question on shower & hvac logic


Can someone explain why on earth some shower controls turn up the cold water 100% - then - gradually start turning up the hot water ???? (ie. you need to twist them in one direction to first get cold going, up to 100%, then it starts to allow hot water to mix)....
This seems like a colossal waste of water (since you're running the cold valve wide open at max pressure) - vs. the pull out type control where you can control pressure & hot/cold mix by pulling out for pressure and twisting for mix).
Which brings us to HVAC systems - i've seen building "environmental" controls that run the AC continously - and when a target temp is reached - instead of turning off - they turn on the heater !!
WTF !???? wouldn't it make more sense to just turn off the air conditioners ?? (instead of turning on the heater - while the air conditioner is still running).
hell of a way to waste energy.
is there some thermodynamic reason behind this (on the surface) idiotic logic ???
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TEK1 wrote:

Don't know about the HVAC but for tubs & showers it is for safety. Harder for kids to burn themselves if they have to turn past full cold to get hot water. Most now have an adjustment also. I have theirs set so that even if they turn it all the way to the left they can't get burnt.
New showerheads have a restrictor the limits the amount of flow to save water. Kramer can get you one w/o the restrictor but it could cause problems.
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Safety? I don't know . . . It seems that a child, or someone like my confused old mother, would panic on getting hit by the cold water and quickly push the control around to full hot. And in a nursing home there is no one who cares about stuff like hot water temperature. But at least it's an excuse. It's the first I have heard.
I hate them. It's good to see I'm not alone! I had encountered them only in hotels and assumed that was because they were cheap for some reason. It was a disappointing surprise to find them in my new house! Joe

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TEK1 wrote:

1. If a hot water recirculator is running, then the hot wter comes on at nearly full temp within a very few seconds of turning the tap on 2. See reason #1, recirculators are becoming more popular and the scald risk with 125F-135F water is real. Set you water heater for temps lower than this and you risk bacterial infection from Legionnella (sp)

1. Lots of people inside that dissipate heat and humidity to the building 2. Lots of equipment inside the conditioned space that generates heat (computers, TVs, Lights)
Even with the need for heat, there is a significant need to dehumidify, so the AC runs all the time.

or above the desired temp. When heat loss to the outside is low (moderate weather conditions outside), AC load drops enough that heat is required.
AC gets sized for expected max occupants, max equipment thermal load, heat gain/loss thru windows/walls and doors and for max outdoor temp. If anything is less than max (people, equipment, heat gain from outside), the AC loafs along, and may overcool the building. So add heat to get the outlet temp back to spec.
Should be noted that these systems run the fans 100% of the time to circulate air in all areas.
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We did a multifamily building that very nearly had the AC situation you described.
If you are running a "blower" unit, you have your choice of a 2-pipe or 4-pipe system. 2-pipe systems are cheaper and the whole building runs either hot or cold. So on a day when it's 50F in the morning and 80F in the evening, you system is still running 1 or the other. So some people are always uncomfortable, esp when one on the sunny side wants AC and one on the shady side wants heat.
The 4 pipe system solves that problem by providing everyone with heat and cooling and uses controls and fans to provide 1 or the other. Near the set temp you should get neither. It should hold the room at about the temp you set.
In our case, it wasn't really wasting heat or contributing to pollution because both the heat and the cooling were on district loops. The heat came from excess heat at a local power generation facility. The cooling came from another industry. But if we didn't use it, it would have been wasted into the air. So it really wasn't inefficient.
As for other buildings, so have quite complex systems that in mild temperatures act like heat pumps. An AC doesn't magically create cold, it removes heat from the air and blows the heat outside via the coil. Some large buildings use the same concept. The run a system that removes the heat from the air, puts it into one system to provide heat to the building and uses the cold for the cooling loop. It then uses supplimental heating and cooling when it gets beyond the limits of what the system can handle.
You also have to remember that large buildings, unlike houses, have "make up" air that gets introduced into the system so you get a certain percentage of fresh air. Usually makeup air gets introduced in the hallway. When you turn on the exhaust fan or whatever, the air goes intot he unit for make up for what you are venting. The makeup air needs to be heated/cooled as part of the buildings mechanicals. Often these use heat exchangers to try to use some of the heat in the vent air to pre-heat the incoming air.
Large buildings are very complex beasts. But they have good staffs and consultants to make sure they aren't wasting money. So if they are doing something, there's usually a reason for it.
TEK1 wrote:

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Rare that people don't run the shower at 100%. One factor in the design is safety. You get to temperature gradually. Water should be adjusted so that it is not so hot that 100% hot will scald you. Many new shower controls have safeguards for a maximum temperature. I'd bet there is realy very little waste, if any, with that type of setup.

If it is a true environmental setup, it is not just controlling temperature. Air has to be filtered and cleaned, humidified or dehumidified, depending of the season and location. Your co-workers are farting all day long and unless the air is freshened, you get to breathe their farts all day. It may or may not be the AC running, but blowers bringing in fresh air as needed. In large buildings, it is possible that some areas need heat at the same time as others need cooking because of variables like sun load, people's body heat, etc.
My first experience with a large AC system was in a printing plant. The AC was not put in for the comfort of the workers, but for the control of the air for the paper. It was often used to control humidity by running the huge chillers and the boilers at the same time. Similar to the defrost cycle on many automobiles that turn on the AC for a quick moisture removal.
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Sounds dumb. If a house needs dehumidification, we might run an AC if it also needs cooling or a dehumidifier if it also needs heating, but what do we do if it needs neither? Maybe run both, but not for long.
Nick
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