Here's an update - now X axis covers 8 hours.

HW from direct-connected gas-fired HWH continues to drop steadily due to usage and thermal losses via HWH tank.  HW temp is low enough at the moment, and DSH is running frequently enough at the moment, such that DSH is providing benefit at this time.  Thus, HWH tstat hasn't turned on gas burner yet (I've got a gas on/off indicator that will show up as a black line).

I probably don't have the HWH's tstat turned down low enough to prevent reverse heat flow from the DSH water loop to the R410 gas loop after the HWH's burner turns off, but, we'll see.

The second chart shows the maximum temp the R410A refrigerant got to today.  Looks to be about 124°.

Best regards,

Bill

OK, after about a day of looking at the numbers, it sure looks like what I noted a year ago, when I too wanted to know the answer to whether or not it works to have a DSH direct-connected to a gas-fired HWH.

(This is specifically for (and limited to) a water-to-air (WaterFurnace Envision series) R410A 2-speed 3 ton GSHP, in cooling mode, where it's DSH is direct connected to a 50 gal. natural gas burning HWH.  It may or may not apply to heating mode, R22 systems, and/or something other than water-to-air GSHPs.)

As seen from the below 4 hour chart, when the HWH's burner kicked on, HW temp was raised above the temp of the R410A refrig going into the DSH.

So, my observations is:

To make the DSH provide economic benefit, the HWH's tstat has to be turned down enough to minimize the amount of time the GSHP runs in a less than normal efficient manner.  Turning down the tstat will maximize DSH performance / minimize GSHP efficiency impact.

And obviously personal comfort and geographic region will substantially affect tstat setting.

I've got a 20 hour chart now set up to accumulate, to check this observation.

Hope this has helped on this subject.

Best regards,

Bill

I collected more data, enough so that I'm ready to summarize my observations.

I realize these summary observations do not agree with well stated and respected contributions here, from either owners who are satisfied with their DSH's performance when direct-connected to a gas-fired HWH, or professionals who do install GSHP DSHs direct-connected to gas-fired HWHs.  I don't have an explanation for why my observations are not consistent with these contributions.

My summary observations (see charts below):

(1) The amount of heating contribution to Domestic HW from the DSH is minimal.  Most of the time the HW temp increase contribution is just a few degrees.  It's tough to add heat to the HW when the HW incoming to the DSH is on the order of 110 - 120°.

(2) During the time when the gas burner is on, and for a time period after the gas burner turns off, there is a reverse heat flow.  Heat flows from the HW to the refrigerant.  This results in a small amount of cooling of the HW, and a small amount of GSHP efficiency reduction.  It's tough to avoid this situation since the HW is being heated to a higher temp than the R410A refrigerant.

(3) The situation can be improved favorably with a substantially lowered HWH tstat setting, but loss of considerable comfort is a result.

My specific conditions:

(1) Water-to-air (WaterFurnace Envision series) R410A 2-speed 3 ton GSHP
(2) In cooling mode
(3) DSH is direct-connected to a 50 gal. natural gas burning HWH

I don't know if my observations are applicable to heating mode, R22 systems, or something other than water-to-air GSHPs.

The charts below, and other charts, are available in real-time at http://welserver.com/WEL0043/ . 

Best regards,

Bill