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  1. #1
    Join Date
    Jul 2008
    Location
    DeLand, FL
    Posts
    37

    Heat Recovery Units

    For my new house, I asked my A/C guy (his own company, long time friend) to install a heat recovery unit for my domestic hot water like in my old house. He told me that it would require using an 'old tech' R22 system since the R410 units run too cool a temp at the compressor discharge. He installed an SEER 13 R22 system with the heat recovery unit and as in my last home, I get free hot water and switching the HRU's circulator pump off and on shows a 11.6 to 9.7 amp change in compressor draw. Obviously it's on the right track to save me considerable money...I run a summer power bill of $130 for a 2000 sq ft house here in cent FL. My question is, are HRU's becoming obsolete with the lower temps of an R410 system? If so, will the increased efficiency of the R410 HVAC compensate for the loss of free hot water? Still seems to me that since you are moving heat around anyway, you should 'put it in your hot water'. Thanks, Gary.:

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,123
    A 13 SEER R410A system has about the same compressor discharge temp as a 13 SEER R22.

    Your friend is mistaken.

    A higher SEER unit will have a lower compressor discharge temp.
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  3. #3
    Join Date
    Jul 2008
    Location
    DeLand, FL
    Posts
    37

    Discharge temp

    I wonder why the compressor discharge temp of higher SEER units would be lower? Wouldn't the temp difference be at the end of the condenser coil rather than the compressor? After all, you should still be removing the same amount of heat from the house and once the gas is compressed, I would think the temp would be about the same. Am I not understanding something about the refrigerant cycle?

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,123
    It is close.

    Higher SEER units use smaller BTU rated compressors then the rating of the condenser.
    So they aren't moving the same volume of gas, and the compression ratio is lower, so the compressor discharge temp is slightly lower.

    Remember, for every plus there is a minus, for every minus there is a plus.
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  5. #5
    Join Date
    May 2007
    Location
    Naples, Fl
    Posts
    889
    gary

    There are guys on here that are great with refrigeration theory and will give you the technical answer. I'll offer up field experience with a few high efficiency systems both R22 and R410a.

    The hot gas discharge temps on these high efficiency units will not get hot enough (normally) to close the NO furnace/fan klixon that is in series with the pump for heatpump applications. We have measured the temps with our clamp on thermometers. HRU aren't something we recommend but some customers insist on them. We aim to please.

  6. #6
    Join Date
    Jul 2008
    Location
    DeLand, FL
    Posts
    37

    Temp of compressor discharge

    Here in DeLand (cent FL), the compressor discharge temp is 195 degrees on an 88 degree temp day. The data from the HRU mfg states you need an minimum of 180 degree discharge temp so I'm ok there. Since the install my hot water heater has been turned off with no problems of hot water storage (I installed an 80 gal water heater). I also see compressor current draw drop from 11.6 to 9.7 amps when I switch on the HRU circulator pump. Apparently the HRU has about a 20% decrease effect on the head pressure...how this actually effects cooling capacity is something I'm not sure how to measure. I assume it probably does lower the system capacity but I don't know how to judge how it would balance out cost-wise. Hope I'm making my concerns clear....Thanks, Gary.

  7. #7
    Join Date
    Jul 2008
    Location
    DeLand, FL
    Posts
    37

    Thanks again!

    I'd like to thank the two of you for sharing your knowledge on this subject with me...I've had positive results with 3 HRU's but realize you guys have seen vast numbers of systems over the years and I really appreciate your valuable input...Gary

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,123
    At lower outdoor temps (where its too warm to open the windows and cool the house, and just hot enough inside to need A/C) it can have an adverse effect on system capacity.
    At outdoor temps at the mid range of outdoor design and above, it helps.
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  9. #9
    Join Date
    Aug 2006
    Location
    Dallas (Plano), TX
    Posts
    168
    This is an interesting discussion. Thanks for the contributions.

    I have two R410A 2-speed heat pump units (WaterFurnace Envision GSHP, 3 and 5 ton) operating in a hot attic in the Dallas hot weather heat at the moment. Here's a chart of the temperature of the compressor's discharge for the past 20 hours:




    These 2 units have enough capacity, relative to my residence's needs, such that they rarely operate in anything other than 1st stage.

    In cooling mode at the moment, it's my observation that, for 1st stage operation, neither compressor's discharge temperature gets much greater than about 125. In 2nd stage it's my observation the output temp gets up another 10 (F) higher.

    And here's a reference to R22 versus R410, written by DuPont, summarizing the performance differences (see Table 3): http://refrigerants.dupont.com/Suva/...pdf/h47125.pdf

    where, in summary, in cooling mode, for R410, EER is about even, discharge temp is about 10 cooler, and pressure is about 140 psi higher; in heating mode, for R410, EER is about 1% higher, discharge temp is about 14 cooler, and pressure is about 120 psi greater.

    Best regards,

    Bill

  10. #10
    Join Date
    Jul 2008
    Location
    DeLand, FL
    Posts
    37

    Discharge temp

    I'm so amazed at the relatively low discharge temp your system runs. You wouldn't think it could shed enough heat at the low a temp differential although I guess your water temp is around 70...still I'd expect much better transfer with about a 100 degree differential.

  11. #11
    Join Date
    Jul 2008
    Location
    DeLand, FL
    Posts
    37

    temp

    Thinking about it some more, do you think your systems are actually grossly oversized? I would think any system would run even more efficiently if it was operating at full design capacity..I'm suspecting that my 3.5 ton system has been effectively 'oversized' as I've added solar shades on the windows, HRU, solar attic ventilators, and more shrubbery on the west side of my house. The run cycles seem to be shorter (no real recorded data on this...just seems like it to me) than when we first got in our house..

  12. #12
    Join Date
    Aug 2006
    Location
    Dallas (Plano), TX
    Posts
    168
    Quote Originally Posted by garymunson View Post
    ... I guess your water temp is around 70 ...
    With a 3rd straight day expected here in the Dallas area of near or at 100+F heat, at this moment my Entering Water Temp is a nice, cool 76.

    And longer term, my EWT is staying below 80 so far this summer:




    A good rain (hasn't happened for a while) will knock back down the EWT a couple of degrees.

    Best regards,

    Bill

  13. #13
    Join Date
    Aug 2006
    Location
    Dallas (Plano), TX
    Posts
    168
    Quote Originally Posted by garymunson View Post
    ... do you think your systems are actually grossly oversized? ...
    I don't know.

    My residence is a 3400 sf, single-story, in the Dallas climate, with a huge poorly ventilated heat generating attic. The house has a lot of glass windows, and a lot of air infiltration via the glass, 6 ventilation fans, a fireplace, and poorly-sealed doors. I also don't have good attic insulation, and I have close to 50 non-air-sealed recessed ceiling light fixtures.

    Perhaps all of this was taken into account when my HVAC system I currently have was professionally designed and installed, resulting in 8 tons total of capacity - a 5 ton and a 3 ton unit, both with 2-speed compressors and variable speed blowers.

    The current operating result includes that I almost never run on anything other than 1st stage. So I'm essentially cooling the home on 5.6 tons with a water loop designed for 8 tons.

    From a humidity point of view, I don't have difficulty keeping RH at or near 40%.

    For the first 6 days of July (hot outside), I've averaged running the 5 ton unit 6.9 hrs per day. While the 3 ton unit averages running 9.7 hrs per day, I have zoning on this unit, which prevents an apples-to-apples comparison.

    Using GSHP technology, the run-times translates to about 38 KWH total. My electric rate is $0.11, so I'm spending, this month (July), about $4 per day for cooling. Or restated on a 30 day basis, about $125.

    You can see these numbers, as well as other operational numbers here: http://welserver.com/WEL0043/ .

    And here is thermostat / environment performance, for the 5 ton unit, to illustrate capacity versus demand:




    I don't have difficulty dropping the temperature 4 - 5F pretty quickly. Unfortunately, the house characteristics cause room heating to occur quickly.

    The tstat / environment performance for the 3 ton unit is more difficult to interpret because of using zoning technology:




    In summary, again, I just don't know if I have too much capacity, noting that I have a GSHP system, and noting the characteristics of my residence.

    Best regards,

    Bill

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