Quieting the Suburban furnace model NT-16SE, same as the NT-20SE

[Frederick L. Simson]

Originally Posted by DavidSo

Leaving out the model was a rather glaring error, sorry! I tried to go back and edit my first post to include that but couldn't.
It is a NT-16SE, same as the NT-20SE

I added the model info to your Thread Title.

[DavidSo]

Gene,
I can't view it without joining. Maybe I will.

Sherry,
Yeah, the housing acts as a reverberation chamber. Every noise that is produced is magnified. You could take an electric shaver that makes a nice low hum and touch the housing . Suddenly you have a very loud and annoying shaver! That part is actualy very easy to fix and I know exactly how to do it. I should have the materials for that in a few days. The mounting was disturbing frankly. I doubt that it would become an issue for most people, but I drive some aweful roads! Sooner or later that rear of the furnace would give up. The installation manual clearly states that the bottom of the unit must be supported. It is not. Aside from the mechanical strength issue, securing it will help quiet it as well. Stay tuned

Truthfully, now that I can see the issues more clearly I'm less annoyed about the unit itself. It's not as big a pile of junk as it sounds . For about $20 more Suburban could have fixed this problem though. On a $500 unit that should have been done IMO. That part IS annoying, but I know how it happens. To go in after the fact as I am and re-engineer it will probably cost me $75 to $100

David

[DavidSo]

Originally Posted by Frederick L. Simson

I added the model info to your Thread Title.

Thank you Frederick!

Ok, here is some additional info that I have gathered that may prove useful. I found a PDF of the service manual for these furnaces. This is what the service guys get. Even if you never service your own furnace, you should download this! In the back it has a list of estimated service times for all the common repair procedures. This might keep you from being over charged by an RV repair shop. The time they listed for the motor replacement is 1.6 hours, which sounds right on to me. These are the times that Suburban will reimburse for warrantee work, so you should not pay more either unless some circumstance warrants it. Here's the link: http://bryantrv.com/docs2/docs/ntseries.pdf

Another bit of strange info that I have gathered is this. Suburban makes two furnaces based on the NT design. They are the NT-16se, and the NT-20se. They are identical except that the 16 is 16,000 BTU and the 20 is 19,000 BTU. When I say identical, I mean they share the EXACT same parts! . Everything right down to the gas jet is the same, so how the heck is one 16K BTU, and the other 19K ?? No where is this descrepancy discussed. I'll try and dig up an answer, but one positive implication is that the unit is designed for 20-25% more heat generation than the 16 produces. It's always good to have a cushion.

David

[Donna D.]

Originally Posted by DavidSo

Gene,
I can't view it without joining. Maybe I will.

You won't be sorry if you do. These molded trailers are all more alike than different and there's tons of good technical stuff on the CasitaForum. Ask me how I know

[Gene L]

Originally Posted by DavidSo

You wouldn't happen to have the rated CFM of the room fan would you?
David

According to the Suburban service manual that I have (available on my forum) it says the "Max CFM" for the NT-12SE & NT-16SE is 140 and the NT-20SE is 150 so it looks like the only difference between the models is the airflow?
Gene

[Byron Kinnaman]

One of the big problems with attempting to re-engineer something like this is not knowing all that was taken into consideration during the design process and all the information gathered since the initial release. Generally speaking when something is released for consumer use there's been lots and lots of hours spent in making the design work within the design requirements. It's just one person either, it's several. Have no real idea why the air flow is set where it is, but I'm sure there's a very good reason for it. The over heating was a speculation on my part knowing a bit about air flow, heat management, and other interesting engineering things.
Here's where a bit of research could come in handy. I suggest that an attempt to contact any furnace manufacturer's engineering department. See if you can find an amiable engineer that might shed some light on why the air flow is set where it is. There is a reason.

It's usually quite difficult to have a successful re-engineering of a product that's been around for as long as these little furnaces have been around.

Good luck and be safe.

[DavidSo]

Originally Posted by Byron Kinnaman

One of the big problems with attempting to re-engineer something like this is not knowing all that was taken into consideration during the design process and all the information gathered since the initial release. Generally speaking when something is released for consumer use there's been lots and lots of hours spent in making the design work within the design requirements. It's just one person either, it's several. Have no real idea why the air flow is set where it is, but I'm sure there's a very good reason for it. The over heating was a speculation on my part knowing a bit about air flow, heat management, and other interesting engineering things.
Here's where a bit of research could come in handy. I suggest that an attempt to contact any furnace manufacturer's engineering department. See if you can find an amiable engineer that might shed some light on why the air flow is set where it is. There is a reason.

It's usually quite difficult to have a successful re-engineering of a product that's been around for as long as these little furnaces have been around.

Good luck and be safe.

Byron,
It is entirely possible that the solution will be a fan that produces the same amount of air, but at a lower noise level. Steeply pitched metal blades like the one used in the furnace are cheap but noisey. Motors with brushes are noisey. There are fans that turn significantly higher RPM with shallow pitch, produce the same CFM, and are quiet. My comment about air flow was that the scamp does not need 140 CFM to move the air around that small space. If lower air flow results in an overheat, that will be revealed in the testing.

It's also possible that my work could result in a SAFER operation. Here's one example that I'm looking at. To get 140CFM out you have to have 140CFM in. Where is that air coming from? There is no dedicated air vent to supply the heater with room air. The air comes from passages left around cabinets, around the doors, stovetop, and other places. Nobody would block a dedicated air vent, but various modifications or cargo could easily block passages that normally supply this air. If for some reason, the passages were blocked enough that the fan produced a vacuum in the cabinet, it could possibly suck CO out of the exhaust connection. Most people probably don't know, but the exhaust connection is not a solid connection. It is a loose slip joint that depends on the scavenging effects of the passing exhaust and a neutral cabinet pressure to take the gas out. I really don't like that. Such an air restriction could also result in overheating. The unit can be rated for whatever you want, but the airflow will not be greater than what is available to move. Restrictions increase noise too. Put your hand over the hose on your shap vac and listen to the motor.

The final operation and safety of these units is the product of the original design and the installation.

What I'm saying is that my work is getting a lot of attention because it is on the furnace itself, but people could be endangering themselves more with something as seemingly innocent as a cabinet modification. I'm trying to de-mystify some of this for the general good. Any modification to a trailer, whether simple or major could effect things that impact safety in unforseen ways.

I should also restate that what I'm doing is 100% for my own use. I'm sharing the process for the sole purpose of information exchange. It's not an attempt to convince anyone of anything. People can (and should)judge the competency of what I do for themselves and act accordingly. I could post my credentials, but what use is that on the internet? It would be no more believable than anything else I'm doing.

David

[Rob and Linda]

Originally Posted by DavidSo

<<< snip >>> The modifications will include sound deadening of the sheet metal housing, <<< snip >>>David

David...

Damping the vibrations of the sheet metal housing is going to give more bank for your buck that you would ever imagine. I have applied high-temperature "sound deadning" sheet to selected portions of the housing with remarkable results. My intentions are to pull the guts out of the furnace housing and judiciously apply additional vibration damping materials... so much of the furnace and its housing truly ring like a gong! Every bit of vibration from the fan(s) is efficiently transfered into the sheet metal, exciting resonances and vibrations all over the place... dang!

I don't know if you have sniffed around the other [Casita] Forums about this topic... several of us have been fiddling with (albeit pretty sporatically) trying to quieten the furnace for the past year and a half or so.

An "ideal" solution for me (and one that would not modify the basic engineering of the furnace) would be (1) proper vibration damping of thesheet metal, and (2) a "soft start" electronic circuit that would ramp up the fan over aone or two-second period (from OFF to FULL ON)... it's that jarring rrrrrrrooooooooaaaaaarrrrrrrrr when the fan turns on that drives me bonkers.

Cheers!

Rob

[DavidSo]

Rob,
Yeah, I wish I still had my sound measurement equipment to quantify the effect that each step has. If you have read through my posts, the metal housing is high on my hit list. I do not believe that additional sound deadening mat on the inside will gain you anything though. It would if the problem were noise transmission through the housing. The problem though, as you stated is resonance. Once the resonance is killed, that magnification of the vibration is done. I'm pretty sure the outer coating accomplishes that. To prevent the noise from the fan from exciting the housing it would have to be enclosed (impossible). The sound shield would have to include a suspended septum. This type of sound containment would be more useful on the inside of the cabinet than the inside of the housing.

My mat just arrived today and it is a new product to me called Boom Mat. It is high temp compatable and metal backed. I'll let you know if it's any good once I get it on.

I agree that the start up is jarring. That's what wakes me up. You are out in the woods in total silence and then CLANNNGGGGGROAAAARRRRRR!

At present I'm only a member here, so no I haven't looked around much. Donna and Gene are working my elbow joint pretty hard though

David

[GeorgeR]

Did you consider separating the two functions from the motor to one motor driving combustion air and another one for recirculating air in the trailer. You could then place the recirculating air unit in a place allowing for easy reduction of transmission motor vibration into a cabin and use input and output mufflers to muffle the fun noise. It would require a lot ducting work, but, good sleep is like gold just don’t engineer it to be a permanent one. See Truma side (in American English it should be spelled Wallet Trauma), somewhere there I have seen mufflers in their installations. I like their idea of combining a water heater with a space heater, unfortunately it is 220V and it is not approved on US roads. I while back my 1985 Westphalia VW had Eberspacher heater (gasoline powered) installed in Germany during my European delivery. This was a very quiet heater as well, but I don’t think they have a LPG model. Eberspächer UK
Truma - Gas-electric heating systems
Good luck,
George.

[DavidSo]

George,
Here's a quote from my post #8:
" Well now I have to do some research to find the right fans to fit the spaces I have here. Luckily there is plenty of space for the room air fan. The combustion fan may be a little harder to source. Instead of one motor driving two fans I'll have separate motors/fans."

2 fans was always the plan.

I understand your point about remoting the room air fan but that is not practical with the existing unit. The housing is the air duct and it is mounted very close to the wall of the trailer. I do plan to mount the room air motor on rubber isolators.

David

[GeorgeR]

Originally Posted by DavidSo

George,
Here's a quote from my post #8:
" Well now I have to do some research to find the right fans to fit the spaces I have here. Luckily there is plenty of space for the room air fan. The combustion fan may be a little harder to source. Instead of one motor driving two fans I'll have separate motors/fans."

2 fans was always the plan.

I understand your point about remoting the room air fan but that is not practical with the existing unit. The housing is the air duct and it is mounted very close to the wall of the trailer. I do plan to mount the room air motor on rubber isolators.

David

I missed that,

Good luck,

George.

[Roger C H]

Reading this thread I have a couple of thoughts:
1-Could the fan be replaced with one designed for electronics or computers?
2-Maybe the only difference between the two BTU rated models is a slightly bigger gas nozzle orifice on the higher rated one, something that wouldn't be obvious. A few thousandths larger diameter would increase gas flow enough. 16 to 20 is only a 25% increase. A 25% increase in area would be achieved by an orifice just 10% bigger in diameter. Changing anything else in the design would not be economically feasible. Probably designed for the 20 and reduced the orifice for the 16. Hmm. I wonder if they offer an even smaller orifice to reduce the BTUs to 12 or even 10? Maybe the burner fan changes a little for the different ratings, or a flow restricter for the burner air?

BTW: Lots of good information at: http://www.propane101.com/index.htm

Read especially: About Propane, Propane Tanks, and Regulators

[DavidSo]

George,
No worries! your ducted air idea has merit too. It could be done through the side but I'd sure like to avoid it if possible. It would be nice to have it all inside. An air duct to supply air from outside the cabiinet is also a possibility. I appreciate all the thoughts because i don't know where this project will go once I get into it. It could be that the ideal fan can't live in the housing because of heat. I hope that is not the case.

Roger,
I've thinking along some similar paths. The brushless 12V DC fans that I'm looking at are used for all sorts of cooling, including electronics. the smaller more common versions are the "muffin fans" that most of us are familiar with. I've used these fans for a number of projects over the years with good results. I have never had to replace one either. They seem to run forever and they consume little power.

As for the BTU, my suspicion is as you stated. Even though they list the jets as having the same part number, I'll bet they are different. They do make an NT-12se as well! That means that we could just swap jets and get a lower heat output. I have felt from day one that the heater in my trailer is too big. I think it would be better if it ran longer per cycle at a lower output. That would warm the trailer more before it shut off. As it is, the temp rises so quickly that the thermostat kicks off because of the spike in air temp. I would like to experiment with jet sizes. If we can get a 12K BTU jet we can always ream it if need be to customize the output. I'm guessing that 12k would work well.
David

[Rob and Linda]

Originally Posted by DavidSo

<<< snip >>> To get 140CFM out you have to have 140CFM in. Where is that air coming from? There is no dedicated air vent to supply the heater with room air.

Actually, there is just such a 'dedicated' return air grille in our 2010 Casita Spirit Deluxe.

Originally Posted by DavidSo

<<< snip >>> If for some reason, the passages were blocked enough that the fan produced a vacuum in the cabinet, it could possibly suck CO out of the exhaust connection. Most people probably don't know, but the exhaust connection is not a solid connection. It is a loose slip joint that depends on the scavenging effects of the passing exhaust and a neutral cabinet pressure to take the gas out. I really don't like that. Such an air restriction could also result in overheating. The unit can be rated for whatever you want, but the airflow will not be greater than what is available to move. Restrictions increase noise too. <<< snip >>>

Not only is there a grille, the backside of the furnace is in space common to the sink/stove cabinet and to the space beneath the [street side] rear bench. A year or so ago, I did a practical 'test' to see what would happen in I were to completely block the return air grille... nothing... the fan still blew like crazy with no apparent change in the 'sound' of the fan. Apparently, the whole 'under counter' area is so 'leaky' adequate air can easily be supplied even if the grille is 100% blocked. Of course, this was a sample size of one, which, in the grand scheme of things means nada!

[Rob and Linda]

Originally Posted by DavidSo

With two fans, the room air fan could continue to blow the sail switch while the combustion fan has failed. I think I know a way around this .

<<< snip >>>

David... I'm eager to see how you are going to work around this show-stopper of a problem. Failure here could ruin your entire weekend (well, at least it would probably be a short weekend ).

Rob

[GeorgeR]

If you decide to go with an inline blower Attwood makes a couple of units. Not knowing how this blower would function in the hot air I would place it upstream. Marine Blowers, Boat Blowers, Boating Lift Blower : Attwood Marine
George.

[Herb Sutton]

I have the same furnace in a Casita SD 17'. I had to go back and see that yours is in a Scamp. Casita has the output end resting on a piece of 2"x2" wood fastened with 2 screws. The other end rests on top of the wheel arch, but has no fasteners. I positioned it carefully when I installed it before boring the holes for the stacks.

Mine is out and apart at this time because the room air impeller came loose, slid off the shaft and was bent up. I do recommend using Locktite on the fan set screws.

I found that the guts will come out of the outer housing without difficulty. They are held by a metal screw adjacent to the propane fitting. Greatly enhances the accessibility.

[campfrugal]

David, I am watching your educational furnace upgrade with high interest. Mainly because I hate noisy electric fan furnaces along with my dislike of the catalytic type.

I am glad that you are making sure the 'Over Heat' switch is operating so you will have a 'Fail Safe' aka Fire Safe unit.

Your photos are really well done and appreciated.

I really wish the old 'Gravity' type Hydro Flame & Coleman furnaces were still available. They were quiet and did NOT drain the battery.

Thank you Dave,
Doc the boonDOCer.....of campfrugal

[DavidSo]

Thanks for all the replies and ideas! I'm still researching fans, and I got sidetracked by my real job . I'm pretty sure I can just add another sail switch in the blower plenum and wire the two in series. Some adjustment of the sail may be needed to match the lower output of the combustion fan. That way a failure of either fan will shut the unit down. I consider defeating any of the safety features as a failure of the project. It's not an option.
David