I've taken some factory training classes where they indicate that desuperheaters should not be connected to gas water heaters due to the elevated water temps, marginal payback, and low eff. of the water heater.

Has anyone ran a desuperheater directly to a gas water heater?  I get asked this question every time we do a retro, and I need a more solid answer.

Thanks

I have my desuperheater connected to my A O Smith Power Shot gas hot water heater. I do get some benefit from it.

I have a customer with an independantly metered electric water heater who's bill went up with a DSH and no buffer tank. We're months away from a legitimate test sample, but I'll post it when we have real data. With all do respect to Dewayne and his experience, I'm convinced that an un buffered DSH fed tank suffers greater standby loss.
J

I posted a graph of my gas consumption over 3 years here: http://www.greenbuildingtalk.com/Forums/tabid/53/forumid/13/tpage/2/view/Topic/postid/42787/Default.aspx (about half way down the page). Gas consumption in winter is about 1/3 of the shoulder seasons. Tank is Rheem powervent and the DSH is connected through a coaxial fitting in the drain hole. During the heating season, not only does the gas consumption go down, but the water is hotter - this is because the DSH output temperature is higher than the thermostat setting on the gas tank. Heat pump is a 3.5 ton ClimateMaster R22 unit so your mileage will vary with R410a.

Many people on this forum said that my setup couldn't work, but I do have actual data to prove that it does work. In fact, the local gas company came to investigate, suspecting meter fraud, as my gas consumption declined so much in winter (works out at about Can$4 (four) per month during the coldest weather - total including all meter charges and taxes for January was Can$20 ($4 of this was the actual gas consumed).

Paul.

Does the Rheem Powervent not deliver heat to the very bottom of the tank? Coventional old-tech center chimney gas storage water heaters apply the flame to the very bottom of the tank. If the Rheem does not, that could allow a chunk of cold water to exist there and give the DSH something to work off of.

Another explanation is R22. That plus another clue - that your hot water temperature rises during heavy DSH seasons suggests that the DSH has much hotter compressor discharge gas to work with. Is your CM a single stage? I've seen / heard of single stage R22 units with hot gas temp close to 200 F. Lack of a TXV in older systems may give rise to more superheat, depending on charge, and that contributes to higher compressor discharge temp as well.

In low stage my 410a unit's hot gas discharge line is not much hotter than what I can hold onto with bare hands - maybe 140 or so.

I've never doubted your numbers and success with single tank DSH, just trying to understand why it flies in the face of experience of others - now I think I understand better.

The coaxial heat exchanger in the drain tap is definitely a few inches below the tank's burner and feels cold when the heat pump isn't running (spring/fall etc.). The GSHP contractor selected the tank presumably based on prior experience with this setup and, for us, it has worked very well.

The heatpump is single stage, but it definitely has a TXV and the DSH is limited to 125F - I believe the thermostat on the hot water tank is set at 50C (just over 120F). The water is hotter when the DSH is running (and we had pretty much continous heatpump running in January with the average temperature in the -13C range), though it's not in the 200F range!

Paul.

200F was in reference to the refrigerant gas discharge temp. Idea being greater waste heat available than on a 2 stage unit.

I am no expert on any of this - I am only a homeowner. I can only tell you how my system works.

I have a Rheem Powervent 75 gallon gas hot water heater with no buffer tank. The Rheem's thermostat is set to heat the water to 120 degrees. The Rheem unit is connected to the desuperheater of my Waterfurnace Envision.

Water from the Rheem water heater is drawn from the cold water pipe (which goes down into the center of the tank) and sent into the Waterfurnace desuperheater and then returned to the bottom of the hot water heater. The desuperheater raises the temperature of the water it receives by about 5 degrees when it returns it to the bottom of the Rheem hot water heater.

During the winter and summer (the times when the Waterfurnace is running often) the desuperheater easily keeps the water in the Rheem hot water heater above 120 degrees, thus eliminating the need for the hot water heater to heat up its water supply to maintain a 120 degree temperature (to satisfy the thermostat)

The only time the hot water heater has to heat its own water supply is during the times that hot water is being used up quickly (baths, showers, laundry) that is, when the demand for hot water exceeds what the desuperheater can provide. Once it has it up to 120 degrees, the water heater stops heating and the desuperheater continues its job.

Because the water from the desuperheater is heating the water being circulated through the hot water heater, the hot water heater does not have to come on to heat the water that would normally cool off if left standing in a static tank.

Since I've had my gas Rheem Powervent hot water heater hooked up to my Envision's desuperheater, my gas usage has gone done by about 8-12 ccf per month - less during the months when the furnace doesn't run as often.
Your mileage may vary.

The only time I have seen the desuperheater pump shut off was around a midnight when the outside temp was around -5 degrees F - and that was because the water in the hot water heater had reached 135 degrees.

btw. there are some differing theories as to whether the water should be taken off the top or the bottom of the hot water heater. - I believe Climatemaster draws it from the bottom - the opposite of Waterfurnace.
I believe Waterfurnace's method is better. Water is taken from the cold pipe of the tank (which extends down into the center of the tank and then it is returned to the bottom of the tank where the cooler water is (it settles because it is more dense). Hot water is drawn from the top of the tank as always. All this applies when the hot water heater's buner is not on of course.

Good data. I also like the mileage may vary comment. I think we are seeing cases where the system may work without a buffer tank.
If that's true, would it be fair to say that in most cases it will work with a buffer tank?
I'm thinking like a contractor again who can't afford to go break promises.
J