I'm curious, mostly because my system (due to the unusually cool summer so far) has been running maybe 4 hours total per day, which is spread out over the morning, afternoon, and into the evening.

I have temp probes on the DSH in/out and I can guess that the buffer tank temp is also the temp going into the DSH.

Given the tanks have standby losses over the day, it's a 40 gallon buffer tank, we use very little hot water, but certainly 2 adults, daily short showers, sink of dishes every day or two... 

According to the WEL, over the last 7 days (of intermittent runs), the water in the buffer tank has never gotten past 69 F, even though the DSH out is producing water that's reached 96 F. 

I know it takes time to circulate all that water and get it up to 96, but unless the weather gets hotter, I can't picture the DSH ever producing enough hot water to make any kind of different on this set up. 

Does this sound right, given the short run times, smallish buffer, and daily use? 

www.welserver.com/WEL0487

It would be nice if geo systems offered a ->domestic water heat exchanger large enough to run in a "heat domestic water only" mode. Also, be sure to clean the DSH if your water has hardness or iron.

Right...I mean, I know the DSH won't get the temp up to 130 F, but it seems to be making minimal contribution. It would be cool if somehow the system would first heat as much hot water as possible, then switch over somehow to the ground loop. But it would mean a bigger exchanger and some pipes/valves/etc.

Posted By jonr on 28 Jun 2011 08:07 AM
It would be nice if geo systems offered a ->domestic water heat exchanger large enough to run in a "heat domestic water only" mode. Also, be sure to clean the DSH if your water has hardness or iron.

Geo systems do offer what you describe- they are called hybrids and available from many minor players such as Hydro Delta, Econar, Hydron.......and DX companies like Earthlinked. Only one major player (WF) is into them so far.
I have yet to see a Hybrid that doesn't offer extra trouble along with the extra feature.

"According to the WEL, over the last 7 days (of intermittent runs), the water in the buffer tank has never gotten past 69 F, even though the DSH out is producing water that's reached 96 F." 

You have to think of this in terms of BTU's or units of heat. Every BTU you put into the tank is beneficial in some way.
First the entering domestic water to your home is likely not 69F. So at the very least you save the units of energy it would have taken to raise the water to 69F.
Next you modestly reduce the LWT going to your ground loops. Helping to keep EWT from rising.


One of the pitfalls of 100% loaders is very short AC cycles. Even in modest weather a good fitting system should run a little while in first stage.

j

I thought the desuperheater ran the whole time the unit was running, since it really can't get the water temperature up to a level that would be considered hot, there no point on having a shutoff for it when it reaches a certain temperature. It's meant more as way to take advantage of the byproduct of cooling a house, and doesn't flip the system on just to pre-heat the hot water.
 

Something is off with your numbers. The entering water temperature into the heatpump DSH should be close to the buffer tank temperature (since this is where the water comes from). That would mean you are dealing with a 27F delta, which indicates that your flow is way to low. Check the entering water temperature into the DSH.

Doc
The temp probes are on the in and out line of the dsh. So the delta in the 20s is correct. I'm assuming the buffer tank is about 68 because the temp going into the dsh is 68.
I know that the DSH is connected with 1/2" PEX to the buffer, approximately 10 feet in one direction. Today, I will take a water sample directly from the buffer and measure the temp, and also check the probes.

You're saying that the DSH out should be cooler, because with more volume going through the DSH, it would result in less temperature rise?

Yes, depending on your piping, you should see between 7-13 degrees in heating mode, a bit less in cooling mode.
You have twice as much.

My delta is only 3-4 degrees in first stage cooling mode (good bonded probes with thermal paste and insulation) on a 5 ton WF unit!

Wasn't much better than that in heating mode 1st stage (4-5 degree delta).

During the heat of the day (low 80s) my DSH tank got to 78dF today during no use until early afternoon.....

Hmmm...I removed the probes and reattached just to be sure I wasn't getting false readings. We'll see today what's happening (temp in the low 80s...may or may not get much runtime) and I'll graph the delta of the DSH on my WEL page (WEL0487).

FYI, the DSH threaded connections appear to be 1" (?) - on my system they immediately reduce to 1/2" with an adapter, make a 90* turn, then run to the buffer tank through 1/2" pex, approx 10 feet each way. The manual states that 5/8" is the minimum, but when I called Heat Controller Inc, they said 1/2" was ok.

Is there a way I can bleed the system (just in case) or is this best left to my new repairman?

I also checked the temp of the buffer tank through the t-valve at the bottom of the tank...it's about 68....which happens to be the basement temp!  I don't actually think the DSH is doing a single thing. 

I plan to move the temp probes to where the pipes enter the buffer tank.

A few days ago, I moved the temp sensors to where the pipes ENTER the buffer tank (rather than where they exit the GSHP/DSH). I also changed my WEL to do a "sample & hold" of the delta between DSH in and DSH out (for those of you without a WEL, it means that I only record the temperature difference when the system is running, otherwise, it "keeps" the last recorded temp before the system cycled off).

It appears (and I think this is logical) that the delta when the system is on is typically between 8 and 12 F, and that short system runs (even though I have CPH set to 1) do not allow the DSH pump to activate (I observe no temperature delta when the system runs for only 5 minutes or so, but when the system runs for about 10-15 minutes or more, a delta does occur)

Bright side is you are saving money as cooling is not required for your home.....you just have to pay a little more for hot water.
j

Stuart, what unit do you have again, and what year. After switch over to R410 manufacturers had issues with desuperheaters stealing heat from the hot water tank when the loop temp was too cold, e.g. the compressor discharge temp was too low (below 125F). As a result, DSH pump only comes on if compressor discharge is over 130F. So in your case the DSH is always running, but the pump is not turned on. What you see is thermal siphoning, which means very low flow and high delta T through the DSH. If you check it, the circulation pump is not running. Do you have a Climatmaster or clone (e.g. Heatcontroller etc) built before 2009?

I'm with Doc on this - though your system particulars may vary, some manufacturers incorporate some sort of "smarts" into the DSH design so that the pump does not run when conditions are not favorable for water heating. A two stage system operating in single stage with fairly cool loop water fits this scenario well.

I also agree that the 27*F delta likely stems from thermal siphoning, not active DSH operation.

A couple suggestions to consider:

1) Economy AC clamp meters are now sold at the likes of HomeDepot for sub$50. Use one on ONE (not both) of the leads to the DSH pump motor and you'll get about 0.3 amps if the pump is running...I can't guarantee an economy clamp will go that low (mine, since upgraded, did)

1a) Even easier - DSH pumps get hot when running for awhile

2) "laying on of hands": Check temp of compressor discharge line leading into DSH heat exchanger against one coming out after system has been running for 10+ minutes. Line out will be much (20-40*F) cooler than line in if DSH is running.

If pump is hot but ref lines have low Delta-T, then waterside is likely airlocked. If so, disable pump until this is resolved.

All my DSH installs incorporate both isolation valves and bibbs for flushing DSH. Bibbs also serve to purge air during commissioning.

Thanks. I'm going to have to do some more detailed checking. I believe the discussion so far is in the right direction, and it only confirms what I was guessing. I have a GeoMax 2 (Climatemaster) with the new R-410 and it's only 2 years old. I know there is both a thermostatic switch on the discharge line, as well as on the DSH hot water pipe. My runs, even when the system is set to 1 CPH, are only 10-15 minutes max, then off for 45 minutes or so. At that rate, the discharge temp isn't getting high enough to activate the pump (I will triple-check this with amp-meter or laying on of hands). The deltas in the pipes right by the unit probably are high because of siphoning, whereas when I moved the sensors next to the buffer tank, the delta became less, and I think I have a more accurate picture of what's happening.

There was one day last week when the system ran combined total of several hours during the day, and the buffer tank got up to 76. Otherwise, it's currently hanging around 74 F.

Posted By decafdrinker on 16 Jul 2011 07:39 AM
Thanks. I'm going to have to do some more detailed checking. I believe the discussion so far is in the right direction, and it only confirms what I was guessing. I have a GeoMax 2 (Climatemaster) with the new R-410 and it's only 2 years old. I know there is both a thermostatic switch on the discharge line, as well as on the DSH hot water pipe. My runs, even when the system is set to 1 CPH, are only 10-15 minutes max, then off for 45 minutes or so. At that rate, the discharge temp isn't getting high enough to activate the pump (I will triple-check this with amp-meter or laying on of hands). The deltas in the pipes right by the unit probably are high because of siphoning, whereas when I moved the sensors next to the buffer tank, the delta became less, and I think I have a more accurate picture of what's happening.

There was one day last week when the system ran combined total of several hours during the day, and the buffer tank got up to 76. Otherwise, it's currently hanging around 74 F.

Stuart, you have a Heatcontroller unit which does not turn on the circulation pump unless the EWT is warm enough to ensure compressor exhaust temp over 125 F. They made the switch in March 2010 to a more sophisticated control which takes into account the entering water temp into the DSH. None of our pre 2010 units make DSH hot water yet here in Buffalo. Loops are still too cold, around 60 F so far. What you are seeing is designed that way in a panic when they realized that DSHs were actually taking heat out of the DHW tanks after the switch to 410a.

Jeez...I'm ok with this, I just want to point out to someone (anyone) that there's no way in my situation that the unit could generate 50% or more of my hot water! Ok, I said it. Now back to the data.

For the FIRST time this year (mid July) I guess the temps outside and the temps of the ground loop got high enough that there was a HUGE jump in the buffer tank/DSH arrangement. Over 2 days, the temperature when from about 75 to 105 in the buffer, and the following day, went from 105 to 110 in the buffer tank. I wasn't watching EWT, but did notice that the jumps occurred on two of the hottest days so far when the system had long Stage 1 runs (at least an hour) with several switches into Stage 2 to maintain cooling temperature.

I will have to go back and examine EWT. I'm happy to hear others with similar units aren't making hot water yet, because I know it's not just mine, but feel slightly cheated by the original installer promising endless free hot water.

Thank you to everyone for their help/input/suggestions. I'll check into the other temps and post, just in case future people find this link and want the info.

I'd look into retrofitting the more sophisticated control.

On my deep energy ASHP retrofits I add a DSH and a tank. It works well, but I've had to bypass a similar 125*F threshold to starting the DSH pump. A high efficiency 410a ASHP in low stage will not provide refrigerant anywhere near 125*F. I went with a 113*F threshold, and I'm about to try 100*F.

The manufacturer of my favorite add-on DSH is coming out with a smart circuit to assess refrigerant and water temps.

This retrofitting of smarter controls would be to replace the thermostatic sensors on the refridge coil, and possibly add one to the ground loop EWT, and use a combination of those signals to trigger the dsh pump.

I can't, for example, just bypass the refridge thermostatic switch for the summer, or replace it with a lower temperature version only?

Engineer, it sounds like you simply swapped the thermostatic switch on the refridge line....where did you find a 113F or a 100F? I'd definitely like to run that by my installer so he's kept in the know. I'd certainly like my geo unit to start generating HW earlier in the season.

Thanks everyone.