DHW temperature

Discussion in 'Open Loop' started by clockwalk, Aug 2, 2011.

  1. clockwalk

    clockwalk New Member

    My system is a Water FurnaceEnvision 049 four ton water to air heat pump using a open loop from my well and then dumping back into it. The well is my domestic water source also.System was installed in October 2010 here in New England. So far this summer I am waiting for all of that "Free" hot water but my electric bill for june was more than last June when I had a conventional electric AC system and an oil furnace for hot water. Here are some facts. Last week the outside temperature was in the 90s . I have the thernostat set for 75 degrees which is very comfortable and the same as what I used to set it at last summer.The EWT is 61 degrees. During the winter it was in the high 40s. The LWT is now 75. Water temp going to my storage tank is at most 70. I am using alot of electricity to get my DHW up to 130.
    During the winter I adjusted my GPM to 8 and I am wondering if I should reduce this to around 3 or 4 to raise the temp of the LWT? What should my LWT be and what should I expect the temperature to be as water is entering the storage tank?

    Thanks for any help,
    Ed in Rhode Island
  2. ===========
    Ed, There are LOTS of variables, so I will try and stay with principles. You didn't seem to say you have a desuperheater, thus two separate heat exchange coils, but I will assume it.

    Different heat pump manufacturers will control the water circulation for heating hot water differently. (Control strategies) This may be a simple temperature klixon switch on a discharge line. Could be carefully placed thermister probes on 3 different lines.

    This much is sure. Unless you have a circuit board with thermisters, or a carefully calibrated temperature switch, you can remove temperature from the compressor discharge side that maybe you shouldn't in the winter, or have a situation like you are describing in the summer.

    Normally you should get all the heat you need if it is over 80 degrees outside in the summer and you are trying to cool your house. (cooling mode)

    I don't want to waste time talking about using the two tank hot water system, using the desuperheater as a primary heating tank, feeding the final tank to get the most out of the system. There are probably 12 excellent posts elsewhere covering this. (You can read them on your own time).

    Understand this, you just said that enterring water temperature is 61 degrees. Essentially this means that all your heat is being easily carried away by your primary water flow. There is no heat left to transfer to the desuperheating hot water coil. You are probably not getting anything yet from it. In fact if the heat pump circulating pump is running and the electric hot water heater is turned on, you actually may be transporting heat from the hot water heater to your main water loop lines which is the opposite of what you want.

    You can easily tolerate lower water flows, but you need to use instrumentation like 2 or 3 strapped on probes from a Cooper Atkins temperature meter (or equivalent) and either use pressure gauges in Pete's ports, (Your hvac installer can train you) or (what I prefer) a PERMANANT visual indication translucent flow meter which shows you your open loop water flows in REAL TIME. I have a post on this elsewhere. You need to know what the heck you are adjusting when you turn those ball valves.

    With temperature probes (or some equivalent) strapped to your inlet and outlet main water flow piping, you adjust high flows downward until outlet main water temperatures begin to RISE.

    It is complicated because basic factory charts assume NO desuperheater, thus they are misleading. Also dual capacity Copelands are tricky because you have two DIFFERENT flows going through two separate water flow valves. Sometimes one water valve is open. Sometimes two water flow valves are open.

    The rule of 1 1/2 - 3 gallons flow per minute per ton is also slightly misleading because it should be considered a "guide" or starting point for flow. You rarely need to go higher, even in the winter, you can almost always go lower, especially in the summer.

    I doubt if you will get ANY heating out of the desuperheater if the main water leaving temperature is not at least 88 degrees. I shoot for 88 - 95 degrees on my personal unit.

    Those are my starting thoughts. They are not final. Try and work with your installer. Clear things with the manufacturer of your equipment so you don't mess with warranty.
  3. AMI Contracting

    AMI Contracting A nice Van Morrison song Industry Professional Forum Leader

    Ed you said storage tank, so by that do you mean unpowered buffer tank upstream of a powered finish tank, or hot water storage tank with DSH and electric elements working together?
    When you mentioned 70 degree water going to "storage tank" what was leaving temp (storage tank to DSH)?
    If you do not have a buffer tank, you should start there. If you have 70F water at DSH in and out then your circulator may not be energized.

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