"Bill; still would love to see a description of your mechanical room, on it's own thread to see if we can help you solve your HWG issue.
Joe"

I started this thread here in response to Joe's comment above, from the very interesting horizontal loop discussion thread.

Compared to 'you guys' up North, down South here (Dallas area), there's rarely such a thing as a "mechanical room."  Mine is shown below (don't laugh) - just large enough for a Hot Water Heater (this image shows the top portion of my DeSuperHeater pipe plumbing).

What's also common, down South here, is to put the HVAC equipment up into the (hot) attic.  An image of my 5 ton unit, sitting in the attic, is also shown below.

So when it comes to a GSHP-based retrofit HVAC job down South here, the starting point for the conversion is lots of water pipes up above ceilings, with potential to leak.  And often a gas-fired HWH in a "mechanical room," a HWH that doesn't work well to provide DSH-generated hot water because it's gas fueled.

Right now my DSH is turned off on my 3-ton unit (and not connected on my 5-ton unit), due to confirmation that efficiency of the unit actually suffers slightly otherwise.  This has been well discussed and affirmed in other threads here.  Suffice it to summarize here that a holding tank really is needed to get maximum performance for DSH-generated hot water.  An electric HWH, without a holding tank, is a good alternative, but a gas-fired HWH without a holding tank is not.

So my challenge is how best can I hook up the DSHers from 2 GSHP units together, with a holding tank, in as safe of and cost effective manner as possible.

The options I see are:

1. Convert the gas-fired HWH, shown below, to an electric HWH, and plumb in the DSHers from both units (new electric circuit needed).

2. Add a holding tank up in the attic, as a pre-heat to the existing gas-fired HWH, and plumb in the DSHers to the holding tank (lots of water up in the attic).

3. Add a holding tank in the garage, as a pre-heat to the existing gas-fired HWH, and plumb in the DSHers to the holding tank (lots of HW piping).

4. Replace the gas-fired HWH with a no connection electric HWH holding tank, plumb in the DSHers to the holding tank, and install a tankless HWH (probably the most expensive option, short and long term). 

I've put in parenthesis what appears to me to be the most significant drawback of each option.

With no clear advantage (to me) of any option, I haven't done anything yet.  I'm at the moment inclined to go with the first option.

I look forward to hearing if anyone has other alternatives to suggest, and/or if there's a particular option that clearly stands out from the rest.

Thanks and best regards,

Bill

Bill,
What I'd been wondering was about overhead space (2, 20 gallon storage tanks make a 40 gallon). Even 1, 20 gallon would give you some benefit. An electric water heater, would also move the chimney out of the way. I also like your thoughts about the on demand (though normally I'm not a fan).
Need load calcs, and occupancy to try to calculate operating cost differences.
Joe

Joe, thanks.

I hadn't thought about the space above my existing gas-fired HWH, in my "mechanical room."

My WEL has recent data for HWH usage, but not for this past winter.  I.e., for the month of Aug., I've burned 0.6 hours/day, on average, of NG fuel.

I don't have any other data, so I'll have to wait in order to collect it.

Best regards,

Bill

Exactly Bill,
Let's see what everyone else thinks of that you and I don't.
As they say more than one way........(apologies to cat fans).
joe

What does the .6 hours a day turn in to Therm's wise? I can say we were burning about 5 therms a week before they came back and hooked up our DSH. Items that use gas in our house are, stove, furnace, fireplace and hot water (furnace and fireplace are off for the summer). After the DSH was hooked up we are using .5 therms a week. I am guessing that is mainly cooking, the 50 gallon gas water heater has ran a total of 90 minutes in the last 8 weeks and we only have a 40 gallon (old electric) pre tank. I am pretty perticular about when we do use hot water, like doing laundry or dishwashing when the DSH tank is hot. When possible I also take my shower when it is as hot as well.

Again our family consist of us two adults, 6 boy, 5 girl, 3 boy and 2 girl. Right now they aren't huge shower users, but we have two dishwashers and run about 3 loads a day. Our Wash machine runs at least once a day, 3.2 cu ft LG front loader.

I like the idea of an upper / lower setup. You might be able to stack a 40 low and a 20 high, and then you choose which is the DSH heater. I personally would use the 40 pre and 20 electric hot, but if you use a lot of water at once I might reverse that. The 40 pre tank and instant electric would probably be the best setup, but more expensive to start out and might be more difficult getting that much power to that location?

I like an attic preheat tank, especially if both your Envisions are up there?.

Given the temps of a Dallas attic, I'm not sure you want insulation on a preheat tank up there.

You already have lots of water up there in your geo piping

Put the tank in a pan and drain the pan to somewhere you'll see a leak - most leaks begin slowly.

Variation on Brock's question: how many people live in the house? If just 1-2, payback will be long. If big family, payback much shorter.

What rise is needed? What is typical cold domestic water inlet temp?

In MI we do avoid water in the attic whenever, however I love Engineer's thoughts. You could add solar (attic) gain to your hot water production. Summer benefits could easily out weigh risk.
One thought; do we need to temper/is there a chance of "too" hot water in July?
J

I have appreciated the comments thus far - thanks!

Putting a HWH (either electric/gas or pre-heat) in my attic is certainly the least expensive consideration - by a long shot, setting aside for the moment concerns with having 40+ gal. of water in the attic.  And even with the expense of additional safeguards to prevent accidental flooding, still, the alternative of a tank in the attic is still the least expensive alternative.  The DSHs from both Envision GSHP units can be combined, only one good sized preheat tank is needed, and piping is minimized.

In fact, I think it's doubtful that a reasonable ROI is available for other alternatives.  HWHs have been around too long and are just pretty cost effective compared to the alternatives.

Noting thermal heating in the attic, yes, my guess is we would not have little NG expenditure in the summer months, with a pre-heat tank connected to both DSHs.  DCW, here in the Dallas area, is easily in the upper 80 degree range, thus requiring little hot water for any purpose other than dish washing.  In fact, we're only averaging about 30 minutes of NG usage daily this month for HW generation.

I have WEL instrumentation on the HWH that gives me gas burner on/off time.  I'll have to look up how to convert NG on time to cost.  My guess is the HWH consumes x amount of NG-generated heat per hour, that's in the spec for the HWH.  Since I have the total accumulated time for each month of usage, I can integrate to the total NG-generated heat consumed.  And I'm sure I can look up some conversion factor that correlates heat consumed to cost.

There's probably no such thing as generating too much HW in the summer season, or generating water that's too hot.  The hot water is available, but simply not used.  And since my GSHP units are R410a, I would imagine my preheat water won't get above 90 degrees or so.

The Winter season poses different challenge.  While there's no chance of freezing pipes in the attic (attic temp may get down to 32 degrees maybe once or twice each Winter, and then only for a few hours), still some pretty good insulation would be needed on the any tank in the attic.  I'll need to obtain some instrumentation this coming Winter to get a good read on how much hot water is needed, and what portion could be provided by the DSHs.

Again, many thanks for the helpful comments.

Best regards,

Bill

Not so fast - Joe's gotta good point - Overheated domestic hot water is dangerous, especially to small children and the elderly. Fortunately Envision desupers drop out at an EWT of 130 or so, effectively limiting hot water temp to a bit above that.

130 is still too hot for my tastes - I have very small children and so limit ours to hot-blooded wife's required shower temp (~108) + a few degrees for hysteresis (on / off gap control typical of electric storage water heaters) - approximately 115. My low open loop water temp and 80 gal preheat tank limits my desuper output to about 100 F.

Your low loop temps will likely keep desuper water to well below the 130 cut-out. Dallas attic heat gain, while useful, is likely to be relatively minor into a tank whose contents is changed daily or more often.

Thanks for the reminder on too hot hot water.  We too are quite mindful of this, and thus have always kept the HWH temp setting considerably below 130°F (actual HW temp is on my WEL0043 system summary page, but I don't have any more charting room to show it as a trend/chart - it's running at about 115° these days).  I hadn't fully appreciated Joe's well-placed caution until being hit with the caution twice - not something I had thought of.

I hadn't considered the fact that possibly 2 DSHs connected to a pre-heat tank, combined with the tank being subject to considerable heat gain sitting in a hot attic (highest attic temp this month so far is 136°), might result in HW temps too high to be safe.

I'll have to think about how I might figure out what to expect.  I've got some pretty good data (i.e., R410A going into each DSH is at max 125°,  DSH run times are about 7 hours per day, avg, this month, attic max and min temps are available, etc.).  My gut feel is there's no way I can generate anything close to 130° HW in this scenario, but, I agree, it needs to be carefully examined.

Thanks!

Best regards,

Bill 

With your relatively low EWTs, both btu recovery and max temp of desuper will be limited - a larger preheat tank will help you capture more of those lower temp btus. I'm beginning to think I'm better off cutting my open loop flow down to 1.0-1.5 GPM / ton, raising LWT from low 80s to mid 90s. This raises compressor current by an amp or so but the higher superheat temp increases the recovery temperature and total heat, driving down primary water heater element runtime.

It would be elegant to find a way to automatically modulate LWT so as to maximize hot water generation during times of lower load - less loop flow during mild weather - increased LWT diverting more btus to preheat tank.

Agreed.  I'm confident that big, complex commercial implementations indeed do just this.  But my guess is it's not available to the residential market.

Thanks for the comments.

Best regards,

Bill

Another aside Bill, if you find an excess of preheated water (and HWG's cydling less than they could) you might be able to put it to use with a circulator to speed up hot water delivery to fixtures.
Regarding the too hot question, forgive the inspector in me, but worst case scenarios are my business. Water heaters in the attic are un heard of in MI and not knowing your peak attic temps.......if you took the family out of town in July for a week and no one ran the water untill you came home....
Of course I'm also the one that complains to my manufacturers' that a T&P is required on HWG process piping where shut off (throttling valves) are installed (source of heat, closed circuit......) and yes, I do install them.

Joe, thank you for the additional comments and cautions.

Best regards,

Bill