Hey, I was wondering what kind of temps are people getting in their DHW Buffer tanks?

We have a newly installed 4 ton CM Tranquility 27 with the DHW desuperheater option.  It's a closed loop system, forced air ducts.  Ground Loop EWT is between 60 - 70 so far this summer.  800 foot total on my vertical loops.  I am located in South East Pa.  System works very nicely.

For the DHW Buffer tank, I have a used 50 gallon hot water heater connected to the CM unit with nicely insulated lines, all hooked up per the CM manual.  The buffer then feeds into my new 50 gal. GE Hybrid hot water heater.  The CM unit is set to stop making DHW when it reaches 120 degrees.

What I am seeing:  The buffer tank can get up to about 100 degrees max (during long heat wave), but usually only gets to about 80-90 most days.  I measured the temp with a thermometer wrapped in insulation at outlet while DH water is flowing.  We have 2 adults and 2 small kids in the house, so DHW consuption is not that high.

My main question is... Is this typical for a new R410 unit?  Should I start troubleshooting to see if something is not working 100%?  Maybe it was wishful thinking to think I was going to get my buffer tank fully heated to 120 most days, but I wanted to see what other users were seeing as far as buffer tank temps.

Thanks for any input you have.

I don't know if your buffer temps are is typical, but stagnation at 80-100F is prime-temps for growing legionella. A buffer that only had the potable water in a low-volume heat exchanger would be preferable, to guarantee 100% of the potable water at those temps was purged on every significant DHW draw. (The Everhot EA series, ErgoMax, and Turbomax water heater/buffers used with heating system boilers are built that way, but they're several times the price of an electric HW heater.)

The 120F of your main HW heater is the bare minimum for keeping legionella colonies from growing, but isn't nearly hot enough to kill it, even over very long periods. Large tanks of tepid water present the largest hazard- keeping it either under 80F or over 120F and you're usually fine.

I have the new GE Hybrid water heater set heat to 125 deg. F after it leaves the buffer and prior to going to the house. Is this enough to kill legionella that may have formed in the buffer? Should I up the temp to 130 or higher? I do not want to change out my buffer if I can help it. Can I check for the Legionella hazard in some way to be sure?

I still am looking for what temp others users are getting for in their dedicated DHW buffer tanks.

Thanks for any and all input.

It would stop growing at around 130, but you would need to go above 160 to kill the existing legionella if you are growing some in preheater. quarterly chlorine shock is a good idea.
temps very depending on equipment and usage.....85 to 120*............Eric

Some of the WEL systems show the pre-heat tank temperature and your experience appears to be typical. You might even be better off with a tank in a hot attic. But I wouldn't worry about it - your GE heater is fairly efficient. http://www.welserver.com/

I once had a too low water heater temperature. I didn't notice Legionella but the hydrogen sulphide producing bacteria were nasty.

EWT to DSH along with LWT would tell us more.
Joe

I did not get temps to/from buffer tank, but I can feel with my hand that it is cooler going in and slightly warmer coming out of the desuperheater connections on the CM unit. I figured from that, that it is heating and circulation the water. I would guess that is rises the water temp about 5 - 10 degrees just by feel.

It just does not seem to be providing as much hot water as was promised. I was thinking it would get the buffer up to 120 during these heat waves (long run hours) and the highest has only been about 100. Maybe this is all I can get with R410 for my household consumption??

I am trying to get some input from others as to what they are experiencing in their dedicated buffer tanks to see if I am in the ballpark.

Thanks for any and all input.

Are you checking the temperature after a long period of geothermal/DSH run time AND a long period of no hot water use?

I just finished posting this elsewhere: You may want to check temps after a long pause in hot water usage, but after the system has been running a lot. Maybe late afternoon after work, or before bed on a night without a lot of hot water usage. Winter time is easier - first thing in the morning before showers.

It takes time for the DSH to do it's thing. Even heating the water to 100 is much less energy needed to heat it the rest of the way to 130 compared to heating 50f water to 130, so you're saving money. Also, every time hot water is used, cold water enters the buffer. With some mixing of the water, this can cool the whole tank (and your temperature readings.)

Where on the buffer tank are you taking temperature readings (from water high or low on the tank?) SE PA has been HOT, so I assume run time is not an issue. Is the buffer tank an electric or gas water heater?

We have an R410 system. As long as our DSH-out is slightly hotter than the DSH-in, I'm happy. Just wait until winter. Talk about even longer run times and more DSH hot water production!

Max I see is about 115 in evening on a hot day. I saw 123 once last winter - compressor amps and discharge temps are higher for my system in winter than summer. I expect this effect may be even more pronounced in the case of northern systems with cooler EWT.


Some surfing suggests that most of the danger from legionella arises from situations where warm water is mixed with air over an extended period, such as in fountains or commercial wet cooling towers.

Chris, your EWT is enabling efficient (ie, minimal heat) operation. I'd be happy with that.

Chris!
Where in SE Pa? I'm just south of Doylestown, Bucks County.

I lived near Lansdale for 7 years - couldn't stand the roads in that area

Seems like there is a propensity to size heat pumps a little large in PA which can decrease DSH output. I notice you have a 4 ton unit, do you know whay your man J load was?
j

http://welserver.com/WEL0336/

Buffer tank is between 70-90 degrees depending on the run time in cooling mode, and up to 125 in heating mode.

Climatemaster has a temperature switch in their units, the circulation pump for the desuperheater does not even turn on until the compressor discharge temperature is above 125 degrees.

Hydron and Waterfurnace do not have the switch (to my knowledge) so they preheat the tank no matter what the discharge temp is, which usually depends on the loop temp.

Unless something has changed since I was a CM dealer, the DSH runs whenever compressor is on unless return water temp reaches 125.
J

Same thing with WF, except that I believe the cut out temp is 135*F

Maybe he is referring to the fact that COP drops as the water gets hotter.

My guess is that it'll work fine with any temperature of inlet water. COP increases with decreasing water temp as Jonr wrote, however the figure of merit for water heaters is EF (Energy Factor) gauged against a common set of conditions of inlet water temp, draw volumes and frequency.

A properly installed DSH offsets such a sizeable portion of hot water generation cost that return on investment in an HPWH gets much longer. I expect HPWH prices to drop considerably in the next 2-3 years as competition builds up.

"R-410 systems inherently have a lower hot gas temperature than R-22 systems because the equipment is more efficient (i.e. less waste heat is available). It is possible that energy could be transferred from the water heater to the hot gas line instead of from the hot gas line to the water heater during certain times of the year. To prevent this from occuring, a temperature switch will deactivate the pump at those conditions that typically occur in the cooling mode with entering water temperatures of less than 50°F [10°C]."
CM Tranquility 27 install manual

CM has made that change since going to R-410.

There is a temperature switch on the exhaust line from the compressor which deactivates the DSH pump if exhaust temp going into the DSH is below 125*F, which usually reflects 50*F EWT. This is important in a single tank configuration without a buffer tank since it would steal heat from the tank and transfer this into the ground.
Not an issue with a pre-heat buffer tank configuration.
Waterfurnace and Hydron do not seem to have this switch, they are running continuously, at least in the ones I had installed. They depend on a 2 tank configuration, at least in northern climate. Hope this clarifies it.