Re-duct on mobile home-bad air flow?!?

[airdesignguy]

Hello fellow HVACer's,

I think I'm in over my head on this one. I re-ducted a mobile home with flex duct to get better air flow to each room. It is much better, but not good enough. We installed a new Intertherm downflow unit and paid for the optional 5 ton fan set up. Static pressure went to .4 iwc!! Called Nordyne, they told us to lower the fan speed, basically back to 4 ton volume. Now it is .15 iwc, which is great but the air in the mobile is weak. We added booster fans to the trunk lines and it improved marginally. The owner is on the verge of demanding his money back. I have never failed a re-duct. How can I improve this situation? Who knows the most about mobile home systems? Any web-sites on the topic? I'm never touching a mobile home again.

[bigtime]

If you arent moving enough air, why dont you put the fan back on high speed?

[ISOTHERMAL]

First off I'm not an expert in mobile home airflow. But exactly what all did you change and is the furnace you installed approved for a M/H? A M/H has high static ducting. I personally do not believe that you can do much to improve the airflow in a M/H. The ducting in a M/H is not designed like conventional duct systems in resi. homes.

[r22jjc]

hey bro. i can help you out. how exactly did you reduct?how big is the home?does it have sidewall registers?is it gas or elect. ? i hate to admit how many of these darn things we have done,they can be a real pain in the ........

[beenthere]

So what static pressure is the furnace rated for.

.4" TESP would be great for a regular home.

A drop from .4" to .15" is about a 60% reduction in air flow.

Get rid of the booster fans, and put the fan back to high speed.

[airdesignguy]

When the unit is in high speed it does not have any return pressure at all. High amp draw, no air flow. A piece of paper won't even stay against the return grill. The guys at Nordyne admit that they do not recommend the 5 ton blower for this reason. The blower deck has a bad design, they suggested cutting it open to allow more airflow, I was not comfortable with doing this. They also said that .4iwc was too high. So I guess this is not a high static system. It is a M/H system. Out here in S. Cal Coleman/Intertherm are the top sellers for M/H's. The air delivery is different from a regular home downflow system, it blows at the back of the cabinet, and the only place for the air to go is down. In other words it is not actually blowing down, it is blowing horizontally. Not a very good design. The .4iwc is not TESP, it is just the return static pressure. Yeah total external static pressure of .4 would be outstanding! But I have a TESP of .8, not good!

[airdesignguy]

When the unit is in high speed it does not have any return pressure at all. High amp draw, no air flow. A piece of paper won't even stay against the return grill. The guys at Nordyne admit that they do not recommend the 5 ton blower for this reason. The blower deck has a bad design, they suggested cutting it open to allow more airflow, I was not comfortable with doing this. They also said that .4iwc was too high. So I guess this is not a high static system. It is a M/H system. Out here in S. Cal Coleman/Intertherm are the top sellers for M/H's. The air delivery is different from a regular home downflow system, it blows at the back of the cabinet, and the only place for the air to go is down. In other words it is not actually blowing down, it is blowing horizontally. Not a very good design.

[airdesignguy]

hey bro. i can help you out. how exactly did you reduct?how big is the home?does it have sidewall registers?is it gas or elect. ? i hate to admit how many of these darn things we have done,they can be a real pain in the ........

The home is a double wide, about 1500 sq. ft. I reducted with R-6 flex duct, removing the old and many 10 x 4 floor cans and replaced these with either 14 x 8 x 8 or 10 x 10 x 8, depending on free space under the floor. We eliminated the numerous old supply cans and used one supply per room, just like a regular house. It is a gas, split system, downflow. The return is not ducted, just a grill on the wall of the closet where the furnace is located. The system is not designed to have a ducted return.

I am anxiously awaiting your response. Today we installed solar shades for the customer to help reduce the heat until we can figure this out. He has generously given us 1 week to resolve his issues.

[beenthere]

By return grille, you mean the retturn in the furnace door?

[beenthere]

What size flex did you run.
Did you run a new trunk, or a plenum with teh flex supplies off the plenum.

Your customer is use to the throw of the smaller registers.
The larger regisgters won't give the same throw he is use to.
He is associating velocity with volume.

And the larger registers have volume, but less velocity.

[airdesignguy]

What size flex did you run.
Did you run a new trunk, or a plenum with teh flex supplies off the plenum.

Your customer is use to the throw of the smaller registers.
The larger regisgters won't give the same throw he is use to.
He is associating velocity with volume.

And the larger registers have volume, but less velocity.

Yes we ran an 18" trunk to a supply plenum in the middle of the home so all of the ducts are of equal length to their terminations.

The old smaller registers were not moving enough air to compete with the heat load on the house. That is why we re-ducted it. Apparently, it is not even close to reducting a house. I do not understand why, except for the faulty blower design of the furnace. Even the techs at Nordyne said it has always been a problem with M/H's and the airflow "dead heading" into the duct system with no real flow.

[airdesignguy]

By return grille, you mean the retturn in the furnace door?

Yes

[beenthere]

Yea, MH's always have a lower air flow.

Whats your supply static with the closet door open.

I thnk your main problem is that he is expexting the air to hit the ceiling.

Are all the supply flex runs 8"?

[r22jjc]

did you come right off the bottom of the furnace with 18" round? if your ducting is to large you loose your pressure, you might need to reduce .i've come off the furnace with a tee, with a diverter in the center,with 18"throats,then ran hard pipe out to y's,around here flex wont last long under there.some of these older homes refuse to be cooled,they have such a crappy return system.you can put flat grills at the ceilings on the outside rooms to get more hot return back to the furnace. do yourself a favor and pull the insulation out in the blower compartment,it will come loose.i guess youve looked the furnace over real good,quality control has fell off at nordyne over the last couple years,

[airdesignguy]

Yea, MH's always have a lower air flow.

Whats your supply static with the closet door open.

I thnk your main problem is that he is expexting the air to hit the ceiling.

Are all the supply flex runs 8"?

7" & 8", with a couple of 5" thrown in for the bathrooms

His expectations are reasonable. The home will not cool below 84. I've looked at more modern M/H's, and they seem to have the same airflow as a house. Why can't I acheive this since we removed the old ineffective perimeter duct and went with a room by room duct system?

[airdesignguy]

did you come right off the bottom of the furnace with 18" round? if your ducting is to large you loose your pressure, you might need to reduce .i've come off the furnace with a tee, with a diverter in the center,with 18"throats,then ran hard pipe out to y's,around here flex wont last long under there.some of these older homes refuse to be cooled,they have such a crappy return system.you can put flat grills at the ceilings on the outside rooms to get more hot return back to the furnace. do yourself a favor and pull the insulation out in the blower compartment,it will come loose.i guess youve looked the furnace over real good,quality control has fell off at nordyne over the last couple years,

That sounds like a good approach since I'm dumb founded at this point. But won't reducing the main supply trunk increase my statics?

[beenthere]

What size A/C is it.

[airdesignguy]

What size A/C is it.

4 ton condensor, 5 ton e. coil, 4 ton air handler with 5 ton blower wheel/motor upgrade.

[beenthere]

The coil should only be around ,25 to .3" PD with some where around 1600 to 1800 CFM through it.

The transition from the furnace to the 18" and then to a distribution box could be dead heading to much.

Now, the next thing. Around here. Its Coleman, and Miller mostly, got a few Intertherms.

But, as far as I know, none of them move 2000 CFM with their blowers set to high. The most I know of is 1700 CFM, and that has almost no duct work on it.

Above .6", they don't move much air at all. .6" is about 1500 CFM.

You said you changed most of the reg boxes to 8" taps.
If you have 7" flex running to a 8" tap, you can lose velocity out the register when your using large registers, and the air won't throw far enough to mix, and it will make it that you can't cool the place right.

How much air, did you size your duct system to move?

Next time you go back. If you cacn get, and post pics, it would help.

[Poodle Head Mikey]

That would be my first question.

PHM
--------



Hello fellow HVACer's,

I think I'm in over my head on this one. I re-ducted a mobile home with flex duct to get better air flow to each room. It is much better, but not good enough. We installed a new Intertherm downflow unit and paid for the optional 5 ton fan set up. Static pressure went to .4 iwc!! Called Nordyne, they told us to lower the fan speed, basically back to 4 ton volume. Now it is .15 iwc, which is great but the air in the mobile is weak. We added booster fans to the trunk lines and it improved marginally. The owner is on the verge of demanding his money back. I have never failed a re-duct. How can I improve this situation? Who knows the most about mobile home systems? Any web-sites on the topic? I'm never touching a mobile home again.

[airdesignguy]

That would be my first question.

PHM
--------

Don't mean to be a smart ass, but if it did, I wouldn't be here discussing this, and there would be no need for a re-duct:cool:

[airdesignguy]

The coil should only be around ,25 to .3" PD with some where around 1600 to 1800 CFM through it.

The transition from the furnace to the 18" and then to a distribution box could be dead heading to much.

Now, the next thing. Around here. Its Coleman, and Miller mostly, got a few Intertherms.

But, as far as I know, none of them move 2000 CFM with their blowers set to high. The most I know of is 1700 CFM, and that has almost no duct work on it.

Above .6", they don't move much air at all. .6" is about 1500 CFM.

You said you changed most of the reg boxes to 8" taps.
If you have 7" flex running to a 8" tap, you can lose velocity out the register when your using large registers, and the air won't throw far enough to mix, and it will make it that you can't cool the place right.

How much air, did you size your duct system to move?

Next time you go back. If you cacn get, and post pics, it would help.

Apparently this is where I made my mistake. I sized it to 1800 cfm, assuming .5 iwc static. We used 14 x 6 cans w/7" flex and 14 x 8 cans w/8" flex. How do you recommend I fix this without re-ducting everything?


Side note: Beenthere, I appreciate your professionalism when you respond to posts. I have seen several responses from you throughout this site, and you show experience and well thought out responses in your text. Unlike many others who strike me as arm chair techs who either do not have any real world experience or feel they need an ego boost by writing condenscending responses to less qualified technicians. We need more of you on this site! Thank you.:)

[Poodle Head Mikey]

I was curious if there was an actual performance issue. Or . . . just a complaining customer. <g>

I've only re-ducted one mobile home in my entire working life and all the new ducts were sheet metal and installed on the flat roof. <g>

PHM
--------




Don't mean to be a smart ass, but if it did, I wouldn't be here discussing this, and there would be no need for a re-duct:cool:

[airdesignguy]

I was curious if there was an actual performance issue. Or . . . just a complaining customer. <g>

I've only re-ducted one mobile home in my entire working life and all the new ducts were sheet metal and installed on the flat roof. <g>

PHM
--------

Apparently this has been a problem for 2 years. It is a rental. Another company put in a new 4 ton condenser about 5 years ago, but left the original e. coil from 1979. They also tapped 12" ducts from one side of the perimeter metal duct to the other side of the M/H?!?!?!?!?! So I suggested a larger air handler and coil. When that was not enough, we re-ducted it. It is much better now, but not as good as I was expecting. The renter would like to reach 79 degrees. This is a fair request, I'm just stumped.

[rob in lvnev]

Dont want to afend anyone why dont you go with a package unit and reduct I did with my Trailer and got rid of the split system work perfectly and saved $$ on power and gas by 50%

[beenthere]

At this moment. Not sure how to make what you curently have work.

If you can go there tomorrow.
Measure the temps at floor level, and 4' and 6' above the floor.

Its posible, that the cold air is just basically falling back to teh floor, before it gets 2' out of the register.

If that is the case. A rgister that has a better throw may be able to mix the air better, and compensate for the low air flow.

Thats a shot in the dark from my chair, but worth checking.

[r22jjc]

neck everthing down,thiis stuff is made to blow thru 4 x14 duct,put something in your boots to make them 4x12 or 4x10 .