gethermal buffer tank/desuperheater buffer tank

Discussion in 'Radiant Heating and Cooling' started by scott mills, May 22, 2013.

  1. scott mills

    scott mills New Member

    It appears that I need to buy another buffer tank. I have a 5 ton climatemaster water to water ground source heat pump. I have purchased a 55 gal Marathon buffer tank to be used for my radiant in floor heat. I have also purchased a 85 gal Marathon water heater. After reading about desuperheaters, it appears that I need to buy another buffer tank. I live in Northern Illinois. We ocassionally see lows below -20 and highs greater than 100. So, quite a broad range of temps.

    What do the experts recomend for a buffer tank for my domestic hot water? Both kids are in college, so mostly hot water for 2 people. Do, I need an 85 gallon buffer tank and a 55 gallon hot water heater, or a 55 gallon buffer tank and a 85 gallon water heater. Or, both the buffer tank and the finishing tank at 85 gallons?

  2. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    My recommendation is for a 50 gal tank, the 85F gallon tank you don't get up to temperature quick enough to contribute water warm enough. That is for the buffer tank.
  3. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader


    You type as a student, which is a very good thing.

    You thrust for knowledge is a good thing. We have that here. Most of the posters here make fun of me some times, but I spent New Years Eve fixing a geo loop in NJ by phone.

    Restate what you have and how it is piped.

    Last edited: May 23, 2013
  4. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    I re-read your post on the DWH. Doc thinks 50 for the pre-heat buffer, I agree. The best way to make geo, water 2 water or water 2 air, save you money on energy costs, is to keep the delta T lift as small as possible.

    So a 50 will take less lift than an 80.

    Last edited: May 23, 2013
  5. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    I guess the angle I am coming from is as follows using the following ballpark numbers:
    Lets say you have a 4 ton system in the winter, a DSH makes roughly 10%= 4000 BTU/h in second, and 3000 BTU/h in first stage. Assuming you have 24 hours 1st stage run time, you get 72.000 BTUs out of it in 24 hours.

    It takes 8.33 BTUs to heat 1 gallon of water by one degree, or 416.5 BTUs for every degree in the 50 gallon tank. Now assuming 55 F ground temperature you need about 27,000 BTUs to bring up 50 gallons from 55F to 120F or 46,000 BTU/h for 85 gallons.
    Now a 4 head family needs about 70 gallons throughout a day, mostly in the morning and evening, but also during the day.
    What is key for overall money and energy saving is how much water can you feed to the second tank at temperatures close or slightly above the second tank's setpoint. The higher the temperature, the lesser the energy the second tank has to use (usually electricity).
    A 50 gallon tank will recover much faster and provide hotter water than an 85 gallon tank, although not as much. However, as eluded before, you only need 10-20 gallons at a certain time, and then you want to recover quicker to a higher temperature again. Plus you want that water to be as cold as possible for higher efficiency. A 50 gallon tank will do this much better than an 85F gallon tank. We tested every scenario and monitored it (from 30 to 160 gallon buffer tanks), and did see 50 gallons for everything from 3 to 6 ton heat pumps the best fit.
    Last edited: May 24, 2013
    HVAC Technician and juergen like this.
  6. scott mills

    scott mills New Member

    I have an existing gas water heater that will be removed. So, everything will be a clean slate. From what I have read on this website, the smaller buffer tank is the way to go for domestic hot water (min delta T). If I had it to do again, I probably shouldn't have bought the 85 gal Marathon water heater (finishing). But, it is what it is. Or, I can sell it on Craigslist and buy a smaller one.

    I will pipe the buffer and finishing tank as shown many times on this website. I printed it out. I think it was Becker, but would have to check.
    Thanks All
  7. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader


    I am not sure craig's list is the way to go with the 85. I will give you 50% of what you paid but you need to move it to Columbus. I will pay less if I need to come get it.

    I might look at re piping what you have. Flow is imperative to the success operation of a hydronic system.

    If you want a crash course, go look at Those guys are wet heads. They get moving BTUH with water. They have a very readable and easy to understand series of essays called Idnonics. It is a free down load and can be had as four color glossies. Most of the text is by Siggy, who wrote Modern Hydronics.

    ohio likes this.
  8. scott mills

    scott mills New Member

    I tore the gas boiler, mixing valve, and all the pumps out. I am adding the spac pak chiller and I wanted to start over. Instead of trying to retrofit what was there. The mixing valve and the location just didn't fit all the equipment that I want to connect to the buffer tank. It is all Wirsbo brass manifolds, pipe etc.

    I will study the site you recommended. As my one professor said, "The more you learn, the more you realize how much you really don't know." Good quote.
  9. engineer

    engineer Well-Known Member Industry Professional Forum Leader

    DJ and I have gone a round or two about buffer tank sizing, and I think we worked out that his tendency toward smaller buffers works better up north and my tendency toward larger ones makes sense. IL qualifies as "north" since frozen water appears often and lingers.

    At any rate, I advise keeping the 85 Marathon as your finishing tank. Sure, it is oversized, but standby losses from those are a pittance, and it should last decades. You could somewhat reduce the already small standby loss by setting the upper element at 120*F and the lower at 100*F or so...may need to experiment with that.

    Consider using the gas fired tank you are considering scrapping as the desuper buffer, whatever its size. Its supreme advantage is that you already own it, free of charge. You'll want to block / insulate the center flue outlet and gas burner area.
  10. scott mills

    scott mills New Member

    I have to agree with everyones comments. I think the smaller buffer tank logic makes sense. I guess I have the choice of using the 85 gal Marathon as a buffer tank for my radiant in floor heating or as a finishing tank for domestic hot water. Either way, I will buy another Marathon for the additional buffer I need. I have a use for the gas water heater in my shop.
    Thanks ````````````````````````````````````````````````````````````````````````````
  11. Howard Ek

    Howard Ek Member

    I have installed ~20 geothermal w-to-w units that heat both my domestic hot water and the radiant floor. PM me if you want the scoop - I do it with one tank!
  12. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Why don't you show everyone here how you do the radiant floors and the domestic hot water with one single tank. I am curious too.
  13. OH_Bueller

    OH_Bueller New Member

    I had a 4-ton 5 series installed last year with a single 85gal marathon. As I have seen in several threads here I had the predictable problem of reduced dhw temps in summer (cooling by the dsh). Through some experimentation I have observed that with the marathon powered off it can be heated to about 90 using the desuperheater in summer, and easily to a very hot temperature in winter when the rest of my family was recently out of town and I turned off the Marathon. I have decided it is worth it to install a buffer tank. I see DocJenser recommends a small (50gal) buffer in Buffalo and Engineer recommends a larger buffer in Florida. I am midway between Cincinnati and Dayton in southwest Ohio. Is the consensus that I should get a 50 gal tank to use as the buffer and keep the marathon as the finishing tank? I should also mention this is for a family of four, with heaviest water draw during morning showers, etc.

    Sorry to post this as a reply in the radiant forum, but I am posting from mobile and this was the first relevant thread I found today although I had researched these threads several times since summer.

    Thank you to all the pros on here for sharing your knowledge and experience.
  14. Mark Custis

    Mark Custis Not soon. Industry Professional Forum Leader

    Hey Bueller:


    I am very concerned with DHW systems that do not get hot enough to kill legionella. I would rather run the finishing tank at 140* or what ever and then use a dumb mixing valve to end up with a temp safe for hot water dummies.

    To get what you can from the desuperheater you need that dedicated preheat buffering tank. I am coming around to Doc's 50 gallon mind set.

    Go get a cheap electric tank for the buffer and do not wire it. Finish the pre-warmed water at a high enough temp to keep all healthy. Then add a dumb mixer if you think small children will wash their hands using only the hot water tap at the sink.

    If you have issues with materials let me know I use several supply houses near you when we are on the road. Big Blue and orange stuff is not what the supply house provides. The materials look the same but are not.

  15. mngeotherm1

    mngeotherm1 New Member

    I have a question on what to use as a buffer tank. We currently do not have one and are considering adding one... We have a 105 gal Marathon tank as our main tank and I would like to get another plastic/lifetime tank as the buffer tank. As far as I understand the buffer tank does not need to be powered. Is it possible to save money by simply using some type of plastic tank rated for higher temperatures? Any way to save money and not buy a $750 50 gal Marathon tank as a buffer tank, it seems a waste to pay for the elements and controls if they are not being used.
  16. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    In our case, due to lesser volume at the distributor level, a non powered tank from the same brand is $20 more expensive that the powered one with all the elements and controls. But yes, you can use a non powered tank as well.
  17. geoxne

    geoxne Active Member Forum Leader

    Think spare parts for your powered tank.
  18. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    I would not buy a Marathon to use as a buffer tank.
  19. mngeotherm1

    mngeotherm1 New Member

    Hi AMI, why not the marathon? If I am not using the elements how long can I expect a normal HWH to last?
  20. mngeotherm1

    mngeotherm1 New Member

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