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Thread: Install Electric WH in series with existing tankless

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    DIY Junior Member rootuser's Avatar
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    Question Install Electric WH in series with existing tankless

    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.

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    Moderator & Master Plumber hj's Avatar
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    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.
    Licensed residential and commercial plumber

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    Plumbing Contractor for 49 years johnjh2o1's Avatar
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    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

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    Quote Originally Posted by rootuser View Post
    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.
    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.

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    DIY Senior Member jastori's Avatar
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    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).

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    DIY Junior Member rootuser's Avatar
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    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.

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    Quote Originally Posted by rootuser View Post
    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.
    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.

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    Plumbing Contractor for 49 years johnjh2o1's Avatar
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    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

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    DIY Junior Member rootuser's Avatar
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    Quote Originally Posted by Hackney plumbing View Post
    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.
    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 by rootuser; 01-13-2012 at 06:50 PM.

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