Heat Exchanger - Recover Hot Water From Shower Drain

www.plumboogle.com. Heat exchanger for recovering hot water from your shower drain. Feed an instantaneous hot water unit or a hot water tank. Subscribe and check out my other heat exchange videos.
To see every step, you need to watch my video on How To Make A Heat Exchanger
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[@dietervanhauwaert8128]

This is one of the best how-to's I've seen on the subject. Clear and concise. I have been researching this subject for some time now, and it seems that the best results are obtained when having the exchanger perfectly upright. Apparently the shower water runs down along the side of the waste tube as a thin film. It does not fall down in the middle. I also read that insulating the outer pipe and the piping from output to boiler increases effectiveness. (unless you live in an area where the outside temperature is usually above 30 °C/86F, but then maybe you'll be taking a cold shower) People who are very environmentally conscious with their water consumption and who take very short showers (water down for 30 secs, close faucet, soap up, rince down another 30 secs) shouldn't bother. I have read about savings of up to 40% using such contraption. But that was in commercial brochures ;-).

[@RobThePlumber]

Dieter Van Hauwaert It is quite abit less than 40%. That does not mean that people should not use devices to reduce energy consumption! I appreciate the input and thank you for watching.

[@hiker919]

Rob - would it be more efficient, instead of pre warming the hot water heater's cold water, to pre warm the cold water running to the shower? The person taking the shower would automatically dial back the shower's hot water. I don't know what method is best, but you would have to run a bunch of extra line if you ran water to the hot water heater, depending on it's location.

[@RobThePlumber]

Yes, Rinnai and Eternal. I am very familiar with instantaneous hot water systems. Rinnai is king of the market in Northern Ontario. There are ways to incorporate heat recovery in almost any system. Although it may not make a big difference in the winter, it would make a difference in the summer. So how is your unit piped atm? Does it heat up high temp (180°F) water to the rads and mix it down with a valve for your domestic?

[@mark_osborne]

Fantastic ! Several years ago I learned they do something similar to this in Germany in all new homes and have always had it in my head to investigate the prospect of adapting a similar system to our shower.. thanks this is perfect !

[@DrewHerrema]

This is the third video I watched and the first with the narration. Your voice adds a lot,

[@6969smurfy]

Somptin I forgot to mention in your other vid, Its cheaper (for me) to run cold water supply water through the ex changer & up to shower faucet. You mix your shower water at faucet 2 Positives "slowing down" the cold water heat transfer rate through ex changer. thus i believe more efficient & saving pipe length on supply runs.. this preheating/warms my cold water supply thus reducing my hot water supply at faucet.. Another Great video Rob . PS Pipe insulate all pipping, big help!.

[@butcher0672]

Cool... you've installed them on Rinnai systems, but If I'm using that very same system to heat my home, and the warmer water is allready circulating back to the heating unit, (from my water radiators) then making a heat exchanger from my shower would be redundant would it not? (just curious)

[@DavidAndruczyk]

how much temp gain do you get in practice for the cold water feeding through the exchanger? i.e. provide info on shower drain temp into exchanger, temp out, and cold water into exchanger and warmed water out.

[@RobThePlumber]

Your right! I have about 150 hours of video to edit through this winter. Going to be some great videos for sure. It sucks that I cannot finish them now. I run a plumbing company and summertime is money time. I hope to get all of my knowledge of heat exchangers out onto KZbin. RTP

[@RobThePlumber]

I agree copper can be quite expensive. Having the knowledge of how things work and knowing that you can do this down the road is priceless. Energy is dirt cheap right now. Things can change quickly. Heat exchangers are often a necessity when working with two different liquids. Copper is by far the best heat transfer material out there for the money. Plastic is a great insulator, but horrible at transfer. Thanks for watching and taking the time to comment. RTP

[@klrmoto]

What I may remember from reading text books on heat transfer was that you get the best heat transfer if the flows run counter to each other. Have you considered this? This is suppost to get you the greatest average delta T from one end of the heat exchanger to the other if I have said that correctly.

[@RobThePlumber]

Great point Dan. I should have mentioned the point of insulating all of the lines. You really do not want to lose any heat that you have gained. Insulation is important in any exchange of temperature whether hot or cold. Warming your cold supply is something I have thought of.There will be future videos for a system I have designed for home with multiple showers and exchangers. By feeding the cold supply for each individual fixture. Anyways, thanks for watching and commenting! Rob

[@JamesSchenck]

Nice work RobThePlumber. I have seen some tank type designs that would also capture some of this energy. Cool to see this being developed and people sharing methods of doing this.

[@Justinofalltrades1]

to be honest when i clicked this video i thought forsure it would be a joke of an idea but i have to say, this is an ingeniously simple and efficient design. thanks for sharing!

[@ryanyoungson6762]

Very nice heat exchanger, I'd love to make something similar but UK regs require a double barrier. One thing to consider if insulating the exchanger: commercial exchanger manufacturers insist that you do not insulate the devices because the cold feed water in the exchanger will be kept warm when the flow has stopped and allow bacteria like legionella to thrive.

[@extremelucidity]

So ,I'm sitting here watching the video wondering if the water flowing through the inner pipe from the drain would fill the pipe enough to provide an appreciable heat transfer to the cold water sleeve surrounding it. I realize that any heat recovered is a good thing, but how much of a temperature rise do you think you could expect in the cold water line ? I would think it would be prudent to insulate the whole assembly as well , would it not ?? Thanks for the great videos !

[@RobThePlumber]

The problem of making the exchanger longer than 6 feet is plumbing code. It varies from state to state, province to province..but you need to have a vent installed within 5'.

[@pmclinn]

Could you run the cold water through the center ? 2" PVC to 2" copper drain via T, then 2" to 3/4 that runs through the center? More warm/hot water would be around the 3/4 allowing for better drainage.

[@butcher0672]

the unit is set to its highest setting at 140F, and sends the water to the entire system at that temp. when my water tank let go, I requested to have this system from my rental company, they did'nt want to do it because of heating and domestic, because of water lines colecting setament during summer months when heat is not being used, I informed them I would clean filters and flush lines, and they decided to "test pilot" my house, 4 years now, so far so good.

[@amschind]

Simple design and works great. Thanks for your videos.

[@BornFreeSailor]

Great stuff Rob. I will try to make a heat exchanger from my boat engine to the interior space.

[@moore6363]

Great design was thinking of trying it for myself a couple of quick questions first. Is there a drop in pressure from the input pipe to the output of the heat exchanger? Also is there any reason that the outside of the copper pipe couldn’t be covered with insulation to improve the efficiency further?

[@RobThePlumber]

Very little to no pressure drop. Friction loss equal to two 90° elbows. (which is really nothing)

[@ronnykristensen3056]

Assuming the air under your shower is warmer than the cold water feed, I'm thinking you wouldn't gain anything with insulating it.

[@ronnykristensen3056]

I have thought about doing something like this. I would connect it to the cold water side of the shower though rather than running it back to the water heater. I'm thinking this would increase the temperature of the cold water that you mix with the hot, so you use less hot water to get the desired temperature when you are showering. How do you think that would compare to your idea?

[@ronnykristensen3056]

Sorry. You've answered this question before. =)

[@meguiarsambassadorcertifie7749]

Intressant mannen

[@meguiarsambassadorcertifie7749]

Kompis hur mår du hoops du mår bra har du viber eller henguts så kan vi video chatta de var längesedan vi hördes av 😃😄

[@butcher0672]

very smart idea!!! If I still had a hot water tank I would do this, but I now have a tankless rinnai system that is the cats ass, I also heat my house with this same system as I have hot water base boards and live in northern ontario canada where our winter are cold. The rinnai is saving me almost half of what it used to cost me. I do have a hot water tank in my garage that I use in a laundry tub for washing meat processing equipment, I just may built this for that. great vid!!

[@RobThePlumber]

The great thing about this exchanger is that the drain is only 1-1/2". Plumbing theory says that a shower drain is designed to be 66% full on the horizontal. In part 2 I will be showing the actual numbers on how much heat you recover. And yes, you should insulate as much as you can. More insulation the more efficient. I will make point of that in part 2. Thanks! Rob

[@wilbrus]

Don't know if someone mentioned it yer, but your schematic shows a parallel-flow heat exchanger. If you switch two of the pipes, to make is a counter-flow heat exchanger, you will have higher heat transfer. Could you not use a PVC or ABS outer tube to save on materials?

[@Darfail]

excellent video! do you know how much % of the energy which would otherwise be lost is captured by this heat exchanger?

[@robbhoogeveen]

I was wondering how long to make the heat exchanger? I have up to 12 feet from the shower to the stack. Does it make sense to make it much longer that 6 feet?

[@RobThePlumber]

Tanks for watching and the kind comment. I am not in it for the money. I have many ideas, hopefully some of them will work. If they do They will be shared with all of KZbin. Rob The Plumber

[@RobThePlumber]

In the last year I have learned to shut the shyness out. Thanks for the comment. Rob The Plumber

[@jjohn77700]

I cannot believed any reclaimed heat would be would the effort of building this. I could be wrong but it seems like a heck of a lot of work for bringing well water up a degree or two.

[@RobThePlumber]

Having an exchanger made from pure copper is expensive. There is a new video I uploaded that uses plastic as the outer chamber. The thermal exchange material is still copper though.

[@chopsooy]

Hello Rob, could this type of exchanger replace a plate exchanger I currently have for exchanging water from my wood stove to home (in-floor) radiant heating? I am having trouble with mineral deposits in the expensive heat plate type exchanger and loosing transfer ability. Thank you for any input you may offer.

[@RobThePlumber]

A plate heat exchanger is way more efficient. The problem with plate heat exchanger is the drop in pressure and flow. This exchanger could be used but you would have to bank 2-3 of these. I personally would buy a new plate heat exchanger for your application.

[@chopsooy]

Rob The Plumber Thanks again Rob, I have cleaned the exchanger out and is working better. Helpful information you are offering, very nice of you to take your time for guys like us. Happy Holidays to you and yours!

[@TheRealBuffaloChip]

Stop being shy! Your videos are great! Thank you for taking the time to help us amateurs out. I love the idea of recapturing the heat from a shower or whatever and reusing the energy, especially with the cold winters we have around here. It's so green, but in an intelligent way, not in a "ZOMG THE HUMAN RACE IS GOING EXTINCT" way. I will definitely do this at my next house.

[@tesos2866]

Wouldn't you want the pea trap at the outlet of the heat exchanger to ensure the copper drain is full till the draining water exits the heat exchanger, as it provides better heat transfer?

[@RobThePlumber]

It will work, might want to watch the video that I am posting this weekend. Banking the exchangers in series and parallel. Stay tuned. Thanks for watching. Rob The Plumber

[@Subgunman]

These work great for your condenser on your moonshine still. ;)

[@dhscts]

So much better than the mute video. Hard to see how the application is worth it. Fast moving cold water running past slow moving waste water and air, for 10 -15 min a day. What is the temp difference between in and out of the cold water?? I have a friend who connected a garden hose to a car heater for A/C in his room. Worked good until the hose broke flooding his room. Also a lot more water was used than needed to water the lawn.

[@RobThePlumber]

Your not the first to ask that. The bad thing about it, the system is designed to be a simple as possible. Warming the cold water to that one fixture can be quite complicated. I need to get my video on the dishwasher exchanger done so this can all make sense. Mixing water in a valve is the most inefficient thing we do in our plumbing systems. I will just make a video... Thanks for watching and commenting. Rob The Plumber

[@ryanyoungson6762]

If you think of your design as being one half of the heat exchanger, which only the cold water comes into contact with. The other half would be a separate length of pipe which fits snugly through the 1.5", to carry the waste water only. In between this double wall would be a microscopic air gap and a leak path for if the copper failed in either wall. I hope that makes sense, it is much easier to explain with pictures!

[@AlexMW-1]

Great idea but a solar hot water heater would provide more return when you consider time on this project vs this one. This heat exhanger would be a better project in an industrial setting. Nice work either way though

[@SeanNorsee]

nice, really like this design.

[@massaroclaudio3390]

Have You tested it? I think you would be cooling your waste water instead of picking up some heat..... I do hope I'm wrong.

[@dudeman9701]

Seems to me you might be restricting your cold water supply and lowering you over all water preasure. Maybe it would be a bit less constrictive if you didn't use the street reducer and used a full size fitting instead?

[@ryanyoungson6762]

It may have more to do with the companies covering their backsides. I just thought it was worth mentioning. Instead; if insulated and the cold supply to shower is after exchanger, you could run a brief cold shower to cool the exchanger down if concerned. An example: "When no cold water is flowing up Showersave, the temperature in the pipe should be prevented from becoming higher than 25°C (77°F). The cold water pipe, connection and the outer wall of Showersave may therefore not be insulated."

[@pvanderhaar]

Love your videos rob keep them comming!!!!

[@SimonLeBonbonbon]

Funny, I made this exchanger in 1999. Mine's a bit different though. The shower is located on the second floor, the heat exchanger is on first floor right below. It's vertical oriented and before I assembled it, I made a spiral by soldering a copper wire around the inner tube. The CW input is located on the lower side, approx 25cm above groundlevel, the output approx 250cm above. I connected the output of the exchanger to the CW inlet of the (thermostatic) shower valve.

[@govmentsucks2323]

uhe must shower alot lmao ... is it worth it after the cost of that copper ?

[@zainebaine6221]

Why is the exhchanger installed if you are A using a boiler and B sending the shower water to drain

[@dzappss1]

You could make 2 with the vent in between. I do believe there is very little heat to be gained this way though.

[@peterrobins91]

Great video. It is very informative. Subscribed.

[@pmclinn]

What is the estimated cost to make this?

[@RobThePlumber]

Thanks for watching, commenting and the subscription! Rob

[@RobThePlumber]

If you had to make a double transfer plate or chamber you would lose 85% of the total possible energy transfer. I really like your comment. Please let me know what you mean by double wall. Rob The Plumber

[@RobThePlumber]

I hope you are right, or what I have been taught is completely wrong! Yes it is cooling the waste. Well not really, we are transferring heat energy from one liquid to another. One is being cooled, one is warming up. You cannot heat the incoming water higher than the temperature of the waste, and you cannot cool the waste to a lower temperature than the incoming water. Ahhh, the beauty of thermodynamics. Thanks for watching and commenting. Rob The Plumber

[@RobThePlumber]

Galaxy SII and I love it! RTP

[@Andrewredwards]

I enjoyed your clip.

[@RobThePlumber]

Integrated Digital Marketing Strategist I enjoyed your comment. Thanks for watching.

[@RobThePlumber]

Thank you enjoy the videos! RTP

[@davidmiddleton2751]

with warm grey water?

[@stevem3163]

I really appreciated you muting the dental drill noise

[@BobbyFromNJ]

Given the extreme price of copper these days, the payback on this project in material cost alone would probably be 5 years. If you add the labor time, who knows.

[@CaHeoMapMap]

2022, and I am here! How are you now?

[@ariennehanna1066]

Genius!

[@LionHeart-tx2qk]

You're so right about the fluctuating cost of energy. In Europe it is PHENOMENALLY expensive, with many people during the winter choosing between heating and eating - not both. We are enslaved by the energy companies, and nowadays what hits Europe (ie the islamisation of our countries) is a portent of what America can expect shortly. That's why your video is eagerly enjoyed in Europe, and will continue to be as we become more and more slaves to the energy companies. Thank you Rob.

[@craigbulloch5713]

I can see that saving you about 15 cent over a 20 year period.

[@blondeguy08]

Craig Bulloch yeah this is pretty ridiculous for the application for which he is building it for but I have another application by which this type of heat exchanger will save my $2-3k a month on electric bills

[@tesos2866]

Actually if he was raising the temp of the cold to the shower it's amazing how much hot water you save while taking a shower.

[@RobThePlumber]

I do not agree with you. A double barrier does not make sense at all. The cold water is in a system that water really never stops moving in day to day living. But that is no different than water stopping and sitting in your lines for a couple days. legionnaires needs temperature and stagnant water. You would see this when a sprinkler system is turned on after sitting all winter and there is no back-flow prevention installed, and the old stagnant spoiled water enters the home

[@acman0926]

the heat exchanger would have to be 20' long to actually work efficiently. the cost savings are greatly overwhelmed by the copper and labor investment. i dont believe this will do any good, may as well make a sloar exchange.

[@NickWelch]

Unfortunately this won't pass code, as it is single wall. You need double wall (two separate layers of metal between potable water and waste water) if you want to do it properly.

[@RobThePlumber]

It depends where you live. In Canada, it will not. Many spots in the US, it will as long as it is type L or K copper. I could easily make this a double wall exchanger, but it would be so inefficient.

[@woltar9]

nicely don Thumbs up

[@RobThePlumber]

woltar9 Thanks.

[@timless5157]

Doesn't the drain needs to be at least 3" ?

[@RobThePlumber]

No. Shower drains are typically 1-1/2" or 2". I guess in really extreme cases of having multiple shower heads and body sprays you may run into a 3" drain. But if you can afford a system like that your most likely not going to care about how much energy you are saving. Thanks for watching!

[@voltron63]

love the vid..

[@davidreed9916]

my next solar project will use one of these to exchange solar heated Propylene Glycol to domestic hot water use.

[@RobThePlumber]

David Reed It will work great. Thanks for watching.

[@Tim_Small]

David Reed You may get better heat transfer using a commercial off-the-shelf flat plate stainless steel exchanger, since they have a bigger surface area. A friend of mine uses one in a side-arm thermosyphon arrangement - works great.

[@davidreed9916]

Tim Small do you have a link?

[@Tim_Small]

David Reed Not to his, but here's a study which used a similar setup - sbrn.solarbuildings.ca/c/sbn/file_db/Doc_File_f/Experimental%20Evaluation%20of%20a%20Multi-Tank%20Thermal%20storage.pdf - these stainless exchangers are mass-produced and cheap. No good for shower waste obviously, but if you have clean water both sides, they're efficient and cheap.

[@davidreed9916]

Tim Small my plan is to use this on my solar water heater, I will use anti-freeze to heat the jacket and transfer it into the water for my tiny house use.

[@palhardy4292]

Yer gonna want to mount the leibig condenser vertically bruh

[@christophergruenwald5054]

better idea. Just run your shower cold water through it. it'll warm that and you will adjust the flow of the hot water in the shower as you go. should equalize shortly after starting your shower

[@RobThePlumber]

Christopher Gruenwald I have tried that. It does not work very well. Maybe with a "2 handle" faucet it would. Modern sinfgle hande valves are designed to mix cold water with the hot. It just does not mix well. Thanks for watching.

[@christophergruenwald5054]

Just curious as too what happens why it doesn't work very well? I'm building a new house and was thinking of installing one for each shower. There would be heat loss and resistance running all the hot water for the house through 2 heat exchangers on opposite ends of the house. Any input would be great as I would prefer to only have to do this once.

[@dietervanhauwaert8128]

Christopher Gruenwald modern thermostatic valves are adjusted to the temperature of the hot water, at the time of installation. If you would increase or decrease the hot water temp of your heater/boiler, you would notice that the default setting of the faucet would not give you the desired showering temperature. The valve would need to be opened and readjusted. Similarly, significantly increasing the temperature of the cold water would have the same deregulating effect. These thermostatic valves do not have a computer in them, they are mechanical in nature and their range of operation is not very wide, therefore they need careful adjusting.

[@dinendale666]

you run the cold through the heat hex and to the shower cold and tank feed is the most efficient way

[@christophergruenwald5054]

I did some looking into this. It's not legal. It has to be a double wall between the potable water and sewage. There is a company that makes one that's legal and commercially available called powerpipe.

[@benburton3496]

You must must must mount this exchanger vertically in the shower waste system! !!! otherwise you're wasting your time.

[@johnburns4017]

ben burton Correct. Water dropping down the copper waste pipe *spirals around the copper waste pipe* staying to the edges of the pipe giving maximum heat transfer. Fitting it horizontally is a waste of time and expense. *It has to be fitted vertically.*

[@kalleklp7291]

All this nice copper would make a great still.. What a shame to use it for this..

[@Tim_Small]

Cool video - thanks for taking the time to put it together... I saw some research which found that you get a better efficiency (up to 70% vs. up to 40%) if you can make the exchanger vertical - the water tends to spread out around the pipe better. Some independent efficiency figures for a commercial install here: www.johnsavesenergy.com/DWHR.html This other commercial unit also recommends putting the shower trap downstream of the exchanger (page 6) - www.heatraesadia.com/docs/MEGAFLO_SHRU_36006178_issue_01.pdf - I'm guessing this is to get the maximum "spread-out" around the drain pipe inner surface. I wondered about modifying the design to try and make the flow inside the jacket more turbulent (to increase heat recovery) - maybe soldering some copper wire onto the inner tube in a spiral pattern (just tacking it every so often with solder, not continuous) - maybe just 4mm copper wire in a spiral with a 50mm spacing on the spiral. Commercial stainless flat plate exchangers put ribs in the plates to achieve the same aim... As has been said by others, you can use this to feed the cold of your shower too (best efficiency if it feeds both the heater and the shower cold feed too).

[@garybenton2355]

You need to be careful and warned to never use this on a well pump system. The well water will eat the copper and make it paper thin due to the mineral content of the water. ( effect contaminating the water for the house ) You might want to come up with an option to not use copper for the drain line in that type of set up. If you use copper for the drain eventually the copper will fail. Cause: the oils from the body from washing not to mention soaps, cleaning chemicals and last but not least DRAIN CLEANER. Never use drain cleaner in any type plumbing drain system! Any drain cleaner is an acid either sulfuric or hydrochloric. Both of these will eventually eat through any metal pipe. It’s not safe for user or plumbing system. It will not eat through PVC (Poly Vinyl Chloride) It will however cause the PVC to become brittle over a period of time of use and possibly shatter when drain equipment is used causing water damage to your house. You would be better looping ¾ soft copper around a copper drain feeding the water heater. Not really any possibility of cross contamination. The copper drain will fail and leak before the water line loop will fail. You will notice the leak and repair / replace before any possibility of cross contamination. Any plumber should be ashamed of himself to recommend this. Gary Benton Class II Unrestricted Master Plumber Georgia LIC # MP208127 Feel free to validate at the Georgia Sectary of state web site.

[@32AFuse]

Gary Benton I thought about that as well. Since the subject of more ecofriendly energy usage is still in its fledgling stage (unfortunately) ,it simply needs a bit of ingenuity I guess. There are actually pretty simple ways to protect the copper. My approach would be to coat the inside with a wear resistant PTFE based protective paint and compensate its thermal isolating effect with a longer heat exchanger.

[@RobThePlumber]

Thanks for taking the time to comment. Type L copper will literally take 20 years to get pitted from having a PH level of 6. Most water sources do not come close to this level. If you have low PH, you typically would know from past experience with mechanical failure. With any type of plumbing system you should know what your water chemistry is. If you actually developed a leak it really would be no issue other than losing water down the drain. The drainage system is below atmospheric pressure, and the domestic water is normally around 50psi. The domestic water would leak into the drainage and not the other way around. I hope you understand what I am saying. Sometimes what my brain is thinking does not come out in my words. LOL.

[@RobThePlumber]

32AFuse Thanks for watching and commenting.

[@ClownWhisper]

no rob that is a liebig still condenser

[@RobThePlumber]

Clown Whisper Which is still a heat exchanger.

[@ClownWhisper]

yes technically it is. but seeing as its intended purpose is condencyl vapors we call it a condenser. That is a liebig condenser period. it would never alter the temperature of the drain water from showering enough to justify its creation and expense. When you shower half of the water that drips down the drain does not even come into contact with the sides.

[@Karuvares1]

IN and out vrong side. See the drawing.

[@RobThePlumber]

Karuvares1 Ya got me.

[@AgarwalHimanshu502]

Thanks you muted that dentist drill sound...

[@hawkdaddy64]

I see you have a Galaxy S 3

[@novachevyguy]

vid - ee - oooh??

[@smrettpecca]

how to enlarge the Surface, where " heatexchange " will happen? wind a coil! ;)

[@bitcoinbrisbane]

Great video, I'm looking at capturing the heat from Bitcoin miners.

[@graemeleigh8939]

Again what is "SODDER"? I ISN'T IT SOLDER?

[@RobThePlumber]

Yes it is solder. Where did I spell it "sodder"?

[@RobThePlumber]

If you mean on how I say it? In my area the "L" is silent.

[@graemeleigh8939]

You mention "soddering" a joint - what on earth is "sodder"? Or did you mean soldering? - It is spelt "soLder" not "sodder"! So how should it be pronounced?

[@RobThePlumber]

After watching the video, this is what you ask me? The "L" is silent. I know how to spell soldering. I personally do not like the sound of the word. Most tradesman in my area say it this way. Seriously though, just hit the dislike button and move along. Rob The Plumber

[@Amraks]

As with many of these differences between American English and how you say it in the UK, it's actually a side-effect of the US sticking with an older pronunciation. "Solder" is a modern spelling of the Middle English "souder", which derives from the Old French "soudeur", which itself comes from the Latin "solidare". In the 15th century there was a movement to re-latinize the spelling of words, and the "l" got added back in to the spelling, although the pronunciation didn't change to include the "l" until sometime later. Regarding the dropping of the "l" from the original Latin, that was common in Old French, and is why we say, for example, the word "powder" instead of "pulder".

[@kunjidee]

Ewwwwwwwwww....does that mean you're re-showering with dirty water that's been re-heated????? EWWWWWWWWWWW!!!!

[@RobThePlumber]

kunjidee LoL. No. You are exchanging the heat energy only. That is why they are called a heat echanger.