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# Heating a room with resistors (designing a heater)

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#### kybert

##### New Member
Hi,

Im trying to heat a dog house with resistors.

The resistors are 4°C/W to free air.

So, if i use a 100W resistor, it will heat up to 400°C, right?

I'll spead it over several resistors so each gets to about 30°C

How do i work out the required watts are needed to raise the room temperature?

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I did that once, and brunt down the dog house. Dog barely escaped with his life.

You need to determine the minimum temp and the max temp...then add into the mix the thermal transfer coefficient of air which is approximately 10-100 watts per metre (sqaured)K

Best if you looked up on Google or something...

BTW, the answer depends on alot of factors; size of the doghouse, exterior surface area, insulation properties, etc. Best to experiment, and please, please, please be careful.

Ps, if I had this to do over again, I would use a small forced air electric heater with thermal protection. I'd make an insulated box with supply and return ducts in and out of the doghouse. I'd provide lots of bedding for the dog and a door to seal out the cold night air, but one he can operate himself. I wouldn't try to get the temp up to human's comfort level; a dog with good bedding can be comfortable at lower temps. I'd also insulate the dog house as much as possible, and use an interior t-stat along with the heater's t-stat. Perhaps one that would alert me if the temp goes over or under. And, oh yeah, a smoke alarm.

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The safest heater would be a space heater with a ceramic element. Those never gets hot enough to burn anything.

The dog house is insulated with 10mm polystryne, and its about 0.4 cubic metres.

So, at 10-100W/m3 i need between 4 and 40W to heat the room, but how hot will it get?

The door is sealed etc.

Being from a cold weather region around here many people have rather well insulated dog houses. (probably better than most homes or mine at least.)
If you go with the door method mentioned earlier and good insulation the dog itself can usually keep the inside above freezing from it's own body heat.
For and added warmth a simple heating pad mounted to a wall is more than enough. If you factor in good insulation and a fair door flap the actual heat energy needed for a dog house is rather low.

Also consider that animals need to stay cooler in order for them to properly develop their winter fur. Once they fur up many breeds of dogs will find 40 F too warm for them in the winter. Cold weather breeds like huskies can find 0 F too warm some days!

Im going to place a 40W light bulb in there and see what happens. How do i calculate the power needed to maintain a constant temperature, given the outside temperature?
Odvously if its 10°C outside, the amount of watts required to hold the temp at 20°C will be less than when its at -10°C. There must be a formula for the relationship?

You need to know the rate of heat loss for the insulation and other relative stuff.

If you can put a 40 watt bulb in it and maintain a specific temperature differential you can roughly calculate the energy you will need at different temperatures from it.

Or use a larger heat source and one of those freeze detector devices that have the built in thermostat that turns on what ever is plugged into them automatically if the ambient temperature drops below just above freezing. It would give you a basic thermostat control then too.

Im going to place a 40W light bulb in there and see what happens. How do i calculate the power needed to maintain a constant temperature, given the outside temperature?
Odvously if its 10°C outside, the amount of watts required to hold the temp at 20°C will be less than when its at -10°C. There must be a formula for the relationship?
Put the bulb on a thermostat

I keep my dogs in the house, where they might prove useful someday for protection. Doesn't really get cold enough here to worry about, mostly it's the wind and the rain, bugs and heat. Most furry animals can handle the cold pretty well, without the electronics. wouldn't worry about it much, unless you expect several days of freezing weather.

Why not just get him a...

girl dog to keep him warm?

The question is how much wattage do you need to sustain a livable temperature for the dog. Maybe just use a heat lamp to do the trick. You could even use a base board heater thermostat close by (remove the thermostat knob so he can't turn it up or he'll run it up to 32c and there goes your electric bill!). A wall thermostat would already have the hysteresis built in. You only need about 15 - 20 C for the dog to be comfy right?

Oh and BTW I'm new here so Hello all.

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A heated doghouse is a nice thought but not needed, all you need to do is provide good insulation which you seem to have and try to shield the door from the wind, Leave fleece type material in there and the dog will be just fine well into artic temperatures without a heater.

May I suggest:... I have a rescue that's pretty bad crippled up from puppyhood. He's getting old and feels the damp as much as the cold. Do you have the "Dogloo" in UK? It looks like an eskimo igloo.... Styrene or urethane foam a half inch or so thick, dome shaped. Thomas is about 100 pounds, the dogloo right at 3 feet across.

I bent a piece of thinwall conduit to fit just inside the curve of the shell. Mounted a setscrew box adaptor back to back with a cable clamp on the end. Inserted a 9 ft strip of heat trace (water line heat strip) and made up the splice to S-O cord inside. It's warm enough to warm his old bones but not so hot he can't touch the tubing. I found it easier to install the heat tape and then bend the conduit.

The heat trace I used was industrial salvage and had to have a cord spliced on. The same stuff should be available in a hardware store with a cord already attached. Rating is 240 volt at 15 watts per meter. In the States, they are available at 120 volts. With just under 3 meters, its right at 40 watts, just about right. If you make the cord up yourself, be sure to connect the ground wire to the conduit.

ok, so last night i placed a 40W bulb in the dog house, and measured the inside and outside temperatures.

The outside temp was 2C and the inside was 4C. This was maintained right through the night so im guessing my insulation needs to be beefed up a bit!!!

The dog house has the door boarded up, and a cat flap fitted; its for the cats, not a dog, and they tend to like it warm!

Im aiming for a room temperature of about 22C, but must NOT exceed 27C or i'll kill them.

Dont want the cats in my house any more!! but the misses wont let me throw them in the garden without a heated kennel for them, and it must be the same temp as our house. Thats what she wants.

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That is just crual, you are not living with cats, you are living at your cats' and only allowed in the place as a food providing servant.

That said, as long as it will be open, it will loose all the generated heat. Have you consider that cats like their darkness, and hence the lightbulb might not be too good an idea.

If you ask me, all that is going to happen is trouble with you neigbours, as the cats will try sleeping at theirs, or spend the night crying to be let in.

Thanks,

The light bulb is just a simple way to test if 40W was enough, once i have the correct power, il'll use loads of resistors to generate the heat (lots of them so the heat is spread accross the entire floor area evenly - under-floor heating!)

The cats like the dog house, they sit in it all day long, but at night its just too cold outside. There will be a small light from a cuple of leds to light the room at night, but not too much.

Im beefing up the insulation tonight to 25mm on the walls/floor, and 50mm on the roof to see if that helps.

Other cats cant get in, unless they too have a magnetic collar.

Hi there,

It is nice to see that people care about their animals comfort in the cold winter
months.

Long time ago i built a small animal heater for hamsters kept in an aquarium.
I had to lower the heat quite a bit in winter to save on the heat bill and i
didnt want the hamsters to get too cold.
I built this one out of a coffee can with a metal lid with a hole drilled in the top
for the wire. To keep the animals from chewing the wire, a half inch copper
tube was used that was soldered to the top of the coffee can and the wire
was pushed up through the tube and out the top of the aquarium.
Inside the coffee can i put a socket and a 6 watt night light bulb.
The 6 watt night light bulb didnt keep the entire aquarium warm, but would
keep the immediate surrounding area warm.
When i would get home from work at night the little critters would all be
snuggled around the coffee can

The trick to getting it right usually boils down to careful experimentation.
Try a small wattage at first and monitor the temperature with a wireless
thermometer. If it's not warm enough, use more resistors. If you use
low voltage to power the heater it will also be safer for the dog when it
rains. I dont think you want to fry the little guy

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Dont want the cats in my house any more!! but the misses wont let me throw them in the garden without a heated kennel for them, and it must be the same temp as our house. Thats what she wants.
I'd throw the missus out there with them! If you have to go through all that for animals you really don't want to care for in the end, she can keep them warm. Meanwhile you bring home a new pet to keep you warm, a mistress!

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