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My Stove as an RV Heater


THE NEED FOR AN ALTERNATE HEATING SOURCE

There had been little sun for days, just fog and rain. On the second night of near freezing temperatures, I could see we had a problem. I only have one battery, and it had never been adequately recharged with no direct sun. In the dismail rain, ambient light could only muster a little over one amp: that equates to only about 10 amp-hours a day. That furnace of mine is eating up 30 amp-hours a day! It does not take a rocket scientist to predict a fast approaching cold future. My one battery is an old 100 amp-hour battery that is almost depleted of all reserves.
After a day of miniscule charging it is at 12.4 volts. Just pitiful! At midnight it is 11.9 volts: almost completely dead. In desperation, I turned off the furnace. A deliberate and necessary act, because we can not loose "water". Specifically, we can not loose the bathroom! The bathroom needs water. Every thing now is reserved to the water pump, and it now has top priority untill day break.

And I informed my wife that there was going to have to be lot more cuddling in her future due to the situation... RULERMAR.GIF, 1 kB The problem:

BANASTAR.gif, 1.5kB You, LORD, are my lamp; the LORD turns my darkness into light.
2 Samuel 22:29
The furnace is using too much electricity: 7.5 amps. And when nights are cold and the days have no hope of sunshine, I needed an efficient and safe alternate source of heat.

I decided to vent the oven gasses from the stove to the outside, and in the process use copper tubes and the top stove vent as heat exchangers.


StoveFlame.jpg, 8 kB
The hard points to consider are to not restrict air flow. Other wise deadly carbon monoxide could be produced. Even though all gasses are safely vented to the outside, inefficient combustion is to be avoided at all times just on general principal. It is stuped to even mess around with this possibility! After all, the damper could accidently be closed or something.

Also, another consideration: I do not want the stove permanitly modified: I do not want any holes drilled into it. All of the modifications to the stove and it's vent must be superficial. To my knowledge, no one has done this before - at least not still living people.

And thirdly, the oven and top burners will still function as before. Functionality can not be lost, including the hood vent.


StoveBackB.jpg, 43kB
The idea is to vent oven gasses to the hood vent.
Stove-Silicone.jpg, 27 kB
After the aluminum flashing is rough cut. I must determine exactly where the edges of the metal are messing with the top surface. A gas tight fit is required to have no carbon dioxide, water, or carbon monoxide escaping into the RV. To do this I apply silicon2 all around the edges, and then gently place Syran Wrap or Cling Wrap over the still wet silicone. I obtain an impression when the stove top is placed on top. Give it a day to cure then remove the top and the syran wrap. Observe the flat spots. That is where it is touching ok. Reapply untill all areas of the lip are touching. Then switch to High temp silicone and reapply. Then use high temp tape on the walls, fitting to the established lip contour.
StoveTop-Manifold.jpg, 16 kB
Two one inch aluminum angle pieces are placed together to form a two inch channel. The aluminum manifold is placed on top of the stove over all the oven vent slots.
The aluminum manifold is sealed with reflectix heat tape. This tape is quite strong and also has a good grip. The tape holds it in place.

Four holes are drilled in the manifold to accommodate the four copper tubes.
Silicone2-400F.JPG, 13 kB
Silicone is good to 400F degrees.
Silicone-CopperTop-700F.JPG, 16 kB
High Temp Silicone is good to 700F degrees.
StovePipes-45.JPG, 21 kB

StoveSpecs-7100.JPG, 43 kB
7100 BTU/Hr for the oven.

I can get aprox 95% efficiency with a "room-temperature" measured at the exhaust.
And it consumes no electricity!
In contrast, the Furnace is only 70% efficient, and uses a big 20000 BTUs/Hr.

I love the Furnace: the way it mixes the air, the way it heats the floor, and the speed at which it warms the place. Therefore, the oven-heater will be only a supplemental heat source, and a low output emergency source.
StoveTop-Manifold.jpg, 16 kB

StoveUnderGrill.jpg, 29 kB
Here is the underside of the stove top. Gases flow up through the slots and into my manifold.
StoveUnderManifold.jpg, 22 kB
It took a week, each day adding a little more silicone and ceran wrap to make more of the form.


The four tubes are directly vented above the hood vent, into the duct to the outside. The outside vent door is held open with a clip, and does not depend on the fan for its operation. The TT is so air-tight that we need more ventilation anyway. Of course, if we are cooking we use the fan for added air flow.


RULERMAR.GIF, 1.6kB
StoveHoodExten.jpg, 26kB
Another modification...
Had to add a hood extension, made of clear plexiglass. When cooking fish, odors are more easily vented to the outside with the hood extension. It forms a border about 4 inches wide. I removed the flapper door of the vent, so that all operations are not dependent on quantity of airflow.

I do not like smelling burnt gas, which is CO2. CO2 is kind-of odorless, and makes the air smell (or feel) "stale". But you can realy smell cooking odors. Cooking odors can permeate blinds, clothes, and carpets. This happens to other RVs, but not mine: My RV smells just as clean as the day it was new.

When using the oven for broiling the vent filter may be left open with this hood extension. However, if the oven is being used for heat then the filter is closed off. In this case, spent gases are vented directly to the outside, instead of lingering under the hood. You do not want to breath bad air in an inclosed space; It gives me a real headache. The vent filter is closed off with a piece of reflectix, as is shown in the picture.

When I am cooking with low heat, as when boiling a couple of eggs, the spent gas is quite well vented to the outside naturally, without the vent fan being on. But when I am removing lids for inspection or stirring, I use the fan. It makes fast work of rising steam and odors, although it is very loud.

It is always windy along the coast, and wind hitting the right side of the trailer can sometimes reak havoc and simply require the fan to be on. In contrast, wind from the left actually creates a vacuum on the right side, and sucks out cooking air without using the fan. Be sure to crack open a window that is on the windward side, or open slightly the bathroom ceiling vent.
RULERYEL.GIF, 4.2kB There never has been, nor ever will be, any cooking odors in my RV. Nor, do I want to breathe spent stale CO2 gas from the stove top burners or the oven. I like "fresh".

This stove hood fan has always been an aggravation in the back of my mind. Obscure, but decidedly something that I don't like. But now it has received my direct attention, and clearly something needs be done. The fan is in focus: It is loud and obnoxious. It consumes more power than is necessary, especially if I leave it on for an hour of cooking. I always have to turn it off, only to later turn it back on to lift a lid.

Rheostat.jpg, 30kB
The Hood Fan takes 1.5 Amps at 13.5 volts. That is about 9 ohms, and 20.25 Watts. A rheostat of 9 ohms would be 1/2 speed, and a rheostat of 27 ohms would be 1/4 speed, at the far end. I would like about 1/4 speed to be mid-pot (27 ohms, assuming linear taper). So, far-end would be double that, or 54 ohms. So, a standard rheostat value near this target would be 50 ohms.

A brand new pot arrived from Amazon...
The pot is 50 ohms, just what I ordered. Rheostat is 25 watts. I require half that in a worst case scenario (which occurs at 1/2 speed, or 9 ohms), so this is fine.


I cut the line going to the fan, and drilled a half inch hole in the stove hood, and installed the rheostat. Took less than 5 minutes.
Works great! Whisper quiet! I can now leave it on for the entire cooking sesion. And I can adjust to any speed. Perfect! Simple solution! Simple!

Well, not so simple...
Yes, this is a simple mechanical thing, and simple is to be desired. But the cooking environment threatens my simple toy: There will be lots of steam hitting this pot, from vegetables and meats. Burning gas will produce lots of hot water vapor from combustion. Cooking grease is going to coat the windings, not to mention assorted splatters. And then dirt is going to stick to this residue. How will this affect my rheostat?

I have been an Electrical Engineer for 35 years and have seen lots of rheostats. And I know for sure that I will be spending more time cleaning my rheostat than cooking! So here is what must be done...
Ryeo-Grease.jpg, 30kB Ryeo-Grease56.jpg, 50kB Apply grease

Ryeo-Grease60.jpg, 18kB Inclosed and sealed.
Three things must be done:
First is to solder wire pigtails onto the posts. Because in the next steps, you will loose access to the inside of the rheostat.

Second is to apply liberal amounts of a water repellant grease. Apply on everything that may corrode. After the grease is coated in between the windings, I also apply two drops of synthetic oil right on the windings. The object is to prevent moisture from coming into contact with any metals, especially microscopic powder from friction. Most abrasion particles, except for silver, I have found, will become insulating contaminants. These contaminants will then have to be removed by cleaning. To prevent this, grease can be applied as a boundary layer.

Third, is to inclose the rheostat to protect from non locally generated contaminants. This is mainly dust which would otherwise stick to the grease. I made this one out of plastics, cardboard, and tape. Don't use Duct Tape, use Gorilla Tape. Mine is not too pretty, but it works great.

A well maintained rheostat should never arc because two or more windings are always in contact with the brush at all times. But - if they do arc - microscopic hot spots will burn contaminates into and onto the windings. This will promote further electrical "scratchiness". Cleaning will not help much, and the windings will have to be burnished.


ChickenHead.jpg, 22kB Pretty nob
Found a pretty nob: a clear "ChickenHead nob". It gives a better indication, in feel and sight, of how fast is the fan turning. Can't always hear it.


Hood-Lights.jpg, 26kB Hood indicators RGB: 5,000mcd, 5v, 0.2w, 30deg Blinking Red: 0.2 Lumen, 2000mcd, 0.175 Watts, 10v
I accidently left the hood fan running all day. I knew it was going to happen, it is too easy to do. On the low settings with just enough draft to do the job, I can not hear it. On low, it is not using much current; That is not the problem. It simply does not need to be on!

So here is the solution:
Installed two Hi Intensity LEDs, one on each side of the hood. Drilling through the plastic must be done with care, using a sharp bit and little pressure. The LED on the right is a Blinking Red, and the one on the left is a RGB Changing Multicolor. Something has to draw my attention to the fact that either the hood fan is on or the hood lights are on. Also, it is a pretty way to do it.



RULERBOW