Heating Tapes for Pipes

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Old 04-08-18, 10:40 AM
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Heating Tapes for Pipes

OK. So I am itching to go to my cottage but the darn spring weather, here in Canada, is not cooperating as well this year as it has in the past. Usually, at this time of year, we get a couple of nice 50F high days with a few above 32F lows to help warm things up.

My concern is that in my crawlspace I have a 1.5" poly or PVC pipe bringing water into my cottage from the lake. It is all underground except the 15' to 20' that goes above the ground in my crawlspace. My cottage is left unheated all winter and water is left in that pipe length since I cannot remove it.

Only one year in the last 15 have I come to the cottage in the spring and had a frozen pipe and my guess is it was frozen in my crawlspace. That spring had temperatures like we have this spring. All temperatures close to freezing and many below it. I have always thought about installing a pipe tracing electric heat tape along the pipe in my crawlspace. My plan would be to have this off during winter but use it in the spring to thaw my pipe, if my pipe is frozen when I get there.

My question is: These tapes puts out around 7 watts per foot. With a 1.5" diameter block of ice, how long would something with such low wattage take, to thaw a pipe? Has anyone done this before? I was hoping in about an hour or so but since that tape, when turned on, is probably colder then my refrigerator, I am starting to wonder. I've seen a frozen turkey stay frozen in my refrigerator for quite a few days although that turkey I am sure started at a much lower temperature then what I suspect my supply pipe is at right now. Once thawed, the heat from the cottage above it seems to keep the pipe thawed under any temperature conditions outside. The previous owners lived in it year round without any problems or heat tapes.

Appreciate any thoughts anyone has on how to quickly thaw a long length of plastic pipe if it becomes frozen.
 
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Old 04-09-18, 03:13 AM
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I don't recall ever using heat tape to thaw pipes. I'd expect it to take a long time because the heat tape doesn't put out a lot of heat. It's main purpose is to keep the pipe warm enough so it doesn't freeze. IMO draining the pipe in the fall would be a better plan.
 
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Old 04-10-18, 09:58 AM
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One of the problems of draining the pipe is getting rid of all the air when it comes time to prime my jet pump in the spring. After my crawlspace the pipe is all underground right out into the water, where the foot valve is.
 
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Old 04-10-18, 10:04 AM
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So, I haven't tried it yet and I guess it is not something that people here do or have done very often.

I have a friend of mine that is pretty good with math, so we had some beers last night and did some theoretical calculations, for anyone interested. Might as well make some use of that high school education.

1 calorie of energy will raise 1 gram of water 1 degree celcius. With the volume of ice in a 20 foot PVC pipe, 1.5 inches in diameter equalling 6,950 cm^3 (Sorry for my American friends but this calculation has to be done with metric so I had to do a lot of conversions).

6,950 cm^3 = 6,950 g of water/ice

Assuming my pipe is currently -6 degrees C, it would take 41,696 calories of energy to warm it to 0 degrees C. Now for anyone not sure of the concept, more energy is required to change ice at 0 C, to water at 0 C. It is called the latent heat of fusion in the physics books and that would require an additional 555,000 calories to change the volume of ice in my pipe to water, all staying at 0 C.

So the total energy required to change my 6,950 grams of ice at -6C to water at 0C, is almost 600,000 calories of energy. 600,000 calories of energy equals 697 watt hours.

If I wrap a 30 foot heating line around my pipe that puts out 7 watts per foot of energy, then that is 210 watts per hour.

697/210 equals 3.3 hours of time to thaw that pipe with a 30 foot heating wire. That assumes absolutely zero heat loss, which is not going to happen, even though I would insulate it all. So my guess is that it might take around 4.5 to 5 hours to thaw my pipe with a 30 foot heat tape wrapped around it, once I switch it on.

Even if the starting temperatures are different the real energy gets spent on just transitioning ice to water, so starting temperature variance is not that big of deal. You can see why these things work so good keeping water liquid as opposed to changing solid water to a liquid. That latent heat of fusion is quite a large amount of energy in comparison. That phenomenon kind of works for us, keeping our water liquid and is a real pain on the ice side bringing it back, but that is the way the universe was created.

It's a lot of work for an estimated 5 hour thaw time so I will have to give this a little more thought. This is just so I can go down to the cottage a week or so earlier during cooler springs, so I am not sure it is worth the effort and cost. If I do it and ever have a frozen pipe I will update this thread. I am sure the math is going to be off by a little or maybe even a lot. The real world always throws me a curve ball from the mathematical world, but it is nice to have at least a rough calculation before I make my decision.
 
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