We are installing the Climate Master Tranquility 4 ton with desuperheater in ground loop. My plumber says that I might try one water heater (wh) and then add another if it is not enough. But, I recall the reason for two is that the desuperheater connects to one wh that is just for storage and the other is actually used to further heat the water supply the house. Thanks

You are correct, your plumber is not

Plumber is wrong. Desuper gathers heat over many hours and needs a place to store it. Household use of hot water occurs in spikes, and within an hour or so they typical water heater has recovered, leaving nowhere for the desuper to bank the heat.

Gotta have two tanks or it isn't worth springing for the desuper.

I am not an expert. We have 1 electric tank (85 gallon Marathon) with a water heater timer (Intermatic digital with battery backup). I admit that 2 tanks are better than one, but with predictable hot water usage patterns, much of the benefit of the desuperheater with one tank can be realized. We have two large demands for hot water per day at predictable times. We set the timer to go on for approximately 6 hours per day (3.5 hours and 2.5 hours) just before anticipated hot water usage. This leaves at least 18 hours per day for the desuperheater to heat the colder water in the tank. Since the desuperheaters can run while the water heater timer is making power available to the water heater, the desuperheater can actually run for more than 18 hours if needed. I believe we get approximately 70-80% of the benefit of the desuperheater with 1 tank. For people that have hot water demand at unpredictable times or greater usage than 1 tank will allow for, and possibly other reasons, this setup may not work for them.

The owner of the company that did our geothermal installation has 1 tank and a water heater timer in his house. Of course, a timer won't work with a gas fired water heater.

I don't think your benefit is as great as you figure.

My reasoning is that at the end of an on period the water heater will be mostly or completely recovered, leaving nowhere for the desuper to dump heat. If you found a way to ensure the Marathon is both off and has a substantial volume of cool water inside for several hours, then yes, a desuper could have good effect, but that is very hard to do given that the heater recovers fairly quickly.

Stated another way, turning power off to a large storage electric water heater for 18 hours per day will NOT reduce its electricity consumption by 75%

We have large hot water usage immediately after the water heater timer is turned off. We use hot water for showers, baths, washing machine, dishwasher, etc., around the times that we have programmed. This provides a significant amount of cold water (immediately after the tank is at or near full recovery) in the water heater for the desuperheaters to heat. We have found that two water heater cycle times work well for us, but the timer we have (model EH40 I believe) provides lots of flexibility in programming. It provides up to 6 cycle times with multiple day combinations available (individual days, M-F, SA & SU). I stand by the 75% figure for our usage, but this setup may not yield the same benefit for everyone since usage patterns, storage capacity, and volume of water used is different for everyone. A buffer tank is definitely the best way to get 100% of the benefit out of the desuperheater since it works for everyone under all circumstances (as far as I can tell), but we're happy with 1 tank and approximately 75% benefit.

No one has mentioned that WF suggests a lower heating element setting of 100 degrees F in the water heater for a 1 tank setup. This would also provide some cold water for the desuperheater. I have found that doing this doesn't work well when using a water heater timer. When the desuperheater runs it tends to mix the colder water at the bottom of the tank with the hot water at the top of the tank thus cooling the hotter water at the top of the tank. This may not be an issue for a 1 tank setup without a timer since the upper thermostat would normally turn on when the water temperature is colder than the upper thermostat setting, but with a timer, the upper thermostat won't go on until the timer provides power to the water heater. With a timer we have found it best to keep both heating elements at roughly the same temperature setting.

I forgot to mention that I would agree with your reasoning, engineer, if we let the heated water sit unused in the water heater for long periods of time.  Good that you mentioned this so that people will be aware that this may be a potential pitfall in this type of setup.  Hot water usage must occur very close to the time that the water heater has recovered for this setup to be effective.

My plumber says that I might try one water heater (wh) and then add another if it is not enough.

I have systems installed with high water usage with a buffer upstream of two powered electric tanks. I suggest to folks that they can try a 50 and add a second if need be for DHW. Context of that discussion is downstream of my buffer tank (always included).
Something is missing from your question i.e. why is a plumber designing a system to employ a DSH? Who is the geo installer? Is a buffer tank included in the geo bid and the plumber discussing hot water downstream?


11/06/2009 1:40 PM Quote Reply
I am not an expert. I admit that 2 tanks are better than one

With that said, why do anything else? If we agree that buffers are the best way to go then why don't you have one? You could spend as much on a timer, contactor and installer to set them up as a Depot 50 gal electric tank so why? If space doesn't permit, fine, then we will talk about alternatives.

The owner of the company that did our geothermal installation has 1 tank and a water heater timer in his house. Of course, a timer won't work with a gas fired water heater.

Actually, no reason you couldn't use a timer on a gas water heater. Different heaters would simply need different timer/control configurations. If you wish to support single tank systems, I wouldn't exclude gas DHW tanks

Good luck,
Joe

I never said that this approach is for everyone, but for SOME people 1 tank may make sense. "...why do anything else?" To answer your question, you need to look at the cost and payback of the individual household.

Cost: This can vary from household to household. We like maintenance free, so if we had a buffer, it would be a second Marathon. Not cheap. Please let me know if you know of an inexpensive buffer tank with a lifetime warranty.

Payback: In VA we have 4 or 5 shoulder months when the geothermal system doesn't run much, if at all. So, essentially no payback during 5 of 12 months of the year (for us). We also have low hot water usage with only 3 people in the house that conserve hot water where possible. Lower hot water demand increases the payback period. Mild VA climate also increases the payback period the other 7 months of the year. In VA, our geothermal system and desuperheater don't run as much as in Florida or Maine. We have relatively high thermostat settings in the summer and relatively low thermostat settings in the winter. This also increases the payback period.

Given all of this we estimated our payback to be about 15 to 17 years if memory serves. To us, it's just not worth doing a buffer for 25% more efficiency when the geothermal/desuperheater does run.  We don't want a $400 installed steel tank that will leak at probably the worst time imaginable. :-)  I'd rather give up the 25%. Remember, the payback isn't 25% of our total water heating cost.  It is only 25% of the benefit of when the desuperheater is actually contributing to hot water production.  When the geothermal system/desuperheater is not running, there is no contribution toward payback.

The timer we purchased was $75 (a good price) and a little less than that for installation. This is cheaper than even a cheap water heater installed.  I don't like the older style motor driven timers.  Less programming flexibility and you need to reset it when the power goes out for a period of time.

How would you timer a non-powered standing pilot gas water heater? Don't gas water heaters have high standby losses? Without low standby loss, I wouldn't recommend this setup to anyone.

I don't think you can discuss lengthy pay back as a negative and support Marathon water heater in the same sentence.
But frankly all I did was use your words and ask the follow up question.
I'm not trying to pick on you, but you are talking about tanks that last ~10 years and cost < $300 and noting that they don't have a life time warranty like one that costs 4-5 times as much. You can't have this both ways, we'll talk pay back or preference but not both where a Marathon is concerned.
Controlling an electric water heater with a $75 timer requires a high end heater. Controlling a basic DWH with a $75 timer requires much more, as would a gas heater.
Frankly, and with all due respect, I'm more interested in the answers to my questions to the OP than the debate on your system which you begin by concedeng is more niche than common application.
j

Never meant to imply that lengthy payback is a negative. But at some point, people need to make a decision as to whether things are worth it or not given their particular situation. We opted originally for a Marathon because we though it would be worth it for the efficiency and lifetime warranty. When considering a second one, payback is a different set of calculations (not the same as for the first tank) since it serves a different purpose (as a buffer tank). We have decided for that purpose, the long payback period just didn't justify the purchase of a second tank. For someone that would be happy with a conventional steel tank (and isn't opposed to having a second tank in their house) then the payback would be much shorter. Since the Marathon is what we would choose for a buffer tank, its cost needs to be used in our calculation.

"Controlling an electric water heater with a $75 timer requires a high end heater." Would you please explain this? Couldn't this the timer control any electric water heater?  I have said that with a water heater with high standby loss, I wouldn't recommend this setup.  Is this what you mean?

I have said constantly that this setup (that works well for us) isn't for everyone (i.e. not common). I brought it up because the OP had a question regarding 1 or 2 tanks.

In response to the OP.  There are benefits to having one tank (even without a timer).  Water Furnace thinks so.  At the time of our purchase earlier this year, they suggested a lower thermostat setting of 100 degrees on the electric water heater.  This leaves room for the desuperheater to work.  Why would they (and possibly other manufacturers) suggest this setup if it is not appropriate?  Additionally, none of the 5 other quotes we received suggested getting a buffer tank.  I just exchanged a PM with another person that reported the same (5 quotes and no buffer recommended).

Here are some calculations and assumptions using a standard electric water heater to determine payback in our situation.

Our hot water heater has power made available to it by the timer for 6 hours per day and supplies all of our hot water needs for a 24 hour period even when the geothermal system isn't running.  Note: 6 hours is the maximum time the water heater has power made available to it, but the heating elements may not be on for this this long, so this time may be overstated, but I'll still use 6 hours for argument sake.

Our electric rate is $0.0568 per KwH.

So, our maximum annual water heating cost would be 6 hours x 4.5 kW heating elements x 365 days x $0.0568 = $559.76

5 months of the year our desuperheater/geothermal system is not running (or runs very little), so I will exclude $233.24 for these 5 months in the buffer tank benefit calculation leaving $326.52.

How much could a desuperheater contribute to our hot water production during the 7 remaining months of the year with 2 tanks?  Is 70% reasonable given the usage, climate, thermostat settings, etc., I described earlier in this thread?  If so, the maximum benefit is 70% X 326.52 = $228.56.

My estimate of the buffer tank benefit is 25% more (for us) than we currently receive with 1 tank (see earlier in this thread).  25% of $228.56 equals $57.14 potential savings per year by adding a buffer tank.

The cheapest 50 gallon water heater that Lowe's has online for our zip code is $284 and offer $295 standard installation.  Installation seems high to me, so let's use $175, so the total with installation and 5% sales tax (on the water heater only) would be $473.20.

How long will this water heater last?  It has a 6 year warranty, but let's assume 8, 10, & 12 years. 
If it lasts 8 years, we would save $457.12 (57.14 x 8) - $473.20 = loss of $16.08 over 8 years.
If it lasts 10 years, we would save $571.40 (57.14 x 10) - $473.20 = saving of $98.20 over 10 years.
If it lasts 12 years, we would save $685.68 (57.14 x 12) - $473.20 = saving of $212.48 over 12 years.
These savings are not reduced (but should be) for interest a person could receive by not purchasing a buffer tank and putting the money in a CD for 8, 10 or 12 years.  If the cost of the second tank is financed, the savings would also be lower due to interest being paid.

As a homeowner, I do not agree with the "one size fits all" approach to having a buffer tank (everyone needs one).  Homeowners need to do their homework to determine if a second tank is justified for their particular situation.  A few things to consider are: is the estimated payback sufficient to warrant the expense of a buffer tank, do you want a second tank in your house taking up space, do you mind twice the potential for leaks with 2 storage tanks (one powered and one not powered), etc.  For some people the answers will point to getting a buffer tank, for other people the answer will point to not getting a buffer tank.  For some, money will be the primary factor in the decision making process.  For others, non-monetary factors may be more important.  With us, it was a combination of factors.

No hard feelings to the installers here.  We just have an unresolved difference of opinion on this issue.  The installers here say a buffer tank is always needed.  Many installers quoting jobs (based on people writing in) feel a buffer tank isn't necessary.  I believe that every homeowner's situation is unique and it is worth doing some calculations and consideration of other factors to determine if a buffer tank is right for them.

geo mfgs suggest the single tank setup for those unable or unwilling to install a 2nd tank - they still get the markup on the desuper option.

Sub six cents per kwh is a rarity and substantially skews your numbers.

Severely restricting water heater element operation via a timer leaves the household vulnerable to running out of hot water if family schedules change, guests are in town, a bunch of laundry needs washing, etc.

I don't disagree that every situation deserves individual analysis; I'm just not willing to ask a customer to spring for the DSH option and the additional installation cost and complexity without a buffer tank providing me the security of knowing I provided the most flexible, economic to operate system regardless of household circumstances.

For those that may read this thread 'down the road:'

This is in fact what WaterFurnace says, from it's published installation manual:

" To maximize the benefits of the DSH a minimum 50 gallon water heater is recommended.  For higher demand applications, use an 80 gallon water heater, or two 50 gallon water heaters connected in a series ... Electric water heaters are recommended."

"Under certain conditions, Envision dual capacity units operate with very low refrigerant discharge temperatures, producing little or no water heating capability."

Best regards,

Bill

Okay, here's my questions:
1) hi geome... so where does the percentage number "25%" more efficiency of desuperheater versus without desuperheater come from? It seems to me that the desuperheater would or should do most of the water heating and the electrical heater on the supply to the home would do minimal heating to bring the water to the temperature at its setting. Generally then, if one had the hot water setting really high, that would cause less efficiency from the desuperheater.

2) generally hot water is used at peak periods and I thought the buffer tank would supply enough hot water to prevent the electrical from kicking in so much after peak usage.

3) It seems secondary and really unnessary to get into timers and timing of water usage if the buffer tank(s) are used and insulated well. For example, if my supply water heater has a timer to heat 6 hours in the night, I don't really understand why that would use any less electricity than just letting it heat when it needs to (given proper insulation of course).

- Craig

Yes ^ 3

If the problem with a standard tank is that the electric hot water heater will heat faster than the desuperheater what about this single tank option: A GE or Rheem heat pump water heater set to the most frugal setting. This disables the element and then you've got two heat pumps working on the same amount of water. You can change the setting once the cooling season is over. Only drawback I see to this now is the tank is 50 gallons. Too small for some households I would imagine. The tank also costs about $1200-1500 US but rebates are always available and increasing.

engineer,
"geo mfgs suggest the single tank setup for those unable or unwilling to install a 2nd tank" Is this documented in any WF literature?
"Sub six cents per kwh is a rarity and substantially skews your numbers." Again, a justification for doing the analysis.
"Severely restricting water heater element operation via a timer leaves the household vulnerable to running out of hot water if family schedules change, guests are in town, a bunch of laundry needs washing, etc." Yes, some advance thought may be needed when using a timer compared to using a buffer, but this has not been an issue for us since we spread out our hot water usage throughout the week AND we purchased a tank large enough to hold sufficient hot water at the top, and sufficient cold water at the bottom so the desuperheater can work on the cold water. When guests are here, I add an additional timer setting or extend the current settings temporarily until they leave (I hope timer settings only last a few days when my in-laws are here!). However, with the WF recommendation of 100 degree lower thermostat setting, no timer is needed and 1 tank is still all that is required.
I agree that buffer tanks allow for more flexibility, but some people (like us) may not need that flexibility in the form of a buffer tank. I chose an 85 gallon Marathon to give us the flexibility we need because of it's capacity (large for us) and it's ability to retain heat over long periods of time.

Thanks Bill for listing this. As I have said before, I agree that a buffer may be cheaper for people if water usage, tank and installation cost, utility cost, etc., is right, but it may not be better 100% of the time. For us, and I admit we may be in the minority, having a buffer just doesn't pay in monetary and non-monetary respects. I disagree with the philosophy that a buffer is right for EVERYONE under ANY circumstance.
"Under certain conditions, Envision dual capacity units operate with very low refrigerant discharge temperatures, producing little or no water heating capability." Yes, but a buffer wouldn't help in this event (not that you said that it would).

Hi Craig.
1) Based on our setup and pattern of using hot water immediately after the timer goes off with a single tank setup, the 25% comes from 6 hours of POSSIBLE thermostat on time per day (even though the thermostat may not be on for the full 6 hours because the water may be fully heated before then by the desuperheater) divided by 24 hours in a day. With a buffer tank, the desuperheater can potentially heat water in the buffer a full 24 hours per day if conditions are right. So, I'm conceding a 25% benefit to a buffer system.  However, when our desuperheater runs a lot, I doubt we have anywhere near 6 hours of water heater on time since the thermostats will turn off sooner with desuperheater heated water.

2) Whenever there is a large demand for hot water, a buffer will do as you described assuming the desuperheater has run enough to heat the buffer tank. The timer setup with one tank accomplishes the same thing (for us), and so would the WF recommendation of the 100 degree lower thermostat setting (but possibly not to the full extent of a buffer tank, but this would depend on hot water usage, desuperheater run time, tank size, available cool water to heat, etc.)
Here is how our setup works. After the timer goes off, which is just before our normal hot water usage (showers, wash, dishwasher, etc.) we have plenty of hot water in the tank that is available to use. We immediately use this hot water thus allowing colder water into the tank. Then, as the desuperheater runs, it heats this cold water without the water heater turning on. At the next timer on time, the thermostats finds water heated from the desuperheater and not the cold water from hours before. This results in less water heater on time since the heating element is just topping off (if needed) the already heated water from the desuperheater. If you do not use the heated water after the timer goes off, there will be no room for the desuperheater to heat water since it is already heated. It will just keep the tank at temperature, similar to a fully heated buffer tank. In at least one respect, the single tank system is superior to a buffer tank system. That is, when there is little or no hot water usage in a buffer tank system, the water heater will have to maintain it's own water temperature when there is heat loss through the tank. With a single tank system, the desuperheater can maintain the water temperature in the water heater.
Using the WF method, the 100 degree lower thermostat setting provides an area for the desuperheater to heat the 100 degree water to 130 degrees (or whereever the manufacturer set the desuperheater cut off temperature).

3) Yes. I am not suggesting using a timer with a buffer. For single tank systems, I am only suggesting a timer if your usage can coincide with your timer off settings to make cold water available in the tank for the desuperheater to heat (not as flexible) OR the WF recommendation of 100 degree lower thermostat setting (this provides more flexibility than using a timer). From our experience, using a timer AND a 100 degree lower thermostat setting does not work well (reason previously listed in this thread).

dkubarek, that's why I like our timer. With the timer shutting off the water heater, it allows plenty of time for the desuperheater to work. Just need to get a tank large enough to carry you through demand while at the same time be large enough to allow sufficient cold water at the bottom of the tank for the desuperheater to work. That's why we chose an 85 gallon tank for a relatively small household. I'm not sure if recovery rate would be an issue with the 2 heat pumps (engineer)?

Actually, with R410 units, like the WaterFurnace Envision series, what will happen is heat will transfer from the higher temperature DSH's hot water loop to the lower temperature refrigerant, causing some amount of degradation of heat pump performance (efficiency).  I posted some charts showing this quantitatively a while back.

I don't know if the factual savings of not installing a buffer tank outweight the increase run time cost due to the small increase in heat pump inefficiency.

Best regards,

Bill

From experience my choice is a single tank.

I run a WaterFurnace Envision here in Alabama. We turn on the DeSuperheater in spring when it's time to switch to AC. As the average temps outside rise to where the AC is needed everyday, we turn off the breaker to the lone water heater (80 gal Sears PowerMiser12). Our WaterFurnace provides PLENTY of "HOT" water for three adults until the next heating season. Once in a while I notice the High Limit light on. After some hot water use, the light goes out and the Desuper turns on again.

In my previous home we had a WaterFurnace Premier II w/DeSuper and a single 80 gal water heater. Same (excellent) results there, which is why I installed the same DWH setup here. It's puzzling to learn that some installations require two tanks. Could it be the climate difference? Alabama is pretty hot a good part of the time. But then again. I live in a SIP house with HERS rating of 49. Our average power bill shows about 850-900 kwh. The climate inside my house is not as "aware" of the climate outside as a less efficient house would be. Theory being, the WF unit runs as if it is in a milder climate.

Would two 40 gal tanks bleed off more heat then one 80 gal tank? If so, is it significant enough to be a drawback? Is this thought too simplistic to be relevant?

It would be great to have devices sampling and collecting data for: water temp in and out, flow rates, High Limit status etc. And have that data stored in ASCII format to be used in practically any computer/SW. Anyone know of such a system?


A question just hit me. What additional power is consumed by the Envision unit when the DeSuper runs? There must be "some" increased in power consumption just to run the pump in the DHW loop.

Indeed there are a number of us that have such a system.  See http://welserver.com/WEL0043/ for my system, http://welserver.com/WEL0114/ for Dwayne Dean's system ('geodean' here on this forum), and http://welserver.com/ww/ for many others.

I too have WaterFurnace Envision units (3 and 5 ton 2-stage variable speed).  Each has a Grundfos UP 15-18 85, 1/25th hp, 85 W (0.4 FLA) motor to serve as the DSH pump.

Best regards,

Bill