Propane furnace sending heat to outside vent

[IWally]

Hi,

Running our Suburban propane furnace it seems like the unit is pushing as much (or more) heat out of it's exterior vent as it is to the interior vents. At the vents the flow is a gentle convection of warm/hot air. Standing outside it's like a hair dryer on low. It consumes a large tank of propane pretty rapidly .. one or two days with temps in the thirties and thermostat set to ~62

This can't be normal .. can it?

I'm interested to hear the experience of other cool weather campers before I take it to a shop for what might be an expensive fishing expedition.

[JaxDad]

Sam Spade wrote:
I've dealt with a LOT of gas furnaces over the years, both in permanent structures and house trailers and have NEVER seen one where the combustion air was not gravity fed.

Maybe I will learn something new today. 🙂

Gravity? I suspect you meant to say convection.

Either way, I'm not sure how well a foot tall mostly horizontal furnace would vent when the intake and exhaust are at basically the same height.

There kind of needs to be some appreciable height for the hot air to rise to create enough vacuum / air flow for cold air to be drawn in. If the 'intake' and 'exhaust' were at the same height the heat would just spread equally between them, assuming both ports were equal size.

[12thgenusa]

Check your furnace and make sure the blanking plates for unused outlets on the heat exchanger haven't fallen off. Happened to me this summer on the Alaska trip.

That much propane use in the conditions you describe suggests a propane leak.

[Chris_Bryant]

The end of the pilot was the end of gravity fed- if there is a slight leak in the valve, a pilot will keep the LP from building up. Electronic ignition in a gravity model with a slight leak would be a small boom.
Now the forced air runs a purge cycle before ignition. The downside is the gravity models used to get hot before the blower came on- always blowing hot air. Now they blow cold for 30 seconds, the take time to heat up.


Progress 🙂

[Sam_Spade]

ktmrfs wrote:

my sticks and bricks 15 year old gas furnace has a blower for the combustion air on the flue exhaust side. In fact, likely on almost any gas furnace anymore.

I'm sorry, I forgot about "high efficiency" gas furnaces, which have PVC plastic "chimneys" because the exhaust temperature is so low.

I have no information on whether or not that applies to "almost all" of them these days though.

And I had no idea that a "low end" trailer or RV would have that kind of arrangement. I stand corrected.
(I still haven't looked at mine.)

[ktmrfs]

Sam Spade wrote:

harold1946 wrote:

Sam Spade wrote:

Chris Bryant wrote:

They are both the same motor- one cannot fail without both failing, plus the sail switch is specifically meant to prevent this.

OK on the sail switch; forgot about that.

But since when is there a fan helping the exhaust gasses get out ??

The fan brings fresh air from the outside into the combustion chamber and forces it out through the exhaust port.

Is there really such a thing ?
I don't think my C has this but now I'm gonna have to look.

I've dealt with a LOT of gas furnaces over the years, both in permanent structures and house trailers and have NEVER seen one where the combustion air was not gravity fed.

Maybe I will learn something new today. 🙂

my sticks and bricks 15 year old gas furnace has a blower for the combustion air on the flue exhaust side. In fact, likely on almost any gas furnace anymore. Blower is on the exhaust side, starts sucking combustion air through the burners about 10 seconds before the gas is turned on, and continues for about 15 seconds after the gas goes off. When off closes a flappper valve on the flue.

[Chris_Bryant]

Sam Spade wrote:

Is there really such a thing ?
I don't think my C has this but now I'm gonna have to look.

I've dealt with a LOT of gas furnaces over the years, both in permanent structures and house trailers and have NEVER seen one where the combustion air was not gravity fed.

Maybe I will learn something new today. 🙂

Yeah- there hasn't been a gravity fed RV furnace in probably 25 years- they are all forced air combustion. I wish they made a small gravity one now for boondocking.

[MNGeeks61]

IWally wrote:


Is it normal to go through a 30lb tank in one day?

It'd have to be darn cold, and there must be a large area to heat, and perhaps somewhere there's an opening so cold air is getting in?

We did camp sometimes in the winter in 10 degree weather in our 40' 2010 destination trailer (R value was pretty much nonexistent, prob R-7) and yes our 40k BTU furnace would burn through a 30 lb bottle in about 2-3 days. That is also WITH supplemental electric heat.

Is it normal, not really...something's up somewhere.

[Old-Biscuit]

Sam Spade wrote:

Chris Bryant wrote:

They are both the same motor- one cannot fail without both failing, plus the sail switch is specifically meant to prevent this.

OK on the sail switch; forgot about that.

But since when is there a fan helping the exhaust gasses get out ??

ONE motor with TWO blower wheels on it....
Combustion air wheel which supplies burner and then forces hot air thru heat exchanger and out exhaust port (high temp limit switch)

Room air wheel which flows air across heat and out thru discharge plenum to duct/floor registers (sail switch)

[Sam_Spade]

harold1946 wrote:

Sam Spade wrote:

Chris Bryant wrote:

They are both the same motor- one cannot fail without both failing, plus the sail switch is specifically meant to prevent this.

OK on the sail switch; forgot about that.

But since when is there a fan helping the exhaust gasses get out ??

The fan brings fresh air from the outside into the combustion chamber and forces it out through the exhaust port.

Is there really such a thing ?
I don't think my C has this but now I'm gonna have to look.

I've dealt with a LOT of gas furnaces over the years, both in permanent structures and house trailers and have NEVER seen one where the combustion air was not gravity fed.

Maybe I will learn something new today. 🙂

[joshuajim]

I have a FR Wildwood and it also seemed to have an excessive amount of heat exiting to the outside. I started to do some investigation and looked at the rigid 3x10 duct that runs the full length of the trailer ( no flex on this model). I discovered that the duct was significantly deformed along much of it's length and the kicker was that it ran directly under the table support leg. The in floor support "cup" had crushed it to about 1/2 it's original height. I discovered that the "cup" was at least twice the depth it needed to be to support the table, so I cut it off.

I didn't have access to the rest of the duct from below as I would have to remove all three tanks to get to it. I welded up a series of stents shaped like small I beams and forced them into the ductwork from the grill openings. Fortunately the grill openings were only 4' to 5' apart so I only had to push them about 2'.

I got the duct back to about 90 percent of it's original size.

Now the heater is much quieter, runs far less and heats the trailer better.

Yes, the heat flow from the exhaust port has declined considerably.

[harold1946]

Sam Spade wrote:

Chris Bryant wrote:

They are both the same motor- one cannot fail without both failing, plus the sail switch is specifically meant to prevent this.

OK on the sail switch; forgot about that.

But since when is there a fan helping the exhaust gasses get out ??

The fan brings fresh air from the outside into the combustion chamber and forces it out through the exhaust port.
Inside air does not mix with that supplied to the combustion chamber, if it did one would soon be asphyxiated.
The inside air is recirculated independently.

[red31]

Sam Spade wrote:

But since when is there a fan helping the exhaust gasses get out ??

the fan that sucks combustion air in helps blow exhaust out.

[Sam_Spade]

Chris Bryant wrote:

They are both the same motor- one cannot fail without both failing, plus the sail switch is specifically meant to prevent this.

OK on the sail switch; forgot about that.

But since when is there a fan helping the exhaust gasses get out ??

[wa8yxm]

There is a lot of "Outlet" for the amount of blower you have so yes the flow from any given register is not high, and there is a lot of air flow for the amount of burner and heat exchanger you have so it does not get all that hot.

So yes, Gentle and slightly warm is the norm.

[westend]

And they are still using these 20K/30K inefficient furnaces in a 15-25' TT.

I am using a 20K propane fired space heater in my 22' trailer. This is the third small RV that the same heater has been used. There is a Class B vent stack that exits through the roof. The heater has a tile/masonry surround. It operates at 10 lbs line pressure regulated at the heater inlet to a lower pressure. It is basically a residential water heater sized burner in a steel can. I have piped combustion air and stack make up air to it from the outside.

When in use, I start the heater at medium -> high for 30 minutes or so and it heats the cabin rapidly. If at ambient 10f or above, the heater is then turned down to pilot only. If it gets really cold like I've experienced at deer camp, -15f or so, then after the 30 min. cycle the heater is set to just above pilot/low. At 10f, I will burn a 20 lb cylinder every four-five days. That includes the water heater and the range use.

I know the real vintage rigs had a radiant coil type heater and it did the job. I don't know their efficiency. It just seems like a better furnace/heater could be used in a smaller RV than the typical vented downdraft models. I also see a lot of folks complaining about 12V suckage and the noise of the blower.