50% is more of a worst case scenario, and, admittedly, a SWAG. I believe newer, particularly power-vent gas water heaters have much lower standby loss. Please don't make any economic decisions until you tighten up my guesstimate.

Also, when I mentioned ground water temperature I was thinking in terms of input to the water heater - how much rise it has to provide. Municipal water from surface sources will vary a bit from summer to winter. OTOH well water from down deep should be pretty constant year round - mine is around 71 all year.

A family with surface muni water using the same amount of hot water, summer and winter, will need more heat to produce it in winter. Hot water use may rise a tad in winter as family members linger a bit in a shower on a chilly morning, as well as possibly set shower temp a bit higher for same reason. Standby losses in tanks and lines will rise a bit with lower ambient temps.

Another factor to consider is that geo heat pumps generally have more heat for recovery to hot water while operating in heat mode - system pressures and superheat are generally higher - compressor work / power / energy is on the 'good' side of the system during heating mode.

engineer, thanks for the note.

Until I have a better means of estimating how much HWH burner heat actually gets absorbed by the tank's hot water, I'm going to leave it at a 50% derating factor of my 40 KBTU/hr nameplate rating. My HWH is a standardard traditional design - no power venting for example.

Plus this keeps the analysis on the conservative side when looking at how well a direct-connected DSH really is performing.

I have a long range 50 week chart keeping track of the compressor's discharge temperature on a daily high basis. If indeed there's a higher discharge temp for the DSH to use, in heating mode, it will show up on this chart ( DailyHighTempsR410ARefrigerant ).

Normal high compressor discharge temp, for my R410A system, in cooling mode, is consistently at 125° F (1st stage only).

Best regards,

Bill

I wouldn't assume winter versus summer without exact information. I'd be inclined to think I take more showers in the summer, with several days doing 2 showers just to cover the more sweating I do.

Senecarr:  Studies do show the increased energy use for hotwater in the winter.  I was surprised also until I thought about it.

Example:  HWH = 130F, incoming summer water = 70F, incoming winterwater = 55F.  Nozzle shower temp = 110 F providing you with 106F water (loses 4F due to evaporation).     

For each 10 gallons of water used:

Summer shower requires:   6.6 gallons hotwater and 3.4 gallons unheated water
Winter shower requires:     8.7 gallons hotwater and 1.3 gallons unheated water.

As you can see, the winter shower takes much more heated water.  In addition the water heater is usually in an unconditioned space so the HWH loses more heat to the air.  This is probably a bigger culprit than the 120 extra BTUs it takes to heat a gallon of winter water.

Alex

But at 6.6 gallons of unheated water, if I shower 30% more in the summer, I'm coming out about even in number of hot water gallons used. My point isn't that it's one way or another, but that I wouldn't make assumptions without data for the specific family. For example, I have 3 kids, 2 of them under 3. I find they tend to make some of their own warm water with most baths. 2 teenagers won't do that.

Good points, though I don't plan to adjust my estimates of domestic water use by an "incontinent toddler factor" - hard to reliably quantify - my two-year-old wouldn't hold still for measurements to be taken.

A household using same amount of hot water was a stipulation (I wrote "using"), not a statement of opinion or fact (I would have written "uses") for purposes of example calculation.

I too take an extra shower or so, sometimes three per day during hot summer months.

I haven't measured my system's compressor discharge temp in cooling other than via brief finger contact. 125 Deg F is probably about right for mine in low stage. My older R22 system used to run around 160 Deg F. I'm not sure how much of that is due to greater 410a efficiency - it may be more a result of tighter evaporator superheat control by newer system's thermostatic expansion valves. My understanding is that Older systems with fixed orfices (cap tubes or pistons) were charged so as to have higher amounts of evaporator superheat to protect against compressor flooding in lower ambient temps. That higher superheat is carried through the compression cycle and results in higher discharge temps.

 

I changed my efficiency factor for my HWH to 60%, based on this reference: http://www.aceee.org/consumerguide/waterheating.htm .  I have a 'conventional gas storage' HWH.

For my 40 KBTU/h burner, my WEL data will now show 24 KBTU/h of heat actually going into the water, to be compared to the amount of heat also going into the water from my GSHP's DSH.

Best regards,

Bill

Summary of questions:
- Is a desuperheater a worthwhile investment in Vancouver, BC (no need for summer cooling)?
- Is there value in using solar panels to recharge the ground loop in summer?
- Are there bacterial growth concerns when using a DSH with a preheat tank?
- Does anyone know of an auxiliary heat pump that can supply DHW year-round using the main system's ground loop?

I'm a mechanical engineer (though not an HVAC engineer) and am planning to buy a ClimateMaster Tranquility 27 2 ton unit for my relatively small house in Vancouver, BC.  Because of the moderate climate here, the primary use is for heating (we have about 2-5 days per year when A/C would be a nice bonus, so it's hardly worth mentioning), though I understand that it's helpful to run the unit a bit in the summer simply to help recharge the ground reserves.

My main question has to do with the desuperheater (ie, whether I should buy one or not).  My understanding is that the 'free' energy in the summer is essentially the heat that would otherwise be diverted back to ground.  Since that's the main reason I'd be running the unit in the summer, I'm not sure it's a good use of the energy.  Does anyone know what the effective COP would be for heating water in the summer if I ascribe no value to having cool air?  Ie, COP = hot water contribution / electrical energy consumption.

Actually, I'm also wondering what the value would be in using a solar panel plumbed directly into the vertical ground loop to charge the ground with heat energy year-round.  My current yearly space heating requirement using a 93% gas furnace is about 75GJ, and I estimate that a single solar panel could provide about 6GJ/year;  at about $700/panel, I could put 18GJ into the ground for a capital investment of about $2k.  Any thoughts about whether this is worthwhile? 

Going back to my question about buying a DSH - I understand that during the heating season here (Oct to Apr or May), any energy supplied from the DSH is being delivered at about the COP of the heat pump overall (roughly 4.0), but it appears that the amount of energy delivered is related directly to the usage of the heat pump and that it effectively supplies only 30-50% of the HW demand, which means that the contribution to my year-round demand will only be about 15-25% and the bulk of my heating will be via a direct heating option (eg, electric).  Since the DSH requires a fairly significant investment, I'm wondering if I have better options.

One possibility would be to add a preheat tank to capture more of the energy (ie, if there is no competing heating element in the tank being heated by the DSH, there will be more net energy transfer), but my concern is that this tank will end up sitting at a warm temperature which could encourage bacterial growth.  Current recommendations seem to be that water tanks should be kept at above 55C and that mixing valves should be used to step down the temperature to <50C for delivery, which would seem to rule out preheat tanks.  However, solar systems must deal with this issue all the time - does anyone know how they resolve this conflict?

Alternatively, I've also read references (http://www.faithheating.com/geothermal.html) saying that "manufacturers are beginning to offer "triple function," "full condensing," or "full demand" systems that use a separate heat exchanger to meet all of a household hot water needs."  This would seem to be exactly what I want - I'd like to use a small heat pump running off my ground loop to heat ALL my hot water.  Unfortunately, I haven't found any references to a specific piece of equipment which could do this.  Does anyone know (a) if ClimateMaster offers such a heat pump, or (b) if anyone else offers one that could be used in this way and tied into my setup?

Thanks in advance for any help you can offer.

Knowing that it's a climatemaster means that yes, a DSH would help in the winter too (in DX geothermal systems, DSH only operate in cooling mode and require a much more expensive hot water assist). At that point, it's not "free" heat, but it is heat made with the much higher COP of the climatemaster system than that of a normal system. Preheat tanks are almost a necessity to get good efficiency out of a DSH.
If you are looking for pure hotwater needs, and you are already working with a climatemaster installer, you might want to look into the climatemaster tranquility water to water series. I'm just not sure anyone outside of the ancient Romans would use enough hot water to make the system pay back in a timely manner using a system for hot water alone.

Thanks for your comments. Does anyone have any documentation of the amount of energy I could expect from a DSH operating during the winter season in the Pacific Northwest either in a preheat-only mode (eg, with a tankless heater following the preheat tank) or in a less expensive single-tank with electric heater mode? My concern with the electric single tank heater option is that both the DSH and the electric heating element will be functioning simultaneously (eg, in the morning after showers), so only part of the replacement heat will come from the DSH, and then once the tank is back at full charge, no more energy will come from the DSH even though the heat pump might be operating for much of the rest of the day.

Actually, would it be possible to put the electric elements on a timer such that they can only come on for the last hour before waking and before dinnertime? That would ensure that the bulk of the heating came from the DSH operating throughout the day to replenish morning use of water and throughout the night to replenish evening use.

Also, I'm presuming that there's no way to set up controls on a Tranquility unit so that it produces heat only for the DSH during times in the spring, summer and fall when there's no call for space heating. If that ability were available, it would markedly increase the attraction of the DSH - effectively turning it into a water-to-water heat pump when space heating was not needed.

I checked out the CM TMW line, but that's a 28 ton unit! I definitely don't use that much hot water. I'm looking for a unit that will be able to handle just the relatively small amount of energy needed for domestic hot water (about 25GJ/year, based on my current summer gas consumption records). If anyone knows of such a unit, I'd appreciate hearing about it.

The closest I've come to hearing about something like this is the AirTap unit (http://www.airgenerate.com), but this is an air-source heat pump rather than one that takes advantage of the higher COP available from the Tranquility refrigerant cycle. However, it's only $700 and offers a COP about 2.5 year-round, so that's fairly easy to justify. Of course, the heat in the air that it pumps to the water is generated originally from the GSHP, so that means the 2.5 units of heat delivered costs 1.0 unit of electricity to pump from the air and 1.5/4 = 0.4 units of electricity to heat the air before it enters the AirTap unit (at least during the 6 month heating season), or 2.5 units of heat for 1.4 units of electrical input (net COP of about 1.7 in the winter; 2.5 or better in summer), so it's nowhere near as efficient as getting 3 to 4 units of hot water for 1 unit of electricity as we would if we could use the GSHP directly. However, if it works year-round, it sounds as though it might end up being cheaper than the DSH. Any thoughts?

TMW? I'm refering more to the new THW series. http://www.climatemaster.com/index/res_watertowater_page

A DSH is only designed to supplement your DHW. There is no way to make it an on demand unit.

Attached is  a page from the Climate Master Manual.    The HWC column gives you the capacity of the desuperheater.

Thanks - I'd looked at that manual and missed that table.  For an 026 unit, I'm reading that the hot water capacity for 30F entering water is 2 thousand BTU/h or about 500W, which means that it would take about 2.5h to heat a 200 litre (50-60 gallon) tank by 10C;  to increase the temperature from 10C at entrance to 60C (use temperature) would take about 20h, so it could fill this tank about once per day.  Does this sound about right?

As mentioned above, I've been trying to think about how to combine solar and heat pumps, and I'm wondering if the following would work:

1. assume that I have an indirect heat tank available
2. charge the DSH with the same closed loop fluid used in the ground loop and set up the DSH to pull from the bottom of the indirect loop in the tank and return to the top of the loop
3. for the solar, pull fluid from the rising in-ground line (ie, after it's reached equilibrium with the ground), feed it through the solar panel, send the hot fluid to the top of the indirect loop and discharge back to the incoming ground loop pipe before it enters the heat pump.  Add a bypass around the tank if it's already at 60C so that the hot solar fluid mixes with the ground loop fluid before entering the heat pump, which should improve the COP and elevate the temperature of the loop fluid returning to the ground.  During summer, the heat pump could be bypassed, thereby using the solar only for hot water and recharging the ground.

Does this make sense?

Sorry - in the above, I meant to say that 500W would produce a 2.5C temperature rise in one hour, or 10C in 4 h, hence 50C in 20h.

For my WaterFurnace hot water capacity (HWC) is based on an entering domestic water temperature of 90 F. I don't know, but strongly suspect that HWC is quite dependent on entering domestic water temp. Be sure to distinguish between entering source water temp and entering domestic water temp.

Moreover, I read here from others and can corroborate with my own estimate that, at least in cooling mode, that the refrigerant temperature coming out of the compressor is around 125 F, or about 52 C. That means it is physically impossible for the unit to produce domestic hot water at 60 C. As a practical matter my preheat tank has yet to exceed 40 C. Do you really need 60C domestic HW?. My water heater is set for 45 C for both conservation and safety of small children. Lower temps also help water heater live longer. I don't yet have experience with lengthy operation in heating mode - system has only been in since spring and we are only now just getting into what passes for a heating season in North Florida. In my circumstance of year-round 71 EWT I expect both higher superheat temps to recover from and higher HWC in heating mode.

Also, I suspect that if a system were to be running 20 hours out of a 24 hour day, it would be substantially undersized. I think you understand that with these systems DSH is only available when the system is running to meet house's space heating / cooling load. In other words the DSH is NOT a separate feature independent of main system operation. There is a geo heat pump variant available that runs specifically and solely in response to calls for hot water - a worthwhile capability, but the only brand I know of that does that has not enjoyed a reputation for reliability here.

I think (other folks help me out here) that on a design day a system should run no more than 12-15 hours - much longer and occupants will notice thermostat set points not reached and be uncomfortable.

You have hit upon one of my favorite talking points - size a preheat tank to hold approximately one day's hot water use.

As for the workability of your integrated solar panel, I read it three times and my head began to spin. Seriously, it is probably doable but the cost, complexity, and payback might all be prohibitive. Still, as an ME you could work out the necessary controls and limits.

The larger question of whether summer heat can be deliberately transferred and stored underground for use during the following winter is interesting. My opinion (no data) is that it might work in the case of dry relatively low conductivity soils (sub-optimal geo conditions necessitating long ground loops) but in more typical circumstances aquifer movement would carry off the extra heat before it could be recovered.

I have seen a discussion of long term multi-year loss of loop field capacity in a large (hundred + wells) loop field. As I recall that kind of system serving big commercial buildings is cooling dominated owing to internal loads. That implies (undesirable in that case) long term storage of heat underground. I suspect that case is irrelevant to typical residential conditions, though it could pose a potential problem in a dense subdivision with lots of geo systems. We'll have to keep an eye out for that if geo takes off.

Thanks again for your comments.

Re: entering vs source water temperature - my understanding is that the most efficient use of the DSH is to pull water from the bottom (relatively cold part) of the preheat tank and return it to the top (hot side). Thus, early in the reheating process, the temperature difference will be relatively large and will asymptotically approach zero as the tank warms up.

Thanks also for clarifying what the refrigerant temperature is (52C). I suspected as much, but didn't have that number at hand. The reason for wanting the tank temperature as close to 60C as possible is to decrease the risk of bacterial growth - the consensus recommendation appears to be to keep the tank above 55C and to use a mixing valve to drop the delivered water temp to below 49C for safety. A higher temp also increases the energy density in the tank and decreases the overall tank size required, though obviously at some performance penalty if pumping heat into the water (apparently the increased temperature differential and decreased surface area roughly cancel out, so energy losses are approximately equivalent for a large tank at a lower temp and a smaller tank at a higher temp).

I don't expect the heat pump to be running 20h per day - I was just trying to estimate how much recovery I could actually expect in order to decide whether it makes sense to pay for the DSH (about C$900 / US$700) or to simply add a separate heat pump like the AirTap for the same price that could be used year-round. My total HW costs are likely to be about C$400 (20 GJ/year) if I use a tankless electric water heater, so the total annual savings potential is not terribly high anyway - if the DSH contributed 20 MJ/day (2MJ/hour x 10h/day in winter) for 100 days, that would be only 2GJ/year, or 10% of my total annual usage, so the $40/year savings might not warrant the price of the DSH (15-20 year simple payback). If anyone has any data on what the actual savings might be in real use in the Pacific Northwest, I'd greatly appreciate hearing from you.

Re: the solar augment - the reason I started thinking about this was that my installer said that they recommend running the heat pump in the summer even if we don't need the cooling simply to recharge the ground. I had the same concern you expressed about ground water flow taking the heat away, but the installer seems to be of the opinion that the heat from the pump in summer is needed to recharge the ground, which implies that his experience is that the field experiences net annual declines in temperature and does not fully recover over the summer. I'd really appreciate seeing any data on this.

I first came across the basic idea when I read about the Drake Landing project in Alberta (http://www.dlsc.ca/how.htm) where they use solar panels (about 16 per house on average) to charge the ground; they expect that over 5 years, the temperature will build to 80C at the end of the summer and that this will enable them to supply the heat required with an effective COP of about 9!

As for the cost of doing a solar augment, I thought it would be relatively cost effective, but maybe I just don't know how expensive some of the components are. I was thinking that the main costs would be the indirect heating tank (about $1500), the solar panels (about $2000), a circulation pump, a controller and a couple of valves. If this could provide two thirds or so of my annual hot water, that would be worth $250-300, so the simple payback would be in the range of 15 years, not including any benefit from recharging the ground.

I've kept mute for a time but I'm a little skeptical of the notion that "recharging" the ground has significant long term benefit. We all know heat migrates which means that if you heat the ground around your geo heat exchanger, it can not stay warmer than the adjacent earth. Therefore unless your plan includes total "global" warming, the benefits will be short lived.
Regarding the desuperheater, the rule of thumb is that a hot water generator offers about 10% of it's btu's when the appliance is running, therefore I wouldn't expect much out of a 2 ton geo.
That said if the demand is so small for your home, solar does have possibilities (where geo may not). Check with the solar forum.
J

I concur with Joe. There is no way to store heat in the ground. Plus the notion that the ground needs "recharging" is with out basis. The sun does a great job of recharging the earth. Just make sure that the loop is properly sized to meet the needs of your system.

Joe, that 10% rule of thumb is helpful.  Since my space heating needs are currently ~75 GJ/year, that suggests that I could probably expect about 7.5 GJ out of my DSH (about 30% of the total), which would be worth about $150 at an electrical cost of about $20/GJ.  The cost to obtain this heat would be about 1/COP of the amount delivered, for a net benefit of about $112.50/year, which I'd have to offset against the cost of the DSH itself.  This would make for a more attractive simple payback period of 8-9 years.

Re: whether or not the earth can store energy for months at a time - the fact that you can deplete the area around the ground loop over several months of the heating season does suggest that the time constant for replenishment is on the order of months, so to my mind, it's at least plausible that this could be done (though the Drake Landing info also suggests that the charging time constant is on the order of years).  I'll check with my installer about his experience with this and see if he has any concrete data.