof. Operating efficiency is typically higher on equipment of 5 tons and under since the primary market is residential, and the equipment falls under federal residential efficiency requirements. Because this equipment is produced in such large quantities, the reliability tends to be better than that of small commercial equipment. The condensing units are available in both single and three phase. I'd go with 3 phase assuming you have the service in your shop, since it eliminates the start and run capacitors on the compressor, and they tend to be the most common trouble spots over time. Not much difference in cost from single to 3 phase for the unit itself, and not much difference in installed cost for the power wiring.
My guess would be that you can get 2 systems like this for a total installed cost in the $10K range. Normal comfort cooling is typically based on 400 sq ft per ton. 10 tons would give you 250 sq ft per ton, and when the additional loads of motors, controls, and evaporating coolant are considered, I wouldn't go with any less than this.
Don't waste your time looking for used equipment. Its no bargain even if its free in equipment this size. 15 yr old units with an EER in the 8 range will cost you about 60% more to operate than current equipment with a minimum EER of 13.
Assuming your shop is already heated, don't let anyone talk you into heat pumps over straight cooling units. The reversing valves in heat pumps tend to be one of the most troublesome parts of a heat pump, and they're a part that doesn't exist in a straight cooling unit. If your shop is like most, there's not much heat required when the machinery is running anyway.
I think all hvac salesman are crooks.
06-14-2007, 03:43 PM #11
A few things to condsider. If you want to "take it with you" (might not be worth the labor) you could consider installing a spit system. This would require only a small hole in the roof, for refrigerant lines and power, and will help keep the landlord happy without tearing up the roof.
On the other hand, in the SF bay area, something you REALLY should include with a roof unit, is one with an economizer--a device which can cool by using ONLY outside air run through the building, without running the compressor, or on warm days, can automatically close and use mixed or all indside air--a combination of the two.
So far as ductwork, most units can be configured to run with minimal ductwork. Even many spit systems can be "hung" from above and run with only a filter box in the return, and minimal diffusers from a supply box. However, some units are designed with direct drive blowers which can draw too much current when run without duct restrictions. A unit that big will probably have belt drive, which will allow you to change the blower load and speed.
06-14-2007, 03:47 PM #12
Thanks, that's a boat load of good info. I'll be better educated when asking for more quotes. The split system with the smaller roof hole will go over better with the LL.
06-14-2007, 05:25 PM #13
Check your local energy codes before forging ahead with a split system. Usually, there is a cutoff of when a 100% OSA economizer is REQUIRED (Read - big hole in the roof or wall) depending on the tonnage of the equipment. In some areas this can be bypassed by installing more smaller units but in others (California for one) all units serving one area are viewed as a system and require an economizer for over two tons of cooling. Also, if an economizer is required, they might also require economizer relief to give the 3000 CFM a way to escape the building envelope. This is usually accomplished with a barometric damper or fan on a pressure switch. It might be sufficient to open a large shop door since when the unit goes into economizer, the outdoor temperature is comfortable. I don't know how strict VA is.