My GeoMax 2 (4 ton) has the desuperheater option hooked up.

The blue "cold" pipe that comes from the water heater and is connected to the DHW IN on the Geomax is registering 101 F.
The red "hot" pipe that comes from the DHW OUT on the Geomax and goes to the water heater is registering 86 F.

Am I missing something????  The temp reading is using an infrared non-contact thermometer on the PEX pipes.

The water heater has an AirTap for heating, but that's set to 105 F.  The electric resistance heaters inside the tank are not connected.

The water heater is plumbed correctly to the GeoMax.  I can post a pic if desired.

The GeoMax is currently running with the compressors on...I just don't understand why the IN is hotter than the OUT.

The manual cautions installers to make sure the circulation isn't "backwards", but how this would ever happen with an a/c circulator motor on the DHW lines?

With your installer, pictures may be a good idea.

Posted By stuart.wyss on 27 Aug 2010 07:03 PM

Am I missing something????

Yes. You're missing a DSH buffer tank.

Your AirTap has heated the contents of the water heater to a higher
temperature than the DSH output. Apparently, that's common when
the EWT remains relatively cool -- even while in cooling mode.


http://itl.chem.ufl.edu/2045/audio/FirstandSecondLaw.mp3

Heat won't flow from the cooler to the hotter
You can try it if you like, but you're far better not-er
- Flanders & Swann

Stuart, I remember your model may not be the most recent. Does your manual show the following?

"The HWG is controlled by two sensors and a microprocessor control. One sensor is located on the compressor discharge line to sense the discharge refrigerant temperature. The other sensor is located on the HWG heat exchanger’s “Water In” line to sense the potable water temperature." "The microprocessor control monitors the refrigerant and water temperatures to determine when to operate the HWG. The HWG will operate any time the refrigerant temperature is sufficiently above the water temperature."

Mine's the older one, I think. The manual only talks about 1 sensor on the Water In line.... The schematic shows a *second* sensor in series with the one on the Water In line, but the manual makes no reference to it. The second sensor *does* exist.  I can trace the wires.  I should have shown it on the diagram.  There is a second sensor back on the refrigerant lines.  The 2 sensors and the circulation pump are in series.  One must detect that there's enough heat in the exchanger, and the other cuts off the pump if the incoming water is above 125 F.

I need to figure out how to tell if the pump is operating...I can't tell by putting my hand on it.

I did it in MS Paint, but it's accurately labeled.  No water in the house was being used at the time, nor was the AirTap operating.  The Geo was operating.
Internal Workings of the Geo are my observations with the cover removed.  Not sure if/where 2nd sensor is. (not obvious).

Your plumbing arrangement has several shortcomings:

1)Configuring the DHW inlet from the boiler drain risks it ingesting sediment. It is best to discharge to the boiler drain bibb.

2) Cold supply should enter the CW inlet at top of tan, not boiler drain

3) With proper plumbing and flow, I expect DHW to add ~5 degrees to circulating water and subtract 20-30 degrees from compressor discharge refrigerant temp.

Since you already demonstrate willingness to delve into the unit cabinet, a quick laying on of hands should show if hot gas off compressor discharge is substantially cooler (or warmer, which would be bad) after it transits the DHW coil, indicating it has given up heat to domestic water

If cold supply entered the Cold In inlet at the top of the tank, where would the DSH Cold be connected? Same place? Then DSH Hot would push water into the bottom of the tank through the drain?

It is plumbed exactly as pictured as "typical" installation according to the manual I have.

The laying of hands on the gas off discharge pipe would be the pipes going into and out of the secondary heat exchanger? I'll try and post a photo of the insides...

Posted By engineer on 28 Aug 2010 02:02 AM
Your plumbing arrangement has several shortcomings:

1)Configuring the DHW inlet from the boiler drain risks it ingesting sediment. It is best to discharge to the boiler drain bibb.

I know this is an opinion shared by many, so for give me for asking, has anyone ever had a DSH fouled by water heater sediment?
I haven't and I run this configuration. I however always employ a buffer tank (now).
Is it possible that without the catylast of the burner or heating elements, "sediment" is far less likely to form in a buffer tank?

Stuart, the reason for the parenthetical now above is that I once "drank the kool-aide" on the unbuffered DSH and a customer with an independantly metered HWT notified me that his energy consumption increased with dsh.
After we installed a buffer tank (at my expense) he advised me that the energy consumption was close to half the pre dsh readings.
We've had reports of R-22 systems performing well with no buffer, but I think it's safe to say you are more likely to regret the absence of a buffer than the presence.

Now that I am dispelled of the "no buffer" baloney, I can get back to looking forward to the Detroit Lions having a winning season this year
j

Posted By stuart.wyss on 27 Aug 2010 07:03 PM

The red "hot" pipe that comes from the DHW OUT on the Geomax and goes to the water heater is registering 86 F.

Is it correct to say that under current conditions, 86f is the maximum possible benefit that can be achieved from this DSH/system, (buffer tank or not)?  I assume the situation would be worse earlier in the cooling season due to lower EWT's.

Whatever the number is, I suggest doing a payback analysis on a buffer tank, considering the benefit you can achieve with your particular DSH/system.  Maybe it's better in your situation to get a buffer tank, or maybe it's better to turn off the DSH (if possible) when in cooling mode - the analysis will help guide you.  Fortunately, you have an AirTap that can help with hot water production.

Posted By geome on 28 Aug 2010 06:53 PM

Posted By stuart.wyss on 27 Aug 2010 07:03 PM

The red "hot" pipe that comes from the DHW OUT on the Geomax and goes to the water heater is registering 86 F.

1) Is it correct to say that under current conditions, 86f is the maximum possible benefit that can be achieved from this DSH/system, (buffer tank or not)?  I assume the situation would be worse earlier in the cooling season due to lower EWT's.

2) Whatever the number is, I suggest doing a payback analysis on a buffer tank, considering the benefit you can achieve with your particular DSH/system. 
3) Maybe it's better in your situation to get a buffer tank, or maybe it's better to turn off the DSH

1) no it would be correct to say there is a 16*f delta T between entering an leaving water at the tank. it is a useful number only in determining current configuration is moving heat away from DHW.
2) If Stuart's installer used the Geo Design software there is a quantified contribution (based on the 20 year average).
3) Generally the benefit of DSH is per person and per op hours of geo. A grossly oversized heat pump may contribute less due to short cycle times. A buffer tank can be an unwired 50 gal elec from orange store (~$350) and plumbed in series (upstream of finish tank) by most "handymen".
Is it worth $1,000?...maybe not. $500? usually.

I agree, turning the DSH off may be superior to the current configuration.
J

Under the current conditions, are you saying the 86f output could be higher with a buffer tank? If so, would you please explain how? I'd like to understand this better. Thanks :-)

Ok,
if we clarify "current conditions" as:
Constant EWT at geo
Constant EWT at dsh (meaning no change in DHW temp in tank/ elements aren't energized, water isn't used, no standby loss)
Constant EAT at geo
Constant humidity
Constant demand (outdoor air temps and infiltration to home causing constant operation in whatever stage unit is in....)
Original measurement was taken deep into cycle (not at start-up)
etc
then I agree you may suggest 86* will be constant, but only until heat from DSH is lost to ground loop (not long).

One of the biggest problems for young refrigeration techs is that pressures/charges/temps, seem to be a moving target.
In reality, it is the surrounding conditions that change.

So.... while you are correct in your "current conditions" assertion, I would tell you 86* will not be constant (as all other conditions are necessarily in flux).
The number is predictable, but not without glazing the eyes of the average tech.
Make sense?
J

Thanks Joe. I understand that a buffer tank may help at other times of the year too and I see your point that variables are constantly changing. I just wasn't sure how much contribution a buffer tank could help him in cooling mode with his system (and sensors, thermostat settings, weather, etc.) If I understand correctly, at a minimum, a buffer tank would prevent heat from being transferred to the loop from the water heater.

I was away for the day, and found so many replies to this thread when I returned! Wow. This morning, the AirTap was on, but the Geo has been off overnight, due to cooler weather. My wife and I had both taken showers, so the water in the tank had dropped below 105 (AirTap thermo setting). The AirTap came on. I checked the temps (by hand) of the red and blue pex pipes at the entry point to the GeoMax, they were both the same temp, cool to the touch.   I turned up the AirTap this morning to 115 since the Geo hasn't been on all night and the water at the shower/sink was warm, but not nice and warm.

Tomorrow, I will attach some thermometers to the pipes themselves (using the insulated foil wrapped cooking thermometer probe technique described in another post) and see what kind of delta I get from "contact" thermometers vs. non-contact. I'll also feel around in the cabinet underneath and try to determine what the DHW exchanger is doing. I'm also going to attach thermometers to my manifold in/out to get a good reading on EWT and LWT of the ground loops. I'll report back.

We're only 2 people, and I know my showers are probably 5 minutes max...I'm a Jack Reacher kind of guy...(reference anyone??)....my wife takes longer, but still about 15 minutes. We can't possibly using much hot water on a daily basis. Laundry's mostly done with a front-loading washer with cold water, and dishes are done by hand with a dishpan of hot soapy water with little running of hot water.

The buffer tank option has always intrigued me, but I have to weigh the pros and cons of extra plumbing and an extra tank that could rust out at some point and need replacing. I could certainly permanently (or with a switch) disable the DHW circulation pump, but I keep thinking the thermostatic switches should be "disabling" the pump if the water in the water heater is already at/close to temperature. So, maybe no benefit of having the desuperheater, but certainly no problem leaving it connected and letting the sensors do their thing. The bad thing would be if somehow the GeoMax is removing heat from the DHW lines and having to process it into the ground loop somehow, but that would mean the pump is physically running backwards, or the thermostatic switches are faulty.

I pondered on another threat about my winter heating and whether the disabling the desuperheater in winter would be a good idea, since it's taking some of the btus from the ground to heat water (my AirTap could do it during winter, it's an unheated basement), and the Geo was struggling to heat the house. Certainly, I've since identified duct issues, possible loop issues, etc., that probably contributed to the winter struggle. But in theory (?), would the desuperheater *not* switch on if the sensor on the heat exchanger lines determined there wasn't enough heat coming from the ground to be effective at heating water?

I never did install some sort of desuperheater manual cut-off...I didn't want to void any warranties.

I'll report back tomorrow some time (It's supposed to get to 91 F here tomorrow) and let you know what kind of "contact" readings I'm getting.

What would be cool, too, would be smart thermostats on water heaters. Imagine, the water heater calls for heat, and the AirTap goes on first (Stage 1). If the smart thermostat realizes the water temp isn't heating fast enough (delta/time?), it kicks in resistance heating (Stage 2), like a heat pump with auxiliary heat.

Our total water usage is low too, 130-140 gallons per day per the meter in our water softener. I believe that at most 1/2 of that usage is hot water. We have one 85 gallon Marathon. With your front load washer and shorter showers, your usage may be less than ours.

Based on postings, it sounds like you would get benefit from the DSH in heating mode. At the point aux heat would engage, maybe it would be better to turn off the DSH in heating mode. Of course, your system may not run aux heat this year if issues are addressed and the winter is milder.

It would be intriguing to connect a DSH shut off switch to the Aux Heat call. If Aux was called, DSH would turn off.

Posted By stuart.wyss on 29 Aug 2010 09:52 AM
It would be intriguing to connect a DSH shut off switch to the Aux Heat call. If Aux was called, DSH would turn off.

WF's Envision series turns off DSH in stage2. Other brands may do the same.

Geome,
I realized I only answered half your question.
If there is a buffer employed 85* is still warmer than ground water or even room temp making it useful in the replacement of DHW and keeping DHW costs down.....oh and as you said keep heat moving in the right direction.
j

Posted By Looby on 29 Aug 2010 10:02 AM
WF's Envision series turns off DSH in stage2.

Or stage 3 in a dual capacity unit.

Posted By stuart.wyss on 29 Aug 2010 09:48 AM
What would be cool, too, would be smart thermostats on water heaters. Imagine, the water heater calls for heat, and the AirTap goes on first (Stage 1). If the smart thermostat realizes the water temp isn't heating fast enough (delta/time?), it kicks in resistance heating (Stage 2), like a heat pump with auxiliary heat.

Since you mentioned controlling resistance heating in a water heater, we have an EH-40 digital water heater timer with battery backup from Intermatic installed.  About $75 + installation.  If you try it, make sure you only control power to the heating elements in the water heater and not the AirTap.  A timer would not work well without regular hot water usage times.

Thanks Joe.