The BIG tankless water heater post

If you’ve read our blog before, you know that we have had some ups and downs with our tankless water heater. You can read about our noise problems in this post, and our joy in receiving a replacement unit from Bosch here. The long and short of the story is that I rebuilt the supply gas line twice, trying to resolve a problem that turned out to be a defective gas valve in the heater. Given the massive number of comments we received on those posts and the numerous questions I’ve gotten from friends, I’ve been wanting to put together a new post that shares our opinion on the pros and cons of tankless water heaters. Also, I’m going to share our overcomplicated but effective plumbing layout. First, I’ll start with a picture of our Bosch 635ES (aka 250SX) in all its glory:



Tankless Heaters – why use them?

I think most are familiar with tankless heaters, but just in case you’re not, I’ll touch on this topic. There are several reasons to consider using a tankless heater:

  • Energy conservation – since a tankless heater heats water only when needed, they use less energy than tank heaters. Although tank heater insulation has gotten much better over the years, a heater that doesn’t have any standby heat losses will always be more efficient. Also, I would argue that since a tankless can be set at a lower temperature (since you don’t have to jack up the temp as you might with a tank in order to extend your supply), there will be slightly less heat lost through your pipes and insulation to the surrounding air due to the lower temperature differential. Our heater is set at 110° vs the 120-240° most people set for the tank heaters.
  • Endless hot water – as long as your tankless is large enough to meet your needs, it can supply infinite hot water. This is great when you’ve got visitors staying at your house, and the fifth shower is the same temp as the first.
  • Smaller footprint – being able to hang the heater on the wall saves a lot of floor space. Ours sits above our washing machine, so it’s not taking up any valuable space.
  • Direct venting – many of the gas tankless heaters support direct venting out the side of the house. This was a big plus for us, since we were removing our old chimney to gain extra space in our kitchen. An electric tankless wouldn’t require any venting at all, but I’ve not read too many good things about them.
  • Longetivity – since tankless heaters are mostly made of copper tubing, as opposed to steel tanks, they will not rust through and should outlast tank heaters by a wide margin.

Why not use a tankless?

Although opinions will differ widely on this topic, you probably already know if you are a tankless person or not. Some people, like us, get so committed to the idea that we kept at it until we had a system that worked for us. Others don’t want any hassle or to spend more money than they have to, so I wouldn’t recommend them if that describes you. I often compare them to hybrid cars.  You buy one because you want to use the least amount of energy possible, not because you think you may even out the extra cost with lower gas usage.  While the payback of a tankless really isn’t all that bad once you factor in rebates and tax credits (see below), I think the inconveniences would scare off someone who wasn’t really motivated to save energy just for the principle of doing it.  And don’t think that there weren’t many times during our troubles that I came very close to ripping ours out and plunking down a big ‘ol tank. There were moments where I didn’t care how much we had invested in it…it just wasn’t worth it. Imagine how I felt after upgrading to 1 1/4″ gas line because our gas run was a tiny bit longer than Bosch recommended for 1″ line (to which I had upgraded from 3/4″), only to find that we still had noise problems, since the unit was defective. I don’t believe that our situation was the norm, though, so please don’t take our initial bad experience as a deterrent. However, there are a few tankless traits that make them not suitable for everyone or their homes:

  • Intermittent/low-flow issues – this is my least favorite trait of tankless heaters. While they excel at providing unlimited showers, they pretty much suck at the little stuff, like washing hands and doing dishes. Imagine someone washing their hands in the bathroom when the water in the pipe is totally cold. It may take 1 or 2 extra seconds for the hot water to reach the sink (compared to a tank), since the heater has to kick on once it senses the flow, but that person will not even know that you have a tankless heater. But now imagine that a second person goes in to wash their hands. At first things will be fine, because the line is already full of hot water from the previous use. But there will be some cold water introduced into the line due to the startup delay, and if they wash long enough that cold blast will make it to the sink. And that will be followed a couple of seconds later by the “new” hot water. And all tankless heaters need a minimum flow rate to even turn on, so too low of a low will always come out cold. We learned new behaviors, such as filling a sink for dishwashing or shaving, instead of using little bits at a time. But eventually we got tired of having to think so much about “gaming” the system that we eventually added a mini-tank heater under the kitchen sink. That worked out so great that we installed another one to serve the bathrooms a year later. You can read about our hybrid system below.
  • Gas line size – if your gas line (from the meter to your heater) is too small, the heater will not be able to run at full output. If you don’t meet the manufacturer’s recommendations, you may have to upgrade at least part of your line. That’s not a big deal if you can do it yourself, but imagine it could get pretty spendy if you’re paying a plumber. Our unit is one of the higher capacity units, and its 160,000 BTU capacity dwarfs the 80,000 BTUs of our furnace. A tank heater doesn’t really care about the gas supply, since it’s heating a large amount of water at a slower rate. Of course, that’s also why they run low on hot water after a shower or two has depleted the tank.  They just can’t recover quickly enough to keep up with lots of showers.
  • Expense – there’s no escaping the fact that tankless heaters cost 2-3x as much as tank models, but there are many federal and state tax credits and rebates available to help offset the cost. The direct vent models also usually require some fairly expensive stainless pipe, which can add $100-200.

Our funky but effective hybrid system

As mentioned above, we eventually grew dissatisfied with having to always out-think the heater when it came to lower flow or intermittent uses. Eventually, we added in two mini-tank heaters to solve this issue, as well as minimize water wastage, since we now have true “instant” hot water at most of our sinks. Although they are reducing the efficiency of our hot water system somewhat, they seem very well insulated. It does mean that some of our heating now comes from electricity, which costs more than gas, but it is definitely nice to waste less water. And if you’re curious about the energy usage, I used my Kill-a-Watt meter to average out their consumption. The 4 gallon heater uses 25 watts, and the 6 gallon uses 37. These are averaged out over several days, so they suck this energy down around the clock. Although it might be slightly more or less depending on your usage, in our case it’s like having a 60 watt bulb running 24/7. I’d rather not be doing that, but some amount of that is offset by water that does not have to be heated by the tankless anymore. And just as important, we don’t ever have to run more water through a faucet than we actually need, since we don’t have to get enough flow to force the big heater to activate.

We have a 4 gallon Ariston under the kitchen sink and a 6 gallon in the basement that feeds the basement bathroom sink, the main bathroom sink just above as well as Chloe’s sink upstairs. It will also feed our (future) bar sink in the basement.  In retrospect, we might have used a 6 gallon for the kitchen sink, because it does sometimes run out if we have to handwash a bunch of pans while running the dishwasher. But even when it does, it’s inline with the tankless heater just below, so it’s easy to “recharge” it quickly. Here are pictures of the heaters, as well as a diagram of our expensive and overcomplicated, yet effective hot water system. This is not for the faint of heart or cheap or wallet, and while we are very happy with it now, I can’t honestly say whether we would have gone this route knowing what all lay ahead.







19 thoughts on “The BIG tankless water heater post

  1. Interesting! I would LOVE to install one so that I could greatly reduce the size of the closet that holds our tank now. It’s right inside the doorway to the bathroom… and takes up a lot of floorspace from our tiny bathroom!

    The cost is what is driving us away from it. I love the energy savings, but we don’t have a lot of money to output right now. Perhaps I will investigate it first, before “killing” the idea, though.


  2. We installed a 199k btu Paloma water heater (rebranded here as Reem). I like it as it is pretty darn quiet but I can see how waiting for hot water can be a pain. I have to wait right now in our current house as we have a tank heater but it is far from our sinks/shower. The Paloma was much closer to the sinks/shower so I think that helps our case.

    We decided because our door would have been very difficult to open in the utility room:

    So because we switched, we had to really put a lot into the stainless piping ($400):

    We also had to rejigger the water lines in the walls but it turned out OK:


  3. Sorry to post randomly– the way blogspot works, I can’t reply to comments except to post a comment of my own and hope that the commenter is tracking it…

    But I wanted to thank you sooo much for the bathroom advice and lesson in tile! Since you don’t have to get abstracts for homes anymore (and they’re so expensive) I have very little idea about the house’s physical timeline and info like yours just gives me that much more insight. Doesn’t matter if I can’t pinpoint an exact date– it’s just really interesting and makes me feel like I understand the house a little better…Sounds silly, but…oh well!

    And double thanks for the bathroom color and lighting suggestions. Several people had suggested painting the walls black (maybe too severe for me), and I had been scared to try matching to the existing tile, but it’s only paint– and not that much of it to boot, so it’ll be easier to play with until we get it right. I had been in love with lights like that for a while, too, but didn’t know whether they’d work there, so the confirmation that they probably will is exciting. It’s our first house and I think I kind of forget that we can let loose a little– it’s ours now! No landlords to say we can’t.

    We’ve been reading your blog for a little while and we both love it. It’s really been a treat–

    Thanks again!!


  4. Reading your comments about your tankless heaters makes me again think about getting one for the first floor bathroom. I hate running the water while waiting for it to get hot. Interestingly, hot water comes out of the upstairs bathroom faucet more quickly than it does in the first floor faucet. But the significant issue is wasting that much water. It’s too bad that water standing in the pipes can’t be recycled into the heater, rather than being lost down the drain.


  5. Can’t tell from the photo but the PRV discharge for the heater under the kitchen sink appears to discharge into the sink drain line. I do not believe this is code compliant as safety discharge lines of this type need to discharge into an indirect waste receptacle or outside the house, which may be the case here. Also, the heater in the basement does not have the PRV connected… which may have been the case at the time the picture was taken. It’s an interesting installation.

    I just finished installed a large Noritz tankless unit in my garage attic. Depending how well it works I may end up installing a small 15 gal Marathon electric water heater in series to avoid the “cold slug of water” scenario. The only time the electric heating element would run would be to satisfy standby conditions, which should not be often since the Marathon is super-insulated. I think this would provide the best of both worlds: the ability to provide small quantities of hot water without activating the tankless, along with an endless supply for showers, etc.

    The tankless + small tank installation is detailed in an article at It’s a free PDF download.


  6. Tom,
    Regarding your comments, the release valve on the tankless heater was not yet fully plumbed at the time of the picture. The line was later extended into the utility sink.
    The under-sink heater does not have it’s pressure valve plumbed into the sink’s drain line. That’s just an optical illusion due to the camera’s point of view. It actually extends down into the basement and exits into the same utility sink.
    I am sure that your 15 gallon in-line electric heater would work great.


  7. I experienced similar unpleasantness with regards to the ‘cold water slug’ and thought to myself that there has to be a way to make some kind of buffer to reduce the effect of this. What I came up with is quite simple and works beautifully.

    I added a small 2 gallon tank at the exit of the tankless system. The big difference here is that it isn’t plugged in. The hot water from the tankless fills the tiny tank and will remain hot all day and night as it appears to be well insulated. The beauty of this set-up is that the ‘cold water slug’ gets mixed into the hot water residing in the tank and is un-noticable. Because there is always hot water available, it reaches the taps quickly just like my old tank. Since I am not running electricity to this tank I have not added to the operating costs of the system. Washing dishes by hand is a piece of cake now.

    The tank did cost $200–a small price in my opinion to smooth out the irregular temperatures of a tankless system.

    Some people might think that 2 gallons isn’t enough, but that is all you need to eliminate that short blast of frigid water. The tankless system will quickly heat it back up within seconds.

    Another benefit I have noticed is that I can run my tankless heater at the lowest flame setting of 29,000 BTU (instead of 119,000 BTU) which will save even more money.

    With regards to the noise, I selected the Bosch model 425 which does not have a power vent–it simply goes up the chimney silently. All I hear is a little tick tick tick and a quiet whoosh as it fires up.

    All in all I am thrilled with the set up as I now have unlimited efficient heating of the water while still enjoying the benefits of having quick delivery to the taps.

    Costs were as follows:

    Bosch 425 tankless heater
    Type B Pipe to connect to chimney
    Total $1615

    Considering that replacing my old 40 gallon tank with a new one would be $600 plus $200 for installation, I think I am much farther ahead.



  8. You are just who I am looking for. We recently installed a tank less heater and also further down the line we have a 50 gal traditional heater that is heated by geothermal and if needed the electric heater in the tank. My question is in the installation should the 50 gal tank have the cold intake fed by the hot line from the tankless and then hot outlet connected back to the hot line, or should the cold intake be fed by the cold line with the hot outlet going to the hot line of the house? Thanks, Greg


  9. Addition to above request for information. Since finding your blog I have been reading all of the info on it and also the links. I think I have answered my question and the tank should be connected in line with the hot water line only, correct me if I am wrong. Also I realise that my adding the extra tank on my own is in line with the thinking of all of you for more consistency of hot water, but is the fact that I used such a large tank instead of the 5 to 10 gal size you all have used going to give me problems? Should I replace it with a smaller tank? I dont want to unless absolutely nessary. Please advise. Thanks, Greg


  10. Greg,

    I can’t really answer your question, because I don’t know much about geothermally-heated water heaters. If the geothermal heat was enough to meet your needs, then I’d connect the input to the cold line. Assuming that it’s not able to keep up with high-flow uses such as showers, I’d connect it to the hot line after the tankless. There is no reason you can’t use a 50 gallon tank in this way, but it’s a different approach with a different result. Ours are set up to both buffer the “cold slug” and to give instant hot water. I wouldn’t classify a 50 gallon tank as a buffer. The big tank would be the primary heater, but you would get the advantage of having pre-heated water flowing into it from the tankless. It will ultimately be less efficient, because you’re keeping a much higher volume on constant standby. But again, I know nothing about geothermal heaters, so that may not be an accurate statement. One things for sure…you will never run out of hot water or be surprised by cold slug!


  11. The problems with tankless water heaters are endless not to mention that they take longer to provide hot water. Think of the inconvenience of having to wait longer every time you need hot water! A great alternative is a Hot Water Lobster Instant Hot Water Valve! It’s a recirculation system that conserves water while also providing instant hot water throughout your entire home! It’s easy for anyone to install, works off of the homes existing plumbing, and only costs $179.95!

    The Hot Water Lobster uses no electricity and is pump free, so it creates no noise. It is made in the U.S.A., has a 10-year warranty, and can be easily installed in under 10 minutes.

    Check it out at:


  12. I have 5 Takagi units installed at my home and rental units. All have been running flawlessly for the last three years BUT… we encountered many installation problems that had to be resolved to get things working properly.

    I’ve put up a web page to discuss what we learned, as well as some additional tankless installation tips I’ve picked up at my Home Inspection business here in Chicago at:


  13. When taking a shower, the water temperature will be hot initially, than cold, then really hot, then warm, then back to regular temp. What’s up?


    1. We have that problem occasionally too (much less so now that the unit is working and not whistling!) I can’t tell you WHY – maybe Julio could – but I’ve noticed that this mostly seems to happen when it is really hot outside or really cold outside and my hypothesis is that when we’re experiencing temperature extremes outside it messes with the water heater’s thermostat. When ours is working the way it’s supposed to, it starts out cold unti the water that’s been sitting in the pipes has been flushed out and the new hot water makes it to the faucet. It’s really consistent after that, unless, like I said, it’s a really hot or really cold day and then fluctates between freezing and scalding (and makes me crazy!!!)


  14. Amber,

    I think what happens is that even though the minimum flow rate is met (0.75 gal/min), our heater seems more “comfortable” when working hard (raising the temperature of the water a lot.) Its performance is much more sporadic at lower flows or temp increases. Our issue seems to happen at the peak of summer, when the incoming water will be at its warmest, meaning the delta between in/out temps is at its lowest. At all other times of year, it seems to do OK. As Stephanie mentioned, our first unit would cycle constantly during showers.

    The hot/cold/hot/cold temps are really just a symptom that your heater is turning off during the shower. The cold slug is a result of the time it takes the heater to kick back on. Even if it only takes a few seconds, that’s a lot of cold water flowing through. Not very fun when you’re in the middle of a shower!


  15. tankless hot water heaters DO NOT end up being cheaper than the old 40 gals because they’re under $100 used now for something that’s only 10 years old. I did it for the space and because I was remodeling the basement and it was my only good time to reorganize the plumbing. My break down
    Takagi flueless – $700
    installation – $300
    slickness – priceless


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