Where’s my free hot water in cooling mode?

Discussion in 'General Discussions' started by kkl, Dec 5, 2011.

  1. kkl

    kkl New Member

    This past summer, I was looking forward to getting the free domestic hot water from my GSHP that I’d read so much about. I was gone on a long trip, so I had turned off my hot water heater. When I returned, it was a hot day, so I turned on the A/C and left the water heater off. After several hours, I found that the water was still cold, so I started to investigate.

    I found that the incoming vs outgoing temperature through the DSH was only 3 deg F. I called my HVAC contractor who spent several hours taking the DSH apart and making sure that there was no sediment or air in it. At most, he got a 7 degree difference, but the following day, it was back to a 3-4 deg difference and it stayed that way pretty consistently.

    I kept asking what I should be getting. A vague reference in the manufacturer’s literature said 120 deg. The proposal for the system said 130 deg. Eventually, I was told that there wasn’t an absolute temperature, but that I should expect an 8-10 deg difference between incoming and outgoing. I never did achieve that in cooling mode.

    Now that we’re in heating season, I checked again. Surprisingly, I’m now getting an 8-14 degree difference and it’s actually heating up the water to above 120 deg. I would barely reach 100 deg in cooling mode under the best circumstances.

    So…, where is this free hot water I thought I’d be getting when using the A/C? Does it make sense that the DSH would work better in heating than cooling mode? I would think that the unit would be trying to dump as much heat as it could through the DSH in cooling mode.
  2. tstolze

    tstolze Member

    It is considered "free" because some of the heat that would be transferred to your ground loop or well is now being stored in your hot water tank for the cost of running the circulation pump. In the Winter you will likely have a better experience with your hot water, the unit diverts approximately 10% of the heating capacity to heating the hot water.

    I have a small Geo unit, 2 tons, during the summer my buffer tank only exceeded 100 on the warmer days. It's currently sitting at 105 after a cloudy day in the low 30's and will likely be around 115 in the morning.
  3. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    First you do not appear to have a buffer tank, so by design, you do not get anything useful out of the DSH.

    It is likely that your source loop does not get warm enough to allow high enough discharge temperatures from the compressor, thus not producing enough desuperheat. The good part is that at those loop temperatures your A/C will run very efficient. It is quite normal that your water gets only preheated to 90-100 degrees under those conditions.
  4. kkl

    kkl New Member

    Thanks for the replies. Almost everything I read about desuperheaters in operation seemed to talk about fairly high water temperatures. It's good to know that my situation is not that unusual.
  5. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    That is in heating mode.
  6. GCI

    GCI Member

    In southern, cooling dominant locations, you should get a lot of "free" hot water in the summer and not so much in the winter. The opposite is true in northern, heating dominant locations.

    The amount of hot water you get out of a desuperheater is all about run time. The more it runs, the more you get out of your desuperheater.

    If you have a two-stage unit, you won't get much hot water when it runs in part-load (low speed).
  7. kkl

    kkl New Member

    I have an oversized 8-ton unit that has fairly short run times and has never used the 2nd stage in over a year, so that seems to explain part of it as well.
  8. zacmobile

    zacmobile Guest


    Yes, I find the more undersized the heat pump, the longer the run time, the more desuperheater heated water.
  9. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Curiously you might also say, the more "right-sized" the unit, the longer the run time and the more hot water from DSH.
    One more reason bigger is not better in geo.
  10. zacmobile

    zacmobile Guest

    that sounds better

    I suppose "undersized" has some negative connotations attached to it.
  11. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    It does sound wrong from the beginning......:cool:

    Manny contractors build units which are too big, cost more upfront and run less efficient. it really is a rookie mistake
  12. CentralVA PremierE

    CentralVA PremierE New Member

    Oversizing is a rookie attempt to cover up for lame duct work IME. Assuming that it was not just an attempt to sell something, which does happen occasionally.

    Not saying that any of this happened in this case, but I have seen it on jobs repeatedly, so something to look out for.
  13. =====================

    I absolutely agree, hence the importance of accurate Manual "J". My fellow professional HVAC technicians who installed my system hit the calculations perfectly on my house. And adding insulation just improves things. But one of the neat things is one, the existing two part capacity of current 2 stage Copeland compressors, which adjust VERY WELL to differing load conditions (without any start and stop) and new variable capacity Variable Frequency compressor drives which are just coming out.

    As a geo fan I have to say that the current Japanese air to air units are PRETTY DARN GOOD. Mitsubishi and its sisters blow away traditional american brands. (I have four of these at work) (read the specifications and actual heat output) and of course the newest geothermal stuff from ClimateMaster / Carrier / Waterfurnace is (at least in concept) a "class act". COP numbers are unbelievable.
  14. Jerry_NJ

    Jerry_NJ Member

    "the newest geothermal stuff from ClimateMaster / Carrier / Waterfurnace is (at least in concept) a "class act". COP numbers are unbelievable."

    Do you have any numbers handy, for a 3 ton if that matters?

    I have posted on this forum before about my thinking it may be time to replace my existing Waterfurnace Premier 2 Stage that is almost 20 years old. It has good COP numbers and appears from my measurements to still be delivering about 4 in Stage I and 3.5 in Stage II. It is not a two-speed Copland, some "lesser" compressor. I assume for the same capacity unit I can continue to use the existing ground loop, which I hope doesn't disqualify the upgrade from tax incentives. Thinking, if a new unit has significantly higher efficiency COP would that have any impact on the loop design? I'd think not as the source/sink of the loop remains the same, a higher COP/EER simply means it takes less KWH to move the BTUs. That is with the same heat out (heating mode) or size the new unit would run as much, but at a lower KW consumption to get the higher COP.
  15. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Yes, you might be able to use your old loop, however, more efficient units use lesser compressor heat and extract more heat out of the loop. But generally it should work.
    Tax incentives are linked to the heatpump.
  16. ====================

    Start at the beginning. Professionals do calculations. For anyone to help you they need the original numbers, the original design capacity and the exact loop design, etc, how well it worked, etc. Then you know what you had at the beginning. As I remember newer COP numbers are like eight or 10 points higher than existing numbers. But REMEMBER the rule: Let somebody else test something brand new first before you try it (or else make sure you have the best warranty and servicers on the planet) ! (Older does not automatically equate to bad or inferior ! My two - stage Copeland draws 4.5 amps most of my winter), producing 80 - 90 + degree hot water at the same time.

    Most of these units are going to variable speed to get those numbers. How well made are those electronic boards ? What is the warranty on them and the whole unit ? As I have said elsewhere electronics are destroyed in microseconds by lightening and power surges. You repair electronics by buying new electronic boards. Out of warranty, they will be pricey. You probably will need decent surge protection for the unit if you want it protected.
  17. docjenser

    docjenser Well-Known Member Industry Professional Forum Leader

    Higher COP/EER means more heat extraction/rejection for every unit of energy used for the transfer. At the same output, more BTUs must be extracted/rejected to and from the loop.
    So a 3 ton units has lets say 30.000 BTU heating capacity and 36,000 cooling capacity at certain operating conditions. COP is lets say 4.0, meaning that 7500 BTUs are coming from the compressor and 22.500 BTUs from the ground (assuming simply that all the electricity is converted to heat, ignoring the power factor).

    Now a unit with a COP of 5 at the same output of 30,000 BTU now get 6000 BTUs from the compressor, and 24,000 BTUs from the loop. So the load on the loop does not go up much. Not a big deal.
    However, if your 20 year old unit has a COP of 2.4, and your new one a COP of 5.0, that might change things more....
  18. Jerry_NJ

    Jerry_NJ Member

    Some attractive numbers... 4.5 amps at 230 is about a KW, mine in low speed (highest efficiency) is about 6.5 amps, so if we're taking about similar heat output mine cost 50% more, or yours 33.3% less energy cost. That being a rough estimate I could save a third on my heating cost or about $30 a month, that would take some time to earn back $30K, say (shouldn't be that much but the numbers are easy to work in the head and the results at no more than 30% over), or about 1,000 months, taking it to be just have that long, 500 months isn't a reasonable payback period. That's about how much time I have on my existing unit, and the reason to consider replacing is if I replace on a preventive maintenance basis I can shop around and schedule for a convenient time. My 20 yo unit has a COP of about 4 at 32 degree EWT. I never see the temperature that low. I may buy some better thermometers just for accuracy. In my case I have to measure the EWT outside the pipe going into the HP cabinet - between the metal pipe and an insulation wrap, not a lot of insulation, just the typical pipe sleeve...maybe 3/8" thick.

    Doc, you say in part: "
    Now a unit with a COP of 5 at the same output of 30,000 BTU now get 6000 BTUs from the compressor, and 24,000 BTUs from the loop. So the load on the loop does not go up much." which I take to mean you are accounting for compressor heat. This is something I hadn't even considered, I always figured the heat in/out was due to an exchange in the circulating loop heat content. In my example on Stage I I measure (VA approximately Watts) 6.5x230 = 1.5KW (5K BTU), but I assumed the heat contribution trivial (for heating, a unwanted load for air conditioning). Are you saying if I get about 20K BTU (my spec is closer to 22K) that 5K is coming from the compressor heat? This heat is mostly going into the basement, the compressor sets below the plenum (the air flow to the house). That being the case I am getting only 22-5 or 17K BTU into my living space. I have made some measurements I'll spare you on here that suggests that is not the case, my tests seem to confirm the spec of 22 K BTU delivered (actually my measurements were made in Stage II as I needed to keep the airflow constant and measure the effect of switching 5KW of resistive heat in/out and measuring the temperature difference in that air flow stream, the Stage II COP is more like 3.5 - going from memory).

    My elementary and poor memory of thermodynamics tells me if a system is getting 1.5KW input it has to deliver an equivalent output which can be in the form of mechanical energy and heat loss in the case of a compressor. I take the mechanical to be the compression of the refrigerant.

    As for new/old technology it is my belief that the Copeland compressor is well tested (has been in wide use for more than 10 years) and is more efficient than older designs but just as durable, maybe more durable. As for the variable speed, seems like a great idea to save on stop/start cycles, my two speed comes to a complete stop when gong from I to II or back down, lots of stop/start wear, still it is still running after 20 years, so I'd not be inclined to pay more for a variable speed unless it had significantly higher efficiency and/or longer life.
  19. mtrentw

    mtrentw Active Member Industry Professional Forum Leader

    It seems to me, energy in is energy in. When I put watts into my system, either with the compressor, or my circ pumps, it will end up as heat. My 300 Watts in the circ pump is going to turn to heat. If it stayed pure mechanical energy, my loop would continue to accelerate to infinite velocity. It doesn't. Pipe resistance/friction will convert that mechanical energy back to heat. The compressor, like any mechanical device has a heat byproduct as well.
  20. ssmith

    ssmith Member

    Here's the method I use to measure the COP of our system:


    It measures the heat extracted from the loop and adds the compressor portion after adjusting for power factor. I also use a tc to measure temp of the loops brass fittings under the insulation, and measure voltage and current in the main panel. I haven't measured flow. Just use the number that our installer said he set the flow to. I initially thought you needed to know temp and flow of the air you got out of the unit to calculate COP, but I guess this way works too if you figure what is extracted from the ground becomes heat in the house. Maybe those who know a lot more than I do can comment on this.

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