Yes, two young kids.  Currently their biggest impact is the volume of laundry, but as they get older, the showers are going to take a hit.

Well, I understand that the primary tank is almost always going to have some work to do.  But during peak seasons of GSHP usage, it seems like 80 on the buffer is a reasonable tradeoff of losing heat to standby loss vs giving the DSH a bigger bucket to dump heat.

So maybe I'm wrong here, but it looks to me that by upsizing the buffer to 80 all I'm out is $150 in up-front costs.  From there, I'll have the ability to extract more efficiency from the DSH during peak times, and it's likely that any additional standby losses from the extra 30 gallons will probably not offset the efficiency of the DSH. 

When the DSH is not running at peak (or for very short amounts of time), then the upsized buffer tank isn't going to be heated, so standby losses aren't an issue in the scenario.

As always, sanity-checks to my logic are welcome...

Occasionally you might get the Hotwater generator to cycle off depending on heat pump size and usage. By and large though, you will have the same amount of btu's in either buffer tank you will simply have 80 gallons of warm or 50 gallons of warmer water.
j

A large buffer makes sense to me. I would go with a tankless water heater for the primary and not have any worries about running out of hot water.

So that's the essence of the question: is it a problem to dilute the BTU's?  So you go too big on the buffer, and your BTU's get diluted more often.  Go too small, and the buffer will occasionally max out on how many BTU's it can hold. 

I could see it a couple of ways:

• Dilute the BTU's, and you always have a place for the DSH to dump heat.  The downside is that the buffer water may almost never be up-to-temp with the primary, so the primary always has some work to do during recovery.
  
• Concentrate the BTU's, and occasionally don't get the full benefit of the DSH.  The upside is that the primary potentially spends less energy during recovery, since the buffer water doesn't drag down the primary water as much.
  

Seems like this would also have an impact on the First Hour Rating. 

• With the BTU's more concentrated (lets say 80 + 40), it would take less water for the buffer to be tapped out and start feeding colder water into the primary, which then starts a decline in output temp. 
  
• With the BTU's diluted, it would take more water for the buffer to be tapped out, but since primary is being fed cooler water, the output temp decline starts earlier, but more gradually.
  

Thoughts?

So how about it folks?  What's wrong with the buffer+tankless design?  I'd be looking at an electric tankless (I presume such a thing exists), would those units have problems being fed hot water until the buffer starts to get tapped out?

The other concern I would have is shoulder seasons, and so I would want a tankless to perform well without DSH assist.   My current 50 gal propane has an 86 gal FHR.

David,

There's even more homework for you to do!! Tankless water heaters DO have limitations and liabilities. Not all tankless can handle 120* EWT they will shut down on limit. Make sure you choose the right one. You can not assume the tankless will be supplied 120* water, so it must be designed as if the desuperheater is not there. Tankless systems will only put out a limited number of GPM at temp. There are maintanance issues also. The water heater MUST be back-flushed at regular intervals to prevent scalling.

I don't mean to discourage you from a tankless water heater...Just informing you. Knowlege is power.

Bergy

I would consider:

Is hot water important when the power fails?
Do you have sufficient amps to run it at the GPM you want?

Scaling depends on water quality and happens in all water heaters. In electric ones, they lose output capacity but not efficiency (there is nowhere to lose the heat to).

Whole house electric tankless will consume massive amounts of current in many applications - have to CAREFULLY calculate required temperature rise and max simultaneous flow. Unit could require 120 amps of service or possibly even more. Heaven help whoever owns one if demand based electricity billing makes it to residential tariffs under the aegis of the 'smart grid'

Electric tankless units have a phalanx of very high watt density heating elements and frequently-cycled high amp contactors - neither is a recipe for lengthy trouble-free service. I have begun hearing of a contactorless design but have yet to lay eyes or hands on one. That may cure the durability issues but not the current demand.

Tankless electric may be more efficient than conventional storage electric, but not by much, and the difference may never make up for higher first and possibly maintenance costs.

Ditto Bergy - just informing of issues.

Know what,
this is less important than so many other concerns as you shop.
here's a tid bit for ya; your DSH is like open loop geo and will matter very little without regular maintenance.
so...pick whatever you like and if you don't keep the coil clean....forget about it.
j
ps not trying to make this topic sound trite, but we're really splittin hairs.

Besides using soft water, how do you keep DSH coil clean? Thank you.

Regards, Masoud

Quality of the water of course has an impact.
It may not happen as quickly as open loop, but stuff in the water can coat a DSH coax as well. Syptoms would be a reduction in Delta T in the DSH entering and leaving water. Initially you could compensate by opening throttling valves. Cleaning would be similar to any other copper coax (white vinegar to iron out).
Good to hear from you,
J

Well for better or worse, I went with 40/80 Vaughn Hydrastone-lined tanks.  Given the climate in NH, my contractor claims that it would be an unusual summer where even a 40 gallon tank would get heated to max temp for any length of time.  The GSHP simply doesn't have to run that long to keep the house cool on the average NH summer day.

So 40 on the buffer, with the small risk of underutilizing the DSH, and 80 on the primary to give me the same FHR as my current 50 gal propane.

An 80 gallon tank with a 40 gallon buffer you say? Who suggested that I forget:) (well ok I said 50 gallon buffer but if your buying the cheapie tanks it's the same price as the 40).
I'm closest.....
Hail Joe king of the thread...

How about one tank and a instantanious heater, seems a lot less fuss, space, piping etc.

Incidently I'm far from sold on the extra value of the deluxe storage tanks.
Kimko, welcome.
A recent straw poll on this and another site had most contractors state they wouldn't have tankless in their own homes. Purpose of that thread was to solicit input from contractors. Please find that thread and add your thoughts.
J