Attic furnace install: Floor mount or suspend from rafters?

MysticCobra · Jan 26, 2008

I am considering bids from three contractors regarding complete replacement of my A/C and heating in Houston, TX townhouse. The inside equipment will be in the attic, which is typical for this area. The original equipment installed when the home was built was supported on the attic floor.

Two of the contractors propose installing the new equipment on the floor like the original system was installed.

One proposes suspending the new furnace from the rafters.

Both camps claim that their installation method will reduce noise into the house. All three offer a Lennox G60V (this is Houston, remember--not so cold in winter, so the G61V is not really worth it), which is supposed to be about as quiet a blower as you can get. Since even my current 25-yr-old Magic Chef isn't audible in the living space of my house, I'm really not too concerned about noise coming through the floor.

Unless I've missed a thread, I've gathered that as long as the install is done competently, both methods can be acceptable. Do I have that right?

I'm a little skeptical of the roof mount. I don't know how to judge the integrity of the truss system, and don't know if it could be compromised by hanging a few hundred pounds off it. Since I haven't had a rafter-mounted system before, I don't know if that might introduce sound to the house where there wasn't any before.

The floor-mount guys also say that rafter suspensions introduce the risk of metal fatigue at the gas line connection, if the furnace vibrates or sways. Is that a real concern, or just scare tactics?

Thanks,
Jim

HVACSvc08 · Jan 26, 2008

I'm in N.C. and most of the furnaces that I install, they mount on the floor on blocks vibration pads b/w the blocks and the furnace. Also they could brace it to the rafters so that would keep it from vibrating when it's running.

t527ed · Jan 26, 2008

we do them on platforms all the time, last one we did the customer wanted it suspended. construction official said if we wanted to hang it we would need an engineers report that the roof rafters would hold it...........

.
that one went on a platform to..

g60v furnace is so quiet you will never hear it, especially if you never heard the old one. we use rubber pads between unit and supports and have never had a complaint.

dash · Jan 26, 2008

Funny how things vary around the country.

Here in Florida ,code requires them to be hung from the top chord/rafter of the truss,to floor mount you need engineered design,as they say the bottom of the truss is only designed to hold the weight of the drywall.

t527ed · Jan 26, 2008

most of what we do attic systems in are older stick built houses, last truss house we did was new construction and builder beefed things up for our platform.

Catpower · Jan 26, 2008

It really doesn't make alot of differance, when they first came out with inshot burners they would get rather noisy if installed on the deck, but they're alot quieter now.

If there is concern about whether the rafters or deck will hold the weight of the unit,you have alot more problems than deciding on how to mount it.

skippedover · Jan 26, 2008

We generally install on a platform. We're in snow country so nobody really cares which way it's done as both the ceiling joists and roof rafters are plenty strong to suppor the weight of a furnace and we do find them suspsended. But out issue is mainly the overflow drip pan. If the unit is mounted in the pan and raised up on blocks or other non-conbustible materials, you know the pan is not going to move and be misaligned with the furnace or AC coil. When they're suspended, we usually find the pan installed on the collar ties, under the unit and subject to being knocked out of position. I did see one recently in a commercial applications where the was not attic above and the installers had suspended the furnace and left the threaded rod long enough to include a second layer of Unitstrut beneath the first holdling the overflow pan. That was a good solution but they never put anything under the drip pan to provide support. So if the pan had ever filled with water it would have creased in the unsupported area. We replaced the furnace, used the same arrangement but put a 1/2-inch plywood sheet under the drip pan.

I'd got with a Goliath drip pan mounted on the floor personally. But honestly, there are cable hangers that will not transmit any noise. Gas piping, well that's another issue entirely. I've never seen gas piping suffer from a unit being suspended but there's always a first time, eh?

MysticCobra · Jan 27, 2008

skippedover said:
We generally install on a platform. ... But our issue is mainly the overflow drip pan. If the unit is mounted in the pan and raised up on blocks or other non-combustible materials, you know the pan is not going to move and be misaligned with the furnace or AC coil. When they're suspended, we usually find the pan installed on the collar ties, under the unit and subject to being knocked out of position.

That's an interesting consideration. In the installs I've looked at here in Houston, I've never seen a unit "mounted in the pan". Even for floor-mounted furnaces, the evap coil is usually strapped to a roof member for support (so there's no bottom support to interfere with the drip pan), and the drip pan is arranged on the floor underneath with a tilt to encourage flow to the overflow drain.

I've never given a drip pan a "tug test" to see how much it could move. Now that you mention it, I suspect a good swift kick to the overflow drain would send most of those drip pans flying. I guess that's one reason why I generally watch where I step around an A/C!

cem-bsee · Jan 27, 2008

MC-- what is the closest city, in which state?

hard for us to judge your present rafter or truss loading from here!

but, one can always beef- up either framing member & 'spread' the load.

MysticCobra · Jan 27, 2008

cem-bsee said:
MC-- what is the closest city, in which state?

From my original post: "...in Houston, TX townhouse. ... (this is Houston, remember--not so cold in winter, so the G61V is not really worth it)".

cem-bsee said:
hard for us to judge your present rafter or truss loading from here!

I understand--as I mentioned in my first post, I'm right here and it's hard for *me* to judge my rafter / truss loading. But I didn't really ask for an evaluation of my specific situation; my questions were very general:

I've gathered that as long as the install is done competently, both methods [floor mount or rafter suspension] can be acceptable. Do I have that right?
...
The floor-mount guys also say that rafter suspensions introduce the risk of metal fatigue at the gas line connection, if the furnace vibrates or sways. Is that a real concern, or just scare tactics?

I've posted several questions on this forum now, and gotten some fantastic help from many of the professional members. Many thanks to all who have replied--this forum has been a fantastic source of information, and I really appreciate it!

Jim

neophytes serendipity · Jan 27, 2008

MysticCobra said:
One proposes suspending the new furnace from the rafters.

Roof truss systems are usually speced for the minimum weight and wind load requirements needed. Drywall, OSB sheathing and asphalt shingles weigh a lot.

That means there isn't much extra capacity for the weight of an HVAC system, especially if there are only 3 pairs of anchor points distributing the weight (two on the air handler, one on the cased coil).

The weight is distributed better if a length of Unistrut spans four or five trusses, and the equipment is hung off of that... but that costs more.

If the original location worked, then that's where I would insist on having it installed.

A plywood/extruded foam/plywood sandwich does wonders for noise isolation.

Make sure the pan under the equipment is large enough to do the job.

a/c-harris · Jan 27, 2008

In dallas they have to be hung, and I would suspect in houston it is the same. Whenever I see one sitting on stands in a attic it is usually a very old installation. Having it hung is usually quieter. I guess if I was a installer I would rather mount it on the floor using stands because it is easier. Have it hung man.

classical · Jan 27, 2008

In Houston they do not need to be hung and either process is acceptable however many Townhomes have 2"X4" rafters ot 2"X6" so flor mount is acceptable.

I also now know who one of your bids is from and if I am correct they have dropped in quality of work over the last few years.

My furnace is hung and I occasionally hang furnaces but it genrerally is not the best option or worth the extra expense or effort.

Most furnaces today make more noise than the old Magic Chef unit you currently have because they have so much more going on. The main reason for hanging the furnace is to eliminate vibration from transfering to the ceiling joist. As long as the blower wheel stays clean and doesn't through a blance weight that should not be a problem.

As to another statement made in another of your post flex duct in Houston must be supported every four feet to pass inspection.

As far as a potential problem with the gas line that is just bunk!

Wittyfull · Apr 8, 2010

I am in the exact same scenario - townhouse in Houston.

The rafters in my roof look really weak and I am concerned about hanging the evaporator/air handler unit from there. I'm thinking I should place the unit on the floor of the attic. Would any HVAC person know how to do this? What parts are needed? What construction techniques are needed for the support to minimize NVH?

Thanks,
David

neophytes serendipity · May 16, 2010

The "floor" of your attic is no different than the "roof" because neither is engineered to support more than the expected loads.

The "floor" is designed to hold up drywall ceiling below and not much else.

The roof is designed to hold the sheathing, roofing material and additional loading from wind, rain and snow as required by location.

The air handler in an attic can easily exceed 200 pounds once stuff is attached to it.

Locating it above a wall below is always a good idea.

Locating it parallel to the framing between (ie: center span) load bearing or partition walls is a bad idea.

Bridging 2 rafters for each anchor point will divide the added load by 2 compared to using a single rafter as an anchor point.

The above 2 items are more for stick vs truss framing.

Truss framed roofs do not typically have any "extra" capacity. They are not designed to support a load from the bottom of a roof framing chord or anywhere along the "floor". They can be made for this, but typical "to code" residential construction is the legal minimums.

I would locate the equipment above a wall whenever possible.

One solution to NVH is a wood/extruded foam/wood sandwich.

bell3156 · May 16, 2010

ya, here in phoenix. just did a new cut in, made platforms for 2 airr handlers, looked great, set up on 3 4x4's each, iso pads, the pltaforms spaned 5 trusses... the customer is wants a tile roof, had an engineer do a roof load, engineer wants me to hang them on 3 rafters a each, using strut and all thread... I think the platform is of better quality... the strut worked out, looks good... the drain pans have great platforms to sit on

Wittyfull · May 29, 2010

Thanks for the valuable input.

1maytech · May 29, 2010

Never had issues

Never had any issues with both ways of installing them. On the floor make sure you have vibration issues addressed.

havdaddy · May 29, 2010

Same here. We always set them on blocks w/ pads. A lot easier to level for drain issues also. We have also had to reinforce platforms to accept the weight.

nvr2old · Aug 21, 2011

I wish every attic was as neat and pretty as the ones in the quick sling ad.

nvr2old · Aug 21, 2011

A/C Harris, hanging a unit is not manditory in Dallas

genduct · Aug 21, 2011

guy that wants to "hang unit" is going the extra mile

Hanging the unit not only helps the acoustics but also gets the unit above the insulation so that there is no chance of condensation.
Also with the extra height there is less of a problem with condensate drain.
Oh, gas pipe concern handled with flex connector

catmanacman · Aug 21, 2011

i like to hang them but i have installed many furnaces on cinder blocks with isolation pads also on the g60 furnace with ac if you go with the honeywell iaq thermostat and have it wired correctly it will slow the blower for dehumidification

hvacvegas · Aug 21, 2011

Floor vs roof mounting:

Are either designed to hold the weight of the unit?

Is so:

Is there a good place to put the unit? Yes: floor. No: hang it.

precision hvac · Aug 21, 2011

We hang most of ours.
Just looks better. We use 1 5/8 unistrut w/ lags to span several rafters & hang unit w/ 3/8 rod. We use strut under the furnace as well to aid in supporting the middle of the unit. We then sway brace all 4 corners of the S&R plenums since this is earthquake country.

Never had one fail inspection being hung.

motoguy128 · Aug 22, 2011

Snow loading is typically desinged to 20lb per square foot and roofs can easily handle 3 layers of asphault shingles... and some folks are actually concerned at all about a little 200lb furnace? As long as the weight is dstributed equally from 4 points, I can't imagine any issues on the attic floor or hanging.

For the record, floor joists in attcs are not just for holding the weight of the drywall. their primary funciton is acting in tension to supports the roof supports.

Older homes (those built before WWII) are often built far, far more substantial and often use 2x6's rather than 2x4's for all framing... still on 16" centers. Floor joists are often 2x8's. Before 1930's the wood is older growth and significantly stronger as well with fewer knots.

Just something to keep in mind.

genduct · Aug 22, 2011

For the record,

ceiling joist are designed for 10, 20 or 30 lbs per sq ft even the 10 lbs with the load near the sides will not compromise the structure
As far as the roof rafters are concerned, under the theory of "plastic design" if one structural member becomes a little over loaded the load automatically begins to find other avenues to self support. Just like we have cooling load diversity, structurally we have the same thing so NOT TO WORRY it won't fall down

RoBoTeq · Aug 23, 2011

Truss roofing is designed for the roof supports to hold weight with the rest of the structure designed to support the roof joists. As has been mentioned, the ceiling joists of trusses is rated primarily for the weight of the ceiling and insulation and is not recommended by truss manufacturer's for supporting HVAC equipment. That said, I've never seen a problem with having units supported by the ceiling joists of trussess.

Personally, I built a platform at a working level or as high to a working level as I could for attic installs. As mentioned for hanging units, I wanted the indoor unit at a comfortable level to work on, high enough to never have a condensate issue and out of the way of insulation.

What would you guys think if there were an easier way to hang a unit? Would you be more inclined to do so?

t527ed · Aug 23, 2011

the problem we have with hanging units is the inspection depts are requiring an engineer to sign off on the trusses ability to handle the load.

its just easier to build a platform over an interior wall and not deal with the engineering report.:.02:

that and to many goofballs that do hang them do it in such a way that it is impossible to service the unit years down the road.:gah:

RoBoTeq · Aug 23, 2011

t527ed said:
the problem we have with hanging units is the inspection depts are requiring an engineer to sign off on the trusses ability to handle the load.

its just easier to build a platform over an interior wall and not deal with the engineering report.:.02:

that and to many goofballs that do hang them do it in such a way that it is impossible to service the unit years down the road.:gah:

This fascinates me since the entire purpose of a truss is to provide roof support. Every part of a truss is designed to allow the roof joists to carry more weight for roofing and snow and ice load.

This is why all of the bridges to New Jersey only charge a toll to leave New Jersey; they know that everyone will be willing to pay.

genduct · Aug 24, 2011

Really tickled by this focus on structural capacity

The Building inspector is really getting you to get an engineer to sign off for the incidental load of a unit? WOW have they no common sense?. How about the incidental load of a person even getting into the attic to access the unit?
Can't put your snow tires or Christmas decorations up there without an engineering review!
Spanning several joist with unistrut that really gives you more options with the hanger locations, any other common sense approach sounds good to me

We are NOT talking about installing something the size of a small pick up truck, just a stinking little unit which as I think someone mentioned is structurally best when hanging from the rafter/ top chord and not the bottom chord of the truss.

Again the choices are 10, 20 or 30 extra pounds per square foot of extra load is built in to the truss design.

Just shows that we need to broaden the education of all trades to include simple structural understanding.
Of course, my problem is I started as, and will always be a carpenter.
There, now you know my dirty secret which explains all!

precision hvac · Aug 24, 2011

genduct said:
The Building inspector is really getting you to get an engineer to sign off for the incidental load of a unit? WOW have they no common sense?. How about the incidental load of a person even getting into the attic to access the unit?
Can't put your snow tires or Christmas decorations up there without an engineering review!
Spanning several joist with unistrut that really gives you more options with the hanger locations, any other common sense approach sounds good to me

We are NOT talking about installing something the size of a small pick up truck, just a stinking little unit which as I think someone mentioned is structurally best when hanging from the rafter/ top chord and not the bottom chord of the truss.

Again the choices are 10, 20 or 30 extra pounds per square foot of extra load is built in to the truss design.

Just shows that we need to broaden the education of all trades to include simple structural understanding.
Of course, my problem is I started as, and will always be a carpenter.
There, now you know my dirty secret which explains all!

Truss or rafter- I still would rather hang with strut than lay it on a platform. Especially if it's a 90+ furnace (condensate drain exits on left side of furnace when laying down- which puts it at bottom). Now for example- I'll use a 5ton Rheem for example. Furnace & coil are 68" long. Add an EAC or media filter & supply & return plenums & that package is now close to 10 ft long. I use 2 pieces of 1 5/8 unistrut running parallel to system bolted to each rafter w/ 3/8x2 lag bolts (pre-drilled to prevent splitting). If rafters are on 16" centers, I'll be lagging & spreading that load over 8 rafters per strut. On 24" centers it would be over 6 rafters per strut.
Trust me- I've seen some horrible jobs hung w/ nylon straps that got signed off. Inspector wouldn't want to go there with me.

So I guess when working in attic w/ a helper I need to rig a zip line & harness to span the middle of attic & attach where structure is over a bearing wall & harness up. Oh brother.

Attic furnace install: Floor mount or suspend from rafters?

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