Hello everyone.  I am new to the forum and to geothermal.  We have just decided to have it installed.  It is a four ton GeoComfort Serenity with a desuperheater.  The question I have is what size of buffer tank would be better.  I have an 85 gallon Marathon for the powered tank and need to buy a new tank for the buffer.  I am thinking of getting another Marathon in the 50 or 85 gallon size.  The 85 gallon appeals to because I would have an extra set of elements should the powered tank need one.  I was unsure if too big of buffer tank would not get warm enough because of the large volume.  I am located in Iowa and have hot humid summers and very cold winters.  My wife and I both work during the day and both children are in school, so most of our hot water usage in morning and evenings.  If anyone has any input on what size tank would work the best I would appreciate it.

Have you contacted Marathon to see if the part numbers are different for a 50 gallon vs an 85 gallon tank? Elements for both size water heaters can be had in 3000, 3800, and 4500 watts. The people at Marathon have always been extremely helpful when I have called with other questions.

for what you are describing, 50 gallons as a buffer should be plenty.

My advice has been and continues to be, until some bright spark persuades me otherwise, is as follows:

Size the main finishing tank for the expected worst hour of hot water consumption; err a bit on the side of caution.

If feasible (space and budget) size the buffer tank for a typical day's hot water use. My thinking here is to account for the fact that the geo desuper produces warm water slowly over many hours, and times of peak production (late night in winter, late afternoon in summer don't generally coincide well with times of peak use.

I obviously don't know your family's schedules and habits, but my guess is that the 85 Marathon might be a bit oversized as a primary tank but might be just right as the finishing tank. Conversely a 50-65 gal tank might work as a primary tank.

Given Marathons' very low standby losses, I can't see how you's go wrong, space and budget permitting, with a 2nd 85.

I checked the Marathon web site and found that the upper elements are the same for the 50 and 85 gallon heaters, but the lower one is different.  It looks like the thermostats are the same for both sizes.  For the price difference between a 50 and 85 gallon size I could buy a set of elements to have on hand in case one went out.  I went with an 85 gallon size for my powered tank because we have a 75 gallon whirpool tub in the master bath and my wife likes to use it.  My biggest concern was if I used an 85 gallon tank for a buffer tank that I would just get luke warm water out of it because of the volume of water the desuperheater needs to heat.  I know 50 gallons will heat alot faster.  I just don't know what to expect out of the desuperheater.  I have never owned one before and want to make the best choice for my buffer tank.

Posted By Jeremy on 26 Aug 2010 12:13 AM
My biggest concern was if I used an 85 gallon tank for a buffer tank that
I would just get luke warm water out of it because of the volume of water
the desuperheater needs to heat.  I know 50 gallons will heat alot faster.

Suggest you re-think it in terms of BTUs, rather than temperature.

So what your saying is there will be the same amount of BTUs in a 50 or an 85 gallon tank.  The 50 will have a higher temperature because it is a smaller tank.  Is there a right or wrong answer on sizing the buffer tank if you get the same BTUs regardless?  At what point is the tank to large?  I can see the disadvantage of the tank being to small if the desuperheater will only heat to a certain temperature.  Once the tank is up to temperature it wouldn't heat any more.   Is a 50 gallon tank large enough that I would take full advantage of the desuperheaters output when the geo is running full capacity on a cold winters night?

There is a balance point. If the tank is too large, it will not get up to temperature especially in the summer and shoulder season. If too small, you run the risk that the heatpmp heats it up too fast and then shuts off, so you do not transfer any more BTUs into the tank but you are running cold too quick again after hot water use. For what you describe, 50 gallon buffer sounds about right. There is not much data out there, just experience. The amount of BTUs is limited by the the desuperheating capacity and by the running time.

Posted By Looby on 26 Aug 2010 09:35 AM
Suggest you re-think it in terms of BTUs, rather than temperature.

I concur and would add that the 85 gallon will let you move more BTUs

Estimating your hot water use may provide valuable information. Doing this helped us with our decision. You may have access to specs on dishwasher and washing machine water use. Our dishwasher is 100% hot. Figure a percentage of hot vs cold on the washing machine. Length of showers and manual dishes x estimated hot water percentage x 2.5 gallons/min (or whatever the flow rate is.) You get the idea.

Thanks for everyones input.  I ordered the 50 gallon Marathon today.  The drillers are supposed to come on monday to do my wells.  I am excited to get this all installed.