Hi all.... We live in western Oregon, are building a new home and are installing a Waterfurnace Envision Geothermal Heat Pump (NDH049 variable speed with desuperheater storage tank and intellistart features, 4000' of HDPE piping and 200' of 1 1/4" piping). We will have a 50 gal. Marathon, (MR50245), for our water heater, but have been given the option of upgrading the desuperheater to a Marathon as well. The standard desuperheater tank is .85 efficient with a 6 yr. tank warranty. The Marathon desuperheater tank is .94 efficient with a lifetime tank warranty. Upgrade cost is $583.00. Worth the additional cost?.....Thank you!

Probably not.

1) Desuperheater recovered heat is free in summer, cheap in winter, so it is harder to justify an expenditure to conserve it.

2) the efficiency rating you refer to is "Energy Factor" and is largely dependent on amount of insulation around the tank. For less than 1/10th of $583 you could upgrade the insulation around (and underneath - don't forget that) the 0.85 tank, bringing it up to 0.90 or higher.

The EF is arrived at by a complicated sequence of hot water draws and standby periods described in the DOE WHAM model.

3) In my experience, 6 year tanks routinely last longer than 6 years. I further believe, but do not have data to support, that unpowered tanks last longer than ones with powered heating elements - less localized thermal stress and slower rates of thermal expansion / contraction.

All that said, I did not, but sometimes wish I had, gone with Marathons for my setup (twin GE 80 gal 12 yr tanks). I like having the best available setup, but the price adder / payback is not justified.

Curt--
Thank you for your thoughtful and timely response. You answered my question perfectly. And without making me feel like a nincompoop for inquiring about saving $500.00 when spending tens of thousands!....(you, I think, grasp that it is precisely because we're spending so much on the system that we want to spend wisely)...
If I might pick your brain a little more......the desuperheater will be in our garage on an interior wall. When you talk about the insulation for it, are you referring to a blanket type wrap, or if not, what kind of insulation would you recommend?
Also, how can I tell if the superheater to be installed (Bradford White 50 gal.) is unpowered or powered with heating elements? Please forgive the ignorance.....
Finally, along the lines of upgrades, we're considering upgrading to either a Honeywell F300 Electronic Air Cleaner or their TrueClean Air Cleaner for +$950.00 and +$1,350.00 respectively. Thoughts?

THANK YOU for taking the time to educate me!

Posted By diane mcenany on 05 Aug 2011 07:30 PM
The Marathon desuperheater tank is .94 efficient with a lifetime tank warranty. Upgrade cost is $583.00. Worth the additional cost?.....Thank you!

Is the added efficiency worth the cost - perhaps not.  Is the lifetime tank warranty and potential for fewer leaks worth the cost down the road - that's a decision you will have to make yourself.  For us -  YES!

Blanket wrap

The desuperheater buffer tank will almost certainly be a conventional storage electric water heater complete with elements. That's our cheapest source for an insulated water storage tank. The elements should not be energized.

An unpowered buffer will simply not have conduit to it.
With few exceptions, electronic air cleaners require monthly maintenance and the performance falls very dramatically in as little as a week or two. Most folks aren't that diligent about an air cleaner.
We prefer the Aprilaire media air cleaners or the 5000 electronic. With annual maintenance requirements we feel they deliver greater benefit to our customers.

Price a 50 gallon electric water heater at the big box store. You might find you could buy 2 more buffer tanks before adding up to the price difference of the Marathon.

Joe

When considering any type of filtration equipment one must pay close attention to the static pressure drop across them.The total cfm will be reduced and the hp of the fan will increase .I prefer using electronic air cleaners anytime over a media filters in residential applications.Duct sizing is critical for proper performance of hvac sytem.

Posted By diane mcenany on 05 Aug 2011 07:30 PM
Hi all.... We live in western Oregon, are building a new home and are installing a Waterfurnace Envision Geothermal Heat Pump (NDH049 variable speed with desuperheater storage tank and intellistart features, 4000' of HDPE piping and 200' of 1 1/4" piping). We will have a 50 gal. Marathon, (MR50245), for our water heater, but have been given the option of upgrading the desuperheater to a Marathon as well. The standard desuperheater tank is .85 efficient with a 6 yr. tank warranty. The Marathon desuperheater tank is .94 efficient with a lifetime tank warranty. Upgrade cost is $583.00. Worth the additional cost?.....Thank you!

I recently installed a 5 ton Climatemaster, 50gallon cheap Rheem depowered electric buffer going into Rheem Powervent (combustion air intake and exhaust) gas water heater.  I too wish I had gone with a LARGER DSH tank.  In the summer, given the relatively low delta from inside to outside and shorter run times I see typical DSH buffer tank intake temps of 75-90 degrees (average close to 80-85dF) but in the winter with much longer run times I often see the buffer tank maxed out at 125dF, but with the relatively small buffer tank and very cold intake water temps from my well it quickly falls with any intensive hot water use.

A 80 gallon Marathon would have not been that much more expensive and would have allowed much more effective use of the desuperheater advantage, especially in the winter.  I would upgrade if I was you.

cschmelz,
Diane's installer would likely be using a 50 gallon marathon with a 50 gal. finish tank.
Your advice to go with a larger buffer is not the same.

Usage determines the size of hot water requirements and for lack of any contrary information, I agree with Curt that generally one would size the buffer the same as the finish tank (though mismatched can work fine).
If one's advice is an 80 gallon buffer tank, could that not be a big box store brand?....it does not have to be a marathon.

My strategy is actually to size the finish tank for the home's worst (highest use) HOUR of hot water use and the buffer for a full DAY's use of hot water. As a practical matter we supply 2 80s in nearly all cases or a single 80 feeding two or more electric tankless heaters. I might drop that to an 80 feeding a 50 or 65 in a small house.

I wonder, but have not yet run the numbers, whether in case of a small system, 2-ish tons, one might be better off with a smaller buffer...perhaps tis better to heat less water more.

A big box 80 runs from $500-$700 (6-12 year warranty) and, last I checked, a Marathon would be $1100-ish, a sizeable jump.

Trouble may lie ahead...My understanding is that as of 2015 there will be no more conventional storage resistance electric water heaters greater than 55 gallons for sale. We may have to gang a pair of 50s or seek alternate (probably higher-priced) sources of buffer tanks.

I have read that electric tanks and DSH do not always require a holding tank. Can the DSH simply supply a larger Marathon, say a 100+ gallon one?

Posted By lzerarc on 16 Aug 2011 11:35 AM
I have read that electric tanks and DSH do not always require a holding tank. Can the DSH simply supply a larger Marathon, say a 100+ gallon one?

Unfortuanately manufacturers continue to endorse the one tank configuration while the pros continue to offer hard evidence that it offers little (if any) benefit with R410 systems.
I personally had a customer with a seperately metered water heater whos bills went up with the addition of assistance from a dsh (per instructions). Introduction of buffer tank cut pre dsh bills in half.
j

I will not recommend, order, install, configure or in any other way be involved with a DSH that is not plumbed to a dedicated upstream buffer tank.

Why not?..because it simply does not work.