Heating pad element sizing

[santeri1992]

TL;DR Summary

True amateur here. I am planning to make a heater pad for a rabbit enclosure that keeps their water bowl thawed during winter. Need help to figure out if my plan is feasible

Three rabbits will be wintering in a 2m x 2m x 2m wooden hut.

Outside temperature does not go below -30c (-22f)

Planning to put a 50cm by 50cm 1mm thick stainless steel sheet on the floor of the hut (inside off course)

My plan is to heat the sheet with a 50w PTC heater element https://fi.rsdelivers.com/product/d...5mm-50-w-100-240-v/dc/2995922?backToResults=1

Is this heater (50W) enough to keep the pad warm enough to sustain liquid water in a steel bowl on top of the sheet?

I am going to apply thermal paste to the contact point of the pad and sheet and control the temperature with a simple outlet thermostat.

The rabbits will produce some heating inside the hut, but that might be negligible.

Thanks in advance to anyone who finds the time and effort to help me out as i like to find DIY solutions to as many of my problems, but thermodynamics is too difficult for me

 

[.Scott]

I had the same problem about 15 years ago. What I did was to run a water pipe heating element through the inside of a copper pipe - about 1" diameter - and then ran about 30 inches of that pipe through an enclosed portion of the hutch. I kept a metal watering bowl in contact with that pipe.
But the bunnies had different opinions. First, they did not spend much time in the enclosed area - even when the temperature got down to -2F. Second, they weren't crazy about having either the water bowl or the food bowl in that area either. Third, the *love* snow.

Outside the enclosed area of their hutch was a caged-in "run" - about 70cm square. Snow could not fall directly onto that area - but plenty of snow would get blown in. My plan was to clean up the snow and the poop from that run whenever it needed it. I was using a dust pan and whisk broom to accomplish this.

Between the time that I had whisked the snow into a pile, and before I had time to reach for the dust pan, one of the bunnies had plopped himself belly down on the snow pile and was using his front and rear paws to push the snow back to recoat the floor. They were 2 black Belgium bunnies and they were remarkably expressive.

So I switched tactics. From that point on, I made sure the bunnies had plenty of fresh snow in their run area. As for the water, that pipe kind of worked - to the extent that the bunnies tolerated it. But I also continued to bring tepid water to their bowl daily. Even without the pipe at 0F, the water stayed liquid for plenty of time.

As far as cold temperatures are concerned, even at -2F, the bunnies showed no preference for the enclosed semi-heated area.

Based on your design, I think you already know this. But for other readers, it is worth mentioning that bunnies will chew through any wire - it's what they do. So, in my design, the copper pipe kept the heating element safe and in the OPs design, it is that metal plate.

 

[Baluncore]

Planning to put a 50cm by 50cm 1mm thick stainless steel sheet on the floor of the hut (inside off course)

My plan is to heat the sheet with a 50w PTC heater element

Stainless steel is a very poor conductor of heat. Heating one point on the thin sheet will be insufficient to heat the full area of the plate. You might use a sheet of 1.6 mm thick copper. That could work.

I would consider a flat, shallow box, then fill it with a glycol antifreeze. Heat the antifreeze in the middle of the bottom sheet, so the thermal siphon circulates the fluid, to warm the upper plate. To reduce conductive heat loss, place a thin sheet of polystyrene foam under the outer parts of the plate.

 

[santeri1992]

Stainless steel is a very poor conductor of heat. Heating one point on the thin sheet will be insufficient to heat the full area of the plate. You might use a sheet of 1.6 mm thick copper. That could work.

I would consider a flat, shallow box, then fill it with a glycol antifreeze. Heat the antifreeze in the middle of the bottom sheet, so the thermal siphon circulates the fluid, to warm the upper plate. To reduce conductive heat loss, place a thin sheet of polystyrene foam under the outer parts of the plate.

Thank you for your answer. Unfortunately antifreeze of any sorts is out of the question as the rabbits would inevitably find a way to get in contact with the antifreeze. I would be able to tig-weld a rabbitproof sealed container, but in that case the scope of the project escalates too much for me.

I think copper is allso a hazard as the rabbits will pee on the plate and there is a risk that copper urate will contaminate their food or water.

would increasing the power of the PTC (50-100W) help this issue?

 

[Baluncore]

would increasing the power of the PTC (50-100W) help this issue?

The heater power needs to overcome the thermal losses from the plate. Those losses are proportional to temperature difference between the plate and the environment. Without a thermally conductive or convective plate, increased power will cause a local hot spot. The temperature of that hot spot will be regulated by the PTC characteristic, not the available power

You could heat a stainless plate from above with an IR lamp.
You could epoxy, or powder coat, a copper plate.
Best would be a TIG welded, flat stainless steel box, with an external tube to keep the antifreeze away from the animals. If glycol is too dangerous, use a vegetable oil or winter diesel.

 

[.Scott]

Thank you for your answer. Unfortunately antifreeze of any sorts is out of the question as the rabbits would inevitably find a way to get in contact with the antifreeze. I would be able to tig-weld a rabbitproof sealed container, but in that case the scope of the project escalates too much for me.

I think copper is allso a hazard as the rabbits will pee on the plate and there is a risk that copper urate will contaminate their food or water.

would increasing the power of the PTC (50-100W) help this issue?

I had two Belgian rabbits. When I started using the copper pipe, they were about 1 years old. One lived to ten, the other to twelve - both longer than average. They did not attempt to gnaw at that pipe. Also, the pipe was not in direct contact with their water. The water bowl was metal - as I recall, stainless steel. I used metal clad cable to get power from outside the cage to the heating unit.

I made sure that the copper pipe I bought was wide enough to hold that somewhat bulky thermostat. Then I ran that pipe on the floor of the enclosed part of the wooden bunny cage, along the walls.
That Frost King heater will put out 7 Watts per foot. As I recall, the one I used was just 3 feet. But they have a 6-foot version (42 Watts).

 

[erobz]

TL;DR Summary: True amateur here. I am planning to make a heater pad for a rabbit enclosure that keeps their water bowl thawed during winter. Need help to figure out if my plan is feasible

Three rabbits will be wintering in a 2m x 2m x 2m wooden hut.

Outside temperature does not go below -30c (-22f)

Planning to put a 50cm by 50cm 1mm thick stainless steel sheet on the floor of the hut (inside off course)

My plan is to heat the sheet with a 50w PTC heater element https://fi.rsdelivers.com/product/d...5mm-50-w-100-240-v/dc/2995922?backToResults=1

Is this heater (50W) enough to keep the pad warm enough to sustain liquid water in a steel bowl on top of the sheet?

I am going to apply thermal paste to the contact point of the pad and sheet and control the temperature with a simple outlet thermostat.

The rabbits will produce some heating inside the hut, but that might be negligible.

Thanks in advance to anyone who finds the time and effort to help me out as i like to find DIY solutions to as many of my problems, but thermodynamics is too difficult for me

I don't think anyone has asked about the material construction of this hut...is there some reason we don't have to characterize it to make an assessment? Is it almost fully enclosed, with just a small entryway; how thick are the walls etc...?

 

[.Scott]

I think the main purpose is to make liquid water available. Generally, the warmest spot will be a small area (perhaps 10x40 cm)) with a hole large enough for the bunnies - but not much bigger. And it will be filled with hay or something similar. For my cage, the heated pipe did pass through this area. But it is not where the watering bowl would be. For my cage, most of the time the water ended up in the run - almost out in the open. At best it was in a semi-enclosed area protected from wind - but not much more.
The colder it gets, the less the bunnies drink. So, for as long as there is some water on the surface, it will suffice. But if the surface is fully iced over, the bunnies will not attempt to scrape or break it. The OP is saying that his outdoor temps can get down to -22F. At that temperature, I am guessing that the bunnies will finally be hunkered down waiting for morning sun. I had a thermometer in the cage and a CC camera with that thermometer in its view. Down to -2F, those bunnies stayed in the run area looking very comfortable.

 

[Baluncore]

Why not heat a stainless steel water container, that is fixed-in-place?
If heated from below, the water will circulate and mix, so ice will not form.

 

[Rive]

Why not heat a stainless steel water container, that is fixed-in-place?

A small hot water boiler may fit that (set at low temp), but then you need to heat the outlet pipes too.

..on second thought a salvaged electronics itself still could be re-purposed to keep the temperature an enclose water body below the steel plate: and so the whole matter about the required average power is moot.

Ps.: well, too many issues with safety. Maybe a heating element from a towel dryer?

 

[.Scott]

Why not heat a stainless steel water container, that is fixed-in-place?
If heated from below, the water will circulate and mix, so ice will not form.

If this can be toppled or moved, the bunnies will topple or move it.
And, of course, the electric power cord to that heater will need to be replaced with something armored.
Also, even if you refill the bowl daily, you will also need to flush it out a couple of times a week - so it will need to be removable - but by humans only.

In my case, there was another requirement. The outdoor bunny cage was a compromised. My family's initial notion was that these were going to be indoor bunnies. So, it was thought, those poor bunnies need a heater. Well, eventually everyone agreed that, at least for the temps we were talking about, they did not. But when I first set things up, they wanted more than just the water warmed up.

 

[.Scott]

Ps.: well, too many issues with safety. Maybe a heating element from a towel dryer?

Home depot sells that Frost King pipe heater (with preset thermostat) for about $23 for the 3' and 12' version and $20 for the 6' version. That's the one I pictured above. Under normal use, they are rated to keep water flowing through a 2" copper pipe down to -40F. That black pad on the side of the thermostat is suppose to be flush to the pipe. For my purposes, it was flush to the inside of the pipe.

What that meant in my case is that I spent more on the copper pipe than I did for a nicely sealed up thermostat and heating element combination that, except for the unarmored cord, was an ideal match for my bunny application.

 

[sophiecentaur]

TL;DR Summary: True amateur here. I am planning to make a heater pad for a rabbit enclosure that keeps their water bowl thawed during winter. Need help to figure out if my plan is feasible

Is this heater (50W) enough to keep the pad warm enough to sustain liquid water in a steel bowl on top of the sheet?

Surely the whole problem should be considered. The bowl will absorb heat and lose heat in proportions that correspond to the temperatures all around the bowl. If the bowl is in an ambient temperature just above 0C then it won't freeze. Are you intending to keep the bunnies from freezing too? What would the minimum temperature need to be to keep them 'alive' / 'happy'.?

What running cost would you accept? A big enough insulated box with a 'cat flap' would be cheap to run (all relative) The animals would spend time in the box, according to what they wanted. You'd need sufficient change of air in the box, of course.

Bottom line is you need a small pet forum, rather than relying just on PF. You are after a complete engineering solution here.

 

[erobz]

Well one thing, decrease the volume of the box if that is possible? I can’t see why the bunnies need a 2m tall box. Minimizing the volume will elevate the steady state temperature inside the box when the heat source is applied.