Question on Proper Plumbing of DSH pre heat tank

Discussion in 'General Discussions' started by GorgeFarmer, Sep 29, 2020.

  1. GorgeFarmer

    GorgeFarmer New Member

    Hello All,

    I recently made the decision to transition from a propane furnace that came with the house to a waterfurnace 7 geo system. This site was hugely valuable during the evaluation process.

    Install is nearly complete, but I've run into a snag. The lead technician seems to disagree with my plumber on how the Desuperheater is plumbed to the pre heat tank. I'm hoping you all can help me work through it.

    The technician is saying he needs a boiler drain on the line supplying pre heated water from the geo unit to the pre heat tank in order to properly bleed/purge air prior to initial DSH startup. My first priority is getting this right, but if I were to make this change it would be another hour or two from my plumber.

    In the first picture below the pre heat tank is on the right, you can see the warm water supply coming to it with a check valve, ball valve then boiler drain. The technician is asking me to install a boiler drain on the geo side of the ball valve. He is suggesting I don't need a check valve there and to replace that with a boiler. He is saying that is the only way to properly purge air out of the lines. He is also saying during yearly maintenance he wants it to flush out the lines to prevent build-up. He is saying this is what the instructions call for and seems to want to die on this hill. I included a screen shot of the diagram from the instruction book and it looks to me (very little plumbing experience) that it matches more the configuration I currently have.

    The sales guy/designer owned a geo focused company that this technician worked for prior to it being purchased with them both working for the new org. This gentleman is saying he believes it's good as is. So I'm a little frustrated as Im getting conflicting signals.

    Do any of you folks have experience install DSH from a waterfurnace unit? Would appreciate if you can weigh in on this.


    20200928_124313 (1).jpg 20200928_124321 (1).jpg

  2. gsmith22

    gsmith22 Active Member Forum Leader

    waterfurnace 7 series 700A11 installation manual has a plumbing diagram for this on page 11 with the pre-heat tank. The hot water out of the desuperheater goes to the bottom drain valve of the storage tank. Cold water into the desuperheater comes from a tee off the top of the cold water input to the pre-heat tank. It also shows locations for vents/drains - I've attached the manual. With only one heat pump/desuperheater, I don't think you need a check valve on the copper plumbing as water can only flow in one direction. I have two heat pumps and I had check valves installed on the output side of each desuperheater to prevent one desuperheater backfeeding the other desuperheater (in the event only one heat pump desuperheater is running).

    Attached Files:

  3. GorgeFarmer

    GorgeFarmer New Member

    Thanks gsmith22 for the response. The diagram I posted is the one you refer too from page 11 of the manual. To me it looks very similar to how I'm plumbed and vented now. Do you see any reason why a boiler drain on the unit/supply side of the ball drain that is on the hot supply line coming out of the unit? That is what my technician is asking for. I'm not really seeing why and I'm trying to avoid an un-needed plumbing retrofit. But if that is the best practice I'd be glad to do it. Thanks.
  4. gsmith22

    gsmith22 Active Member Forum Leader

    i can only see two pictures of your basement in your post. the "diagram" comes up as an image symbol (and not the actual image) so I didn't realize that you had posted the same waterfurnace diagram. In your first picture, I don't understand why the position of the boiler drain would need to move to before the ball valve. In fact, I think the ball valve at that point is a waste and would be better placed on the incoming cold line to the pre-heat tank that I don't see in the picture.
  5. ChrisJ

    ChrisJ Active Member Forum Leader

    Don't know if you have seen this diagram Bergy DSH piping.jpg

    Closing the ball valve that is on the cold water in on top of the preheat tank forces the cold water through the DSH plumbing. By turning on a hot water faucet it purges the DSH.

    I have a check valve on the DSH "IN" just after the "T" on the cold feed to the preheat tank.
  6. SShaw

    SShaw Active Member Forum Leader

    I think ideally you want two things:
    1. Ball valves and drains that isolate the DSH circuit so you can easily flush the system with a descaling solution. This requires a drain between the buffer tank and the ball valve at the input to the buffer tank. Your technician is right on this point. You also require a similar setup at the top of the buffer tank, where it feeds back into the DSH. See photo for how mine is setup.
    2. Plumbing that allows easy purging to remove air from the DSH. The diagram ChrisJ posted covers this. That diagram does not include the drains needed for flushing though.

    Attached Files:

  7. Tom Bradford

    Tom Bradford Member

    Question about the manufacturer’s recommended plumbing connections to the buffer tank:

    Shouldn’t the pipe from the desuperheater connect to the hot water connection at the top of the buffer tank instead of what’s shown? That way the warmest water is introduced to the tank at the top of the tank where the water is the warmest, and the coldest water is drawn from the tank at the bottom where the water is the coldest. This will keep the tank water stratified and will assure greater heat exchanger effectiveness.

    In other words, introducing tempered water at the bottom of the tank will increase the temperature of water returning to the desuperheater, decreasing the effectiveness of the heat exchanger.

    I have come to trust and follow manufacturer’s recommended installation instructions, but I’m doing some head scratching on this one. Please help me understand the rational behind WF’s recommended arrangement.
  8. geoxne

    geoxne Active Member Forum Leader

    I used to think the same thing. However, I was very wrong.

    The hottest water is at the top of the tank and and is drawn from there when you open a hot water faucet.
    The desuperheater works best and can provide the most BTU's by circulating the coldest water in the tank. It does this by drawing from the cold water dip tube and pumping into the bottom of the tank maybe only a few degrees warmer. The warmer water will rise above the bottom of dip tube. Eventually it has the potential to heat the whole tank over time, depending on HP runtime, whether heating or cooling, source temperature, and hot water use.
    Keep in mind when there is a hot water draw, the bottom of the tank will quickly drop in temperature. The DSH will see it immediately as the cold water flows through the DSH, and dip tube.
    If you pumped into the hot outlet of the tank your hot water would immediately turn cold.
  9. Eric Kurtz

    Eric Kurtz Member

    I really don't understand the logic here. The dip tube only goes half or maybe 2/3 of the way to the bottom of the tank. Also most have holes to diffuse the cold water into the warmer water. It would seem to me that you would have a sizable portion of tank that would theoretically never get circulated thru the DSH. (However the introduction of the cold water when water is drawn probably takes care of this problem.) But I still fail to see how drawing the coldest water from the bottom of the tank and running it thru the DSH and piping it somewhere into the top of the tank is an inefficient arrangement.
  10. SShaw

    SShaw Active Member Forum Leader

    As I understand it, you do not want the tank to stratify, as that will reduce the amount of hot water present in the tank. The less water in the buffer tank that you heat with the DSH, the less efficient the overall system will be. So, you inject the heated water from the DSH into the bottom of the tank, and the heated water rises to the top, mixing the water. Also, I would think that if hot water is in use during DSH operation, and cold water is flowing into the top of the buffer tank, then that cold water will flow from the tee directly into the DSH, bypassing the tank and preheating the water before it goes into the tank.
  11. Eric Kurtz

    Eric Kurtz Member

    But once the top half of the tank is 100*, you are pulling that water into the DSH, so heat transfer will not be as efficient. The tank will stratify when water is not being used and the DSH is not running. If you pull the water off the bottom of the tank you will heat all the tank.

Share This Page