Install Electric WH in series with existing tankless

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by rootuser, Jan 12, 2012.

  1. rootuser

    rootuser New Member

    Messages:
    5
    Location:
    Silicon Valley, California
    Hello all,

    I have learned a lot from this forum and I have a few questions:

    I have an existing tankless HW heater, the Bosch Aquastar 125HX (Yeah it's pretty bad). The wife loves the endless hot water and wants to keep it but we have most of the typical problems, including short cycle problems with our front-load washing machine and water miser dishwasher, pressure problems if there is more than one appliance pulling hot water etc. The run from the tankless water heater to the bathrooms is extremely short (less than 12 feet) and not much more to the kitchen sink and washer. I have room in the closet with the tankless to add a 10-20 gallon electric tank.

    My question is:

    Can I just add the tank WH in series after the tankless? Will I run in to any pressure problems or anything that I need to account for?


    Thanks a lot in advance.
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    27,055
    Location:
    Cave Creek, Arizona
    If you have pressure problems when using more than one faucet, it is in the heater and adding a second heater "inline" will do nothing to cure that. The electric heate will operate almost the same way it would if it were the only heater, (i.e., it will not use power to heat the water initially, but will use it to keep the tank hot), until it runs out of water and then you will start using the tankless hot water. Until that point, the tankless will just be refilling the electric heater.
  3. johnjh2o1

    johnjh2o1 Plumbing Contractor for 49 years

    Messages:
    1,143
    Location:
    South*East
    Yet another problem with a tank less water heater. I just don't understand all the hype about these things. There may be some energy savings but they are very expensive, making the payback almost unachievable especially with the maintenance that is required to keep them running. Just my opinion.

    John
  4. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    Your unit is made to deliver 3.2gpm @ 55 degree temp rise. When you exceed its capabilities the water heaters flow valve chokes the flow to allow the heater to deliver water to its temp setting.

    Your tankless is too small....adding a tank water heater in series after the tankless will correct the problems with the dishwasher and washing machine BUT will not improve the volume of water....what your refering to as "low pressure".

    Installing a larger tankless(properly sized) would solve the low volume but not the issue with the washing machine or dishwasher.

    I suggest a larger tankless combined with a 5 gal tank type water heater in series after the tankless.
  5. jastori

    jastori New Member

    Messages:
    118
    Location:
    Illinois
    As others have stated, you cannot solve the low-volume problem with a tank in series.

    How many bathrooms do you have? Do you need to supply two simultaneous showers?

    How about a small tank water heater in parallel to supply the washing machine and kitchen (dishwasher and sink). Leave the small tankless feeding the bathrooms. Then you can continue to have endless showers (one at a time) and your dishwasher and washing machine will function normally (and not affect the showers).
  6. rootuser

    rootuser New Member

    Messages:
    5
    Location:
    Silicon Valley, California
    Thanks for all the responses. Excellent feedback.

    To johnjh2o1: It is the endless hotwater that we are after, it's not an effeciency thing and from what I understand there is not a standard tank that can offer endless hotwater yet.

    To answer the other questions:

    I have 2 bathrooms, with a total of ~12 feet of plumbing for both of them. They share a common wet wall and are back to back and the tank is in a closet that goes right into the shared wet wall. The run to the closest sink is only ~24 inches or so for example.

    I would like to solve the short-cycle problems first, so my guess from the feedback is adding the electric tankless will work.

    My volume problem seems to be an issue with the tankless not being able to keep up. Is there a way to allow the water to flow albeit at a cooler temperature (less rise) through the tankless so there is less loss in volume? I do understand it looks like I need to replace the tankless with a larger sized one (Noritz and Rinnai seem to be popular). I would like to supply up to two showers.

    Another thought (I have no clue what I'm talking about so please shut me down if I'm way off): What if I install a T from the incoming water going into the tankless and the tank. Then the tank would be feeding the house with a direct connection to the incoming water supply (volume issue solved), but ALSO there would be a feed into the tank with the tankless hotwater when needed for quick heat-up and endless hotwater? Not sure what tank would allow me to bring in two sources however. And what would I use to trigger the tankless to turn on. I'd need a pump of some sort, and then a way for it to kick on to pump water through it (to fill the tank) when the tank got low. Not sure about that part either. Is this just not doable? While I am an engineer for a living, I realize fully that you guys with the real world experience are really the ones to talk to about how to make things actually work rather than pie in the sky.

    Any feedback would be appreciated.
  7. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    The tankless is getting plenty of water,the problem is it doesn't have a big enough burner to heat it. You need a bigger tankless and maybe a larger gas line to feed the bigger burner.
  8. johnjh2o1

    johnjh2o1 Plumbing Contractor for 49 years

    Messages:
    1,143
    Location:
    South*East
    My point is why not get the proper sized tank type gas heater to supply all your hot water needs. And eliminate the need for two sources to produce hot water. In other words KISS.

    John
  9. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    Filling large tubs or needing more than 50 gal of storage is the deal breaker. The price of water heaters goes way up when you get over a 50gal. I have often suggested two tank type heaters instead of one larger if a tankless is not practical.
  10. rootuser

    rootuser New Member

    Messages:
    5
    Location:
    Silicon Valley, California
    I understand it is getting the water it needs but is it there a way to force it to flow better regardless of rise? Thus fixing the volume problem?
    Last edited: Jan 13, 2012
  11. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,269
    Location:
    New England
    Essentially, no. The thing is working as designed - lowering the flow to try to maintain the set temperature. Not sure how much your water temps change winter/summer, but it will be worse in the winter when the water is colder. The thing can only raise the water so much...if it's 10-degrees colder in the winter, it'll be 10-degrees colder on the outlet, or it will reduce the flow so it can add more heat while it goes through. Some of the tankless units are meant to be daisy-chained, which gives you a larger volume and greater temperature rise.
  12. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    Install a 40 gal electric water heater that is fed by hot water from the tankless. Crank the thermostats all the way up to very hot.........then install a tempering valve on the outlet side of the regular tank type water heater. Set the tankless at its highest temp setting.

    I posted this for discussion as I know its not the proper way to do what needs to be done......but would it work? Of course if enough water is used for long enough you would eventually lose the volume again because the tempering valve would no longer be feeding in cold water to adjust the temp giving you the volume you need.
    It would be very very inefficient.
  13. Hackney plumbing

    Hackney plumbing Homeowner

    Messages:
    1,174
    Location:
    Alabama
    If you had a target shower temp of 105 degrees and the incoming cold water temp of 50 degress 55% of the tempered mix would be 150 degree hot water.

    Assuming you used a showerhead that delivered 2.5 gpm you would need to deliver 55% of that 2.5gpm at 150 degrees to the mixing valve. That calculates to 1.3750 gpm of 150 degree water.

    Your Bosch Aquastar 125HX will deliver 2.0 gpm@90 degree rise. I suspect the Bosch would deliver 1.5 gpm@100 degree rise,just above the requirment for an endless 105 degree shower with 2.5 gpm with a cold water inlet temp of 50 degrees. Clothes washers and dishwashers do not use enough water fast enough to deplete a 40 gallon water heater even if used at the same time......so I would change from the 40 gal to maybe a 30 gal. tank.

    I couldn't find the max temp setting for the Bosch......anyone know?
    Last edited: Jan 14, 2012
  14. rootuser

    rootuser New Member

    Messages:
    5
    Location:
    Silicon Valley, California

    I don't know if I have room for a 40-gallon. I think I only have room for a 20-gallon in the closet. A 20-gallon turned all the way up with a tempering valve I think would handle the short-cycle issues plus be enough water for a 10-15 minute shower or so before being fed from the tankless.

    During the winter with the Bosch turned all the way up, even 1 shower isn't quite hot enough. It is definately under-sized. I'm going to have to figure on replacing it sooner rather than later. but I'll focus on the tank first.

    I am still wondering about adding a T inline and feeding the tank and the tankless at the same time and some how kicking on a pump through the tankless to re-fill the tank when needed. I'll have to think about what I would use as a control to do that.
  15. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,269
    Location:
    New England
    A typical showerhead is a little over 2gpm - the Feds limit it to 2.5, and most are less. On a WH, you get about 75-80% before it cools off. But, you'd be feeding yours with warm/hot water rather than cold, so it would last longer. As noted earlier, though, the flow/volume problem you have won't be fixed by adding a tank in series - it would never be better than what you get through the tankless. The combination of the tankless' heat output and the tank may keep you going for quite awhile.
  16. rootuser

    rootuser New Member

    Messages:
    5
    Location:
    Silicon Valley, California
    I suppose it's a good start until I get a new tankless that can handle more volume. If I'm good for a 10-15 minute burst at full hot that will be enough in most situations right now. At some point I'll look at running them a little different to better address the volume problem. At this point I'm doing one thing at a time, and fixing the short-cycle problem would be a win right now.

    Anyone have a suggestion on a decent 10-20 gallon electric tank waterheater? Do you know what kind of circuit I will need to run? Is a 20-amp dedicated enough or should I run a 30? I can do either as this will be a new circuit I'm installing just for the water heater.
  17. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,269
    Location:
    New England
    Run a 30A, 240vac circuit, then, you can install almost any residential electric WH. On a 20A branch, you limit your choices. The wire will cost you more, but the flexibility is worth it.
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