Minnesota Is it possible that my DSH is cooling my Marathon water heater?

Discussion in 'Maintenance and Troubleshooting' started by mngeotherm1, Sep 24, 2014.

  1. mngeotherm1

    mngeotherm1 New Member

    I have a closed loop geothermal system with a DSH and a 105 gallon Marathon water heater (no buffer tank). This system was installed in a new home and I had no say in the design (and at that time wouldn't have known any better anyways). It is just 2 people living in the house and we have off peak electric for the water heater (on for 7 hours in the middle of the night and 4 hours in the middle of the day). We average 10 showers a week and use a dishwasher and laundry a few times per week. Nonetheless we run out of hot water a couple times per month. I called our electric company to get off of off-peak thinking that was the problem and they had an energy auditor look into it. He said that it is highly unlikely that based on our energy usage that we are actually running out of hot water and that there may be something awry with our system - and the DSH may be the culprit.

    My simplified view is that if the DSH outputs at 120-130 and the water heater is at 150, isn't the DSH actually cooling the water heater?

    What are the optimal settings in this set up? Should I turn down the water heater? Should I shut off the DSH valve to the water heater?

  2. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    thoughts? Yes, several:

    1) DSH may well be removing heat from the Marathon as it is now set up. My own system's compressor gas discharge temp runs around 130, so absent smart controls, it would GAIN heat from a tank of 150*F water absent controls to keep that from happening.

    2) As has been hashed out here many a time, an obvious solution is for the DSH to have its own tank. That would be best.

    3) Why is the water heater set at 150*F?

    4) Given only two people in the home and such a large primary tank, it is possible this DSH could work with a single tank:
    a) Plumb DSH to circulate only water from bottom of tank (dip tube and boiler drain)
    b) reduce lower element temperature setpoint; ideally turn it off
    c) reduce upper element setpoint to something more reasonable; given the off peak service, 125*F or so might suffice

    5) Don't shut DSH valve to Marathon without also turning off DSH pump
  3. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    "5) Don't shut DSH valve to Marathon without also turning off DSH pump"

    Don't shut off valves to DSH without draining the water from the DSH. It will make heat weather you move the water or not. A closed vessel of water with a source of heat is a plumbing no no.
  4. geoxne

    geoxne Active Member Forum Leader

    Strange DSH quirks-
    -Some geo models have logic controlling the DSH pump that shuts it off when conditions are unfavorable to making hot water. A lot don't. With a cold loop field it is possible to transfer heat from the DHW tank to the ground.
    -The hotter the entering DSH water is, the less it can produce.

    A DSH is connected to the entire domestic water system. Pumped water will follow the path of least resistance. In our local very humid climate, plumbers usually install mixing valves on the toilet supplies to prevent the tank from sweating. In one case the check valve failed and the pumped DSH water found it easier to run a 30' loop to the mixing valve and back rather than go through the restrictive dip tube in the DHW tank. This effectively lost most of its heat radiating to air.

    Point is you have to figure where the heat is going and a preheat tank is important to make best use of DSH capability.
  5. mngeotherm1

    mngeotherm1 New Member

    Thanks Guys!

    Another thought, running out of hot water is more common in winter.

    I turned up the waterheater shortly after the install because we were running out of hot water (I first turned the mixing valve as far as it would go towards hot).

    I'm not sure how to tell if the hot water heater is hooked up with a dip tube, on the outside it looks like it was installed correctly when compared to the Geocomfort manual, thoughts?

    So I guess I will turn down the hot water heater top element, and turn off the bottom element.

    Any idea if the Geocomfort system has logic to turn it off when conditions are unfavorable to making hot water?

    We don't have mixing valves or any loops out of the water heater (other than to the DSH). So, as far as I understand, the DSH is pulling warm water from the HWH and putting it out in the loop field? If not, I don't understand how our waterheater is getting below the DSH temp of ~120*.

    Thanks again!
  6. geoxne

    geoxne Active Member Forum Leader

    Those statements are contradicting. Make sure mixing valve has operating check valves.
    Any place where hot and cold pipes meet has the potential to cross flow when opened to each other (check valve failure, open hot and cold valves (like when you turn the shower on), etc).

    If your DSH pump is reversed (drawing from the bottom of the tank and returning to a tee at the cold inlet), during heavy hot water usage the lowest pressure point will be at the open hot water outlet. At times when the DSH pump is not running, the cold water will reverse flow through the DSH loop, through the tank and find its way to the hot water outlet (lowest pressure point). This will quickly mix down the temperature of your stratified hot water tank and result in low hot water outlet temps.

    ALWAYS make sure you are drawing from the cold water inlet of the tank and return to the bottom of the tank. A DSH loop has 4 states of operation and they all have to be taken into account when diagnosing flow issues. So when PROPERLY piped and pumped-
    1) DSH Pump OFF with NO DHW draw- No flow, no problem
    2) DSH pump ON with NO DHW draw- The DSH loop will draw from the lowest point of the dip tube and return to the bottom of the tank.
    3) DSH pump OFF with DHW draw- The DSH loop will immediately "see" the coldest water at the cold water inlet tee. The tank will be getting cold water from the dip tube and DSH loop at the same time (balancing pressure between tank cold inlet and DSH loop)
    4) DSH pump ON with DHW draw- The same as #3 with increased flow to DSH loop

    Check the arrow on your pump (or HP DSH connection labels) and trace piping to tank inlet and outlet to determine correct flow direction.
    Double check for cross connections between hot and cold water systems.
  7. geoxne

    geoxne Active Member Forum Leader

    I can look it up if you pass along a model number.
  8. mngeotherm1

    mngeotherm1 New Member

    Hi goexne,

    My apologies on the mixing valve/no mixing valve. What I meant was that there is a mixing valve immediately exiting the tank, and then nothing after that (no loops for toilets etc.).

    The model number is GST060A11AAACSS

    I just switched the system over from cool to heat yesterday and already running out of hot water... So frustrating.
  9. geoxne

    geoxne Active Member Forum Leader

    Have you determined proper piping and flow direction?
  10. mngeotherm1

    mngeotherm1 New Member

    Hi geoxne, piping goes from the DSH to the bottom of the HWH then there is a connection a the top of the HWH with a tee where the cold water comes in and the DSH goes out.
  11. mngeotherm1

    mngeotherm1 New Member

    Another thought occured to me today. The geothermal system is not running for very long or often. Is it possible that the DSH is not getting hot enough and it is sending luke warm water to the HWH instead of hot water and thus cooling the HWH down?
  12. parrisjr

    parrisjr Member

    I fought similar issues along with others until I installed a buffer tank. After installing the buffer my DSH has performed flawlessly and has produced lots of hot water. I believe that no DSH should be installed without the buffer tank.

    I believe you will find similar sentiments from the experts here.

    Just my two cents.
    Stickman likes this.
  13. geoxne

    geoxne Active Member Forum Leader


    If, the HP isn't running you will get no hot water. If, it is barely running you will get barely any hot water water. If it is running half the time you will get half the hot water. If it is running most of the time you will get mostly hot water. This is why you need a buffer tank, to isolate the less than hot water (most of the time) from the Hot Water that you paid to get HOT. A DSH doesn't make hot water on demand.

    The GST has no logic to compare hot gas temps to incoming water to DSH. When entering DSH water is higher than Hot gas temps your powered water heater is in reality heating your ground loop.

    All this in it self should not keep hot water from getting to the tap or shower. I believe you have a second issue with cold water bypassing to your hot water piping when drawing hot water. Make sure you are piped like the attached diagram paying particular attention to trace pipes and confirming the labels match. If, they do match then the cold water is bypassing some where after your water heater.

    Attached Files:

    Last edited: Oct 5, 2014
  14. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    A quick check would be to compare hot water temp at a nearby tap (such as a laundry or utility sink) to pipe temp leaving water heater.
  15. tonyg

    tonyg New Member

    Is the use of check valves need. I noticed that even with an 80g buffer and 80g HWH I notice a 10 degree dip in shower temperature during the winter.
    Then summer time, its back to normal. I confirmed the plumbing is correct according to your picture, but Im wondering if I need check valves?

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