I am new to the forum, pardon if this has been covered but I could not find it.  We are in the planning stages of a new home in Central Oregon. The climate is quite cool most of the year, and dry.
We are planning a very well insulated, tight home with triple pane windows. We are planning on a GSHP with Humidifier, and estimates are we will need a 2 ton unit. (we will do energy modeling during design stage)

I am trying to decide on the hot water system, and thinking:
 
DSH with storage tank, and Marathon heater
DSH with Tank and Heat pump Water heater
Or eliminate the DSH?

The system will be used the vast majority of the time for heat (freezing nights are not unheard of 10 months of the year.)
Only two full time residents, with most of the hot water use first thing in the morning, will the DSH build much heat in the tank after running all night?
Is the HPWH a better plan even though
the Water heater will be in conditioned space so any heat will have to be replaced by the GSHP. and the room would need to be insulated for noise and temp.


Is the marathon with superior storage capabilities (less heat loss) a better choice?
and of course there is a cost to all these options.

I await your advice.....

A 50 gallon HPWH isn't much more $ than a Marathon 50 gallon conventional heater. HPWH can be switched to mixed or fully resistive operation if overcooling space is a problem.

New GE HPWH fairly quiet

A household of two may only use $200-$300 in electricity for heating water resistively, depending on electricity unit cost. DSH would offset maybe half of that, so looking at maybe a 10 year payback. (cost of DSH option + install + buffer tank)

With DSH in a heating dominated climate, you get maybe half your heat at the underlying COP of the geo heat pump, at least 3.0 in most installs. COP for DSH heating is arguably a bit higher than system COP since DSH merely adds a minute or so to an on cycle, so benefits from higher steady-state COP, but that is splitting hairs.

The argument against HPWH located within the conditioned envelope in a heating-dominated climate is that heat has to be "pumped" twice - once into the HPWH's room, then again into the HPWH tank. This likely serves to halve the proportion of the Energy Factor >1.00. Actual EF may be in the 1.5 - 1.8 range when everything is taken into account.

HPWH also tends to dehumidify wet rooms, an unquantifiable benefit which may not matter to you in a dry climate.

In the end, I'd probably go with the DSH / HPWH simply because the HPWH is little more $ than a Marathon, and the HPWH does provide useful cooling during summer, and it can be made to operate conventionally if needed in winter.

Rebates and tax credits may render marathon and HPWH cost same or HPWH even less than Marathon.

I'm sorry, but I have never been a huge supporter of the Marathon tanks. I am a supporter of the KISS principle.
DSH and a electric finish tank is relatively inexpensive. Remember just the opposite of a HPWH btus lost from the storage tank help heat your home.
Savings difference with a Marathon or HPWH would likely have a 20+ year payback horizon.

Modelling shows that for marathon tanks you save about $40 per year due to lesser heatloss, but keep in mind that they loose the heat into the building, which is made with the efficiency of the geosystem about 60% of the time. Save the money, with two people and your heating patterns, put in a DSH with 2 eletric tank with sprayed on foam, one as a buffer, one as a final tank. I would question if the solution with ASWH is more economical. For sure it is more money upfront.

Thanks for the advice, The Marathon heater and the ASWH both carry similar Rebates/Tax Credits so the cost is down to about $500 for either. Joe, you do have a point, I hadn't thought of, Scrap the Marathon, the heat loss isn't a bad thing.

OK DSH with finishing tank, ASWH still up in the air.....

I'm not sure the Marathon's advantage over a conventional amounts to as much as $40 per year since non-Marathon tanks have improved.

The point that water heater standby losses in a heating-dominated climate are not so bad, is well taken.

Why are standby heat losses of a water heater in an unimproved basement ok?  In the living areas of the house - how much does this heat loss increase the cooling load in cooling season, and how much does this heat loss decrease the heating load in heating season?

A well insulated conventional tank operated at 50 degreens above ambient might dissipate 100 Btuh. Marathon will shave a bit off that.

Depending on circumstances, heat in a basement is not completely lost to the home.

.....and heat in a basement does not automatically impact cooling load or comfort.
There are reasons to buy a marathon (i.e. subsidies and kick @$s warranty), but saving money isn't one of them in my book.

whirnot, if you plan to be in the house a long time, you may want to factor in water heater longevity (i.e. parts and labor replacement cost) and tank warranty into your decision. The Marathon has an advantage in these areas. Also, with a Marathon you will never have water damage due to leaks from a rusted out tank. All of these factors as well as lower heat loss influenced our decision to buy one.  Short term, I agree it's not the best bang for the buck, but long term is another story, especially if you need to hire someone to install a replacement water heater for you (maybe 1 to 3 times.)  Personally, I hate insurance claims and deductibles.  Piece of mind...priceless.

Modelling shows that for marathon tanks you save about $40 per year due to lesser heatloss,

Compared to what? Don't nearly all electric hot water heaters have energy factors in the 0.90-0.94 range, just like the Marathons?

Each hundredth point in the EF range is worth about $5 at DOE conditions.

Marathons do tend to exceed conventional heaters of similar volume by several hundredths.

A Marathon may really shine when used as a preheat storage tank. Conventional tank water heaters do not heat the bottom 4-6" of water, so losses downward are minimal. Prehat applications generally evenly heat the entire tank, so downward losses could be more significant. Marathons feature full thickness insulation on the bottom of the tank.

Not sure where this info about Marathons' performance is coming from, but to save $40 a year, they'd have to have a substantially higher energy factor than the other conventional water heaters. A quick review of the Marathon lineup shows that they have an unweighted EF average of 0.924. Even the GE heaters at Home Depot with the lowest rating "Good" have an unweighted EF average of 0.915. The difference is LESS than one hundredth of a point. The heaters in that category go for as little as $200 which is a small fraction of what I think the Marathons are. It would take a very long time to make up that difference at $5/year in energy savings. The fact is that you can get low cost electric water heaters with the same EF as the Marathons have.

That's why the pros have not been extremely hot on Marathons ICF. You can find electric tanks available with .87 or less EF. It's like you are trying to start an argument while saying the same thing.

Plastic lined tanks will fail to Geome, I've been working with them for 20 years and seen lots of them go. The yucky part is between failure of the plastic liner and rusting through of the outer shell on the old Amtrols. Folks potable water was coursing through the insulation!
We came to recognize a failed liner on these with a touch test (saturated insulation makes tank much hotter to the touch) during annual checks.

An honest question I'll ask is how many BTU's is a DSH really going to contribute with a two ton system and a well insulated house? I'm thinking that the finishing tank will seasonally take on the vast majority of the water heating duties. Also, I'm familiar with Central Oregon, isn't there also a fairly substantial cooling season? If you need to cool in the summer, a heat-pump water heater in the conditioned space isn't a bad thing. BUT my first choice for water heating in Central Oregon with its 300 days a year of sunshine? SOLAR!

Joe, how many Marathons have you seen fail with the problem you described? When purged (at Any time pre-failure) were Marathon's draining instructions followed (i.e. open a hot water spigot, etc.)? Most plumbers are not aware of this, nor was my installer until I mentioned it to him (pre-installation).  I would like to see longevity figures from properly installed and maintained Marathons.

Thanks for the tip on checking for leaks between the shells.

Posted By geome on 28 Jun 2012 10:11 AM
Joe, how many Marathons have you seen fail with the problem you described?

None- put simply I don't sell them so I'm not called upon to service them.
The liner is like that of a plastic milk jug and one of the most convienient places for hot water expansion or water hammer to dissapate.
For Amtrol longevity we began installing bladder tanks in city water applications.

Posted By joe.ami on 28 Jun 2012 10:34 AM
For Amtrol longevity we began installing bladder tanks in city water applications.

Another good tip, thanks.    We have a well and pressure tank for potable water.  I assume the pressure tank would serve the function of the bladder tank in our application?  There is no backflow preventer on the water heater.

Posted By Blake Clark on 28 Jun 2012 09:47 AM
An honest question I'll ask is how many BTU's is a DSH really going to contribute with a two ton system and a well insulated house? I'm thinking that the finishing tank will seasonally take on the vast majority of the water heating duties. Also, I'm familiar with Central Oregon, isn't there also a fairly substantial cooling season? If you need to cool in the summer, a heat-pump water heater in the conditioned space isn't a bad thing. BUT my first choice for water heating in Central Oregon with its 300 days a year of sunshine? SOLAR!

Blake, although GSHP will give us cooling I anticipate we will almost never use it. Central Oregon has such cool nights, and we have always opened windows at night and shut them in the day, with a properly designed home, I would be surprised if we even turn on the AC.  I wish I knew how much the DSH will output, that is something I cannot get a handle on.  My thought was if it outputted a reasonable amount and a marathon finishing tank would hold the heat well, that might take care of most of our hot water needs as they occur in the morning.
We have a freezer and plan on putting it right next to the Heat pump water heater if we go that way, they should compliment each other.  Solar is an option but due to low sunshine when you need it, and no rebates, low tax credits, / net meetering programs, (REA) it does not seem to be the Answer. The 300 days of sunshine per year is grossly exaggerated. we have been tracking it, the winter and spring sunshine levels are way below national averages.

Geothermal manufacturers (well, at least Water Furnace), publishes detailed DSH information in their specification manual. Of course, DSH contribution is also a function of geothermal run time.