HVAC suspended from rafters. What is the correct method?

[Gene South]

Regarding the steel straps that are used to connect some attic HVAC units to the rafters,I see varying amounts of fasteners holding the steel straps to the rafters. Sometimes I see 3 nails, sometimes 2 nails, sometimes 1, and some time sheet metal screws (See the photos). Also I sometimes see only one sheet metal screw holding the HVAC cabinet to the straps. I recently heard about an attic unit pullling loose from the rafters. My question is there a guideline for the correct amount of fasterers connecting to the rafters and also connnecting to the cabinet? I so far have not located a guideline.

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[Jerry Peck]

Gene,

The answer is: It depends.

It depends on the species of wood.

It depends on how thick the metal is and if aluminum or steel.

It depends on if a nail or a screw.

It depends on the size of the nail or screw.

It depends on if you are referring to pull out or shear.

I did not find a shear/pull out chart like I was searching for, but did find this one:
- Common wire nails

Also, while searching, and makes good reading, are these, and they show one of the connections I described on another thread - the split-ring connector:
- http://www.fpl.fs.fed.us/documnts/fp...chapter_08.pdf (scroll down to page 8-19)
- http://www.woodweb.com/Resources/woo...dbook/Ch07.pdf (different chapter, same connector on page 7-19)

-http://www.awc.org/pdf/NDSCommentaryCompressed/Part12NailsandSpikespp140to149.pdf
- http://www.most.gov.mm/techuni/media...014_chap2a.pdf

[Gene South]

Hey Jerry, thanks. I will check out those links. Gene

[Jerry Peck]

Gene,

Here is an interesting, and maybe helpful, online calculator for loads:
- Connection Calculator

If you select the following:
a) Allowable Stress Design (ASD)
b) Lateral Loading (for shear)
c) Nail (or wood screw, your choice)
d) Single Shear (only choice)

Then click 'Submit Initial Values'

The select the following:
a) Southern Pine
b) 1.5 in (the default)
c) Steel (the default)
d) 20 gage (the metal straps are probably 26 gage, there is no thinner choice than 20 gage)
e) Common Wire
leave all the remaining choices at 1.0 unless you know differently, for example Load Duration Factor for 10 years after the nail was driven is 1.0, then click 'Calculate Connection Capacity',

The Adjusted Stress Design Capacity (ASD) is shown as 76 pounds.

Note that 1 nail in each of of the 4 straps would be 76 x 4 = 304 lbs.

If there were 2 nails in each of the 4 straps, the capacity does not double as nail spacing affects the additional capacity, as does the strength of the metal. With a 26 gage strap the strength of the metal is not going to allow much increase for additional nails as the strap capacity will be exceeded.

How much does that air handler weigh?

When I repeat the above but start with 'Wood Screw' and select a #8 screw, 1" long (which is likely the length used), then click to calculate, the answer is "Inadequate penetration length of the wood screw in the main member. Try selecting a longer wood screw, or a thicker main member, or a thinner side member."

When I go to a 1-1/2" long screw, which has likely not been used, the answer is ASD = 97 lbs.

Now, I wonder what the screw into the metal housing is ... (there is no option on this calculator for metal to metal) ...

[Phillip Norman]

I think support should be from floor joists. Insulate the attic floor, including space under a platform for the HVAC unit. Rest ducts on the insulation, and insulate ducts with tucked-in over-wrap.

Like this with flexible ducts.

Like this with solid ducts.

I don't think 2x4 roof joists are meant to be point-loaded this way. Why would you?

[Harold Doughty]

I recently had a new furnace installed in my attic and the installation manual specifies 3/8" threaded rod and 2x2x1/8 angle iron under the cabinet. See page 9 of this pdf.
http://www.alpinehomeair.com/_viewresource.cfm?ID=2252

Based on this I would think that what you have pictured is woefully inadequate. In any case, if nails into the rafters are sufficient, I think those straps should go all the way around the furnace.

[michael avis]

The HVAC installs I generally see have the airhandlers suspended on all-thread
bolted to the rafters and connected to two pieces of unistrut as a saddle. System sits on rubber/cork vibration isolators. This is a much more robust method. This doesn't really address the OP's question but the system pictured does look pretty flimsy. I would be concerned about the sheet metal screws vibrating out.

[Rich Goeken]

What do you think of this installation?

[Markus Keller]

Can the sheetmetal screws or nails pull out, Yes. So far I have not seen a unit where straps with screws and nails have totally failed. However, I have seen units where the nails clearly were pulling out of the rafters and the sheetmetal screws were pulling out from the cabinet.
That type of install used to be more common around here but has faded away. At least I don't see it much anymore. Most HVAC guys have gone to hanging the units with rod and unistrut. This allows for easier servicing and the easy installation of a pan under the unit.
I write up what you have as a poor install, consider changing it, and installing a pan.

[Jerry Peck]

What do you think of this installation?

Well ... the flexible ducts are required to be strapped within 18" of the metal collar they are attached to where the duct is still straight out from the attachment collar and *before* the duct bends and goes in another direction.

Also, when one reads the installation manual for the flex ducts, it seems to imply that there should be a support within 18" of each side of the bend.

The straps are required to be at least 1-1/2" wide, and look like they are.

I doubt that the spray foam insulation is deep enough to meet the required minimum R-value (I keep a piece of #14 AWG solid copper wire in my clip board so I can poke it into the insulation to see how deep it is - it is seldom, if ever, applied as thick as required for the required R-value - I measure the thickest part so I can state how thick that is, the thinnest part, and the 'average' thickness - if the 'thickest part' is too thin ... ). Is there an ignition barrier sprayed on over the spray foam insulation?

[Rich Goeken]

Well ... the flexible ducts are required to be strapped within 18" of the metal collar they are attached to where the duct is still straight out from the attachment collar and *before* the duct bends and goes in another direction.

Also, when one reads the installation manual for the flex ducts, it seems to imply that there should be a support within 18" of each side of the bend.

The straps are required to be at least 1-1/2" wide, and look like they are.

I doubt that the spray foam insulation is deep enough to meet the required minimum R-value (I keep a piece of #14 AWG solid copper wire in my clip board so I can poke it into the insulation to see how deep it is - it is seldom, if ever, applied as thick as required for the required R-value - I measure the thickest part so I can state how thick that is, the thinnest part, and the 'average' thickness - if the 'thickest part' is too thin ... ). Is there an ignition barrier sprayed on over the spray foam insulation?

JP.. You missed the substantial support provided under the HVAC distribution box.

[Bob Harper]

Those rafters were sized for the expected live and dead loads. Now, you add additional dead load without any reinforcement. Ideally, those rafters should be doubled up but this is seldom done.

There is plenty of strapping available that is less than 20 gauge. You can hardly cut it as it twists in your shears.

Most pros I know use 3/8" all thread rod either directly through-bolted to the cabinets or better yet, to Uni-strut trapezes.

[Jim Luttrall]

Obviously follow the manufacturer's instructions, but we may be over thinking this a bit.
Just for fun, I just googled the weight of a 3 ton attic heat pump unit commonly used here.
At the listed shipping weight of 115 lbs. I doubt there is a need to loose sleep over the weight placed on the trusses.
Say there are 4 straps, all-thread or struts, that is less than 30 lbs. per corner. Even at double the weight (doubtful) for a gas furnace we are still not talking about anything that will really tax the structure of the average house.

[Rod Butler]

Jim has a good point about the weight.

One thing I always look for is minimum three screws at every connection or joint. If strap is used I'd prefer to see it wrapped over the top edge of the cord but if it is the top cord that's going to be hard to do. So the strap should have at least three 1-1/4" screws into the wood and the strap should continue down to underside of the duct with at least three 1/2" screws into the metal.

Never allow sheet metal screws into the cabinet except at the duct and coil flange connections.

Truth is a 20 gauge strap can hold up a lot of weight.

[Brenda Comrie]

I am new here but still have questions on this topic. My apologies up front for the long post.

My suspended horizontal system is pulling loose from the rafters and the attachments on the units don't appear correct to me. Plumbing strap is the only material used for the "suspenders" and none the ends are folded over and screwed through.

For the rafters, the strap is attached with similar hardware using various methods. Some are wrapped over the rafter and some are not. All have 1-1/2" sheet metal screws but their number and location vary. Some are on the side that leads up the rafter, some are on the side where it wraps over the rafter, and one is even screwed into the edge of the rafter. Some have 2 screws and some have 3.

For the system, the strap is attached along the side near the top 3" of each unit with 2 3/4" sheet metal screws. The gas furnace has 4 straps attached, 2 are on the back, 1 on the left near where the coil meets, and 1 on the right side actually between the filter box and the furnace. The coil has 1 strap on the back, 1 strap on the front attached behind the access door inside the unit on the underside of the top, and 2 on the left between the coil and the plenum which 1 is actually inside the plenum. All 4 of these are within 6" of the plenum but there are none on the coil near the coil/furnace attachment.

Straps wrapped around each plenum are attached to the rafters as well.

According to my manual for the furnace plumbing straps can be used but "should be attached to the furnace bottom side with sheet metal screws and to the rafters or joists with bolts". It goes on to say "The preferred method is to use an angle iron frame bolted to the rafters or joists".

So my main question is how do you attach to rafters that are sloped and spaced 24" apart on center. The furnace is 36" long and the coil is 30" long so their alignment with the rafters is minimal. The coil is especially challenging since the access door covers the entire front of the coil which leaves no place to attach to. Through various surfing attempts it seems that the coil needs 4 straps and the furnace needs 6 as shown in Harold's example from his manual though his depicts using threaded rod with angle iron saddles (This confused me as well starting with the fact that his manual is not consistent regarding the 2"x2" angle iron. Is it 1/8" thick per the verbiage or 3/8" per the diagram?) And again how do you attach the rods to a sloped rafter since they are on an angle and with my unit they would be running parallel to the rafters, not perpendicular?

I know that a suspension system is possible using angle iron or unistrut under the equipment but am still confused as to the proper method for attaching to the rafters since they are sloped. This is very confusing especially when I read about using threaded rod for the suspenders.

The best idea I can come up with is to create a system using 2 long pieces of angle iron or unistrut at least 5-1/2' (length of the coil and furnace combined) to sit my equipment on. Then add 2'-3' angle iron saddles underneath everything for bolting to each rafter with the pipe strap in 2 places, 1 front and 1 back. I would also bolt the saddles to the long pieces on the bottom. Would this be against code since these saddles would attach parallel to the rafters? Or the 5-1/2' pieces could somehow be attached to the rafters and then the pipe strap saddled under the equipment and over the long pieces and attached along them where needed. However this method would not resolve the problem with the coil's access door being in the way so I would have to provide saddles for it. Or as a third option, I could combine both methods and put a pair of the long pieces in both locations, under the units and on the rafters giving me the freedom to attach the saddles where needed also. The result would give perpendicular attachments from the saddles to the long pieces, and from the long pieces to the rafters as well. Once more, this still leaves me with the question as to how to properly attach the long pieces to the edge of the rafters.

Please provide any advice so that I can make sure the next person who suspends my equipment does the job properly. And, thank you in advance for your time and help.

Confused homeowner,
Brenda Comrie

[Gunnar Alquist]

I am new here but still have questions on this topic. My apologies up front for the long post.

My suspended horizontal system is pulling loose from the rafters and the attachments on the units don't appear correct to me. Plumbing strap is the only material used for the "suspenders" and none the ends are folded over and screwed through.

For the rafters, the strap is attached with similar hardware using various methods. Some are wrapped over the rafter and some are not. All have 1-1/2" sheet metal screws but their number and location vary. Some are on the side that leads up the rafter, some are on the side where it wraps over the rafter, and one is even screwed into the edge of the rafter. Some have 2 screws and some have 3.

For the system, the strap is attached along the side near the top 3" of each unit with 2 3/4" sheet metal screws. The gas furnace has 4 straps attached, 2 are on the back, 1 on the left near where the coil meets, and 1 on the right side actually between the filter box and the furnace. The coil has 1 strap on the back, 1 strap on the front attached behind the access door inside the unit on the underside of the top, and 2 on the left between the coil and the plenum which 1 is actually inside the plenum. All 4 of these are within 6" of the plenum but there are none on the coil near the coil/furnace attachment.

Straps wrapped around each plenum are attached to the rafters as well.

According to my manual for the furnace plumbing straps can be used but "should be attached to the furnace bottom side with sheet metal screws and to the rafters or joists with bolts". It goes on to say "The preferred method is to use an angle iron frame bolted to the rafters or joists".

So my main question is how do you attach to rafters that are sloped and spaced 24" apart on center. The furnace is 36" long and the coil is 30" long so their alignment with the rafters is minimal. The coil is especially challenging since the access door covers the entire front of the coil which leaves no place to attach to. Through various surfing attempts it seems that the coil needs 4 straps and the furnace needs 6 as shown in Harold's example from his manual though his depicts using threaded rod with angle iron saddles (This confused me as well starting with the fact that his manual is not consistent regarding the 2"x2" angle iron. Is it 1/8" thick per the verbiage or 3/8" per the diagram?) And again how do you attach the rods to a sloped rafter since they are on an angle and with my unit they would be running parallel to the rafters, not perpendicular?

I know that a suspension system is possible using angle iron or unistrut under the equipment but am still confused as to the proper method for attaching to the rafters since they are sloped. This is very confusing especially when I read about using threaded rod for the suspenders.

The best idea I can come up with is to create a system using 2 long pieces of angle iron or unistrut at least 5-1/2' (length of the coil and furnace combined) to sit my equipment on. Then add 2'-3' angle iron saddles underneath everything for bolting to each rafter with the pipe strap in 2 places, 1 front and 1 back. I would also bolt the saddles to the long pieces on the bottom. Would this be against code since these saddles would attach parallel to the rafters? Or the 5-1/2' pieces could somehow be attached to the rafters and then the pipe strap saddled under the equipment and over the long pieces and attached along them where needed. However this method would not resolve the problem with the coil's access door being in the way so I would have to provide saddles for it. Or as a third option, I could combine both methods and put a pair of the long pieces in both locations, under the units and on the rafters giving me the freedom to attach the saddles where needed also. The result would give perpendicular attachments from the saddles to the long pieces, and from the long pieces to the rafters as well. Once more, this still leaves me with the question as to how to properly attach the long pieces to the edge of the rafters.

Please provide any advice so that I can make sure the next person who suspends my equipment does the job properly. And, thank you in advance for your time and help.

Confused homeowner,
Brenda Comrie

Hi Confused Brenda,

Around here, I see a lot of different methods of support in attics. Sometimes, the contractor will support the furnace with lengths of fascia gutter that rest on the plywood platform. Hanging the furnace from the trusses or rafters with threaded rod and Unistrut/Superstrut channel in a "trapeze" configuration is also pretty common.

The one problem that I see is when supported with the "trapeze", the furnace cabinet can sag in the middle. The method that I like the best is when they use a "Hung-Rite" or modify the trapeze by securing an additional two lengths of channel on top of and perpendicular to the "trapeze" so they run the full long distance of the furnace, plenum and a/c coil box (if present). The threaded rods are supported with angle brackets that are typically through-bolted through the rafter. Longer systems that include an a/c coil box might need an additional "trapeze". "Hung-Rite" uses cables rather than threaded rods and bolts directly to the rafter. While I know cables are plenty strong, I am more comfortable with threaded rod. It just seems more secure to

That said, while I recognize that the original install was likely done by a heating contractor, I recommend having a different heating contractor properly support the furnace.

https://www.unistrut.us/product-deta...r-trapeze-kits
https://hung-rite.com/
https://www.youtube.com/watch?v=gjpiUe_wVv8

[Jerry Peck]

Hanging the furnace from the trusses ... is also pretty common.

The magic word was in there ... "trusses" ... and no holes shall be bored through trusses without the the signed and sealed letter of approval by the truss engineer (or a structural engineer when the truss engineer is not available).

Additionally, with "trusses" (that magic word again), no additional loads are allowed on the trusses ... unless that signed and sealed letter from the engineer shows it is approved as it is at the time you see it (it may have been approved at one time, but something may have changed and the approval may no longer be applicable).

That is a good reason that a copy of the signed and sealed engineer's letter should be placed in a plastic bag (for protection) and stapled or otherwise permanently attached to the truss(es) at the area in question.

With no signed and sealed letter there at the time of the inspection, the only thing you can do it write it up and call it out to get one (whether that is done by the buyer, the seller, the agent, or no one is up to those involved in "the deal/transaction", but you have it on your reports).

With rafters, then it is up to what was approved at the time of construction, and if added there later, then someone (preferably an engineer) needs to say it is okay. You should also note anything which 'does not look right' or 'looks like it is failing' in the report as it provides support for the client to have it reviewed.

And this is a dead give away for it to be reviewed:

My suspended horizontal system is pulling loose from the rafters and the attachments on the units don't appear correct to me.

Anything "pulling loose from the rafters and the attachments" is not a good thing.