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Old 02-13-2019, 12:31 PM   #1
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Improve balanced heat output?

Greetings all...I have a 2018 Surveyor 251rks...generally pleased with TT, except for terrible heat balance out of the four heat vents...FR chose to run one 2 inch duct from the 30K Suburban furnace to feed the four vents, with about 80% of the heat coming out of the first vent a couple of feet from the furnace, even though the furnace instructions clearly call for a minimum of 3 four inch duct runs for this size furnace. My only fix is to use the 2 four inch knockouts that I can access on the side of the furnace and run 2 three or four inch duct runs through the floor and feed the four vents in a two and two split with probably an air balance damper on one of the runs to create an optimum balance...this requires dropping the colorplast plastic underbelly for access...my question: who has attempted this project and what unanticipated pitfalls, besides lots of swearing, did you encounter in routing the duct runs around tanks, electrical, etc. Thanks for any comments and tips...Mike C
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Old 02-13-2019, 12:46 PM   #2
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Greetings all...I have a 2018 Surveyor 251rks...generally pleased with TT, except for terrible heat balance out of the four heat vents...FR chose to run one 2 inch duct from the 30K Suburban furnace to feed the four vents, with about 80% of the heat coming out of the first vent a couple of feet from the furnace, even though the furnace instructions clearly call for a minimum of 3 four inch duct runs for this size furnace. My only fix is to use the 2 four inch knockouts that I can access on the side of the furnace and run 2 three or four inch duct runs through the floor and feed the four vents in a two and two split with probably an air balance damper on one of the runs to create an optimum balance...this requires dropping the colorplast plastic underbelly for access...my question: who has attempted this project and what unanticipated pitfalls, besides lots of swearing, did you encounter in routing the duct runs around tanks, electrical, etc. Thanks for any comments and tips...Mike C
Sorry I am no help to you. So far we have not had to use the furnace since we bought the 251rks in 2016. Hoping that will change this year.
I will reach out to you or will look for updated posts from you.
Good luck!!
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Old 02-13-2019, 02:09 PM   #3
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Following this thread. We have a 247BHDS and the bathroom vent blows extremely well. However, the 3 floor vents are mediocre at best. The front bedroom heat is nearly non-existent.

I've thought about seeing if there is any blockage for the floor vents vs. the bathroom vent.
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Old 02-15-2019, 09:07 PM   #4
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Balanced heat output

Hi all...I'm replying to my own thread...I have discovered some interesting things in the last day or so...first, I stated that there was only one 2 inch duct hose exiting the furnace on my 251RKS...this is incorrect...after pulling down a section of the underbelly across from the furnace I was able to see what is not obvious...bottom line: there is a rectangular knockout on the bottom of the furnace, hidden from view, and punched through the floor below where the furnace sits is where three 4 inch duct hoses are connected to run to the three floor registers...pity the poor person who has to pull the furnace out for service, as you will destroy the duct hose connections...you must open up the underbelly under the furnace, which I did, and found that one of the 4 inch duct hoses was NOT connected. Huge PITA to reconnect this duct, as there is zero room to get the hose clamp on and tighten...probably why it fell off in the first place...will try a short sleeve extension that I can press fit on and hope to squeeze some hvac tape around it to hold it, then connect the duct hose to that. This explains the lack of heat coming out of my middle floor register. In addition, the two duct hoses that feed the middle and bedroom register have to travel over the top of the slide out support, and because of this they are crushed down to probably an inch or so of inside diameter, which, again, means very little heat coming out of them. I am going to install an air damper on the duct hose that feeds the register closest to the furnace, in hopes that by closing down to about 50% the other two registers will get better output. At least now I know the problem and I have a reasonable chance to fix it for a few bucks and some Eternabond tape to seal back up the opening I've made in the plastic underbelly to access all this. I can also report that there is NO specific heat duct that blows into the underbelly...the so-called "heated underbelly" simply means that heat lost through the un-insulated duct hose runs creates a very minor amount of heat down below, which is probably immediately lost through the cracks in the plastic underbelly where attached to the frame. There is ZERO insulation of any type down below. Hope this helps anyone trying to get better, more even heat out of their furnace in the Surveyor line, and specifically the 251RKS. MC
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Old 02-15-2019, 09:40 PM   #5
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Thank you for the information. I will have to check this out.
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Old 02-16-2019, 02:54 PM   #6
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Just a FWIW. On a trip early last year, crosswinds caused the front half of my underbelly to disengage, found the issue when we pulled into a campground.
So I took the entire underbelly off at that point.

On my trailer the "ducts" were skimpy mylar runs and were loosely installed. A bit of inner web searching led me to some info that said this kind of duct must be installed taught to get max air-flow. I've also read a number of posts from folks who had mice chew holes in the stuff.

So, while the underbelly was off, I replaced most of the ducting with rigid or heavy aluminum flex and aluminum taped all the joints. I'm now getting much better air flow to all registers.

If you search back on some of my posts, there are a couple of threads with several pictures of the work I did.
While I was at it, I neatened up the wire runs and covered the wires with split loom conduit.
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Old 02-16-2019, 05:59 PM   #7
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No, you don't do it that way.

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pity the poor person who has to pull the furnace out for service, as you will destroy the duct hose connections
They never take the furnace out that way, so no problem. They take off the grille, then remove the front plenum cover, then the single anchor screw that holds the firebox/blower assembly in place. Then disconnect the gas line and four wires, and the firebox,blowers, and control board come out, leaving the entire plenum and duct assembly in place.

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Old 02-16-2019, 10:58 PM   #8
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My Rockwood has floor vents for the heat. I removed the factory installed vents and replaced them with vents that can be adjusted individually for air flow. The ability to adjust balances out the heat distribution. I have not noticed any additional strain on the furnace blower. I bought them at Lowe's.
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Old 02-17-2019, 10:17 AM   #9
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The Suburban installation manual...

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My Rockwood has floor vents for the heat. I removed the factory installed vents and replaced them with vents that can be adjusted individually for air flow. The ability to adjust balances out the heat distribution. I have not noticed any additional strain on the furnace blower. I bought them at Lowe's.
The Suburban installation manual warns against doing that. The risk is that once balancing begins and a few vents are restricted, the remaining room gets too hot. Then, instead of opening the other vents a tad, the user restricts the one remaining vent, too.

Then the furnace gets too hot and the high-limit switch cycles on and off, and the owner complains of cold air from the vents and irregular heating. If the furnace gets too hot regularly, the fire chamber can crack, leading to a dangerous situation.

The manual also has restrictions on the number of square inches of return air vent required. It is surely available on the Library on this website.

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Old 02-17-2019, 05:56 PM   #10
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The Suburban installation manual warns against doing that. The risk is that once balancing begins and a few vents are restricted, the remaining room gets too hot. Then, instead of opening the other vents a tad, the user restricts the one remaining vent, too.

Then the furnace gets too hot and the high-limit switch cycles on and off, and the owner complains of cold air from the vents and irregular heating. If the furnace gets too hot regularly, the fire chamber can crack, leading to a dangerous situation.

The manual also has restrictions on the number of square inches of return air vent required. It is surely available on the Library on this website.

Larry
I am not seeing that in the manual where it is referring to anything except the flow of air into and out of the burner itself, which indeed would be dangerous. There are no restrictions to the combustion chamber.
By changing the operation of the heated air vents does nothing to the airflow to the combustion chamber. The heat exchanger does not run any hotter than normal, the circulation fan pushes the air over a greater distance but that air flow is not restricted either. 5 cubic feet of air per second is 5 cubic feet of air per second whether it comes out of a vent 4 feet away from the fan or 12 feet away from the fan. If the total air movement volume at all of the vents added together is equal to the total air movement volume of the fan, what is the restriction?
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Old 02-17-2019, 07:22 PM   #11
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Perhaps...

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I am not seeing that in the manual where it is referring to anything except the flow of air into and out of the burner itself, which indeed would be dangerous. There are no restrictions to the combustion chamber.
Perhaps you read the wrong manual? Try this one, on pages 19-22.

One of the more interesting sentences from page 19:
  • Suburban furnaces require that a minimum duct area be maintained throughout entire duct system including through the register.

The most common RV furnaces are the SF-series. On page 21, this manual gives the required minimum ducting area including the registers. For an SF-30, for example, 3-4" ducts are required, with bottom duct area of 56 square inches, top duct area of 56 square inches, or left-and-right side duct areas of 36 square inches. (My interpretation is that the last three are alternatives; you do not have to satisfy all three.)

Or try this manual. On page 17 it states
  • Ducting - Suburban furnaces require that a minimum duct area be maintained throughout entire duct system including through the register. It is very important to adhere to the minimum duct area in order to keep the furnace from cycling on high limit and to assure proper operation of the sail switch (sometimes referred to as a microswitch.) NOTE: (Refer to the installation manual for the minimum ducted square inches area for each model.)

This manual also contains tables similar to the previous one, pages 18-20.

Enjoy your reading.

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Old 02-17-2019, 07:33 PM   #12
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By changing the operation of the heated air vents does nothing to the airflow to the combustion chamber. The heat exchanger does not run any hotter than normal, the circulation fan pushes the air over a greater distance but that air flow is not restricted either
This reasoning is incorrect. The single blower motor in the Suburban furnaces is double-ended. The shaft exits the motor housing on both ends. There are squirrel-cage blowers mounted on both ends of the motor. One blows air through the combustion chamber. The other blows air around the combustion chamber.

When you restrict the airflow through the ducts, it loads the heating air blower which slows the motor down. That reduces airflow through the combustion chamber. Reducing airflow both in and around the combustion chamber (both blowers slower), causes the chamber to run hotter. This is the reason for the warning about reaching the high-limit and shutting the furnace down. Contrary to your reasoning, the heat exchanger does, in fact, run hotter than normal.

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Old 02-18-2019, 11:13 AM   #13
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This reasoning is incorrect. The single blower motor in the Suburban furnaces is double-ended. The shaft exits the motor housing on both ends. There are squirrel-cage blowers mounted on both ends of the motor. One blows air through the combustion chamber. The other blows air around the combustion chamber.

When you restrict the airflow through the ducts, it loads the heating air blower which slows the motor down. That reduces airflow through the combustion chamber. Reducing airflow both in and around the combustion chamber (both blowers slower), causes the chamber to run hotter. This is the reason for the warning about reaching the high-limit and shutting the furnace down. Contrary to your reasoning, the heat exchanger does, in fact, run hotter than normal.

Larry
Actually the above bolded statement, is exactly opposite of what happens. If you restrict the outlet of a squirrel cage blower the amperage goes down. And the motor tends to speed up (but only slightly). It's counter-intuitive to what you would normally think. However the rest of the statement is true.

Less airflow across the chamber causes the furnace to run hotter.
The burner firing isn't modulated, so reducing the airflow across the exchanger causes it to run hotter because the burner is still firing full blast.
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Old 02-18-2019, 11:57 AM   #14
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Did some research

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Actually the above bolded statement, is exactly opposite of what happens. If you restrict the outlet of a squirrel cage blower the amperage goes down. And the motor tends to speed up (but only slightly). It's counter-intuitive to what you would normally think. However the rest of the statement is true.

Less airflow across the chamber causes the furnace to run hotter.
The burner firing isn't modulated, so reducing the airflow across the exchanger causes it to run hotter because the burner is still firing full blast.
Thanks for the education. I did some research and confirmed that you are right: if you restrict the airflow on a squirrel-cage blower speeds up. Loosely, the motor has less work to do stirring the air around the housing than it would actually moving air.

But there's one little thing that's confusing me (as Inspector Columbo would say). In these furnaces there are two independent blowers and air-circuits on the same, single motor. You restrict airflow on the interior air circuit and the motor speeds up. Now you have also increased the rotational velocity of the blower on the combustion air circuit. Wouldn't that move more air through the combustion circuit and keep it cooler? Why would Suburban warn of higher temperatures? Or is there some velocity beyond which the slope of the rotational speed vs air velocity goes negative?

(Would be easier if I could use symbols like lower-case omega here....)

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Old 02-18-2019, 12:26 PM   #15
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restricting air flow will increase heat rise across the heat exchanger and increase discharge temperature; if it goes too high the furnace will cycle on high limit, not on the thermostat and the coach will not get warm enough, had that issue on my old toy hauler, added another vent and it worked great, a friend's TH, different brand had the same issue.
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Old 02-18-2019, 02:34 PM   #16
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Thanks for all the posts! Very educational indeed !!
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Old 02-18-2019, 02:47 PM   #17
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But there's one little thing that's confusing me (as Inspector Columbo would say). In these furnaces there are two independent blowers and air-circuits on the same, single motor. You restrict airflow on the interior air circuit and the motor speeds up. Now you have also increased the rotational velocity of the blower on the combustion air circuit. Wouldn't that move more air through the combustion circuit and keep it cooler?
While the motor tends to speed up because of less load, it doesn't speed up much at all, because these motors are designed to maintain normal speed no matter how much load is on them. So the increase in speed (and therefore air flow) is minimal.
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Old 02-20-2019, 06:30 PM   #18
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Balanced heat output

Hello all again...this post is aimed at member Larry-NC...Larry thank you for the clarity on how the furnace could be pulled out without destroying the under-furnace ducting attached. Now that I have all three of those ducts connected back to the furnace (as I noted earlier, one of the ducts was disconnected and laying on the underbelly floor) and am enjoying a measure of heat coming out of all three floor registers I decided to do a little heat gun measuring. The furnace unit, measured at a top back corner of the furnace cabinet, heats up from apprx 90 degrees to almost 150 degrees before the limit switch kicks in and the temp drops back down to 90 ish...this cycle process takes apprx 5 mins (2.5 mins up and 2.5 mins down)...I observed this cycle for about a half hour and it was very consistent. My question to you Larry, as you obviously are pretty knowledgeable on the furnace operation, does this temp range and cycle time seem appropriate. The furnace is a Suburban 30FQ, 3-4 inch ducts to 3 floor registers (two of which are crushed down apprx. 50% due to routing over slide out mechanism) and a 2 inch duct to bath wall sidemount. The plate on the furnace notes maximum (internal?) air temp range, but I don't believe I can accurately measure that with a simple heat gun. Thanks much if you happen to see this post and respond. MC
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Old 02-20-2019, 11:05 PM   #19
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The high limit switch should never kick in

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Hello all again...this post is aimed at member Larry-NC...Larry thank you for the clarity on how the furnace could be pulled out without destroying the under-furnace ducting attached. Now that I have all three of those ducts connected back to the furnace (as I noted earlier, one of the ducts was disconnected and laying on the underbelly floor) and am enjoying a measure of heat coming out of all three floor registers I decided to do a little heat gun measuring. The furnace unit, measured at a top back corner of the furnace cabinet, heats up from apprx 90 degrees to almost 150 degrees before the limit switch kicks in and the temp drops back down to 90 ish...this cycle process takes apprx 5 mins (2.5 mins up and 2.5 mins down)...I observed this cycle for about a half hour and it was very consistent. My question to you Larry, as you obviously are pretty knowledgeable on the furnace operation, does this temp range and cycle time seem appropriate. The furnace is a Suburban 30FQ, 3-4 inch ducts to 3 floor registers (two of which are crushed down apprx. 50% due to routing over slide out mechanism) and a 2 inch duct to bath wall sidemount. The plate on the furnace notes maximum (internal?) air temp range, but I don't believe I can accurately measure that with a simple heat gun. Thanks much if you happen to see this post and respond. MC
I am flattered that you would rely on me. Thanks!

I don't "think" the high limit switch should ever kick off. It is a last-chance safety. The troubleshooting guide does not seem to mention anything that cycles the gas on and off except the thermostat. You don't suppose your thermostat is cycling on and off, do you? Does this happen when you set it way above the current temperature? Two things to try when you have set the thermostat way high above current temperature are below.

Two things to try:
  1. The switch itself is is located in the center of the top front edge of the furnace. It's a two terminal device. If you put a voltmeter on it, you should see about zero when running normally and 12 volts when the limit is reached and the switch kicks open. Put your meter across the switch while the furnace runs and see if it really is opening and closing.
  2. The front cover (just below the high limit switch) comes off easily. Just a couple of screws. Run the furnace with this cover off. Perfectly safe, no fumes will enter the trailer, but all the air will come straight out, not forced through the ducts. See if the same cycling occurs or not.
EDIT: You CAN measure the internal temperature. When you have the front cover off, just point your temp gun at the firebox.
The manual does not identify any other control that cycles the gas on and off. It does hint that if the high-limit switch opens, but then closes within five minutes, the furnace will continue to operate.

By the way, your furnace is 12 volts with fan control board, pages 26 and 31.

If the high limit switch really is cycling on and off, either the ducts are restricted or the switch is defective (opens at too low a temperature). Other forums report that one failure mode is to open at too low a temperature. Note that there are different part numbers for SF-20, (SF-25, 30, 35), and SF-42. Be sure to order the right one. (Maybe the wrong one was installed?)

A new switch is under $10 at Amazon.

Larry
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Old 02-20-2019, 11:21 PM   #20
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