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Plumbing and furnace efficiency

lance_p | Posted in Energy Efficiency and Durability on

I have two questions regarding how products are tested, and the reasoning behind:

1. Supply pressure for water fixtures.  When we talk about a 2.5 GPM showerhead, what supply pressure is maintained during the test?  Looking around the web I found 60 psi to be a common point of reference, but I couldn’t find out if this is how they’re tested.

1a. Does anyone here have any experience with running their water system at lower supply pressures, and if so, how low did you go and what were the results?  My city water is 60-70 psi and we have issues with faucets spraying so hard that water splashes out of the sink.  Our new house will be on a well and I’ll be able to set the operating pressure.

2. Furnace efficiency.  When a multi-speed or modulating furnace is tested, I assume it’s tested and rated at full output.  Would the efficiency of the furnace not improve if operating at a lower output?  I’m thinking if heat input to the exchanger is reduced or modulated to a lower percentage than the air flowing across it (40% heat, 70% airflow for example), the efficiency should improve.  If so, could a 96% furnace achieve higher than that in actual use?

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  1. Expert Member


    The only device or appliance I've ever had problems with due to low water pressure was a commercial dishwasher. Experiment a bit. I'd bet 45 to 50 psi will be good.

  2. Expert Member
    BILL WICHERS | | #2

    A typical well pressure switch is 30-50, which means the pump starts at 30 PSI (the “cut-in” pressure), and stops at 50 PSI (the “cut-out” pressure). That would imply that 30 PSI is an acceptable lower pressure limit. I personally use a 40-60 pressure switch which gives a little higher pressure which I prefer.

    RO (reverse osmosis) filter systems are more efficient with higher water pressures and will stop working completely if the pressure drops too low. You can get small booster pumps for these systems if you don’t want to run your entire well system at higher pressures.

    A furnace, and pretty much anything else, will typically be most efficient at or near full rated capacity. That is in terms of how much useful product you get out for total input energy. The advantage to the modulating systems is that you use less total energy to keep your house comfy (and sometimes modulating systems also allow for more comfort than a simple on/off cycling system) even though the actual efficiency of the furnace itself will vary. Hopefully that makes sense.


  3. Expert Member
    Dana Dorsett | | #3

    The gpm ratings of fixtures are always at a presumptive 60psi, but half that is fine. As long as the static pressure is at least 30psi measured at the highest elevation faucet/shower head in the performance will be fine for any appliance or practical purpose.

    The AFUE test for single stage furnaces traditionally (since at least the early 1980s) presumed a 1.7x oversize factor at the 99% outside design temp, and works backward from there toward a tested duty cycle. With multistage and modulating equipment that has been modified, but it's definitely NOT simply duty-cycling it at high-fire with a presumptive 1.7x oversize factor. I believe the current standard for multi-stage testing is ASHRAE 103-2007, which ends up duty cycling most 2-stagers at low fire, but at a higher duty cycle. Knock yourself out figuring out what it really means from the NOPR comments from manufacturers in 2015:

    It's probably clearest starting at the bottom of page 20:

    "However, the two-stage and modulating calculation method in ASHRAE 103-1993 can result in an AFUE of more than one percentage point higher than the AFUE resulting from the single-stage calculation
    method. The reason for this discrepancy is that ASHRAE 103-1993 assigns different on/off times to single-stage and two-stage/modulating units. ASHRAE 103-2007 resolves the inconsistency between the two calculation methods by calculating the on/off cycle times for two-stage/modulating units while maintaining fixed on/off times for single-stage equipment. The resulting two-stage and modulating on/off cycle times are closer to those specified for single-stage units, as one would expect based upon their operation. "

    So a 96% efficiency 2-stage or modulating furnace basically operates at that efficiency at low-fire, and won't perform better than that in-situ. It can however perform worse than that if it's set to kick to high-fire too soon in an extended call for heat.

  4. tommay | | #4

    Most shower valves have a volume control, learn how to use it. If your vanity faucet is out of control, control it by slightly closing the supply valves under the sink.
    I would believe that the entire duct work would have some kind of effect on airflow......

  5. lance_p | | #5

    Malcolm, yes I can see a commercial appliance suffering from low pressure. Makes sense. I wonder how residential dishwashers would fare with a lower supply pressure

    Bill, part of my question stems from the typical 30-50 well pump switches. My house will be two story with basement and 9 foot ceilings, so about 24 feet of head loss from the pressure tank to the top floor showerheads, or 10-11 psi. If I want 30 psi minimum on the second floor I may have to go with the 40-60 switch.

    I already bought my RO system, and yes it's a booster pump model; iSpring RCC7P-AK

    Dana, figures every time I assume something isn't that complicated... well, this happens. Looks like I don't need to be losing any sleep over a point of efficiency here and there. Kind of disturbing though that the rating is taken at the unit's peak efficiency and not its highest output. You'd think they would have to specify the efficiency at maximum. Oh well.

    Tom, you're speaking a different language than some people use. Control? What is this newfangled word and what does it mean? Everything is just supposed to be wide open all the time, man! Brushing your teeth? You need enough water to just about blast the paste clean off that brush when you wet it. Washing dishes in the kitchen sink? The water needs to be so hot you need gloves to keep from burning yourself, and the tap goes from full on to full off and back again, there are only two settings. Not unlike the heat in the car...

    An oversized filter and low velocity ductwork are in the plans for the furnace to keep the airflow as high as possible through the unit with minimum fan effort.

    1. Expert Member
      BILL WICHERS | | #6

      I’d go with the 40-60 pressure switch. Note that they are adjustable too, you can turn a nut inside to bring the pressure up. ONLY adjust the main pressure nut (it’s usually the one on the biggest spring in the unit), don’t adjust the differential pressure (the difference between the cut-in and cut-out pressures). Don’t overdo any upwards pressure adjustment.

      I recommend getting a good-size pressure tank for the output of your RO system. I used to have a 3 gallon tank, but I was never happy with it and recently (about a month ago) I added an additional 14 gallon pressure tank. HUGE HUGE improvement in usefulness of the system! HIGHLY recommended!


      1. Trevor_Lambert | | #7

        Why not adjust the pressure differential? I've adjusted mine to 40-66psi. It increases the effective size of the tank.

        1. Expert Member
          BILL WICHERS | | #8

          Every pressure switch advises not to change the differential. In my own opinion, widening the differential like you did shouldn’t be a problem, but it’s still not recommended. Reducing the pressure differential could be an issue if shortened too much since short cycling a well pump can damage it.


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