Questions about desuperheater and options??

Discussion in 'General Discussions' started by Joshua Morganstein, Jun 20, 2015.

  1. Joshua Morganstein

    Joshua Morganstein New Member

    Would greatly appreciate any feedback on our situation. I've been reading on this site for weeks and still feel like there is so much more to learn...

    We are currently on propane in Maryland with no natural gas option. A 1950s rancher that was renovated and expanded by 150% into a 5-level split with zoning to each level along with 12 20-year old skylights (and probably other leak areas in the home) has us spending nearly $5k annually on LP to keep the house only moderately comfortable during the cold season. We have insulated the existing attic spaces and done Aeroseal throughout which was exceptionally effective at reducing lost CFM.

    After 7 winters of owning this home, it's become clear geothermal is the best option for us and we are scheduled for installation in July. Because the house is on 2 separate systems, we will have 2 WaterFurnace Series 7 units installed with backup hydronic heating using the existing (and relatively new) Delta Triangle Tube combo boiler with loops run to both Series 7 units. We have a house of 2 adults and one toddler and shower at home 3-4x/week, the other days are at the gym. Our installer offers several desuperheater options, though I remain a bit unclear on a few things after talking with them. The options and pre-30% federal tax credit costs are:

    1. WF DSH ($1500)
    2. WF DSH + 50 gallon tank ($2000)
    3. WF DSH + WF 85 gallon GeoTank ($3000)

    My questions are:

    a. Are DSHs usually a wise investment given our situation?
    b. Is there any significant benefit to having an extra tank added per option #2 or #3 above and what would that be?
    c. We already have problems getting hot water to the furthest two bathrooms and because we do not have a hot water return loop, I had a Grunfos recirculating pump installed which helped noticeably with this problem, but it's still not optimal. Would a DSH adversely effect the time it takes to get hot water to these problem bathrooms? In other words, might I be saving money on LP (even if I'm spending some on electricity) to make my hot water cheaper, only to worsen the problem that bothers my wife the most, which is how long it takes to get water to the master bath?

    Thanks very much for any feedback and if there is relevant related guidance, I welcome that as well.
  2. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    It seems that your hot water use might be rather low to justify DSH from a financial perspective.
    Unless both units are relatively close to the existing water heater, DSH may only be viable for one or the other.
    If you decide to get DSH, you can not do it effectively without a buffer tank. Although manufacturers show such setups, the folks with the most experience on this and other forums will vehemently oppose an unbufferred DSH.
    The long lag at the Master bath should be unaffected. The run from your finish water heater to the Master bath will notionally be unaffected by any DSH or buffer tank piping you install before the finish heater.
    Happy wife, happy life- If you want to make the Mrs. happy look for other options in the master bath. Do you have room for a point source water heater that could be conveniently piped in near the bath. I'd think about a 5 gallon point heater tank close to the demand(ing).
  3. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    They make time and temparature re-circ pumps for portable water. That would fix the MBR issue cheaper than a de-super-heater.

  4. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    With 3 people the DSH probably will save you $300-$400 per year versus propane, so with $2K pre tax credit, payback should be around 3 years. You must have a buffer tank, otherwise do not bother. I assume your installer proposes to use the proposed tank as a buffer, you do not need the 85 gallon tank, 50 gal is fine for a buffer tank. Distance of buffer tank to heatpump is important for performance. DSH performance in 7 series is great due to continuous running.

    Circulation lines are energy hogs, we usually put them on a timer or a motion sensor.
  5. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    Consideration could also be given to a heat pump water heater in lieu of propane. A lot depends on your available space and tolerance for noise and additional cooling. Living in Southern Maryland, I find mine is a great addition to the basement. Excellent supplemental cooling and dehumidification in summer and shoulder seasons. The added cooling in winter is a negative, but the replacement heat from my geothermal still keeps the overall COP above 1.0 for hot water.
  6. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    I 2nd the motion as to a heat pump water heater. 50 gallon model can be had for $1k and has the advantages Trent cites.
  7. Joshua Morganstein

    Joshua Morganstein New Member

    Thanks to all for the feedback. This is very helpful.
    mtrentw, thanks to you and engineer for thoughts on heat pump. We have a pretty low tolerance for noise which is one of the things we are looking forward to with the Series 7... quieter operation vs current system. I think the $2k option with 50gal buffer tank will be reasonable when we take the $600 out for federal credit, so cost is $1400.
  8. Joshua Morganstein

    Joshua Morganstein New Member

    docjenser, one of the Series 7 units will be on the other side of the wall from the DSH/buffer tank/main water tank along with the combo boiler. The other unit is about 40 feet away. Any feedback on how to determine how impacting the distance would be in our situation? Is the issue loss of heat as it travels from the Series 7 to the DSH?

    Also, sounds like 50 gal buffer is good enough, though 85 gal is supposed to be more insulated. Is there just not enough benefit to any extra insulation to justify the extra $1k (or $700 after tax credit)?
  9. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    I am in the camp who promotes smaller DSH buffer tanks, since the tank does not help you if the temp does not get high enough. The thermal mass is too high to get reasonable amounts of hot water in shorter time, thus your secondary tank turns on more.

    So 50 gallons is good, usually 50 gallons have also lesser surface area, thus lesser standby heat loss. I like the bradford white, they are reasonable, and have 2" foam insulation. I would save the extra money, and indeed spend it on a heat pump water heater for the second tank.

    40 ft is kind of long for the line, but you can ensure that it is well insulated, so it performs somewhat. Yes, ultimately you have heat loss in the DSH lines which limit overall performance. Again, insulate well!
  10. Joshua Morganstein

    Joshua Morganstein New Member

    docjenser, thanks. A few more questions if you have time:

    1. Are you indicating that cost to both install AND operate the heat pump hot water heater will be less than a DSH?

    2. How noisy is a heat pump water heater as mentioned by mtrentw?

    3. If I have 2 Series 7 units, would both of these have to connect to the DSH (i.e.; the 40 foot run and insulation issue you mentioned)?

    4. If we do a DSH with buffer tank is this a good option?

    Thanks again.
  11. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    "There are 100 ways to make hot water, and none of them is a good one! " (John Manning)

    You could skip the option of DSH, order the heat pumps without it, skip a tank, and install a dedicated small water-water heatpump with (1) simple tank and call it a day. That way you would make all the hot water with the efficiency of the geo system, all year long. A bit more upfront, but with saving 2 DSHs, the need for the buffer tank, and the piping costs, that option might be more attractive than you think.

    Here to your questions:

    1) I am indicating that it is not worth spending money on super expensive tanks. All they have to do is holding water.
    2) I don't know. But the compressor is small, but not well insulated.
    3) You can order only one with a DSH, or only connect one. Your DSH contribution goes down, but again, with 40 ft lines in there, this might be a wash. You might not get much, if any, DSH contribution out of a 60 ft line.
    4) I do not like the Smith tanks, they used fiberglass in the past, and were not well insulated. While the insulation is not that crucial with the buffer tank, it should have 2" foam, at least.
  12. Joshua Morganstein

    Joshua Morganstein New Member

    Helpful answers. Thanks! We would only have 1 DSH installed. I spoke with our install company to get clarification. We would only have 1 DSH installed along with the AO Smith 50gal buffer tank (on the AO website, the specs indicate "thick CFC-free foam insulation"). When I mentioned the long run from the furthest unit to the DSH, he stated we should be able to just make one run from unit that will be nearby (only a few feet to the DSH and buffer and main tanks) and they always heavily insulated those lines as part of their installation. The Series 7 is the 3ton unit which he thought would be adequate. The cost for the DSH + buffer tank is $2000 (pre-tax credit) and they install these. He indicated they do not install heat pump water heaters which means I'd need to get yet another person to come in and do that and hook it up to our geo system and hope they don't screw anything up. I assume I could get 30% tax credit on it if geo, but not certain. Any additional thoughts pros/cons given the above info?
  13. Joshua Morganstein

    Joshua Morganstein New Member

    doc, also I'm finding different things when I look for heat pump water heater. The first item below looks like what you mention in the beginning of your reply above as it indicates connecting to a geothermal system. How does it save piping costs if it is using geo system all year long, wouldn't there need to be a connection from the Series 7 to the Climate Master water heater?

    The second item below makes me think of how a hybrid or dual-fuel heat pump works for a home by extracting heat from ambient air. Do you think the second item would actually cut hot water cost by 65% and is that realistic? Is this the non-geothermal application people are talking about that totally skips a DSH and just installs this items separately or are referring to another type of heat pump water heater?

    The first option seems to hook up to the geo system and, I assume, just extract any heat that was going to be discarded (at least during times when such extra heat is available) and I have no idea what the cost is to connect it. The latter seems to just replace our existing water heater with something they claim is dramatically more efficient and the MSRP is $1200, and that sounds like a relatively straightforward install. If it cuts hot water costs by 2/3 that sounds outstanding if it lived up to those claims. Can you clarify which item you would be thinking of for our application or any other things to consider?
  14. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    I have a de-superheater loop feed to a buffing tank and then to a heat pump finishing tank near Prince Frederick, MD. I have camped in Silver Spring.

  15. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    To pipe 2 heat pumps into a buffer tank is more work than to pipe a single water-water HP into a single tank. You would completely abandon the DSH option, thus there would be no connection between the 7 series and the hot water. See diagram. Waterfurnace also makes dedicated hot water heaters. They have to have double walled heat exchangers.

    I am not too much in favor of air sourced hot water heat pumps in heat dominated climate, since your source is conditioned space, which already took energy to get heated in the first place. Some people would argue that it is still better than a COP of 1, thus there is net gain. I would argue that a DSH or a dedicated HP is a better solution, but more expensive upfront.

    Yes, dedicated ground source water heaters save about 65%, much higher efficiency than DSHs, and heatpump water tanks, which are air sourced. DSHs make about 60% of your DHW with the efficiency of the geo system, the remainder has to come from other sources, thus the overall savings are about 50%. Less if more water is needed. In addition, a 3 ton unit makes lesser DHW than a 5 ton, since about 10% of the capacity goes in the DSH.
    Clearly, dedicated DHW heat via a hydronic heatpump is the most efficient option, especially in heat dominated climate. But it is also the most expensive upfront.

    Attached Files:

  16. malatu

    malatu New Member

    I'm trying to follow this but want to make sure I understand the abbreviations.

    DSH = desuperheater
    HP= ?
    When you refer to the hydronic heatpump, are you referring to the desuperheater?

  17. ChrisJ

    ChrisJ Active Member Forum Leader

    Yes DSH = desuperheater

    HP = Heat Pump

    Hydronic Heatpump refers to a water to water heatpump. Which just makes warm water and cool water if it has a reversing valve.
    Water to Air heatpumps just make warm air or cool air.

    Desuperheater is also called a hot water generator.

  18. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Hi, Chris:

    The student is now teaching. I love it.

  19. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Hey Chris:

    Did you get what you wanted/needed for your home?

  20. ChrisJ

    ChrisJ Active Member Forum Leader

    Not yet...

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