Joe,

I don't think anyone recommends a gas fired water heater. My comments were in relation to a link posted earlier in the thread. The link lead to a thread where Bill (a0128958) posted a photo on 9/22/08 of a gas fired unit being used for the DSH.

Bergy

The "Bergy" was part of your attached quote. Looking back it does appear that I directed my comment at you.
My "footnote" was in support of your observation.
I think it is worth mentioning to folks again that direct hook to gas fired water heaters could allow someone to worm out of a warranty as it is against manufacturers' installation instructions.
J

Sorry about that Joe...

Now that I'm awake it does read differently!

Bergy

Thank You to everyone that replied to this thread!

I installed my buffer tank tonight, got done about an hour ago. What I have now
is a 60gal pre-heat tank and a 40gal hwh. The 60 gal used to be my hwh and had
an older 40gal hwh which I have installed as the hwh.

I have just run the system for about 20min and have about a 8 degree delta-t,
58 degrees in and 66 degrees out, not sure if this is what I'm supposed to have,
will try to monitor the system in the next few days and pay attention to the
temps in the shower.

Will let you now what I find.

Thanks

John

Desupers recover only about 10% of units total rating. Not sure where you are measuring the delta T, but it should take several hours or more to fully heat the buffer tank

Hi Engineer

Maybe I'm using the wrong terminology, what I was measuring was the entering and leaving water temp to the desuperheater conections on the furmace.

Thanks

John

8 is good.

For the sake of discussion assume no hot water use, the 8 delta first results in raising the water from 58-66, and then after the buffer tank water has completed a pass through the desuper it'll become 66-74, and so forth. As the temp of the buffer rises the 8 delta will drop off as the water / refrigerant temp differential drops.

Hope that makes sense

I am in the process of building a 3400 sq ft house in south jersey.I have a water furnace geo system with a desuperheater,it circulates thru a 85 gal marathon tank.my builder talked to the waterfurnace people and they told him that all i needed was the one tank .I originally was going to tie it into a tankless water heater which my builder and i dicussed at great length.He doesn't like tankless and suggested a Rheem 50 gal heatpump water heater if i insisted on a two tank system.This is the way i decided to go.Now i'm starting to second guess myself.Any thoughts?

Yes. See the "Justifying two electric hot water heaters" thread.

I have the same Marathon as you (if you get the 4500 watt element).  Get the installer to put in a 3/4" full port ball valve drain at the bottom (the internal opening is larger than the standard drain valve so larger debris will be able to pass through it when draining or flushing out a few gallons to keep it clean) and make sure the installer reads and follows the installation instructions since the Marathon can be damaged by improper installation.  There are installation differences between this and standard water heaters.

I believe you will like the Marathon.

A Marathon buffer tank upstream of a heat pump water heater will operate about as cheaply as any hot water system can - that's truly a Cadillac setup. Your return on investment could stretch into decades, and the Rheem HPWH hasn't been out long enough to be field proven.

I have a somewhat similar setup: My WF 038 DSH stores in an unpowered GE 80 gallon buffer. The buffer feeds a 2nd GE 80, recently depowered; that tank is now warmed by an externally plumbed Geyser HPWH. My hotwater costs have dropped to about $6 per month, and I get a fair bit of extra AC and Dehu in the mechanical room from the geyser.

Thanks for the input engineer,i had hoped i made a good decision,only time will tell!

Have another question.

When the heated buffer tank has been used up and the tank is back down to 58 degrees, would the demand for water heating lower the air side delta T?

I noticed this morning after eveyone had a shower that the LAT was down from about 91 degrees to 87 degrees. The delta T was down about 5 degrees from 24 to 19 taken about a week ago at night time.

I will check tonight again and see if things have changed once the buffer tank has heated up some.

The hotest I have seen the buffer to date is 102 degrees.

Thanks

John

Colder water of the preheat tank absorbs more heat going through the superheater, thus lowering the LAT. Another likely contributor to a lower LAT since last week is a lower EWT. A quick change in the weather here in SE Michigan has resulted in considerable run times and a noticeable drop in my system's EWT, and a small decline in LAT.

Regards,

Masoud

Yes, the LAT will drop a bit when the preheat tank is especially chilly. The effect is slight because the desuper heat exchanger is deliberately sized to grab only a small portion of the unit's overall heat transfer, on the order of 5-15% depending on conditions. It is called a desuper heatersince it is intended to skim off only the especially hot superheat portion of the system's heat output.

The good news is that a couple degree drop in LAT should only be a comfort issue in particularly cold weather, and the long daily run times that occur in cold weather should keep the preheat tank toasty, minimizing the LAT hit during cold weather. The system is self-correcting in that way.

Years ago I home-brewed an air source to geo conversion and along the way added a (way-too-big) coax for domestic hot water. Result was that if hot water was drawn the LAT would drop just about to ambient since the big coax nabbed all the unit's heat. Worked OK when I lived alone, not so good after Mrs Engineer moved in...rev 2.0 was quite promptly designed and deployed in the form of a throttling valve on the coax waterside.

Ok took some more temp measurements, all 1st stage.

EWT 43 LWT 40
EAT 67.5 LAT 88.5
DSH IN 96.1 DSH OUT 98.1

My compressor seems a little noisier than normal, might just be my imagination. I opened the cover, alot more noise with it off. The temperature of the compressor itself is too hot to touch, not sure if that is normal?

The Delta T on the loop seems low, was about 5 degrees in the beginning of the heating season down to 3.

The Delta T on the air side was about 25 down to 21.

In second stage the loop Delta T is 4 degrees. And air Delta T is 25.

Not sure if these drops are due to colder weather or I might have a problem.

Thanks

John

The lower the EWT, the less heat the system can extract, though those numbers look OK for now. What was your EWT when heating season began?

Compressors run hot, typically too hot to touch

Compressor amps and its discharge temperature are two of my favorites, but getting them puts you in close proximity to high pressures and lethal voltages - no place for the ill-informed or faint of heart.

Thanks Engineer

Good to know.

The beginning of the heating season the EWT temps were in the low 50's but dropped to the high 40's just a couple days later. Just filled a hot tub a few days ago and water was 48 from the well, so that should be what my deep down temps are at.

Thanks Again

John