2 weeks ago I had a geothermal HVAC with desuperheater installed in my home replacing an older electric "heat pump". The desuperheater is set up using a one tank design, just the old 50 gallon water heater. Hot water is good to go UNTIL the system kicks in, then you're getting cold water. The inlet valve to the desuperheater is piped from the bottom of the water heater, outlet to the top. 

   Is this a common thing? I am thinking a buffer tank would help out with this but am wondering if that should be a necessity? The only thing I can figure is that the DSH is sucking cold water out of the bottom (since I'm 5 minutes into the shower) and warming it a little and dumping it into the hotter water at the top, creating a current in the water heater and thus losing my hot water as it all stirs up.

   Any other single tank systems out there not having this problem? Any suggestions? Is a buffer tank the only solution here or maybe there's a plumbing disorder I need to work out?

   Thanks!

   William

My DSH was hooked up the same way yours was, with the water coming OUT the bottom of a 40 gallon Bradford White water heater and going back into the top. The AirTap (heat pump water heater also on the Bradford) ran almost constantly to keep the water at temperature, and the DSH *IN* pipe was hotter than the DSH *OUT* pipe over by the Geothermal unit! I had plenty of hot water, but I'm pretty sure it was the AirTap that was doing all the work (See the thread "My DSH IN is hotter than my OUT" by me).

A few weeks ago, a different contractor came by, added a 40 gallon buffer tank, and reversed the pipes....Water comes OUT the top of the buffer tank, into the DSH, and then back into the bottom of the buffer tank. Another pipe pulls water from the top (hot out) of the buffer and into the top (cold in) of the "finishing tank" with the AirTap unit.

I can't say positively that it was the plumbing switch or the addition of the buffer tank, but now I have great hot water and the AirTap almost never runs, unless the Geo has been off for long periods of time (nice weather, etc.) Also, the DSH OUT is now hotter than the DSH IN, usually a good 10 degree difference.

covered here many times, with a modern 2 stage R410 heat pump, DSH tends to rob heat from your HWtank (more than it adds).
while I can't swear it's causing the problem you describe, you need to fix it anyway. start by adding a buffer tank.
Since Water Furnace is a frequent contributor, perhaps they can tell us why they (and other manufacturers) continue to suggest an installation that does not work (bufferless DSH). Or did they stop recommending this?
Joe

The order I would do things in to see what helps is:

1) during shoulder months increase the bottom water heater thermostat closer to the final water temperature you desire. Mixing can occur, especially during times of low geothermal use when the system doesn't run long enough to substantially contribute to hot water production. A buffer tank would help with the mixing problem, but during periods of low geothermal use, a buffer tank won't make more hot water than not having one. You need to remember to reduce the the lower thermostat setting again when the system runs more (a reminder on the calendar can help). A buffer tank is certainly requires less manual intervention.

2) change DSH plumbing to see if this helps.

3) get a buffer tank.

Also, make sure the DSH Out is slightly hotter than the DSH In when the system runs. The lines could also be air locked.

My order of priority would be:
1) get a buffer tank
2) see if you need to do more than #1
We have testomonials that unbuffered DSH systems can actually cost you money by taking heat from the tank.
j

Posted By joe.ami on 25 Oct 2010 07:54 AM
covered here many times, with a modern 2 stage R410 heat pump, DSH tends to rob heat from your HWtank (more than it adds).
while I can't swear it's causing the problem you describe, you need to fix it anyway. start by adding a buffer tank.
Since Water Furnace is a frequent contributor, perhaps they can tell us why they (and other manufacturers) continue to suggest an installation that does not work (bufferless DSH). Or did they stop recommending this?
Joe

Maybe circumstances always happen to be right when I observe our system, but the DSH's in our WF units (packaged unit and split) have never failed to run when our geothermal systems run, and the DSH Out never fails to be hotter than the DSH In line (including cooling mode.)  Our Envision system was installed in April 2009.  Maybe WF made an improvement?  If so, I wouldn't blame them for not sharing how they accomplished this.  Has anyone here had an Envision system installed since April 2009 where they have observed the DSH Out colder than the DSH In in a single tank setup?  I recall (hopefully accurately) that Bill had a single tank setup that had the problem you described, but I believe his unit was installed before ours.

I did not deal with waterfurnace, but with climatemaster they initially addressed the issue by not turning the circulation pump on unless the exhaust temp was 125 degrees. (end of 2008/ begin of 2009). Which meant with open systems (50 degree EWT here in the Northeast) the desuperheater never turned on in A/C mode. Not a good solution. There is now adjustments for water temp out and the ability to sense gas temperature and Leaving water temperature (DSH). Piping procedures are the same. That way now the DSH pump is turned on when the delta between the exhaust temp and the LWT DSH the is not more than 30 degrees (I believe that was the number). That way the tank gets at least preheatet. But it also means that with a one tank configuration, the DSH will not turn on in A/C mode, because you do not reach the required 30 degree delta (Tank is already too warm).

Solution: Buffer tank with colder water and no other heatsource

And yes, Waterfurnace, please jump in here and explain more, it is not a trade secret!

Thanks Doc. I recall that WF also has some logic built into the DSH of their Envision system. Now that I think of it, I believe I posted an excerpt from the WF manual in another thread on this. Maybe we're just fortunate that our DSH Out is always hotter than the DSH In (even in cooling mode) and that it always runs when the system runs. I'm not complaining. :-)

Are you more south, when the loop temperature in A/C mode is much higher? No issue in the heating mode, but in A/C mode, the source EWT determines the compressor discharge temperature and thus the output temperature of the DSH. The issue was particular prominent in colder climate, with not much A/C running time. I refuse to install DSHs with one tank configuration. They just don't perform well, at least not in our climate.

We're more south than Buffalo, thank goodness! :-) Close to Roanoke, VA. Our EWT near the end of the heat wave was in the high 70's (big loop for us too.)

I appreciate all the replies! For the time being I just upped the temp on the water heater. Temps are moderate right now so it hardly kicks in to do anything but I'll be feeling the inlet and outlet piping for warmth soon enough. Right now my plan is to go ahead and do the buffer tank as it just makes sense and if the warmer feeling piping is going in, well, reverse it. Preciate all help and will post feedback once I get it done!

Thanks!

William

My HP installer installed a concentric valve to the bottom of our 80 gallon electric water heater. DSH inlet and outlet are piped to the bottom of the tank. No cold water problems. Typically warms the water to 105/110 degrees overnight. Might have better results if I didn't have ~70 feet between my DSH and water heater. :-(

For a WaterFurnace Envision unit, the DSH pump is controlled by the combination of a refrigerant line temp sensor (that's commonly refered to as the Freeze sensor) and a DSH In water line temp sensor.  The DSH pump will run as long as the DSH In water temp is no more than 35° hotter than the refrigerant line.

The refrigerant line the Freeze sensor is on, in Cooling mode, is the liquid line.  in Heating mode, the line becomes the section of refrigerant from the TXV valve to the main coax heat exchanger.

You can see where the Freeze sensor is on the diagram of my energy monitoring system, at http://www.welserver.com/WEL0043/ .  The DSH In sensor is just behind the cabinet on the DSH In's copper pipe.  Both sensors are connected to the central processing unit inside the cabinet.

My DSH is direct connected to a gas-fired HWH, per the hookup instructions supplied by WF.

I have an instrumentation off/off sensor on my DSH pump, separate from my instrumentation on/off sensor for the unit, such that I can monitor if indeed my HWH supplied water to the DSH is too hot for useful 'desuperheating.'  I.e., how much is the Envision's processing locking out the DSH pump while the unit is overall running.

You can see (from the same reference above), that my unit has run 57 hours so far this month.  But my DSH pump has 56 hours.  So while it's not a lot of DSH pump cut out, indeed my direct-connected gas-fired HWH is occasionally supplying water to the Envision's DSH circuit that's too hot.

Another question people have when considering direct-connecting their gas-fired HWH is will it do any good.  The answer is no, or at least not in whatever the 'shoulder months' are.  Here are the numbers.

For this particular month, the entire month, I've transfered a whopping net total of 2 KBTU from the unit's DSH to the HWH.  That's it.  You can see these 2 numbers at the same reference above, left side.  Basically, there's a reverse heat transfer from the DSH to the refrigerant, making the Envision slightly less efficient (lower EER).  During the cycle, most of the time if the water from the gas-fired direct connected HWH isn't too hot, finally, this reverses after some amount of DSH flow, as cooler water from tank starts becoming available.  But this water from the HWH never gets cool enough to really make the DSH useful.

If the HWH water is too hot, then the Envision simply prevents the DSH pump from running.  So overall the whole DSH process doesn't get too inefficient.

The bottom line is, for a shoulder month like it is for me here in Dallas right now, where I'm still running nothing but in cooling mode, the DSH just doesn't provide any material benefit when connected to a direct-connected gas-fired HWH, even with 57 hours of total run time so far in the month.

Hope this is helpful.

Best regards,

Bill

Posted By jpespisa on 26 Oct 2010 04:10 PM
My HP installer installed a concentric valve to the bottom of our 80 gallon electric water heater. DSH inlet and outlet are piped to the bottom of the tank. No cold water problems. Typically warms the water to 105/110 degrees overnight. Might have better results if I didn't have ~70 feet between my DSH and water heater. :-(

Heat Pump manufacturers have gone away from the concentric fittings because they tend to clog with calcium build-up. 50 ft. (25 ft. each way) max length is recommended for desuperheater piping.

Bergy

I gotta wonder if the OP's return to the top of the single tank is via the hot water outlet rather than the cold water inlet. The CW inlet incorporates a dip tube which conveys the inlet water down to near the bottom of the tank so as not to disturb the stratification within the tank.

I would flat out refuse to install a DSH on a single tank system if asked. The client is better off foregoing the cost and complexity of the DSH.

I guess my situation might be a special case. I'm not worried about calcium as I have a water softener installed. Reading the CM manual again I noticed the only configuration where a single tank design is ok is with a dual element electric water heater. Which is the type I have installed.

Electric water heaters are recommended. If a gas, propane, or oil water heater is used, a second preheat tank must be installed (Figure 15). If the electric water heater has only a single center element, the dual tank system is recommended to insure a usable entering water temperature for the HWG.


Also it looks like CM max length limit is 50'. I'm still over the spec but it heats my water tank. I'm planning to reduce the length in the near future. I'm losing about 5° between the HP and HWH.

Keep water piping lengths at a minimum. DO NOT use a one way length greater than 50 ft.


http://welserver.com/WEL0167/

"I'm losing about 5° between the HP and HWH"

Then you should also loose another 5 degrees between the HWH and the heatpump, when the water flows back to the HP. Keep in mind the delta T the desuperheater puts out should be between 5-10, depending on flow. Are you sure you are not stealing more heat from your tank than you put in?

Posted By jpespisa on 26 Oct 2010 10:57 PM
I guess my situation might be a special case. I'm not worried about calcium as I have a water softener installed. Reading the CM manual again I noticed the only configuration where a single tank design is ok is with a dual element electric water heater. Which is the type I have installed.

Electric water heaters are recommended. If a gas, propane, or oil water heater is used, a second preheat tank must be installed (Figure 15). If the electric water heater has only a single center element, the dual tank system is recommended to insure a usable entering water temperature for the HWG.


Also it looks like CM max length limit is 50'. I'm still over the spec but it heats my water tank. I'm planning to reduce the length in the near future. I'm losing about 5° between the HP and HWH.

Keep water piping lengths at a minimum. DO NOT use a one way length greater than 50 ft.


http://welserver.com/WEL0167/

We know what the books say and as a Climatemaster dealer, I installed a system as you describe. My customer who had an independantly metered electric tank reported that his hot water cost went up. After buffer tank was installed customers hot water cost was 1/2 the pre-DSH numbers.

If you have a way to measure hot water cost with and without water circulating through DSH that would be interesting.
The fact that the pump isn't rated for the length of pipe you describe may actually be helpful (by impeding loss to ground loops).

Joe

For some reason, manufacturers and some installers (including our installer that has a single electric tank setup in his house) don't agree that 2 tanks are always necessary. There are also instances here where homeowners are happy with their results with a single tank setup, and don't want a second tank for various reasons. There seem to be benefits to a 2 tank setup, but there are drawbacks too (previously discussed in depth.)

Heat lost to dsh can be subtle (a few hundred KW). For someone who just added a geo system and had an expected jump in electrical consumption it could easily go unnoticed.
I bring to the table before and after hot water kw consumption data. What is more valid? Can you or your installer document the same?
It'd be nice if WF chose to weigh in on this one, but I would expect the "......per installation instructions" hedge.

Hot water is no longer a must for tax credit, so I advise folks not to get DSH if a buffer is not desired. With our cheap electricity savings v cost is not always even $100/yr.

Further, I don't think anyone disputes that higher delta Ts make DSH more efficient, so if your goal is greatest efficiency why shoot your DSH in the foot while dragging down home heating efficiency at the same time (DSH diminishes heat pump heating capacity)?
j