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It seems to be common advice to run the cold water while dumping boiling water down the drain (for example, when draining pasta). A variety of reasons are cited for this, one of which is that it is bad for your plumbing. I assume the reasoning is that the shock to the pipes when they are heated or cooled suddenly causes material fatigue.

  1. Is there any evidence to suggest that pouring boiling water down the kitchen sink causes pipes to wear out or leak faster?
  2. If so, is there any evidence that running tap water at the same time can mitigate this effect?
  3. If so, should you run hot tap water to "prime" the pipes and heat them up more gradually, or cold water to cool the boiling water as much as possible?

Bonus question: if the answer to (1) is "No", is there any benefit to running the tap at all?

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    I've never heard of this. The pasta example might be due to the idea you should run cold water over the pasta immediately after draining - which you shouldn't do, but that belongs in a different SE site. I'm unaware of any reason, but lack a resource to point to. – Evil Closet Monkey Apr 12 '16 at 0:18
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    The maximum operating temperature of PVC pipe is 140°F (If I remember correctly), so continuously running 212°F water through it is likely not a good idea. I'm not aware of any studies dealing with dumping boiling water down a drain. I'd have to guess if it was a problem, it'd be common knowledge by now, as there'd be loads of busted pipes – Tester101 Apr 12 '16 at 3:00
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    Sources for this advice: my mom, my wife, my friend all agreed on this. I tried finding some sources online and found it on a few forums. The forum posters apparently heard about it on cooking shows or Martha Stewart. Also, see the comments on this blog post. I couldn't find anything more authoritative; it appears to all be hearsay. – browly Apr 12 '16 at 4:55
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    While you certainly wouldn't want to install, say, a boiler plumbed in PVC the temperature rating of 140°F cited is the pressure/tension de-rating temperature for long term service. There are several drain cleaners I've used that instruct you to pour boiling water through the drain after using them - I'd trust a company facing potential liabilities from filling a pipe with boiling acid long before I'd trust Martha Stewart. – Comintern Apr 12 '16 at 17:42
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    Based on the source of the information, I'd officially call this an old wives' tale ;p. – Tester101 Apr 12 '16 at 18:11
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April 13, 2016

Purpose: There is some disagreement as to whether boiling water can be poured down a residential kitchen sink without damaging the drain pipe. It might be assumed that if the pipe drains quickly, the amount of time necessary to cause damage would be greater than the actual time that the boiling water would be present in any particular section of pipe. Assuming that this theory is correct, there is a rebuttal, that kitchen sinks can become clogged or partially clogged, or that a cumulative effect of dumping boiling water down a drain on a regular basis may (eventually) cause the pipe to fail, or collapse in areas where the pipe is buried. In fact, collapsed pipes are not uncommon in the plumbing industry; however, it is unknown to the author at the time of this writing, whether any published works exist which cite temperature as the cause of the collapsed pipe or whether exceeding the maximum temperature rating (140 °F) for PVC pipe has any significant, real-world consequences. This experiment was designed and conducted to measure the extent and rate of PVC warping when (a drain pipe is) filled with boiling water, and to measure the duration of time necessary for water to cool, to within the acceptable temperature range of PVC pipe.

Materials and Methods: A section of schedule 40 PVC of 1 1/4" nominal dimension was selected, which had been previously used as a kitchen sink drain. More specifically, this pipe was originally part of the plumbing of a sump pump based drain which had been eliminated. The brand name of the pipe is known but has been omitted from this report to avoid any possible disparagements that might be inferred by the reader. The pipe was constructed in a short U shape with 90° elbows on each end; it was selected from used scrap materials for this experiment because the shape could retain water, because it accurately represented residential sink drain material, and because it appeared to be free of any structural defects or deformations. The total length of pipe from the ends of each elbow was 50 inches. A ball valve was also included in the length of pipe centered at exactly 11 1/8" from the end of the elbow associated with the short arm. The long arm of the pipe was 16 3/4 inches tall, measured from the outside bottom of it's respective elbow; the short arm was 7 inches tall, measured from the outside bottom of it's respective elbow. The pipe was weighed and found to be 1558.5 grams. Because the pipe had extra 9 3/4" length in one arm and the other arm had one half of a union fitting this added an amount of weight to the overall pipe, which possibly makes the total measured weight irrelevant for the purpose of accurately calculating heat transfers.

enter image description here

The pipe was suspended at each end by resting the ends on two chairs of equal height, such that the pipe was level. Straps were not used to secure the pipe. The elevation of the pipe was 25" from the floor to the center of the pipe. No external forces were applied; the only forces known to be present were resultant from the weight of the water and pipe, and the strains produced from water at temperatures above, at, and around the maximum rating for PVC (140 °F). The volume of water used was predetermined by using tepid tap water to fill the pipe, and found to be approximatley 1300 ml. The volume inside the pipe was such that the water level was exactly 1" from the top of the short arm of the pipe (or 6 inches high from the outside-bottom of the elbow(s)). It is interesting to note here, that the weight of the water nearly matches the weight of the PVC which contained it (after accounting for the excess lengths of the arms).

enter image description here

A mark was made with indelible marker at the center of the pipe and a camera was used to periodically record and document the amount of sagging that occured over a total period of 30 minutes. A mercury thermometer was inserted into the short arm of the pipe to monitor the change in temperature over time. The experiment was concluded after the measured temperature fell below the maximum rating for the pipe. This was a one-time test, which was not replicated for statistical accuracy. The collected data is reported below.

At 3:36pm, a flask containing 1.4L of boiling tap water was used to transfer approximately 1.3L into the pipe. Boiling water was poured into the longer of the two arms. A thermometer was inserted into the the other, short arm, at the far end of the pipe.

At 0 minutes the mark was 25" above the floor Water Temp = 212 °F; room temperature, and (by default) the temperature of the pipe was 70 °F. As the liquid was being transferred twisting and warping of the pipe was observed.

At ~1 minute after -0.15625" Temp = 182 °F

At 5 minutes after -0.25" Temp = 176 °F

At 10 minutes after -0.3125" Temp = 166 °F

At 15 minutes after -0.375" Temp = 157 °F

At 18 minutes after -0.40625" Temp = 153 °F

At 20 minutes after -0.375" Temp = 150 °F

At 25 minutes after -0.46875" Temp = 143 °F

At 29 minutes after -0.46875" Temp = 140 °F

At 30 minutes after -0.50" Temp = 138 °F

enter image description here

enter image description here

Results: After 29 minutes the temperature had fallen below 140 °F (the maximum rating for PVC). At 30 minutes the experiment was concluded by emptying the water into another container, in which it was weighed and found to be 1290.1g. Careful measurements were taken to determine that the pipe had twisted approximately 30° clockwise, from end to end (or approximately 7.5° per linear foot). The pipe began twisting and warping as the boiling water was being poured into the pipe. A measurement of the temperature of the water at the far end, at about one minute, shows that the pipe had already absorbed an incredible 30 °F from the (approximately) 1.3L of water. The total sagging was found to be 1/2" inch after 30 minutes.

enter image description here

The greatest amount of deflection was unexpectedly found at approximately 7 inches (toward the center of the pipe) from the center of the ball valve. The maximum deflection was measured to be 7/8 inch (lateral deflection) or a total curvature of about 2.5 inches measured at either end of the pipe. It is also notable that the long arm of the pipe (into which the boiling water was poured, but not where boiling water was present for more than a few seconds, had a deflection of about 3/16 of an inch; the total curvature was 3/4 of an inch as measured at the end of the arm. The depth of the water was measured to be 6 inches from the outside bottom of the elbow(s). With regards to the long arm, the greatest warpage was found above the water line, closer to where the boiling water first entered and made contact with the PVC. The measurements of sagging that were taken periodically, as part of the experiment, were simply vertical measurements of the mark made at the center of the length of pipe. Prior to conducting this experiment, it was expected that the greatest change would be found in the center of the pipe due to sagging; but the unexpected lateral deflection was 75% greater than the vertical sagging; and the actual maximum deflection per linear foot was found at the entrance, where the boiling water was poured into the pipe. A graphical representation of the measured sagging/changes (at the center of the pipe) is provided below.

enter image description here

Conclusion: Obviously the lateral deflection was due to a strain at the joint of the ball valve; the measured values of sagging were likely affected by the twisting and lateral displacement of the pipe. Speculatively, the most probable cause of the lateral deflection was due to a difference in length of the pipe which was concealed by the fitting; in other words, the pipe was probably cut at an angle. It is known that when different materials or different lengths of material have been bonded together, the object will have significant stearic strains when heated, as the two materials will not expand evenly. Consider the following example: length A is 4ft, length B is 4.1ft.; when heated, each material expands 2% in length. So, length A will be 4.080ft and length B will be 4.182. The difference in the (heated) lengths is 0.002ft, which can cause significant "curling" or warping effects.

Further speculations with regard to the cause of observed lateral warping include a difference in temperature absorption at the joint due to an insulating effect, or possibly, latent forces existed from previous usage of the ball valve, which were finally expressed as the pipe became soft enough to allow potential forces to be released (an unwinding or relaxing effect). Speculations like these could be verified or ruled out by futher testing.

Obviously, boiling water can cause deflection in a 1 1/4" (nominal demension) pipe, which was the industry standard for sink drains for many years. It is also fair to assume that the temperature within the pipe is absorbed so rapidly that heating will almost certainly be uneven, resulting in areas that are quickly over heated and more susceptible to failure. Supposing that a pipe was clogged or slowly draining, or perhaps the existence of a cumulative effect of multiple exposures to boiling water, it is reasonable to conclude that pouring boiling water down a drain could cause failure. This would be especially true of pipes that are buried, as pressure from the weight of soil would be present.

In summary, it has been observed here that schedule 40 PVC pipe which has been exposed for less than one minute to temperatures exceeding the maximum temperature rating will deform. This is evidenced by the 3/4 inch warpage found at the area (the long arm) where the boiling water was poured into the pipe; in this area, boiling water only passed through, and did not remain through the duration of the test. The boiling water was only present in the long arm of the pipe for the amount of time necessary to transfer the water, which was approximately 15 to 20 seconds. Also, where pipes are exposed to temperatures above the maximum rating for an extended period of time, they will continue to deform until the temperature dissipates to below the maximum rating. It seems apparent from the graphical illustration above, that the rate or amount of warping nearly parallels the the instantaneous temperature or rate of temperature dissipation.

Discussion: It is important to consider that the amount of water used for this experiment was only about 1.3 liters (0.34 gallons). Often, larger volumes of water are used for cooking, which will necessarily require more time to drain and will likely transfer a proportionally greater amount of heat/energy to a pipe. Also, the length of time necessary for heat to dissipate could be several minutes, or possibly over an hour when larger volumes (like a gallon) of boiling water are poured into a drain, and/or where drain pipes are insulated. The opinion of the Author at this time, is that pouring a whole gallon of boiling water down a kitchen drain would logically have a greater potential for damaging PVC drain pipe than 0.34 gallons which in this experiment, did cause measurable, significant warping, twisting, and sagging. It is also necessary to bear in mind that for proper drainage to occur, drain pipes should have a gentle slope of about 1 inch per 10ft. Since warpage in this pipe was found to be greater than 1/2 inch per foot, it should be obvious that the cumulative effect of warping and sagging is such that boiling water will likely cause improper drainage, which would logically hasten the ultimate failure of PVC drain pipes, because the exposure time in improper/slowly draining pipes will necessarily be greater.

There were some obvious faults with this experiment. Perhaps the most significant difference with respect to a real-world test, is the fact that straps are used to secure drain pipes in residential construction, whereas, no straps were used in this experiment, which allowed the pipe to twist freely. Certainly, proper support would be beneficial for preventing drain failure. Whether the current construction methods, materials, and/or builing codes are sufficient to prevent failure in cases where the the temperature rating for PVC has been exceeded is not known to the Author at this time. Also, because this experiment did not test for a cumulative effect (repeated exposure of boiling water to the same pipe), it was not ascertained whether a cumulative effect actually exists, and more particularly, whether the pipe becomes sensitized or desensitized by repeated exposure. However, strong evidence has been presented here that there is real-world wisdom in avoiding damage potentially caused by overheating a drain pipe.

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    Great work! I would have repeated the experiment with 1 1/2" and 2" PVC, as 1 1/4" is usually only used for the trap assembly. It might also be important to support and secure the pipe in accordance with plumbing code. I might also repeat the experiment using a properly flowing drain, as this experiment only shows what would happen with a clogged drain. – Tester101 Apr 19 '16 at 13:42
  • @Tester101 Check the edit (bold lines near the end) that shows the pipe was damaged within seconds, especially in the area where the water was poured into the pipe- boiling water passed through but did not stay in the long arm. I noted at the beginning and end that straps were not used, and for that reason the warping was surely greater... but the idea that this could lead to failure and collapse of a pipe is evident. Pictures don't do justice to this test. Based on the results with 1 1/4" pipe, I am satisfied with saying that boiling water will damage a 1 1/2" drain. – Ben Welborn Apr 19 '16 at 14:31
  • What evidence do you have that warping leads to collapse? PEX warps as it gets warm, but never collapses... – virtualxtc Mar 22 '18 at 11:23
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First of all, do you in fact have PVC pipes? Lots of older houses have cast iron end-to-end, so nothing at all to worry about in that case.

Even if you do have PVC, I don't think there's any serious concern, with the very slight possibility of an effect the drain trap (if any) right under the sink. While continuous immersion in 100° C water might soften PVC, a brief transient flow will have essentially no effect (and, yes, I am a physicist with background in thermodynamics). Nearly all the hot water will have exited your house within a few seconds, which is nowhere near long enough for significant thermal transfer to the pipe itself. (The drain trap, of course holds water which is why that's the location of greatest heat transfer)

Now, on a purely aesthetic view, I usually save the hot water to soak dishes in or to do preliminary scrubs on things :-)

  • Not to mention that you're also mixing the boiling water with the room temperature water in the trap. – Comintern Apr 12 '16 at 17:28
  • Can you share the calculations? For example, if you have a capped, 1-meter PVC drain pipe filled with boiling water, how long must you wait for the PVC to reach 140°F? – browly Apr 13 '16 at 18:34
  • @Browly, no, I can't: you'll need the pipe diameter, wall thickness, air temp, heat transfer coefficients, and so on. It's too ugly to bother with. – Carl Witthoft Apr 13 '16 at 19:30
  • Take a look at my experiment (new answer). – Ben Welborn Apr 19 '16 at 13:17
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I just had to have the PVC pipe under my kitchen sink drain replaced. When the plumber took it out it was deformed. It looked like it had melted and was crooked and thus developed a leak. Occaisionaly I dumped a tea kettle of hot water down the drain. I thought it was a good thing to maybe help keep it clean. Now I guess I will make sure to have cold water running whenever I pour anything near boiling down the drain.

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The drains in our area are ABS, not PVC. I found this answer on a different website.

Source(s): "The Uniform Plumbing Code (UPC) requires that the water temperature not be any hotter than 180 degrees Fahrenheit (82 Celsius) in ABS pipes. If there is a large amount poured down the drain it could cause your drains to become leaky because the boiling water could distort the pipe."

Cindy

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Jul 19 at 2:46
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PVC gets soft when heated. As Tester101 commented above, 140°F is the maximum temperature for pvc. Boiling water is 212° F. I have bent many PVC pipes with pipe heaters for running cable (schedule 80 takes about 2-3 minutes to get soft in a pipe heater). They don't bust, but they certainly can and will bend, which is obviosly not ideal for joints or maintaining the slope/gradient of a drain pipe that is hanging.

Edit-As a matter of practicality when a pipe is not clogged and is flowing freely, the water should be evactuated quickly, but we all know that's not always the case. If the slope begins to fail, then it may begin to collect water thus increasing the sag each time, leading to reduced drainage and a cumulative effect. I found this video on youtube kind of informative. I wonder how long a joint would last under boiling conditions.

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    When you're bending pipe, I'm sure you hold the pipe over the heat much longer than the 212° F water will be in the pipe. A quick burst of 212° F water, through a properly plumbed drain, shouldn't cause any problems. – Tester101 Apr 12 '16 at 18:09
  • @tester101 I kind of agree about a properly plumbed drain... how about pouring boiling water into a slow drain? It only takes a few minutes for schedule 40 to start sagging in boiling water. – Ben Welborn Apr 12 '16 at 19:02
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    don't think I've ever seen wavy deformed PVC pipe in a house, so I'd say this probably isn't a problem. Plumbers would be pretty busy, if dumping boiling water down the drain broke the plumbing. – Tester101 Apr 12 '16 at 19:07
  • @tester101 indeed, deformed and collapsed drain repairs keep plumblers in the black... I know from experience. The solution is to drag another pipe or a liner through the old one with a re-rounding tool or a pipe expanding tool. Around here, that'll cost about $600. But to be fair, I think that deforming is primarily caused by soil compaction... but once again, if it's not draining so well, then how helpful is boiling water? – Ben Welborn Apr 12 '16 at 19:38
  • Keep in mind that in the video there's a source of heat (the burner), whereas the water going down the drain does not have any additional energy sources. If you want to do a more realistic test... 1. Get a length of 1 1/2" PVC. 2. Install a cap on one end. 3. Boil water. 4. Pour the boiling water into the pipe. To make it even more accurate, add 1 cup of room temperature water to the mix (to simulate the water in the P trap). – Tester101 Apr 12 '16 at 21:13
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I wonder how many of these people run cold water when the dishwasher drains? Dishwasher temp is 175 degrees. If pipes are installed correctly water will not be in them long enough to heat the pipe to failure (the trap being the exception). A plus for dumping hot water into sink is keeping pipes clear of grease. I have snaked a lot of pipes stopped up with grease and have replaced a few that could not be snaked but have never replaced a pipe because someone poured hot water down a sink.

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The ABS DWV runs horizontally under the sink, The ceiling in the basement has no. I discovered many cracking lines along the pipe, oil leaks, water leaks, drop drop drop. It was hot water dumping out from the dishwasher and maybe hot pasta water destroyed it. ABS pipe can handle only up to 140F. They should be ban from kitchen pipe. I check my hot water tank temperature is about 145F.

  • Thanks for the input, but I'm not sure what you're trying to say. What do you mean by "the ceiling in the basement has no"? What are the "oil leaks" you mentioned? Please click "edit" and change your answer to clarify. Thanks! – browly Nov 16 '17 at 17:17
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While investigating a leak at the drain outlet of our garbage disposal we encountered this severe warpage. Our only explanation is the occasional dumping of pasta water or larger quantities required for sterilizing canning jars.

waped garbage disposal drain connector

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