Posted By stuart.wyss on 22 Aug 2010 12:12 PM
So, if I have a single tank (40 gallon) and it's being heated by an AirTap, set at 120 F or thereabouts, and my GeoMax 2 (R410a coolant) is running, chances are the desuperheater is contributing very little to the hot water situation? I don't have a setting for 125/150F, mine's the older version of the circuit board). Hopefully, heat isn't being "stolen" from the tank and, I guess, put into the ground?

I'm depending on the accuracy of both the AirTap thermostat and the DSH circulator thermostatic switch to maintain an ideal balance.

It depends what EWTs you have, because those more or less determine the compressor discharge temperature. If you are down south, not much of an issue since loop temps are high. The issue is in heating dominated climates with cold grounds. Then chances are that with a single tank configuration, the DSH does not turn on in A/C mode. But the low limit switch at the discharge line set at 125 will prevent heat being stolen from the tank.

I'm in SE PA, probably a heating-dominated climate. While I'm glad the 125 cutoff prevents heat being stolen, I'm probably not gaining much through the desuperheater.

I agree that given the single tank situation, DSH may be adding little or nothing to domestic hot water - we've beat this to death so I won't here further elaborate.

Very true in Northern Florida, however if the HP unit runs in heating mode 6 months of the year, even a single tank design can benefit from the desuperheater setup.

Posted By stuart.wyss on 22 Aug 2010 05:03 PM
I'm in SE PA, probably a heating-dominated climate. While I'm glad the 125 cutoff prevents heat being stolen, I'm probably not gaining much through the desuperheater.

You can make your hot water in the heating mode with the efficiency of the geosystem, and if designed correctly, preheat your water in the A/C mode, in our climate to 80-90 degrees F.

I agree with Engineer (Curt) that the subject of single/dual water tanks/buffer tanks has been discussed and debated so many times on this board. I apologize for sending us all down that one again!

For the sake of completeness, it should be mentioned that a DSH is very prone to scaling and may need to be cleaned frequently. You will get less and less heat from it as it scales up.

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I quite beg to differ - a DSH is no more prone to scaling than any other heat exchanger, and reasonable provision for isolation and flushing is easy to include and use.

I ran one for 10 years with very hard water and paid it not a wit of attention - I know it worked fine since I routinely dumped the breakers for the finishing tank every May for the summer.

I want to thank those that offered input about buffer tank temps and how the systems work. I can decern from the responses that my system is likely operating just fine for a R410 unit. I poked around on the WEL data logger site to see what others were acheiving, and my system looks comparable. My ground loops always have a EWT less than 70 degrees so far this summer (and it has been hot here, SE Pa). I learned from your input that the colder EWT limits my DHW generation somewhat in AC season since I have a bit less superheat avail. That is a good thing as my COP is real good for the HVAC side. I will soon see how the buffer tank temps do for the heating season. They should be better with longer run times and more superheat avail.

I do wish the manufacturers and contractors gave more information on the expected performance of the water heating ability of these systems so consumers could know upfront what to expect. They just give minimal data to keep it simple I guess. From what they give, it's hard to make sound decisions.

Again, thanks for your input.

The amount of scaling one gets on a heat exchanger will depend on hardness, temperature and several other factors. No, a heat exchanger on the closed loop portion will not be nearly as subject to scaling as the DSH. Are there cases of DSHs not scaling - of course, that's why one would write "may".

Posted By chrisbiker on 25 Aug 2010 09:22 AM
I want to thank those that offered input about buffer tank temps and how the systems work. I can decern from the responses that my system is likely operating just fine for a R410 unit. I poked around on the WEL data logger site to see what others were acheiving, and my system looks comparable. My ground loops always have a EWT less than 70 degrees so far this summer (and it has been hot here, SE Pa). I learned from your input that the colder EWT limits my DHW generation somewhat in AC season since I have a bit less superheat avail. That is a good thing as my COP is real good for the HVAC side. I will soon see how the buffer tank temps do for the heating season. They should be better with longer run times and more superheat avail.

I do wish the manufacturers and contractors gave more information on the expected performance of the water heating ability of these systems so consumers could know upfront what to expect. They just give minimal data to keep it simple I guess. From what they give, it's hard to make sound decisions.

Again, thanks for your input.

I think you see a nice performance of the desuperheater in the winter time. What Curt is describing for Florida, namely shutting of the breakers for the final conditioning tank, you will see in SE PA in the heating season, namely much more heat generated by the DSH. So at the end it will still make 2/3 of your hot water. And yes, your COP should be very high, so it is a give and take. BTW, Curt, you running this unit for 10 year, I assume it is a R-22 unit. Did you notice any difference in performance for DSH when the switch was to R-410a with the ETW as high as they are down there?

Posted By chrisbiker on 25 Aug 2010 09:22 AM

I do wish the manufacturers and contractors gave more information on the expected performance of the water heating ability of these systems so consumers could know upfront what to expect. They just give minimal data to keep it simple I guess. From what they give, it's hard to make sound decisions.

Again, thanks for your input.

When I prepare a geo proposal with op cost calculations the geo DHW is in there. It is however for the year not by the month. A DSH is a supplement that you will use how you will. BTUs available depend on the weather.
Temp in the buffer tank will depend on the gallons.
J

Just wanted to post a follow up now that it's heating season. Wow. What a difference. The buffer tank in now much hotter. Our GE heat pump water heater is barely running. After the family showers this morning, it did not have to turn on. I do not have data, but the DSH is really doing it's thing now.

I'm not suprised - some systems run at higher pressures, temperatures and power in heating mode than in cooling. Rejecting heat to 70 degree air (heating mode) may be harder than rejecting heat to the waterside (depends on flow and temperature) That results in higher compressor discharge temps and more heat for the DSH to harvest.

I'm in Florida - long cooling season during which my DSH tank runs about 105, more or less. One cold day last winter the system ran 20 hours and the buffer reached 123 F.

I am glad it worked out for you. I keep saying it, DSH without a buffer tank does not make much sense. Again, the problem is that not many people monitor performance. Nor do they realize how much of their energy costs the domestic hot water is. If you do the math, a buffer tank pays for itself relatively quickly. Hello manufactures, when do we get heatpumps with a dedicated hot water circuit to condition a single tank via a heat exchanger in the tank, like the Europeans do it. We would save the whole buffer tank issue, save installation cost, and have all year domestic hot water made with the efficiency of the heatpump. Although there might be applications where the DSH is still a good idea (southern climate).

Posted By docjenser on 03 Nov 2010 12:24 PM
If you do the math, a buffer tank pays for itself relatively quickly.

It depends on lots of factors.  Need to look at individual situations including hot water usage, EWT, sensor arrangement, programming, tank chosen, size chosen, etc.  Can we agree to disagree on this?

Of it depends on many factors. In my home (5 people), a one tank scenario contributed to less than 5 % of hot water BTUs. Then I switched to a buffer tank arrangement, and got about 60% DHW out of it. Now I have a buffer tank and a second tank conditioned by the heatpump, for a total of 100%. So setup factors and design is important. It is n=1, so by no means data. Just an observation.

Posted By geome on 03 Nov 2010 01:01 PM

Posted By docjenser on 03 Nov 2010 12:24 PM
If you do the math, a buffer tank pays for itself relatively quickly.

It depends on lots of factors.  Need to look at individual situations including hot water usage, EWT, sensor arrangement, programming, tank chosen, size chosen, etc.  Can we agree to disagree on this?

I think we can agree that there are remote circumstances where not having a DSH wont cost you money, but many cases where it can.
 I think it is bad advice regardless of the odd occasion a unit performs with out and it is inexpensive to install with a new system. I also think we will see it dissappear from manufacturers' instructions soon (as a viable installation).
j

I continue to be surprised how much difference equipment cycling makes. I have a customer doing one longer cycle per hour, with the buffer tank up to 145 degrees, and one with 3 cycles per hour, buffer tank around 90 degrees. Both 2 person households, same equipment size, same total run time.