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Old 02-15-2018, 03:20 PM   #1
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Question Suburban Furnace Replacement - Any Reason to to move from SF-35 to SF-42?

TITLE SHOULD BE: Suburban Furnace Replacement - Any Reason not to move from SF-35 to SF-42?

Current furnace is Suburban SF-35 (original from 1995). Motor went out, I decided I will replace the entire unit.

Will go with the Q Models. Gas Input is in the Rear (Exterior Wall, have Suburban Access Door).

I see the SF-35Q and SF-42Q are physically the same (and same as the old SF-35). Amp draw is 8 vs 9 which is lower than the original SF-35 from 1995 which is 10amps.

The difference I see between the SF-35Q and SF-42Q is the static pressure, .2” WC for SF-35Q and .25” WC for SF-42Q.

Not sure why but Gulf Stream installed a 35K BTU furnace in our Coach. The 1995 and 1996 Brochures listed the furnace as 40K BTU.

Does anyone see any issues just upgrading to the SF-42Q? And would you upgrade?

The Price difference is only about $80.

I'm not even sure if we would notice the 5000 BTU difference but I think there is a little bit more airflow (430 CFM max vs 375 according to the manual below):

http://docs.renegaderv.com/Suburban/...11-05-2015.pdf

This Coach does have a LPG Generator (vapor) and new Propane tank and High Capacity regulator.

Thank you in advance


.
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Old 02-15-2018, 03:27 PM   #2
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My thought would be how often I get into cold weather. The extra BTU's will give you a bit faster warmup and a bit cooler lower temperature you can be comfortable at. You might take a minuscule decrease in efficiency. I'd seriously consider doing it for $80.
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Old 02-15-2018, 03:57 PM   #3
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We live in the Pacific North West and my wife is the type that is always cold.

It does seem the the 42Q is a tiny bit less efficient than the 35Q if you look at the output BTU. SF-42Q output BTU 30690 vs the SF-35Q's 28K or 27.3K (not sure which is correct).

I know too big of a furnace can also cycle on/off more often reducing the efficiency. Actually, in cold weather I think the current furnace cycled a lot. Sometimes bigger is not better.

Looks like Gulf Stream was installing 40K BTU units in most 36' around this Coach's year, not sure why they installed a 35K (the Manufacturer Label on the coach does list SF-35). Our is Coach is a 36'.

It's closer to an additional $110 difference with tax.

I suppose the existing propane line should be sufficient for the 42Q extra BTU?

Hmm, I now see the output BTU for the 42Q is only 30690, for some reason I thought it was closer to 32000 so it is less efficient % wise. The newer 35Q are 28000 BTU.

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Old 02-16-2018, 12:39 PM   #4
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Quote:
Originally Posted by CoachStreamr View Post
We live in the Pacific North West and my wife is the type that is always cold.

It does seem the the 42Q is a tiny bit less efficient than the 35Q if you look at the output BTU. SF-42Q output BTU 30690 vs the SF-35Q's 28K or 27.3K (not sure which is correct).

I know too big of a furnace can also cycle on/off more often reducing the efficiency. Actually, in cold weather I think the current furnace cycled a lot. Sometimes bigger is not better.

Looks like Gulf Stream was installing 40K BTU units in most 36' around this Coach's year, not sure why they installed a 35K (the Manufacturer Label on the coach does list SF-35). Our is Coach is a 36'.

It's closer to an additional $110 difference with tax.

I suppose the existing propane line should be sufficient for the 42Q extra BTU?

Hmm, I now see the output BTU for the 42Q is only 30690, for some reason I thought it was closer to 32000 so it is less efficient % wise. The newer 35Q are 28000 BTU.

.
For what it is worth, I believe you are spending an extra "C" note that you do not have to. For a little over 2000 btu I dont see the value. You will get a little more air flow but that does not always translate to more air delivery. Restrictions that interfere with the air flow now will still be there. Main thing would be to see how many restrictions you can clear up before you put the new unit in there. I dont see how you are arriving at the conclusion that the 35 is more efficient that the 42, please explain?

Good luck and keep us posted.
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Old 02-16-2018, 01:59 PM   #5
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For what it is worth, I believe you are spending an extra "C" note that you do not have to. For a little over 2000 btu I dont see the value.

Good luck and keep us posted.
I agree, after seeing the output at only 30600 BTU for the SF-42Q and some thought, I ordered the SF-35Q from Amazon (Amazon is the seller).

I'd rather have the furnace run longer than output more airflow which I believe would cycle the furnace even more, better to have the furnace on longer and off longer.

I could have saved $290 (with the sales tax I pay when buying from Amazon) by ordering from eBay (Adventure RV), they had it for under $400 plus $25 shipping and posted that it was Brand New. However, that price is well under what I see.

I called Suburban and spoke to their Customer Service. They told me to be careful online, many times units are sold without the Manufacturer's label showing model and serial numbers. These are freight damaged units that are auctioned off but Suburban requires that label to be removed. The Suburban 2 year warranty is not valid without that sticker/label on the unit.

I did message the eBay seller and he stated the Model and Serial Number label is on them. Reviews on eBay were outstanding but the website Reseller reviews were horrible. Communications with the eBay seller was great, answered my 4 questions on 4 different messages.

I decided to go with my gut and if there are any issues, dealing with Amazon (as the Seller) for any return has always been a great experience.

I am pretty sure the motor went out on my current furnace. While checking the sail switch and activated the thermostat, the motor spun but not like before. It continued to run and slowed down even more, then sped up near full speed then slowed down. It then stopped and then started up slowly again. Finally stopping.

I did check voltage to the motor and it's above 13v.

I estimated the motor is around $100 but for this model, many parts have to be taken off to replace the motor. And, it's 23 years old and was a little noisy.

I'm not sure I'd even keep a 23 year old furnace in my house.

I should have the new furnace on Sunday (Amazon is great for weekend delivery).

I am keeping the old furnace, I may replace the motor, keep for backup or sell it. I may just sell it as is.



.
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Old 02-16-2018, 02:16 PM   #6
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Quote:
Originally Posted by Quincy View Post
I dont see how you are arriving at the conclusion that the 35 is more efficient that the 42, please explain?
Looks like the 35Q is 80% efficient and the 42Q is 76.5% - not a big deal.


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Old 02-17-2018, 04:07 PM   #7
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Have the old SF-35 furnace out of the Coach (including its metal cabinet). Vacuumed up the larger wood cabinet furnace area and will be using Nashua 330X Extreme Weather HVAC Foil Tape to seal up the cabinet and floor duct (old Foil Tape is in bad shape and obviously was loosing air flow around the floor duct.

http://catalog.berryglobal.com/produ...adhesiv579330x






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Old 02-18-2018, 03:56 AM   #8
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Quote:
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Looks like the 35Q is 80% efficient and the 42Q is 76.5% - not a big deal.


.
We upgraded the S&B furnace a couple of years ago. I wish we had the old one back. From what I see on the new non condensing furnace the efficiency answer is to blow as much air through the exchanger as possible to extract as much heat as possible thus putting out heating air only a few degrees above room temperature. In a long run I wonder how much above but have not bothered to measure it.
Another side effect is all the noise from the high volume of air that is being pushed through duct work not designed for the load.

None of this is explained by the sales droids or the folks who specify the higher efficiencies. In part that leaves me wondering if they include the motor power in the equation or just the gas used. It would be interesting if anyone knows.

Looking at the specs on this job the smaller one might be better than I originally thought as battery load is an issue and the gain not nearly as much. It's an interesting thread. Pictures would be nice.
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Old 02-18-2018, 06:25 AM   #9
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if your are looking for a place to spend $$, go for it. Unless your plenum is rusted, you are just wasting money. I suggest you just replace you blower motor, it is amazing at how fast they degrade after just a couple of seasons. Many utube vids on dyi blower motors. 2nd, look at how big you wet bay compartment venting is. A lot of times the furnace will dump huge amounts of heat that is not needed into this compartment. Simply stuff a rag in the end of the heater vent ducting to limit how much heat goes to keep your poop warm. The colder poop is the less stinky it is....
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Old 02-18-2018, 06:38 AM   #10
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Thought I would mention this. There are 2 different motor sizes for these furnaces. One is a 2 1/2" diameter and the other is 3" diameter. best, to look/measure and see which size you have. The only diff I can see between the 35 and the 42 is the motor. Suburban guards the motor specs so..... Also I've run into quality issues with replacement motors. Some of the RV supply store's don't sell quality motors. Pay a little extra and don't do it twice like I had to do.
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Old 02-20-2018, 09:52 AM   #11
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I've always had cold feet about buying my parts through ebay too. Seems a little suspect how cheap some of their prices are. I just ordered a new furnace from Suburban RV Furnaces, Water Heaters, and Parts! | Suburban RV Parts and they were awesome. Prices are a little higher than ebay and amazon but their customer service is great and parts are all direct from the manufacturer. Just my 2 cents!
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