Posted By jonr on 13 Oct 2012 12:40 PM
The ones that allow the lowest min btu rates. But unless it goes right down to zero, there is always an incoming temp where certain output temps are impossible. Plan on some uncomfortable showers with a DSH/tankless combination.

File that remark under "sweeping but baseless generalities"

We routinely feed multiple tankless water heaters with DSH preheat tank outlet. Nary a complaint in years.

File the above under "companies to avoid until they get more experience".

I have routinely used tankless setups that had this exact problem (warm water input to the tankless causes it to cycle on/off). And of course google finds lots of similar cases.

My Navien had a lot of settings on the key pad that I did not really understand.  I assume some of them were for input cause I chose well, and set my temp to 130 and never looked back
Eric

Select a more flexible tankless and experienced contractor to install it.

Hey! I resemble that remark. Unflexible and no prior experiance installing a tankless, lol
Eric

Posted By engineer on 13 Oct 2012 01:06 PM

Posted By joe.ami on 11 Oct 2012 10:17 AM

Posted By engineer on 10 Oct 2012 09:01 PM
In a far northern climate "conditioned" air means heated air most of the year - one pays twice - once to heat the room and again to heat the water. It is not an automatic disqualifier, but must be considered.

Agreed. We did the math on this once upon a time. At the very least found payback horizon out there a ways. Not necessarily a deal breaker, but a HPWH at 3 COP using heat created at 4 COP does not mean you are getting 7 COP .

Of course not! - Coefficients are multiplicative, not additive, so instead of 3 COP + 4 COP = 7 COP, it is 3 COP X 4 COP = 12 COP!

Something like that, anyway...

That's some pretty kewl math y'all are into- I like it!

But let's lay it out for the kids in the back row who weren't paying attention in class, lest we create a new generation of perpetual-motion theorists, eh?

A heat pump doesn't create the heat, it only moves it.  If you move 4x the amount of heat (from the ground, outdoor air, whatever) into the house than you would spending that same energy to heat the space directly, the coefficient of performance (COP) is 4, which is about where purty-good geo systems run.

But no matter how efficiently you then move the heat from room air to the water, the total COP does not (and simply CAN NOT) exceed that of the original COP=4.  Having to use any additional energy at all to move the heat from the air to the water means you've spent more energy for moving the heat without taking any more heat in from the outdoors, reducing the net COP.  But as long as that net COP is still greater than 1 (which is what you get out of a standard hot water heater's resistive elements) you still garner an efficiency advantage.

News Flash: Satire is illegal west of the Susquehanna.

So why not go with a dedicated geothermal water heating system? WaterFurnace uses an 80 gallon storage tank and either an 18K or 25K BTU water to water unit and a small pump and aquastat to heat the water. You would no longer use the desuperheater(or it would not be as cost effective even if it is nearly free heat). Typically it only requires one additional loop added to the system or the equivalent of about 1 ton of loop size to take care of the load. The 80 gallon tank is used to help buffer enough water against a lower recovery, but a 4500 watt electric puts out a hair over 15K BTU's while an 18K BTU unit provides about 14K BTU's of heat at 2.2-2.3 COP at a 30 degree EWT and a 130 degree output temp into the 80 gallon tank. This is during heating seasons worst days, not lets say its a 70 degree EWT during the summer and the unit provides 22K BTU's of heat at a 4.27 COP at 120 degree ouput temperatures.
If you use the NSW025 unit now its 20K BTU's of heat at 30 degree EWT and 130 output temps at 2.48 COP and up to 70 degree EWT with 120 degree output it provides 35,500 BTU's and is a 4.58 COP.
That is the only heat pump water heater that I would use.

Posted By Looby on 15 Oct 2012 05:02 PM

News Flash: Satire is illegal west of the Susquehanna.

I thought cardinal directions were outlawed east of the ...








(oh, never MIND!)

I have no objection to a dedicated geo domestic water heating other than the cost.

It is worth noting that COPs are likely somewhat higher than what Sky describes since during typical "on" cycles the load EWT is lower than set point until the end of the cycle