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Author Topic: Running furnace blower with no heat  (Read 3516 times)
sheff69
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« on: November 24, 2010, 09:40:07 PM »

Not wanting to steal a thread, I'm starting new one.

I quote Admin Jim.
I'm curious about your statement that "nothing will freeze" in cold weather in the Cameo.  On our 34CK3, the holding tanks and plumbing are only warmed by bleed air from the propane furnace, which gets expensive to operate just for that reason.  Our Carrier V heat pump and cube heater keep the interior of the trailer comfortable at least down to 48 degrees OAT, but do nothing for the uninsulated spaces beneath the trailer.

Has anyone wired the blower to stay on all the time to keep pushing air through the underbelly to stop the tanks freezing when using another source of heat, for example electric heat?
Or if the blower is not that reliable, how about adding an auxiliary 110V blower.  But then the question is how and where would you install something like that?  Any thoughts out there?

Sheff
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cameopher
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« Reply #1 on: November 25, 2010, 06:20:05 AM »

I'm not sure what good it would do to circulate cabin air to the basement and holding tanks, with respect to preventing tank freezing. 

The heat pump reverses the air conditioning process and pulls heat from outside the trailer and circulates it inside.  However, as already stated, this doesn't work below a certain ambient outside temp, about 45F, simply because there is not enough heat outside to extract.  Your tanks won't freeze unless they are subjected to temps below 32F for an extended period.  You would have to run several electric cube heaters to keep the trailer warm at those temps and would probably want to run the furnace anyway just to keep up.

Even without forcing air into basement, if you kept the living space air at a comfortable level during freezing weather, a certain amount of heat would reach the basement through convection, conduction, and radiation.

If I was overly concerned about keeping the tanks from freezing without running the furnace, I would look into electric strip tank heaters, installed directly on the tanks as opposed to altering the existing system.
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sheff69
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« Reply #2 on: December 02, 2010, 07:31:51 PM »

Sorry for not replying straight away, been a bit busy.

What I was referring to, was if you use electric heat (as in using ceramic heaters, not heat pump) in the living area of the trailer instead of using the main furnace, you can freeze the basement area because there is no circulation of warm air.  If we could run the furnace blower, it would transfer the heated air in the living area, and move it through the basement, so keeping it from freezing.

I was asking whether anyone had thoughts on how to do that, or rig another fan to do that efficiently.  I don't think much heat would reach the basement "through convection, conduction, and radiation", otherwise.



Sheff
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2012 Dodge Laramie 3500 DRW
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cameopher
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« Reply #3 on: December 02, 2010, 08:00:26 PM »

I understand what you meant.  I don't think it would be that difficult to direct wire the furnace fan to run without the heat.  I've never heard of anyone doing so though.

You don't have to worry about the tanks freezing until outside temps get down around 30F and below for several hours.  At those temps, in order to keep the trailer living space and basement much above freezing would require multiple small heaters, even if using the furnace fan to circulate air.  I don't think that's practical.

I've read about folks putting an electric ceramic heater in the basement to keep that area and the tanks warm, independent of the living space.  I don't know how effective that would be at very low temps.
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zachlaplante
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« Reply #4 on: December 03, 2010, 10:38:57 AM »

I think the threat from freezing depends a lot on the living conditions and environment.  If you are in a winter climate with temps that are below freezing all the time, you would probably need tank heaters and circulate main cabin air to keep the pipes from freezing.  The last issue of Trailer Life had a good article on what you need to keep your RV safe in a winter climate.  If you are in a more moderate climate, say sub freezing temps at night but above freezing during the day, you probably don't need to do anything.  I would think keeping the main cabin above freezing would protect the pipes, and the heat capacity of the contents in the tanks would protect them at night.  That is, there is enough water in the tanks so that while the outside temps may be below freezing, the contents of the tanks don't cool enough to freeze.  During the day, you are using water, so more warm water is added to the tanks, adding more heat.  If you are camped at a full service site, you could make this even easier by draining your fresh water tank and dumping the black and grey tank every day before going to bed.  One other thought, assuming you have electric service and you have a closed underbelly, put a couple of 100 watt light bulbs in the space inside the underbelly.  You would be surprised how much heat a 100 watt light bulb gives off.  Just make sure its not going to melt any plastic, they can get quite warm.  You might also want to consider heat tape for the black and grey tank drain between the dump valve and the tank.

If you want to revise your fan wiring, my suggestion is to short the fan control switch in the control circuit.  Assuming you have access to the control circuit wiring diagram, there should be a temperature controlled switch that turns the fan on when the heater gets up to temperature and turns the fan off when the heater cools back down.  The thermostat normally controls the gas, not the fan.  If you wire an external switch to short out the fan control switch, the fan should run regardless of temperature.  The heater should still work also, as the gas will turn on at low temp and off at high temp, with the fan running continuously.  So the new switch is basicly a fan on / fan auto control switch.
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sheff69
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« Reply #5 on: December 07, 2010, 06:00:01 PM »

<<If you want to revise your fan wiring, my suggestion is to short the fan control switch in the control circuit.  Assuming you have access to the control circuit wiring diagram, there should be a temperature controlled switch that turns the fan on when the heater gets up to temperature and turns the fan off when the heater cools back down.  The thermostat normally controls the gas, not the fan.  If you wire an external switch to short out the fan control switch, the fan should run regardless of temperature.  The heater should still work also, as the gas will turn on at low temp and off at high temp, with the fan running continuously.  So the new switch is basicly a fan on / fan auto control switch.>>
That's what I was thinking of, but hadn't worked out a good way.  Thanks.
What do you think would be the reliability of running the 12V furnace blower continuously?
That's why I mentioned a 110V blower, and if that would be better, where would you put it in line?

Sheff
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zachlaplante
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« Reply #6 on: December 13, 2010, 08:58:01 AM »

Sheff69, sorry for the delay in responding, I haven't been watching the thread.  Anyway, I don't see why there should be a difference in reliability between a 12v blower vice a 110v blower.  Motor technology is fairly well advanced these days and both should stand the duty of continuous use without much impact on reliability.  That being said, the 12v dc motor will have brushes and they have a defined life, meaning that you might need to replace the brushes on the 12v blower motor at some point.  However, this is not a big deal and I wouldn't bother with checking the brushes more than once a year prior to the heating season.

Not sure what your design is, but on mine (2008 Domani) the heater draws on an open area under one of the cabinets.  This area is open to the inside of the RV, as well as the basement and the enclosed underbelly where the holding tanks are.  The blower discharges to the heater heat exchanger, then through ducting to the same areas for heat.  There is no good place to add a second blower.  Also, any second blower would need to be installed such that the airflow when the furnace were to come on would not be adversely changed.  There needs to be adequate airflow through the furnace heat exchanger and through the furnace 12v blower for cooling, otherwise there could be trouble.  My suggestion would be to not mess with a second blower.  Too many unknowns on how the design is changed and therefore too much risk of an adverse and unintended consequence.

If you are really concerned about the holding tanks, they make 12v tank heaters that attach to the tank and have a mechanical thermostat to turn them on and off.  Much simpler and your tanks will be protected.  The heat in the main living area should keep the plumbing thawed.
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sheff69
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« Reply #7 on: December 13, 2010, 09:28:37 PM »

Thanks.

My concern was sometimes, propane can be a lot more expensive than electricity, especially for short stays where electric is part of the site fee.  In those cases, running electric heaters is more cost effective than just turning on the furnace.
But if you do that, you may run the risk of freezing the tanks because of lack of air movement through the under body.

Just looking at alternatives.

Sheff
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cameopher
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« Reply #8 on: December 14, 2010, 07:37:12 AM »

Thanks.

My concern was sometimes, propane can be a lot more expensive than electricity, especially for short stays where electric is part of the site fee.  In those cases, running electric heaters is more cost effective than just turning on the furnace.
But if you do that, you may run the risk of freezing the tanks because of lack of air movement through the under body.

Just looking at alternatives.

Sheff

Hey Sheff,

I tried to make this point earlier, but evidently didn't do a very good job.  If I understand your scenario correctly, you are at an RV park where electricity is furnished and want to use small electric heaters instead of the furnace, to save on propane.  And, you want to use the furnace fan to circulate the living space air to the tanks so they won't freeze.  I'm assuming you are living in the trailer at the time and want to stay comfortable.

I always use two ceramic heaters for warmth when at an RV park which furnishes electricity.  Two 1500 watt heaters will maintain a temp between 65F and 70F in the trailer when outside temps are above 40F or so.  When temps outside get down to 30F, the electric heaters will not keep up and inside temps drop to the 50's inside.  At that point, at least one and probably two more 1500 watt heaters would be needed to maintain a comfortable inside temp.

So, now you've got three or four small electric heaters running and maintaining a temp inside of 65F to 70F.  And, the temp outside is only 30F, which is not a threat to your tanks freezing anyway.  Outside temps must be below 30F for several hours before you risk tanks freezing.  And, if the inside of the trailer is at a comfortable temp, it doesn't matter if air is circulating in the basement or not, heat will naturally radiate and convect to that area.  In reality, outside temps of below 25F for several hours would be necessary to threaten your tanks.

Here's my point.................at outside temps low enough to pose a real threat to freezing your tanks (20F to 25F), you're not going to be very comfortable inside the trailer without running the furnace.

YMMV

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glgarlow
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« Reply #9 on: December 14, 2010, 10:49:13 AM »

Being a retired (read frustrated) engineer as an experiment I semi-instrumented our 2010 36MAX1 with wireless thermometers and did some data recording.  Our particular unit has double pane windows and astrofoil insulation.  We are in Fredericksburg, Texas where the temperature has dipped to 25 degrees several times for most of the night (at least eight hours).  In a no wind condition we can maintain a forty-five degree differential (temperature 70 degrees) in the main living area where two 1500 watt portable heaters were located; both were still cycling at about a 70% duty cycle.  The basement area (right by the tanks) maintained about a 20-25 degree differential to the outside (in this case 47 degrees).  So for us we had plenty of spare capacity, especially if we had turned on the fireplace heater.  The bedroom was cooler (60-65ish) but we like the bedroom cool anyway.  This is somewhat optimum as usually there is some wind which would lower the insulation factor.

If this is a concern why not just get a cheap wireless thermometer (I saw one at WalMart for $10.00) and put the remote unit in the basement.  If it gets close to freezing for mental comfort you just put a cheap electric heater in the basement and forget about it.

Greg
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RAN
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« Reply #10 on: March 10, 2011, 06:53:23 PM »

I'd be hesitant to do anything to the air heating system.  It's balanced with a certain amount of airflow through the heat exchanger to keep it from being damaged.  Now if you could find a 110v blower with the same CFM pushing against the same back pressure of the installed system you may be alright.
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Ggup
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« Reply #11 on: May 26, 2011, 08:34:31 AM »

We have the 37' Cameo and use 2 1500 watt elec heaters when temp outside is 30 degrees or colder.  I would like to have the furnace blower on without the furnace running so the heat would be distrubuted via the duct work.  Seems we would have better even heat if this was done.

So this would also draw the cold from the tanks and solve that problem also.  Like others have said...propane is much higher than free or included electric for the unit.

I have also thought of the CHEAP HEAT unit at about $800 installed.  Anyone have one of these?

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Fiverdragger
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« Reply #12 on: July 31, 2011, 08:14:02 PM »

I think you need to think about how the tank heat works as running the blower with no heat might cause heat loss problems in the cabin. First of all the heater only supplies air to the tank compartment. It Does Not have a return duct in it. This keeps any smells from being pulled back into the cabin. When the heater is running it is taking air from the cabin, heating it and supplying it to the ducts in the cabin, basement and tank area. The heated air going to the the tank area is bled off through any gaps or cranks in the outer shell. This air comes from the cabin and has to be replaced as the cabin air pressure is reduced slightly. The pressure is equalized from air leakage in to the cabin from the outside. If you ran the blower wild the outside air bleeding in would have to be heated and thus would require more BTU's than would be required to heat the space without the blower running. This may require another heater to compensate. My point is if it gets below 20 degrees the propane heater should be running at least part of the time. This could be accomplished by setting the gas thermostat 10 or so degrees below the electric heaters thus minimizing the gas consumption.
Ron 
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JamesD
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« Reply #13 on: August 03, 2011, 09:08:31 AM »

I thought about adding a thermostat in the storage area to run the blower if it got below 40 degrees down there. 
That way, you aren't running the blower all the time and the heat can kick in if it gets too cold in the living area.
But if there's not return vent it may just be easier or more efficient to put an electric heater with it's own thermostat down there.
I'm kind of leery of doing that because of the potential to start a fire if something goes wrong with the heater.

I've seen people talk about CHEAP HEAT on other forums but so far, I haven't seen anyone besides a new poster say anything about it.
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yuanfang
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