Best way to heat Domestic Hot Water -??-

Discussion in 'Quotes and Proposals' started by tonyinvermont, Nov 21, 2011.

  1. tonyinvermont

    tonyinvermont New Member

    So; I have been browsing the forum and topics but cant find much info on the differences and benifits of different types of hot water finishing-

    I am going with a water to water system and plan on heating only as in the northern part of VT we really only need a couple of days of cooling.

    my general questions are pertaining to the best ways to get water to domestic temp; such as
    1) a electric water heater tank fed by a zone valve from the pump and then used with heating element set to proper temp - Delta T about 30' ?

    2)Hot water storage tank such as solar or boiler tank that has a heating loop inside fed by the GSHP and then finished with a tankless LP heater or electric type so again not alot of temp rasing needed.

    3) Desuperheaters seem to make sense only when you have cooling loads and not sure if that will benifit in our heat only senerio ?

    4)hybrid water heater or one of those 97% lp models? i read reviews of the hybrids that didtnt sound promising (recovery time,fluxuation temp)

    our high Killowatt hours might mean better for the LP but not sure?
    some of the heat pumps are boasting 130' leaving water temp- isnt that hot anough for domestic?

    my childeren are growing so i expect hot water to be a big portion of our heating requirement as they come to shower age and am thinking of 50-80 gallons of hotwater.

    Thoughts please you proffesional's out there!
  2. waterpirate

    waterpirate Well-Known Member Industry Professional Forum Leader

    Hi and welcome,

    I installed a navien on demand hot water heater 6 months ago and have never been happier:D. I did not utelise any preheating via the geo. The added parts just did not make it on my cost/maintanance break down. So far the unit has opperated flawlessly and reliably and it sips propane. I was in the process of determining how much it really consumed when a foul up at the southern states filled my 500 gallon buried tank:eek:. It may be as many as two years before the gauge moves at all.
  3. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Again not a valid assertion. With geo DSH you will cut hot water bill more than you add to heat bill regardless of what you use as a finish tank.
  4. ChrisJ

    ChrisJ Active Member Forum Leader

    To the Pros,

    On the water to water heat pump couldn't there be 1 coil to heat DHW and 1 coil to heat radiant water? With a priority for DHW.

    (QUOTE) "installer says that 120' water is not hot anough and it will need to have some added boost for DHW use?"

    I have been using 120* water from my heat pump for DHW for almost 2 years, seems hot enough.

  5. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Anti scalding laws prohibit water above 120 degrees at the faucet. I keep my tank between 110 and 120.

    Once you have a w-w heatpump, you can design your system for DHW priority, and the rest of the time it will condition your radiant buffer tank.

    2 key things:

    1) system must be sized to consider the extra DHW load
    2) Heat exchanger for the tank must be large enough to transfer the full BTU capacity of the HP to the tank. Stiebel Eltron SBB 400 plus works well.

    Here is an example.
    Temperature and Energy logging by: Web Energy Logger
  6. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    My DHW issue is


    Doc I get the code issue, but that is why they make three way mixing valves. Here in Medina County Ohio if I install a roman tub style fixture filler I have to mix down at the point of use to meet "local Code".


    Read the above.
  7. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    I do question the need to keep your hot water tank above 120 degrees in order to prevent Legionella.
    In pretty much all the reported legionella outbreaks the source of contamination were the cooling towers.
    In drinking water, chlorination keeps bacteria at low levels, not so much temperature in hot water tanks.
  8. tonyinvermont

    tonyinvermont New Member

    yes but

    my question was to
    zone the output of the load side of the pump to go to the storage tank/water heater
    heat that same tank with a desuperheater...
    which is best?
  9. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    It depends a bit on your usage.To set it up as a zone take some skills and more expensive equipment. More upfront costs. If you are a couple users, the DSH is fine and adds to the efficiency. If you have a few teenage daughters, and go through a lot of water, running it as a zone is much more elegant and efficient.It takes some skills to set it up correctly, with a heat exchanger, high CV zone valves, and dedicated controls.
    Challenges are a different temperature for the DHW than for the radiant, enough flow and large enough heat exchanger to transfer the heat into the tank, and to get the prioritization right.
    Here is an example, we got more if you need ideas.
    Temperature and Energy logging by: Web Energy Logger
  10. JeffInCO

    JeffInCO New Member


    A few questions about the setup on your WEL0267 page, as I'm looking to do something similar.

    1) You recommended the Stiebel Eltron SBB 400 plus tank for DHW... Did you use that tank in this design? I'm assuming that you've connected the top and bottom coils in series to roughly double the heat transfer surface area? Have you tried any other DHW indirect/reverse-indirect tanks with Geo (Triangle Tube Smart Tank, Turbomax, ???)

    2) Do you heat the SBB 400 tank primarily/solely with the Synergy? Do you top it off with the boiler? If the WEL picture is accurate, it looks like you can't top off the DHW with the boiler unless you heat the radiant buffer tank to temps that are probably much higher than needed based on outdoor reset?

    3) Does the cold domestic water really enter the DHW tank as drawn? I would think that your desuperheater preheat tank output would be the sole "cold" input to the DHW tank?

    4) Do you think the desuperheater preheat tank is really worth it? I guess it provides free heat when running the Synergy in cooling mode... It doesn't really provide any benefit when heating as you're just siphoning off some of the BTU's which would otherwise have gone into the DHW or Buffer tank.

    5) How high a temperature can be achieved in the DHW tank (the Synergy's COP gets pretty low at 120+ degrees ELT)?


  11. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    That should keep

    Doc busy for a few thoughts.

    Since you asked, DOC and he and I do not always think the same, I will aweight his replay.

  12. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    1) Yes it is the SBB 400 Plus. Yes, the upper and lower element is connected in series.I looked at other tanks, but the Stiebel is the only one with 3" foam insulation. The triangle tube tank only has 2". I am also not sure about the amount of heat which can be transfered. The Stiebel I know works. The turbomax would not work. It is not a storage tank. It is essentially a big heat exchanger.

    2) Yes it is only the Synergy, the boiler is the second stage, but does not heat the DHW. Buffer tank has outdoor reset as the graphs show. Be aware that there are some high CV zone valves directing the flow to the tank, and making sure the Synergy prioritizes on DHW. That way you can run the Stiebel tank at different water temps as the buffer tank.

    3) Yes, the buffer tank output is the sole "cold" water input into the stiebel tank. The tank only has one port on top. There is no cold water flowing into the top of the Stiebel. I need to correct that on the drawing.

    4) No, I don't think it has much of a benefit. I am just playing with it. Yes, it captures the "free" hot water in the summer, but I am not sure if that is worth the effort. However, the heat coming from the DSH is not so much associated with a COP penalty than going through the load coil of the heatpump. In the heating season, the entire water is made by the DSH. The Stiebel never gets activated when the HP is running continuously. But I also loose heat through the piping.
    So I figured the only way to get data and end the guessing is to measure it over a season. But again, my gut tells me it is not worth it.

    5) I keep it between 110 and 120F. Now this is with a 6 ton heatpump, single stage HP (in hydronic mode) which is transfering over 50KBTU/H into the tank in a very short time. So the 120F tank means 130F Leaving Load Temp. However, we have another example where we throttle the heatpump back to first stage above 125F leaving load temp (never thought 2 stage W-W would make sense, but it does here), giving the tank a bit more time to catch up.
    Temperature and Energy logging by: Web Energy Logger
    There we did not go with a buffer, it is heat only. But the house has no basement, and the tank is in the garage, in unconditioned space. It was 14F out there last night, and look at the tank performance. That is where the 3" foam comes into place.
    It also depends where you measure the temperature. The port for the probe is in the middle of the tank, the top of the tank is closer to 125F.

    Hope this helps
    Last edited: Jan 5, 2012
  13. JeffInCO

    JeffInCO New Member


    Thanks for your insight! A few more thoughts/questions:

    I would think the Turbomax to be quite ideal for this application, due to its large ports and the fact that it is reverse-indirect. Being reverse indirect (so the whole tank is filled with boiler water) should avoid any issues of short cycling due to inadequate heat transfer between the boiler water and the domestic water. With that said, I don't think it's insulated as well as the Stiebel Eltron and the Turbomax looks like its priced about 50% more than the other contenders given the same tank capacity.

    One thing that I don't see on the SBBxxx Plus spec is the boiler (solar?) port sizes... are they 1" or larger?

    In my case, I have a 4-ton Synergy 3D (no boiler backup), so I think I need around 80 gallons to keep the minimum cycling compressor run time to around 5 minutes with a reasonable tank differential. So the SBB300 plus should work for me.

    I'm looking at a zone valve with a Cv of 8.0 Should be adequate...

    I've been running the Synergy about a 1 1/2 years on forced air only for heating/cooling. I'm still finishing up my in-floor radiant system, so I haven't used the hot water capability of the Synergy 3D at all, with the exception of the desuperheater. My desuperheater setup is currently a basic 2-tank, with a 30-gallon tank for the desuperheater feeding into a 50-gallon electric tank. So far, I've found that the desuperheater provides almost all of my DHW needs during the coldest winter months. However, I'm spending a lot of kWh to make hot water using the electric elements during the shoulder seasons, and the desuperheater is helping only a little during the summer when running the Synergy for cooling, hence my questions.

    One thing I've noticed about the desuperheater is that it doesn't cut off until the desuperheater tank hits almost 140 degrees... So theoretically, it could be used to "top off" the DHW tank to a higher temp than is possible with the Synergy's main hot water output, assuming that there is some living space heating/cooling demand. However, this would require either giving up the use of the desuperheater for more efficient heating of cold water in the summer, or more valves and controls to swap the desuperheater back and forth between the DHW tank and the dedicated desuperheater tank.

    Thanks again,

  14. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    The turbomax is designed for a high capacity boiler (almost like a tankless boiler) and I would question it would work. Your 4 ton unit would make about 35 kBTU/H at less than 130 degrees, so the tank would be at 120-130 degrees, and you would have around 50F water entering the coil.

    Ports are 1"

    Your cycling time should be longer than 5 minutes, I have a 12 F differential and use a Tekmark 150 to control the tank. The price difference between the 108 and 85 gallon SBB is not that much, but the heatexchanger in the 300 SBB plus is much smaller. However, with a 4 ton, it should work fine.

    CV of 8 is poor, I am using the white & rogers 1" with a CV of 37. Keep in mind, you are pumping 4 ton of heat through there, ad you don't want to create any flow obstruction.

    It is an interesting thought to use the DSH to top off the final tank. But I don't see much additional advantage, especially if, as you state, almost the entire heat during the winter comes from the DSH with a 2 tank setup anyway.
    Sometimes it is better to keep it simpler.

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