What prevents heat tape from getting too hot?

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Old 12-21-17, 05:13 AM
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What prevents heat tape from getting too hot?

"CPSC estimates that each year about 1500 fires, 10 deaths, and 100 injuries result from faulty electric heat tapes."

Looking at one here, which I had nothing to do with installing, I noticed large deep char marks where the pipe contacted wood (which is said to contain chemicals which make it char at 150 F to prevent ignition). Obviously the pipe got too hot, so I've disconnected the heat cable, and may or may not want to replace it with another kind.

Looking at one sold at HD, the Q&A seems inconclusive about temperature.

Manufacturer answered about max temp that "It will not go above 88 degrees when on. You can hold onto it with your hand and it will not burn."

However I'm not sure if that means 88 degrees celsius (190 F), because another question indicates that it may be: "Pex max. working temp. is 200 degrees and you state this product opertates well below 120 but that it can melt pex? Once again another short vague and poor answer... Void Warranty? Of what, the Pex, the heat cable, what? What does the heat cable temp really get up to? Will it actually melt the pex that can operate up to 200 degrees? You didn't really answer the question. Your answer is uninformative, lazy and straight up sucks."

So anyway, I was looking for a heat cable that could not get that hot (even if it malfunctioned), or heat the pipe enough to char and burn wood. But I don't know if I can trust the numbers, I might as well try insulating the pipe better, and see how that does. The temperature in winter here doesn't drop too far below freezing for that to be impossible with insulation alone. Speaking of which, they also said that fiberglass was best to use with heat tape, because it is not flammable, however there are codes which restrict insulation around lighting, if it isn't certified for insulation, so apparently it is flammable too.

Although, in my situation there were no char marks where foam insulation covered the pipe (near where it wasn't covered), and it didn't melt the insulation, however they don't recommend foam, or wrapping the heat tape around itself. Maybe that would answer the question too. How much heat can electrical cable handle without being damaged? If heat tape is too hot for wrapping around itself, I doubt it runs cool enough for that or its heated pipe to contact wood safely, even if it doesn't malfunction and get hotter somehow. Because the charring on this wood was radiating well above and below the pipe itself, it doesn't necessarily have to physically contact the wood to char it, and CSPC says "do not cover the heat tape with insulation unless advised by the manufacturer", so that could possibly char through as well.
 

Last edited by electrialdocius; 12-21-17 at 06:51 AM.
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Old 12-21-17, 07:07 AM
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The maximum temperature of electric heat tape depends on the surroundings. If the heat tape is wrapped around a pipe (full of water) that remains shut off for winter and that in turn is covered with insulation, the temperature can go much higher compared with the heat tape hanging in free air like a strip of flypaper.

Just insulation around a pipe that remains shut off offers limited protection from freezing. Theoretically the pipe will eventually freeze and only these obscure phenomena (not always sufficient) prevent the pipe from bursting. 1. The pipe itself conducts heat from the portion extending into heated space out to the portion in cold space to keep the latter part of itself above freezing (less effective with plastic pipe). 2. The far end in cold space freezes first so the water expanding due to freezing pushes the still liquid water back into the building rather than burst the pipe (less effective with a pipe that emerges and runs alongside the exterior wall for a few feet before getting to the hose bibb)..
 
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Old 12-22-17, 12:00 AM
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Apparently they can get too hot for metal pipes to be attached to wood when their heater is left unattended. I'd only plug those in long enough to thaw frozen pipes, and leave them disconnected at all other times. Maybe I'll put a smoke detector there too.
 
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Old 12-22-17, 09:48 AM
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Maybe not, smoke detectors don't seem to work in freezing temperatures. Maybe a fire retardant wood treatment or fire block foam sealant then... that says it's heat resistant to 240 F. Sounds about right.
 

Last edited by electrialdocius; 12-22-17 at 10:11 AM.
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Old 12-22-17, 10:37 AM
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I only know of two applications where electric heat tape is used; the first is in cold climates where it is often used in gutters and over the lower portion of a roof in a zig-zag pattern to preclude the formation of ice dams. In this application the inherent temperature limiting properties along with the exposure preclude it from getting so hot as to be a fire hazard under most normal conditions.

The other application is for heat tracing of piping that carries a fluid that may freeze under certain conditions which would in turn cause the pipe to burst. In these applications the heat tape is generally laid parallel to the pipe and taped in place. Then a layer of insulation is used to contain the heat. The tape is controlled by a thermostat that has its sensing bulb exposed to the atmosphere so the tape is only energized during freezing weather conditions. Sometimes, not all that often, the conditions are such that the heat tape is actually wrapped around the pipe rather than simply laid on one side of the pipe. These require a bit of engineering to make certain they do not overheat when in use.
 
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Old 12-22-17, 12:17 PM
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What prevents heat tape from getting too hot?
Not enough in my experience. These things malfunction all the time and burn themselves or something else up as your CPSC stats show. Sometimes they have a thermostat cube you plug them in to, which always fails in a couple years time. It's pretty common to go into a crawlspace and find the heat tapes running in the middle of summer. Or find some mystery circuit pulling more juice than it should and there's a hidden 20 year old frayed heat tape in a crawl or attic that the current homeowners know nothing about.
 
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Old 12-22-17, 12:45 PM
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Ben may be referring to consumer grade heat tapes. My experience is limited to industrial heat tape such as made by Raychem. https://www.pentairthermal.com/produ...lf-regulating/
 
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Old 12-22-17, 10:39 PM
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Looks like I have a mixture of cables there. One side uses a thermostat cube, which may have failed, and the other has it built in. Anywhere the pipe is fastened to wood though there is some kind of charring (more or less). I'll try to add fire protection there, and treat the cables like a space heater on pipes, only keeping those on for the minimum time necessary to get the water running. It's only for a utility room, so no big deal. I just don't want to screw with reconfiguring pipes and all unless they actually burst. Then I'd use some other material and maybe recirculate the water to prevent freezing.

Hmm, even looking at "fire..." sealants is sketchy. They say the residential kind is only a draft block, and there are videos of people doing flame tests on that vs standard foam, and the fire rated type burns more readily! I'll have to look for the commercial types, because I wouldn't be using that as a draft block, but more of a heat shield, and those supposedly expand to prevent the spread of heat or flame onto the other material (for 2-4 hours). I woulldn't rely on it for keeping the heat cable plugged in all the time though.

Yeah, there's a chapter in a book about wood discussing intumescent fire barriers, and supposedly those begin expanding at 300 F (according to the data sheet of one I looked up), so something like a fire barrier strip behind the pipe that is against the wood should prevent heat transfer through wood for long enough to thaw pipes with heat tape, I gather. https://books.google.com/books?id=ld...+transfer+wood
 

Last edited by electrialdocius; 12-23-17 at 01:39 AM.
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Old 12-23-17, 02:49 AM
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Now I don't know about that either, it just keeps getting better: "Some intumescents, however, are susceptible to environmental influences such as humidity, which can reduce or negate their ability to function." The proof is in the fire

So I guess a silicone hot pad (pot holder) for handling oven ware would work as a heat barrier there. Those are said not to deform between temperatures of -40 to 500 F. I use those with my bare hands on 400 F pans without a problem, and so I might have thought of this sooner, as it seems like a simpler solution to the problem. The intumescent expansion of commercial materials wouldn't do too much more if it came into play, because that's primarily meant to seal off openings into other areas, which isn't the issue here. As far as heat resistance, the hot pad will do as much, and seems to be more stable in open air.

Reading about tests that were designed to ignite wood in a furnace, for example, a study notes "It can be concluded then that if a wood specimen is ignited under external heating barely sufficient to ignite it, it will ignite at ca. [approximately] 250C regardless of the type of heating arrangement". That's around 480F, so it would seem that a hot pad rated up to 500F could absorb enough heat to prevent the wood from auto igniting if not charring all together, for some period of time (some say silicone can withstand use at 200 C for 10,000 hours or more, and shorter periods at higher temps, so that would correspond to at least where intumescents expand or deform, and some of those include silicone for water proofing, so it must not take away from heat resistance in the commercial applications). Apparently the charred wood I found didn't quite get hot enough to ignite from the heat tape, although it seems like this is possible, since they are known to cause fires, at times if they malfunction (or maybe the wood is decayed).

Anywhow, I was just trying to get an idea of what could work well intermittently between heat tape and heat absorption materials to make this safer, without getting too industrial and ordering space shuttle parts (which don't always work either, go figure).
 

Last edited by electrialdocius; 12-23-17 at 05:31 AM.
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Old 12-23-17, 06:09 AM
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I apologize if this was discussed and I missed it but with a quick read I saw a lot of comments regarding charred wood. It has been years since I used the product, but "self regulating" heat wire would be my choice for most applications. The warmer it gets the less heat it produces and that regulation can vary along its length. Here is a link with a short explanation, but there are many on the internet.
https://www.warmlyyours.com/en-US/po...ing-Cables-525

Bud
 
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Old 12-23-17, 06:46 AM
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Thanks, I'd seen it mentioned elsewhere, but I don't know (the same manufacturer of this wire makes those too). If they can't make a thermostat that works, I doubt self-regulating heat wire could be worry free.

I looked up the wires here and they are mostly the braided type. The other flat type of wire with a built in thermostat isn't near those wood parts, and appears to be the same brand. It's UL listed, but the separate thermostat got over 70 negative reviews online, for either not heating, or not shutting off below 50 F. Anyway it appears to be hooked up as pictured by them, and looks new enough. Someone said they used a furnace thermostat instead, and it worked better. Regardless, I'd still want to put a heat barrier between the pipe and wood, but if I had to go to the trouble of replacing the whole thing, I'd use something other than heat cable after looking into it. Especially seeing as how poorly residential grade stuff is allowed to perform for what it is labled as (like "fire block", which makes something burn better)! In that case self-regulating could mean you have to replace it more often, because it self-destructs. I hope it works for you though.
 
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Old 12-23-17, 07:36 AM
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As far as charring goes, it's obviously an indication that something else is necessary to prevent overheating there (the way metal pipes are often strapped directly to wood). I don't think it is extensive enough to affect the structure in my case, and I even missed it at a glance, going back and forth there working on something else (but it is larger than the pipe diameter, so I don't want it to continue at all). Another study by the USDA Forest Products Laboratory indicates that long term heating would discolor the entire piece of wood to a slightly darker shade (not so much like charcoal in one area), and make it lose weight and shrink. Then it may ignite at a lower temperature than expected (and/or not support weight very well). So it seems the the longer unprotected wood is exposed to heat tape or its heated pipe, the more likely heat tape would ignite it (and this may include self-regulating tape as well). It doesn't have to reach high temperatures to become a long term hazard, as I read it. I'll definitely keep an eye on that, if heating the pipe is necessary (and have a heat barrier in place between the pipe and wood).
 
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Old 12-23-17, 07:38 AM
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The property of these self regulating heat cables is a result of the positive temperature coefficient of the material between the two conductors.
Google "positive temperature coefficient".
As the temperature rises the resistance increases thus the current and heat generated goes down. Instead of a thermostat controlling the current through an entire heat cable where a confined section can overheat, a self regulating cable is powered by two parallel conductors. This means that any section that gets warm the power will decrease in just that section.

It has been many years since I played with temperature coefficients at the laboratory where I worked but I ran into these cables about 20 years ago at home Depot, both 3 watt and 6 watt per foot. I used them in strips to melt through some 6" thick ice I had on a low slope section of my roof. They melted through and provided relief channels to drain the water that had accumulated behind the ice. If I were to pound the ice off it had a habit of taking my shingles with it. The heat wire approach was safe and gave me the time so I could wait for a warm spell where the ice would release from the shingles and could be knocked off.

Since then (ice problem eliminated so no longer need the heat wires) they have taken this technology and used it in several brands as heat tapes. Worth considering IMO.


"In that case self-regulating could mean you have to replace it more often, because it self-destructs". Its self regulating property has nothing to do with "self destructing" just a positive resistance coefficient of the material.

Bud
 
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Old 12-23-17, 08:06 AM
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Sounds good. Personally, I'd have to set it up the same way though, to protect the wood with something that prevents transferring as much heat there from the pipe (as prolonged heating can lower the ignition point of wood without even charring it). In a way I'm glad it came up (sooner than later), because I didn't know this could be such a problem. I'm not saying it is certain, since those who study it say it isn't entirely predictable. It doesn't have to be a heat cable either, I think it's just a general rule (as they mentioned wood near steam pipes igniting at lower temps after prolonged exposure there, among other things).

So I want to limit the use of heat cable to when it's too cold out for the pipe insulation to work, and keep as much of the heat off of the wood as I can while the pipes are being heated, then disconnect the heat source when they are working again, so there is not much of a chance for the wood to be adversely affected (especially since it has been already). Well, if I were to run new pipe, I'd try not to strap it against wood, that's for sure. I'll have to see how often the heater is necessary, since I think they were insulated at the same time. So far so good, without heat on (and it's slightly below freezing at night).
 

Last edited by electrialdocius; 12-23-17 at 08:48 AM.
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Old 12-23-17, 09:24 AM
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Otherwise, there is something unusual about the charring that I hadn't noticed before. Using a spot light I see there are disconnected char marks on another piece of wood, without pipe, well above the first (which was concentrated behind a pipe elbow). So it may be that these were caused by a torch when sweating the pipes together. Coincidentally though there is all this other information which indicates that there could be a danger of charring from a pipe heater, etc. I wouldn't guess that it burst into flames and only marked those two areas without connecting the dots however. So I'm thinking that was a bad use of a torch without anything protecting the wood, especially in that there don't seem to be char marks where there's just straight pipe attached to wood (and the rest are tending upward around fittings, like torch marks—well I don't use torches, and haven't seen this often as such) . Silly me, I'd have to dismantle the whole thing to put a layer of protection against the wood (since I didn't see all the points of contact under the foam initially), but then I wouldn't want to leave it there, so inspecting it more carefully will have to do. I'll still take it seriously either way (and not over use a heater myself).
 

Last edited by electrialdocius; 12-23-17 at 10:46 AM.
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