OK--here goes.  2 Water Furnace Envision units, 3 tons each (vertical loop), 1 split system, both dual stage.  Hooked up to desuperheater with buffer tank.  HWH is propane (existing when geo was installed). Installed in November 2010, never been thrilled with kwh/HDD ratio (around 2, just taking into account the geo kwh) , but right now will limit this post to the desuperheater plumbing.

I have always thought the plumbing on the DSH looked strange, and finally figured out what was bothering me:  the WF manual shows the DHW in should draw directly in from the cold water supply line.  Our installer instead has the DHW coming off the line from the buffer tank to the propane HWH.  Does anyone know why?

Let's start there.  (and thank you!!!!)

Noob Kim

It should be plumbed per the WF instructions, else DSH effectiveness is reduced.

2 units may share a common buffer but there need to be check valves on each unit - backflow preventers in the DSH loop

Thanks. Check valves are there, as are back flow preventers. In this setup, however, we get lots of thermal siphoning. Installer is coming out Monday to fix. I'm wondering if change will affect kwh usage.

I really appreciate your response and thought!

Lots of ways to plumb a DSH some are inconsequential differences some are not.
If you are burning through considerably more KW's than you expected I'm reluctant to blame the DSH plumbing.
It could cause a smallish spike, but a large one????
j

Kdbelvin,

We apologize for not responding earlier. We see that you have been in contact with your WaterFurnace dealer to discuss your concerns. If our assistance is needed, please have your contractor contact our technical support staff, as we will be more than happy to work with you and your contractor in finding a resolution.

WaterFurnace International, Inc.

Thanks folks. DSH has been plumbed properly now, but today I noticed that with the DHW* off* for 24 hours (for the purposes of data collection and comparison), DHW out from the unit is about 20* hotter than the DHW in after about 10 minutes of operation on first stage. I'm guessing this is not OK. I read a thread from jstelmack where the DHW was off and the DHW out was 164*. Obviously this isn't as severe, but I'm thinking it's still bad.

I doubt that's a significant problem - it likely results from natural convection within the DSH lines caused by the high water temperature in the DSH coil while the compressor runs with the DSH pump off. Know that cutting off the DSH pump does not change the hot refrigerant gas path through the system. The essentially stagnant chunk of water sitting in the DSH coil likely reaches 140-160 degrees, and upon becoming that warm, it expands and moves a bit without the pump's action.

There is also conduction down the copper tube walls.

Assuming I understand the situation properly from your post, there is no problem.

Thanks, engineer. I had noticed that it wasn't happening on my split unit, so was starting to worry.

Posted By engineer on 24 Feb 2011 10:14 PM
Know that cutting off the DSH pump does not change the hot refrigerant gas path through the system.

So, does turning off the DSH switch make the 5-15% of the geothermal unit's DSH capacity usable in terms heating air?  Or is this a use it (the DSH) or loose it situation?

Posted By geome on 25 Feb 2011 10:39 AM
So, does turning off the DSH switch make the 5-15% of the geothermal unit's DSH capacity usable in terms heating air?  Or is this a use it (the DSH) or loose it situation?

If the DSH is turned off, then the heat goes into air.  You do not lose it.

Posted By geodean on 25 Feb 2011 10:53 AM
If the DSH is turned off, then the heat goes into air.

Can someone explain this in more detail?  Is it because the heat that is not being transferred to the DHW tank is available to the air coil even though the heat still passes through the DSH's heat exchanger? 

Posted By geome on 25 Feb 2011 03:33 PM

Can someone explain this in more detail?

Hot refrigerant from the compressor first goes to the DSH, but with
no water circulation, the temperature on the water side of the DSH
quickly rises to match the refrigerant temperature.

Without delta-T, no heat exchange is possible -- so the hot refrigerant
just continues down the pipe and gives up all its heat to the air coil.

Thanks!  I like your explanation much better than mine. 

Jeez, that's what I was trying to say...oh well.

I followed what you said about the DSH, Curt. I was just trying to figure out how the heat gets transferred to the air side. No offense intended. :-)

I wasn't at all offended - just noting we are all essentially saying the same thing.

As to how the heat gets to the air, here goes.

Refrigerant leaves the compressor at high pressure and high temperature. The temperature is the saturation temperature matching the pressure PLUS considerable superheat, perhaps 30-60 deg F. In other words the refrigerant is a superheated vapor.

From the compressor it goes to the DSH heat exchanger.If the DSH pump is not running, the small amount of standing water in the DSH coax quickly warms to the refrigerant temperature and no further heat exchange occurs. Refrigerant temperature leaving the DSH is essentially equivalent to its temperature coming in, so all its heat is available for the main refrigerant-to-air heat exchanger, AKA air coil.

Hope that helps...