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indirect with tankless for domestic hot water?

Dirk Wright
Dirk Wright Member Posts: 142
in THE MAIN WALL
The concern is emissions, soot and stress on the unit. Reducing short cycling is important to reduce emissions and soot, and making the unit last longer. Thanks for the suggestion that other tankless manufacturers have considered this problem. I'm going open loop by taking an electric tank type and removing the heating elements and plumbing it in with the heater and using the thermostat in the tank to run a circulation pump:

Comments

  • Dirk Wright
    Dirk Wright Member Posts: 142
    indirect for tankless hot water heater?

    I was wondering about using an indirect water heater to go with my tankless water heater to reduce short cycling on the tankless. What kind of controls and/or pumps would be required? The manufacturer does not mention doing this as an option, but I think it would help with fuel consumption and/or emissions. I know it kind of defeats the purpose of the tankless, but the intermittant operation of tankless heaters is not good either. Maybe there's a happy medium? Is there some book or information available that I should study? thanks.
  • Sean_16
    Sean_16 Member Posts: 4
    Copy and paste this link...

    http://forums.invision.net/Thread.cfm?CFApp=2&&Message_ID=172545&_#Message172534

    Ken has a diagram attached to one of his posts.
  • Dirk Wright
    Dirk Wright Member Posts: 142


    Thanks. I need to study this idea some more, come up with a plan, and then get a parts list together.

    Not only does the tankless heater short cycle, I had to locate it much farther away from the points of use than the old hot water heater, so there is a long delay in getting hot water when you first turn it on. If I can locate an indirect as close as the old hot water heater, then I'd solve that problem completely.
  • Dirk Wright
    Dirk Wright Member Posts: 142
    need help

    I need help figuring out how much hot water an indirect domestic hot water tank would produce in the case that a tankless water heater were used as the heat source instead of a boiler. The tankless I have can put out 150F hot water whereas boilers are assumed to be in the 180-190F area I think. Obviously, the output would be less with the tankless, but is it a simlpe proportional reduction? I need to figure this out before buying anything of course. Thanks in advance for any help offered.
  • Plumdog_2
    Plumdog_2 Member Posts: 870
    Why be concerned

    they are supposed to cycle each and every time you draw hot water; thats what they are engineered to do. If you don't get the volumn of hot water you desire, then you could pipe in a storage tank. See the diagrams from Rinnai or Bradford-White. But then you are defeating the concept of the device.
  • hvacsale
    hvacsale Member Posts: 19
    why go through all that?

    It's usually better to use each device in the manner they are designed for. An instantaneous water heater is specifically designed for heating potable water instantly and cycling according to the typical usage patterns for DHW. If you do not have enough hot water output you should consider a properly sized instantaneous unit. If you are thinking that storage is the way to go, why not use a storage type system? Rather than using a water heater as a boiler, why not use a boiler?

    Let the boiler do the work of a boiler, it will be MUCH more efficient and easier to size pumps and controls for. Let a water heater be a water heater. It is a VERY rare situation where reversal of roles is a good idea, most often it is suggested purely to save money on the install side.

    Not only is that often an incorrect assumption (that it will be much cheaper to use a water heater as a boiler) but it is the kind of decision that only makes sense once in the service life of the system. That being when the first check is signed and handed over.

    This bears some consideration. In the end, who's needs are being serviced by installing equipment for other than its intended usage when other equipment will do a better, more efficient job and last longer at the same time? If we the designers are choosing this, have we given the customer the opportunity to make an informed decision about it? It's their money not ours!
  • Dirk Wright
    Dirk Wright Member Posts: 142
    open loop tank system

    First, this is my own system, I'm not a professional working on a clients water heating system.

    Second, I already have this Toyotomi water heater ($1200 a couple of years ago), and have since realized that it could be better. The Toyotomi is basically a boiler with a small heat exchanger. It keeps about 5 gallons at a constant temperature. This is different than the gas and electric tankless models. The problem with using the Toyotomi in a closed loop indirect water heating system is that it doesn't have a high enough temperature to provide effective heat transfer in the indirect tank. So, the solution is an open loop system like this:
  • Dirk Wright
    Dirk Wright Member Posts: 142
    open loop with tank

    First, this is my own system, I am not a professional working on a client's system.

    Second, I already have one of the most powerful tankless heaters out there: the Toyotomi OM-148. Typical tank type water heaters have much lower effeicencies than a tank type, but a tankless can be improved by adding an open loop tank, as described by Takagi and others. My experience with this heater is that it can't provide enough hot water for a dishwasher and a shower at the same time, for example. An added difficulty is that I had to locate the Toyotomi farther away from the point of use than the old electric tank type heater. Adding a tank to the Toyotomi means less short cycling, which is bad for emissions. Short cycling is bad, no matter if it's gas or fuel oil, since startup and shutdown produces the most pollution out the flue. Since the Toyotomi does not produce water at high enough temperature to effective drive a closed loop indirect heater, then the solution is to go open loop. That's my plan right now. I'm not throwing away a perfectly good and expensive water heater if I can adapt it to make more hot water. I don't want it to turn on every time I open a tap. That's crazy and stressful on the unit. It's much better to let it run for longer periods and let it rest in between. I can locate the tank much closer to the point of use, thus reducing my water usage also, which is important when you're on a well.
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